NGUEGANG WAMBA ERIC
Dissertation submitted to obtain the Master’s Degree in
AEROSPACE ENGINEERING
performed at : CRANFIELD UNIVERSITY School of Engineering Department of Aerospace Flight Dynamics
Cranfield Supervisor Pisa Supervisor Dr. ALASTAIR COOKE Prof . EUGENIO DENTI
NGUEGANG WAMBA ERIC
Dissertation submitted to obtain the Master’s Degree in
AEROSPACE ENGINEERING
performed at : CRANFIELD UNIVERSITY School of Engineering Department of Aerospace Flight Dynamics
Cranfield Supervisor Pisa Supervisor Dr. ALASTAIR COOKE Prof . EUGENIO DENTI
ACKNOWLEGMENTS
This thesis project was undertaken at Cranfield University, School of Engineering and sponsored by the Erasmus Programme. It is submitted in fulfillment of the requirements for the Master’s degree in Aerospace Engineering at Pisa University .
I would like to take this opportunity to express my gratitude to the following people for their contribution in making it a success:
Thesis Supervisor and Senior Lecturer in Flight Dynamics at Cranfield University, Dr. Alastair Cooke, who suggested the project and offered assistance and guidance.
Professor Eugenio Denti, my Supervisor at Pisa University for his support throughout the duration of the course (I will never forget his enthusiastic teaching and devotion to solving issues that arose).
I additionally wish to mention my friends, Leonardo Pino and Arsene Romeo Njousse, for their continuing support.
Finally, a special “thank you” goes to the Associazione Sante Malatesta (and its President, Professor Giuseppina Barsacchi in particular) for the contribution it has made to my development since first embarking on a course of academic study in Pisa.
TABLE OF CONTENTS ... IV LIST OF FIGURES ...VI LIST OF TABLES ...VIII GLOSSARY OF ABBREVIATIONS ...IX
1. INTRODUCTION ... 1
1.1 THESIS OVERVIEW ... 1
1.2 AIMS OF THE CASE STUDY ... 2
1.3 THESIS STRUCTURE... 3
2. LANDING MANEUVER ... 5
2.1 LANDING PHASES ... 5
2.1.1 The base leg ... 7
2.1.2 The final approach ... 8
2.1.3 The Flare maneuver ... 11
2.1.4 The Touchdown ... 13
2.1.5 The landing roll ... 14
2.2 LANDING ACCIDENTS ... 15
2.2.1 Accidents prior to touchdown ... 15
2.2.2 Accidents at the instant of touchdown ... 16
2.3 STABILIZED APPROACH CONCEPT ... 18
2.3.1 Common errors in the performance of normal approaches [4] ... 19
3. HARD LANDING MONITORING ... 20
3.1 REVIEW OF HARD LANDINGS ... 20
3.1.1 Definition... 20
3.1.2 Causes ... 21
3.1.3 Identification ... 22
3.1.5 Inspection and Maintenance in case of Hard Landing ... 23
3.2 PARAMETERS FOR HARD LANDINGS SEVERITY ASSESSMENT ... 24
3.3 METHODS TO COMPUTE EARTH VERTICAL ACCELERATION AND SPEED ... 27
3.3.1 Method to computer earth vertical acceleration ... 27
3.3.2 Methods to compute earth vertical speed ... 28
3.3.3 Summary of the analysis ... 32
3.4 THE NATIONAL FLYING LABORATORY CENTRE ... 33
3.5 THE NFLC MOTIVATION FOR THE LANDING SEVERITY MONITORING ... 39
4. SYNTHESIS OF THE LANDING SEVERITY MONITOR... 41
4.1 VELOCITY AND ACCELERATION IN ARBITRARY MOVING FRAME ... 41
4.2 OVERVIEW OF THE LANDING DATA RECORDED ... 44
4.3 THE MATLAB CODE ... 46
4.3.1 The IRS relative position to the mass center ... 46
4.3.2 Aircraft mass at touchdown ... 48
4.3.3 Right and left landing gear earth vertical speeds ... 50
4.4 TOUCHDOWN DETERMINATION ... 51
4.4.1 Previous approach ... 51
4.4.2 New approach ... 53
4.5 IMPLEMENTATION ON BOARD ... 65
4.6 MATLAB IMPLEMENTATION ... 68
4.7 MATLAB CODE OUTPUTS ... 71
5. CONCLUSION... 72
5.1 SUMMARY ... 72
5.2 CONCLUSION ... 73
6. REFERENCES ... 75 7. APPENDICES ... 76
7.1 MATLAB CODE ... 76 7.2 GRAPHS OF FLIGHT ’LANDING_WED120709’’:CG EARTH VERTICAL SPEED AND
LIST OF FIGURES ...VII Figure 2.1 Phases of landing
Figure 2.2 Phases of landing , top view Figure 2.3 Base leg
Figure 2.4 Final approach
Figure 2.5 Forces acting on the airplane at landing Figure 2.6 Flare maneuver
Figure 2.7 Touchdown of Boeing 737-300 of Lufthansa company Figure 2.8 Undershoot A and Overshoot
Figure 2.8 Case of tail strike Figure 2.9 Case of hard landing
Figure 3.1 Abnormal contact with the runway of A380-841 of Lufthansa company Figure 3.2 Landing severity graph , Airbus
Figure 3.3 Relevant parameters for landing severity, EASA Figure 3.4 Earth vertical acceleration computation method Figure 3.5 Earth vertical speed computation method (1)
Figure 3.6 Confrontation of recorded and computed vertical speed (1) Figure 3.7 Earth vertical speed computation method (2)
Figure 3.8 Confrontation of recorded and computed vertical speed (2) Figure 3.9 The SA bulldog series 120
Figure 3.10 The BAE Jetstream31
Figure 3.11 Jetstream31 flight test cabin Figure 3.12 Main Oleo Leg
Figure 3.13 GPS and IRS location on the Jeststream31
Figure3.14 Effects of flow disturbance on altimeter measurement
Figure 3.15 Step profile of the GPS altitude due to a low update frequency Figure 3.16 Jetstream31 landing gear accident in Greece, on February 2009 Figure 4.1 Arbitrary moving frames : Earth and body frame.
Figure 4.2 Position of CG (x and z component) versus fuel mass Figure 4.5 Extract of [4], determination of the touchdown instant Figure 4.6 Extract of [4], recording of a turbulent approach Fri113305
Figure 4.7 Critical points on aircraft’s flight path in impact zone Figure 4.8 Algorithm for the first critical determination
Figure 4.9 Landing gear wheel altitude for the touchdown determination Figure 4.10 Identification of the first critical point
Figure 4.11 Wrong critical point
Figure 4.12 Critical points at touchdown
Figure 4.13 Time interval for touchdown instant investigation Figure 4.14 Algorithm of the first touchdown instant determination
Figure 4.15 Landing gear wheel altitude at the first critical instant and the
touchdown .
Figure 4.16 Right and left earth vertical speed at the critical instant and the
touchdown
Figure 4.17 Graphical and structural interfaces of Labview Figure 4.18 Example of results displaying
LIST OF TABLES ...IX Table 3.1 Parameters recorded by the IRS and GPS on board the Jetstream31 Table 4.1 Fuel load – CG location and Inertias about body axis
Table 4.2 Implementation solutions
GLOSSARY OF ABBREVIATIONS ...X AAIB Air Accidents Investigation Branch AMM Aircraft Maintenance Manual BEA Bureau d’Enquêtes et d’Analyses CG Center of mass of the aircraft EASA European Aviation Safety Agency EOFDM European Operation Flying Data Monitoring g Earth acceleration Gear_alt Wheel altitude corrected with pitch angle Gear_alt_co Wheel altitude corrected with pitch and bank angle Gps_alt GPS altitude H Vertical distance between the GPS and the wheels L1 Longitudinal distance between GPS and wheels L2 Longitudinal distance between IRS and wheels Lg Lateral position of the landing gear wheel LSM Landing severity monitor NFLC National Flying Laboratory Center NTSB National Transportation Safety Board P Roll rate in body axis [rad/s] q Pitch rate in body axis [rad/s] r Yaw rate in body axis [rad/s] time_td Touchdown instant [s]
Earth vertical speed of the IRS [ft/min] Earth vertical speed of the CG generic position [ft/min]
Earth vertical speed of the landing gear left wheel [ft/min] E arth vertical speed of the landing gear right wheel [ft/min]
w Actual fuel mass [kg]
Longitudinal position of the IRS ( ft) Longitudinal position of the CG (ft)
Longitudinal position of gear (ft) Lateral position of the IRS (ft) Lateral position of left gear (ft)
1. INTRODUCTION
1.1 Thesis overview
Almost half of all aviation accidents occur at the landing phase, making it one of the most critical for pilots and operators.
Among landings accidents, hard landings, (which are defined variously as (a) abnormal contact with a runway; (b) abnormally high vertical speed at the moment of impact; or (c) landings that exceed limit loads in aircraft specifications), account for the highest number of accidents involving large commercial aircraft globally, large financial losses, and in some cases may result in catastrophic events.
The aim of this thesis was to develop a system that uses a suite of on-board sensors to evaluate the severity of a landing and provide a post-landing report that can be used to trigger maintenance actions .
In order to achieve this aim, a combination of signals from a LTN-90 IRS and GPS receiver were used.
This paper includes a definition of a landing severity monitor and investigates the methods and parameters that are needed to synthesize and implement these devices.
1.2 Aims of the case study
The majority of civil aircraft today are fitted with a landing severity monitor which is usually designed with inputs from an IRS (Inertial reference system) as well as sensors so that it can be used to assess flight parameters at the moment of touchdown, for evaluating the severity of landing.
By way of example, on Airbus A320 and A321 aircraft, the LSM will generate a technical report named a “Load Report 15” when landing gear overload is detected which, in turn, requires an inspection to be performed according to the Aircraft Maintenance Manual (AMM).
(Indeed, even when no overload is detected, an inspection must be
undertaken before the next flight departure if a pilot judges a landing to be hard). Landing severity monitoring is a vast new area within the aerospace
industry, however, there is no standardized approach to manufacturing these devices as there are no universally accepted principles upon which their design can be based.
The use of sensors is common in all such devices. Their role is to determine when an aircraft makes contact with a runway. However, these sensors require periodic inspection and maintenance which is expensive; By contrast, light aircraft will rarely be fitted with these devices due to a high maintenance cost, hence the need to find ways of conceiving landing severity monitors that do not require sensors.
1.3 Thesis structure
The aim of the thesis is to achieve the following objectives:
Identify the parameters to be taken into account for the landing severity assessment
Compute these parameters from IRS and GPS outputs, via a Matlab algorithm
Identify the presumed instant of touch down and deduce values of relevant parameters for landing severity assessment
Make the algorithm available aboard the aircraft, in order to run it just after landing
To that end the thesis has been structured as follows:
Chapter 2 Landing maneuver
Landing is a complex maneuver that requires special attention from pilots as errors can result in unanticipated consequences (such as hard landings). The chapter explains the landing procedure with reference to the actions pilots must undertake to perform a secure landing.
Chapter 3 Hard landing monitoring
The Chapter investigates about hard landing to find out methods and parameters currently used within the industry to synthesize the LSM. Furthermore, it introduces the National Flight Laboratory Centre (NFLC), department within Cranfield University, where the project was born.
Chapter 4 Synthesis and implementation of the device
The first part of the chapter defines the structure and describes the
different parts of the Matlab code which is used to assess landing severity . The second part deals with the implementation of the code aboard the aircraft.
Chapter 5 Conclusion
The concluding Chapter is accompanied by further recommendations for improvement.
2. LANDING MANEUVER
In this chapter the principles of normal landing operations will be explained in order to enable an understanding of the factors that influence pilot judgment. A normal approach and landing will typically occur when:
Engine power is available
The wind is light or the final approach is made directly into the wind The final approach path has no obstacles
The runway is firm and of sufficient length to gradually bring the aircraft to a stop.
2.1 Landing phases
The actual landing is divided into five phases: The base leg
The final approach
The flare maneuver
The touchdown
Figure 2.1 Phases of landing , Extract from [6]
2.1.1 The base leg
Figure 2.3 Base leg and approach phase , Extract from [6]
Generally when an aircraft approaches an airport, the pilot has to perform a turn to align the longitudinal axis of the airplane with the runway centerline. However, this turn must be achieved at a set altitude and distance from the runway and end at a determined point that is generally referred to as the starting point.
The positioning of this point is the most important judgment the pilot has to make during the landing phase, as a poor decision can easily result in an accident when there is insufficient elevation to permit a final approach that will allow the point of touchdown to be estimated accurately.
2.1.2 The final approach
Figure 2.4 Final approach
There are essentially three pilot objectives during the final landing approach: o To maintain the flight path, which means keeping the longitudinal axis of
the aircraft aligned with the runway centerline throughout the approach, by recognizing and countering any wind drift immediately .
o To control the angle and rate of descent throughout the approach so that the aircraft will land in the center of the first third of the runway at an acceptable sink rate.
o To achieve an airspeed that will result in a semi-stalled condition just before touchdown.
The equations of motion at landing, from the approach phase are:
Figure 2.5 Forces acting on the airplane at landing
Let’s indicate with the trust available, D the drag , W the landing weight (Gravity) , L the lift , ɣ (Gamma) the flight path angle and V the airspeed.
– D = Wsin ɣ (a) L = Wcos ɣ (b) = Vsin ɣ (c) = Vcos ɣ (d)
For small descent angle, the expressions of descent angle and rate are as follows:
ɣ = (1) = . (2)
The descent angle is affected by four fundamental forces that act on an aircraft : lift , drag, thrust, and weight.
When all these forces are constant, the angle of descent will be constant in a no-wind condition. However, ideal conditions are rarely present, with pilots having instead to make adjustments that take account of the effect of wind on aircraft.
Indeed, wind direction and speed influence the gliding distance and therefore also contributes to the complexity of landing maneuver.
The pilot will constantly control:
o Flaps - to control the drag which means controlling ɣ & o Power - to control the trust which means controlling ɣ & o Angle of the rudder - to control any wind drift
2.1.3 The Flare maneuver
Figure 2.6 Flare maneuver
The flare is the slow transition from a normal approach altitude to the runway altitude, and consists of gradually rounding out the flight path until the airplane touches down on the ground.
The difficulty of this phase is to satisfy the equations of motion (a) and (b) and simultaneously the two touchdown requisites below.
Equations of motion
– D = Wsin ɣ (a) L = Wcos ɣ (b)
Touchdown requisites
o Bring engine power to idle
o Bring the aircraft to a minimum controllable airspeed so that the aircraft will touch down on the main gear at approximately stalling speed.
The pilot will be controlling:
o The equilibrator to increase at a correct rate the angle of attack and therefore the pitch angle.
o The Thrust to bring the engines power to idle.
The first command causes the lift to increase momentarily and consequently the rate of descent to decrease.
The second command cause the airspeed to decrease gradually, and consequently the lift to decrease .
The two commands are meant to be executed at a rate that permits the proper pitch angle and touchdown speed to be reached simultaneously so that the wheels will make contact with and settle gently on the landing surface.
The difficulties of this phase are:
- Determining the proper rate to perform the maneuver - Controlling lift and therefore the airspeed
- Touching down at the minimum controllable airspeed, which means not getting close to the stall either too soon nor too late, but right at the moment of runway contact.
2.1.4 The Touchdown
Figure 2.7 Touchdown of Boeing 737-300 of Lufthansa company
The touchdown is the settling of the aircraft onto the landing surface and (as already mentioned) should be made with:
o the engine idling
o the airplane at minimum controllable airspeed, so that the airplane will touch down on the main gear at approximately stalling speed.
It is extremely important that touchdown occur with the airplane’s longitudinal axis exactly parallel to the direction in which the airplane is moving along the runway. Failure to accomplish this imposes severe side loads on the landing gear.
2.1.5 The landing roll
Once the aircraft has touched down, the pilot gradually puts the maximum weight on the wheels to obtain an optimum braking performance, while observing a set of recommendations to gradually bring the aircraft to a complete stop.
However, that is not relevant for the purpose of this Chapter which has focused on explaining the complexity of the landing maneuver before touchdown and figuring out the factors that might lead to a landing accident.
2.2 Landing accidents
Accidents at landing phase may occur either before, during or after the aircraft’s impact with the runway.
2.2.1 Accidents prior to touchdown
The kind of accident that can happen before impact with the runway are generally undershoot and/or overshoot and cross-control stalls.
Undershoot/Overshoot
When a pilot miscalculates his approach, veers far away from the landing path and realize the aircraft might not make it to the runway because of its distance from the centerline or because it is improperly positioned.
Cross-control stalls
Stalls are a frequent cause of landing incidents and the most dangerous of all is the cross-control stall. They are generally caused by violent
movements of wind during landing, which in turn, may induce a significant airspeed component in the aircraft direction, resulting in aircraft’s wings stall. During a cross-control stall pilots have less time to react because a tailwind increases ground speed.
2.2.2 Accidents at the instant of touchdown
The most common problems encountered at touchdown are tail strike and hard landings . These are not as dangerous as cross-control stalls and undershoot but can nevertheless result in substantial damage, injuries, and other catastrophic events.
Tail strikes
The situation when an aircraft’s tail touches the runway during landing is referred to as tail strike and is more common with longer aircraft where geometry influences pitch attitude.
Hard landings
When the aircraft makes contact with the runway during the landing phase, its vertical speed is instantly reduced to zero; Therefore provision must be made to slow this vertical speed, to manage the impact of touchdown so that the force of contact with the ground does not result in structural damage to the aircraft.
As main focus of this project, these particular landing accidents will be defined and investigated in the next chapter.
2.3 Stabilized approach concept
The concept of the stabilized approach is informed by the theory that a pilot should establish and maintain a constant glide path towards a predetermined point on the landing runway, by adjusting the glide path so that the true aiming point and actual touchdown point coincide.
Factors that influence the stability of an approach and that the pilot is constantly monitoring during landing phase include:
Crosswind
The design and location of many runways often requires aircraft to be landed when the wind is blowing cross-ways rather than in a parallel direction.
Pilots must therefore be equipped to correct wind drifts as it arise Turbulent air
Turbulent air requires a similar approach to the one adopted for crosswinds. However it requires less pitch adjustment to establish landing attitude, and touchdown takes place at higher airspeeds.
Nevertheless, pilots must ensure airspeed is not excessive as it may result in aircraft floating when they reach the desired landing area.
Ground effects
Ground effect is an important consideration for fixed-wing aircraft . It generally involves an apparent increase in performance as a result of induced drag in the area affected by ground effect.
Pilots may not fully appreciate its potential impact on the responsiveness of an aircraft and this may be the cause of the control loss.
2.3.1 Common errors in the performance of normal approaches
In summary, any or a combination of the following errors may result in a landing accident:
Inadequate wind drift correction on the base leg.
Overshooting or undershooting the turn onto final approach resulting in too steep or too shallow a turn entering the final approach.
Failure to complete the landing checklist in a timely manner. Failure to adequately compensate for wind drift .
Poor trim technique on final approach.
Attempting to maintain altitude or reach the runway using elevator alone. Touching down prior to attain proper landing attitude.
3. HARD LANDING MONITORING
3.1 Review of hard landings3.1.1 Definition
There is no universal definition of a hard landing. The International Civil Aviation Organization (ICAO) assigns “Event Code 263” to any report of a hard landing by member states. However, the Code is not defined formally which explains why there is no standardized methodology for determining the severity of a landing.
Here below are three definitions of hard landing:
Event categorized as involving abnormal runway contact (see Figure 3.1 below)
A landing that exceeds limit landing loads specified in European Joint Airworthiness Requirements (JAR) and US Federal Aviation Regulations (FAR) in transport category airplane certification requirements.
In the US, the NTSB coding manual defines hard landing as those involving
abnormally high vertical speeds.
The picture below is an example of what could be a hard landing; indeed the contact with the runway is abnormal, as the bank angle is too high, causing the right landing gear wheel to withstand alone the aircraft’s weight at touchdown.
Figure 3.1 Abnormal contact with the runway of A380-841 of Lufthansa company
3.1.2 Causes
As has already been seen, hard landings can result from a non–stabilized approach.
More explicitly, they are typically a result of: Inappropriate approach speed
Destabilization of the approach in the last 100 ft Incorrect flare techniques
3.1.3 Identification
Flight crew play an essential role in identifying hard landings and are even required to brief maintenance and inspection personnel about the quality of each landing.
LSM are a more reliable source for identifying hard landings – particularly in long aircraft and ones with complex landing gear systems ( flexible structures can influence perceptions of even experienced pilots). Airbus recommends routine examination of the Load Report 15, generated at each landing by its version of the device.
3.1.4 Effects on aircraft
At touchdown, an aircraft’s kinetic energy is dissipated by the landing gear, primarily by struts, typically filled with oil that is forced at a controlled rate through an orifice when they are compressed.
The kinetic energy at landing depends on the landing gross weight and approach speed and explains why pilots must jettison excess fuel prior to landing when an aircraft exceeds the maximum landing weight.
Load placed on landing gear increases as the square of any increase in the vertical rate of descent.
- F ∞ Load on landing gear at touchdown
This means that 20 percent increase in the vertical speed increase the landing load factor by 44 percent.
Landing gear are designed to withstand greater landing loads than those required for certification; consequently loads not dissipated are typically transferred to the landing gear support structure (the wings spar, fuselage structure and skin) – one of the reasons why an inspection must be performed to ensure the safety of these structures.
3.1.5 Inspection and Maintenance in case of Hard Landing
The following is an extract from a presentation delivered in 2007 by Captain
Parisis (who will be introduced below) , during a conference sponsored by
Airbus, exposing the company two-stage approach to inspection and maintenance of any aircraft on receipt of hard landing reports.
Phase (1) general inspection for primary damage and indication of
remote damage
If damage = perform further inspection and required maintenance actions.
If no damage = end of inspection == Phase (2)
Phase (2) No damage = Aircraft can temporarily return to service
Observation period (e.g. 30 days)
Waiting for further elaborated analysis of the event by Airbus Required structural strength is maintained
3.2 Parameters for hard landings severity assessment A first insight on hard landing monitoring was provided by
Capt. Marc PARISIS, head of Flight and Cabin Crew Training from Airbus company at a conference on the subject ‘’Avoiding hard landings’’ on April 2007 at Puerto Vallarta
This conference was focused on parameters for landing severity assessment and methods to avoid hard landings.
Figure 3.2 shows a graph where two parameters are used to assess landing severity:
- vertical acceleration ‘’VERT ACC’’ - vertical speed ‘’VERT Speed’’
Figure 3.2 Landing severity graph , Airbus
As shown on the figure, the limit values will depend on aircraft type and the quality of recorded parameters.
The last condition is due to the fact that devices used to record these parameters are affected by measurement errors and obvious imperfections (for example, vertical speed is measured using Pitot-static tubes based on pressure changes that is altered as an aircraft approaches the ground).
Further insight into how the industry is tackling landing monitoring was provided by GUILLAUME AIGOIN , expert in Safety Analysis and Research Department in EASA, at a conference on the subject
‘’Characterizing hard landings’’ on 12 January 2012
The conference was focused on parameters for landing severity assessment and particularly on how these parameters could be determined. Indeed, the relevant parameters for assessing the landing severity are:
The aircraft mass at landing
The vertical speed right before touchdown
The true vertical acceleration right before touchdown
Figure 3.3 Relevant parameters for landing severity, EASA
He also referred to Certification Specifications 25 which defines vertical speed thresholds, independently of the aircraft mass and provided ways to compute theses parameters.
3.3 Methods to compute earth vertical acceleration and speed
Vertical acceleration and speed can either be computed or recorded, although in either case there is a margin of error due to imperfections inherent in measuring instruments and computers (i.e. altimeter, GPS etc).
These methods and parameters are provided below and for computed vertical speeds, results are compared against registered vertical speed.
3.3.1 Method to computer earth vertical acceleration
3.3.2 Methods to compute earth vertical speed
Figure 3.5 Extract from [2] , Earth vertical speed computation method (1) Objective
The first method consists of deriving pressure altitude to obtain the vertical speed, but this method is affected by errors in pressure altitude measured by the altimeter - especially when the aircraft is in the vicinity of earth surface.
Figure 3.6 Extract from [2] , Comparison of recorded and computed vertical speed
(1)
Analysis
The computed vertical speed curve (in rose) is similar to the recorded one (in green) despite the significant impact of oscillation on results when the aircraft is near the runway.
Indeed, the mere act of deriving signals is known to emphasize the noise inherent to the signal derived, and the discrepancy of computed speed after 60 seconds is due to changes in air flow (and therefore in pressure) around the aircraft when it is close to the runway [i.e. ground effects].
Figure 3.7 Extract from [2] , Earth vertical speed computation method (2) Objective
The second method consists of integrating acceleration data computed above to obtain vertical speed.
The method used to achieve this is shown on Figure 3.6 and implies the selection of a sampling time that defines the degree of accuracy of the final result.
Nevertheless, this sampling time [T] is limited by the processor of the computer.
Figure 3.8 Extract from [2] , Comparison of recorded and computed vertical speed
(2)
Analysis
The above graph shows how similar both speeds curves are, and also how the margin of error between both curves keeps on increasing with time and therefore making the second method unreliable for vertical speed computation. Margins of error are determined by bias inherent in the input and the continued increase is due to the operation of integration.
3.3.3 Summary of the analysis
From the previous analysis, it is clear that vertical speed and acceleration are the main parameters used to assess landing severity.
Various ways of computing these parameters have been presented but have demonstrated that it is preferable to use new generation flight data processors to compute parameters, for greater accuracy.
Indeed the IRS installed on board the Jestream31 is capable of recording earth vertical acceleration and speed in particular - which are the two parameters needed to design the landing monitor.
3.4 The National Flying Laboratory Centre
The National Flying Laboratory Centre (NFLC) is a department within Cranfield University specialized in aerospace research and teaching.
The teaching activity focuses on flight measurement which allows to demonstrate the practical aspect of theoretical teaching in aeronautical engineering courses. To conduct its activities, the NFLC has two aircraft:
The Scottish Aviation bulldog series 120
A two-seater aircraft generally used for training purposes of postgraduate students and often to perform acrobatics maneuvers.
The British Aviation Jetstream31
The Jetstream31 is mostly used by undergraduate students enrolled on masters’ courses in aerospace engineering for performing flight tests. The aircraft has been identified as a candidate for upgrading with the landing severity monitoring device, and is fitted with a LTN-90 inertial reference system (IRS) and GPS receiver which both provide all the parameters needed for designing the landing monitor.
The Jetstream31 contains 18 seats, each of which is fitted with an individual screen on the back. These screens enable to display parameters recorded during various flight tests, allowing real time experience of flight dynamics.
Figure 3.11 Jetstream31 flight test cabin
The Landing gear
The landing gear of the Jetstream31 is an oleo-pneumatic design, with brakes. It is made with two tubes, sliding into one another and two chambers :
- the air chamber which acts as a spring
GPS altitude
Figure 3.13 GPS and IRS location on the Jeststream31
The GPS altitude measurement was used in the project because it is less affected by errors than altimeter altitude data. In fact the pressure altitude computation is based on pressure measurement, but the pressure can be affected by many factors that generally compromise data output. This is shown on figure 3.13 where, at around 50 seconds, the change in flow around the aircraft as reverse pitch is selected causes an increase in pressure altitude which then affects the value of the pressure altitude .
The GPS altitude is measured by a GPS antenna that is mounted on the upper part of the fuselage as shown on figure 3.12
The data measured are sampled at an average frequency of 25 Hz and updated at an average frequency of 1 Hz which explains the step profile of the GPS altitude data shown in figure 3.14 below.
Figure 3.14 Effects of flow disturbance on altimeter measurement
3.5 The NFLC motivation for the landing severity monitoring
During a preventative maintenance inspection of the Jetstream31 in January 2010, a crack was detected on the right landing gear of the aircraft. The size and inconvenient positioning of the crack meant there was no other option but to replace it in its totality.
The crack was old generated and probably the result of a heavy landing but could not be identified because there was no device capable of assessing landing severity. Over time, its dimensions had increased until its final identification during a routine maintenance inspection.
The NFLC has therefore been interested in finding an accurate way for deducing the aircraft’s touchdown parameters and comparing them with manufacturer’s specifications.
The next figure reveals a more developed occurrence of the same problem that caused an accident in Greece in 2009.
4. SYNTHESIS OF THE LANDING SEVERITY MONITOR
4.1 Velocity and acceleration in arbitrary moving frameIn this thesis two frames were considered; namely, the earth and body frame. All velocities and accelerations were computed with respect to the earth frame because the landing severity proper, is evaluated in relation to the earth. It was then necessary to go through adequate transformations between the body and the earth frames, as explained below.
Within the context of the thesis, the earth frame (E, ) is considered inertial and the body frame (O,X,Y,Z) a moving one, with origins in O aircraft mass center and with angular velocity ῳ⃗ . P is an arbitrary point on the aircraft. indicates the earth frame and the body frame.
Position vectors of P ⃗ = ⃗ + ⃗
⃗ position vector of P in ⃗ position vector of O in ⃗ position vector of P in
Velocity and acceleration of P in the earth frame
⃗ = ⃗ + ⃗ + ῳ⃗^ ⃗
⃗ = ⃗ + ⃗ + ῳ̇⃗^ ⃗ + 2ῳ⃗^ ⃗ +ῳ⃗^ῳ⃗^ ⃗ Where:
⃗ velocity vector of P the aircraft relative to ⃗ acceleration vector of P relative to
⃗ velocity vector of the aircraft mass center relative to ⃗ velocity vector of P relative to
⃗ acceleration vector of P relative to
Particular case ⃗ ≅ 0 ,
Then ,
⃗ = ⃗ + ῳ⃗^ ⃗
⃗ = ⃗ + ῳ̇⃗^ ⃗ + ῳ⃗^ῳ⃗^ ⃗
Let’s denote ῳ⃗ = (p, q , r) , ⃗ = (x ,y , z) and ῳ̇⃗ = ( ṗ, q̇ , ṙ ) Therefore, it follows that:
The velocity components of P are given by:
= + qz – ry (a1) = + rx – pz (b1) = + py – qx (c1) Acceleration components = + x( + ) – y(pq +ṙ) - z(pq - q̇) (a2) = - x(pq - ṙ) + y( - ) + z(qr - ṗ) (b2) = - x(pq - q̇) - y(qr +ṗ) - z( + ) (c2)
For the purpose of the thesis, equations (c1) and (c2) will be used to determine the earth vertical acceleration and earth vertical speed of the landing gear at touchdown.
4.2 Overview of the landing data recorded
For each landing IRS and GPS record data (generally) from 1000 ft above the runway are recorded and written into CSV files. The matrix of numerical data contains 40 columns which correspond to 40 flight parameters (35 registered by the IRS and 4 by the GPS). The time vector is independent .
The aim of the Matlab code that will be presented below is to:
Compute the earth vertical speed and acceleration of the CG and the right and left main landing gear
Determine the presumed instant of touchdown
Deduce the values of relevant parameters for landing assessment
Column Vector
Name Explanation
1 Pitch Att Body Pitch angle
2 Roll Att Body Bank angle
3 Body Pitch Rate Body Pitch angle rate
4 Body Roll Rate Body Bank angle
5 Body Yaw Rate Body Yaw angle rate
6 Body Norm Acc Body normal acceleration
7 Body Lat Acc Body lateral acceleration
8 Body Long Acc Body longitudinal acceleration
9 Flt Path Angle Body Flight path angle
10 Pot Vert Speed Geopotential vertical speed
11 Earth Vert Accel Earth Vertical acceleration
12 Earth Vert Speed Earth Vertical speed
13 True Trk True track angle
14 True Hdg True heading
15 Flight Path Accel Flight path acceleration
17 Earth Pitch Att Rate Earth Pitch angle rate
18 Earth Roll Att Rate Earth roll angle rate
19 Earth Along Trk Horiz Acc Track longitudinal angle acceleration
20 Earth Cross Trk Horiz Acc Track lateral angle acceleration
21 IRS Status IRS Status
22 Altitude Pressure altitude
23 IAS Indicated air speed
24 TAS True air speed
25 OAT Outside air temperature
26 Elev Elevation
27 Flap Flap angle
28 TqL Left Torque
29 TqR Right Torque
30 GPS Alt GPS Altitude
31 GPS Lon x 1000 GPS Longitude x1000
32 GPS Lat x 1000 GPS Latitude x1000
33 GPS Track GPS Track angle
34 GPS GS GPS Ground Speed
35 Port Thr Right Trust
36 Stbd Thr Left Trust
37 Time Registration Time
38 Fuel Fuel Consuming
39 Gear Alt Rate Gear Vertical Speed ( Not Recorded – Unknown)
40 Rudder Rudder angle deflexion
41 Aileron Aileron angle deflexion
4.3 The Matlab code
To compute the vertical speed and acceleration of the CG, the equations (c1) and (c2) determined before was used , assuming the hypothesis ⃗ ≅ 0 verified as the fuel consumption rate is low.
= + py – qx (c1) = - x(pq - ̇ ) - y(qr + ̇ ) - z( + ) (c2) Since the earth vertical speed and acceleration of the IRS are known, by using the equations above, the earth vertical speed and acceleration of any point of the aircraft - in particular the mass center - could easily be computed. For this purpose, the only unknown is the relative position between the IRS and the mass center:
4.3.1 The IRS relative position to the mass center
The position of the CG with respect to the IRS naturally varies with fuel consumption. The fuel is stored in a set of wing tanks of rather complex shape, however, the type record for the aircraft gives a tank-by-tank breakdown for the maximum weight configuration. Using this data the variation in CG location and inertia can be determined as a function of fuel mass as shown on table 4.1 . The passengers and crew are situated at fixed locations within the fuselage .
Data provided by the type record was then used to plot in Excel the CG position on the aircraft in term of x and z component, versus the fuel mass (y component was assumed equal to zero).
Table 4.1 Fuel load – CG location and Inertias about body axis, Extract in[9]
Figure 4.2 Position of CG (x and z component) versus fuel mass
y = -6E-07x2+ 0,0017x + 17,694 R² = 0,9925 y = -0,0007x + 2,5003 R² = 0,9965 0 0,5 1 1,5 2 2,5 3 17,6 17,8 18 18,2 18,4 18,6 18,8 19 0 200 400 600 800 1000 1200 1400 1600 Fuel Mass (Kg) Xcg (ft) Zcg (ft) Poli. (Xcg (ft)) Lineare (Zcg (ft))
The position of the CG , IRS and GPS all refer to the CG datum (a fixed referential point on the aircraft) as shown in Figure 4.3.
IRS and GPS positions are provided by the aircraft manual.
4.3.2 Aircraft mass at touchdown
Analysis of fuel consumption data from commencement of recording to when the aircraft reaches the ground, produced an average figure of 4.5 kg which implies that, in order to approximate the fuel mass at touchdown (and therefore the CG position), it is necessary to subtract 4.5 kg from the fuel mass at the start.
≅ - 4.5
Where,
presumed aircraft mass at touchdown aircraft mass at the beginning of landing
Figure 4.4 IRS and CG relative position upward view
At this stage knowing the CG and IRS position, respectively ( , 0 , ) and ( , , ) with respect to the CG datum, the relative distance between both points at touchdown, expressed in components x, y, z will be:
x = – y = z = –
4.3.3 Right and left main landing gear earth vertical speeds
Let us indicate with and the earth vertical speed and acceleration of
the IRS, and recounting the equations (c1) and (c2), the earth vertical speed and acceleration of the CG will be :
= +( py – qx )*60 (c1) = + [- x(pq - ̇ ) - y(qr + ̇ ) - z( + ) ]/32,17 (c2) Note:
is expressed in ( ft/min ) and the parameters in brackets are in ft/s and in g units and the parameters into brackets is in ft/ .
Knowing the CG earth vertical speed, the earth vertical speed of each landing gear wheel can be determined by using equation (c1) and the relative position between the CG and each respective wheel.
= + ( p* - q* ) *60 = + ( p* - q* )*60
where,
is the lateral position of the left wheel is the lateral position of the right wheel Of course , in component = - ( > 0)
= – is the relative longitudinal distance between the CG and the wheels , with representing the distance of the wheels from the CG datum. Angle rate data are expressed in rad/s which explains why the terms in brackets are multiplied by 60 express vertical speeds in ft/min.
4.4 Touchdown determination
4.4.1 Previous approach
The determination of the touchdown instant has already been studied by a previous student, working on the same project. The method he used was a post processing of the vertical acceleration data recorded during the landing, as represented in the following diagram:
Figure 4.5 Extract of [4], determination of the touchdown instant
The algorithm creates a matrix with acceleration maximums recorded during the landing phase with the following conditions:
The altitude must be less than 400ft
the flight path angle must be more than 0 degree.
At the end the highest peak value is supposed to correspond to the instant of touchdown. However, from the analysis of some vertical acceleration recordings taken during landing, the conclusion was reached that the algorithm was not reliable (by way of illustration, the highest peak value can represent either touchdown or a strong gust of wind experienced during final approach). As a consequence, this algorithm could result in the identification of an instant that is
far away from that of touchdown; a high likelihood for the Jetstream31 as it is a light aircraft and sensitive to gusts of wind and turbulence during final approaches (Figure 4.5).
Figure 4.6 Extract of [4], recording of a turbulent approach Fri113305 , affected
by too much noise.
From Figure 4.6, it can be seen that from a height of 400ft , there were several important peaks – with the highest not corresponding to the point of touchdown. The use of the preceding algorithm could lead to a wrong touchdown identification in this and similar cases.
4.4.2 New approach
To remove the possibility of disturbance resulting in wrong touchdown identification, it was decided to focus on GPS altitude measurements that are independent of external conditions. The methodology for this method requires two steps, namely:
Identification of a time interval which contains the touchdown instant
Figure 4.7 Critical points on aircraft’s flight path in impact zone
Such interval is centered in a critical point on altitude curve in the impact zone. This critical point is that where the aircraft tends to regain height after the first impact with the runway, because of the spring effect of the landing gear. The altitude curve in this point changes slope, from negative to positive.
The point needed is the one that satisfies for the first time the two following conditions:
1] The airplane should be close to the runway
Figure 4.8 Algorithm for the first critical point determination
r number of row of any vector
n value of generic row
Gear_alt_co : Altitude of landing gear
Figure 4.10 Identification of the first critical point for a touchdown time interval
determination (red point on the figure)
Explanation of the algorithm
In this algorithm,
- n represents a generic instant between the starting recording time and the end;
- r is the total number of instants recorded by the IRS or GPS
First condition ‘’1] ‘’
The first condition implies possible points which may not satisfy the two last condition are taken out of the equation, since they may result in wrong conclusions as the aircraft will be still approaching the runway. This is shown in Figure 4.11.
In fact during the approach phase the airplane may easily be subject to an increase in altitude. By contrast, the probability of an aircraft increasing abruptly in altitude during the flare maneuver is very low; corresponds to the instant at which condition 1] is satisfied.
80 82 84 86 88 90 92 25 30 35 40 45 50 55 Time [s ] A lt it ud e [f t]
Landing gear wheel altitude
Figure 4.11 Wrong critical point Second condition ‘’ 2] ’’
The minimum variation of 0.5 ft of landing gear altitude has been set with reference to the Jetstream31 by analyzing several presumed touchdown instants. This may vary for different aircraft according to the corresponding aircraft mass and landing gear elastic rigidity.
Finally, due to the fact that there should be at least 1 critical point on the runway , the Matlab command, ‘’break’’, enables the loop to stop at the first one.
∗ in the algorithm is the instant when the first critical point is registered ; On figure 4.12 the point in red is the first critical point on the runway ; the second point is shown by the red arrow.
0 20 40 60 80 100 120 0 100 200 300 400 500 600 700 800 900 1000 Time [s] A lti tu de [ ft ] Gear-alt-co
Figure 4.12 Critical points at touchdown 80 82 84 86 88 90 92 25 30 35 40 45 50 55 Time [s] A lt it ud e [f t]
Landing gear wheel altitude
Determine the presumed instant of touchdown
Logically the instant of touchdown should be investigated prior to the first critical point as identified above ; However, because of the low quality of data recorded – in particular GPS altitude data, and also because of errors of measurement inherent to these instruments, it was decided to extend the time interval where the touchdown instant might be contained beyond the critical point, increasing the probability of touchdown instant determination .
This time interval was set to 2 s, 1 s prior and 1 s after the critical instant as shown on figure 4.13 below.
In this interval of time, the presumed instant of touchdown should correspond to the point with the highest earth vertical speed.
Explanation of the algorithm
This algorithm contains two loops:
The mean sample time period of the GPS is 0.04 s; then 2 s will correspond to 50 sample value of the parameter considered:
- i and j represent the generic sampling instant in the maximum touchdown time for the right and left wheel respectively
- ∗ is the instant of first critical point registered
First loop
Involves a determination of the presumed touchdown instants (for both landing gear wheels) by determining peak values of both vertical speeds of left and right main landing gear wheels.
- The Matlab command ‘’min’’ enables a determination of these speeds V_rt* and V_lt*, as well as the instant when they are registered i* and j* .
The command ‘’min’’ refers to the fact that the peak values of the speeds are the minimum ones , since the earth vertical speed of the landing gear is negative.
- At this stage the possible instants of touchdown are two , n*- i* or n*- j*
Second loop
Enables a selection to be made from either of the two above instants of touchdown in case they are not equal. The presumed touchdown instant refers to the first wheel to make contact with the runway, which means the wheel with the higher velocity peak during the maximum touchdown time.
- The Matlab command ‘’If ... elseif ’’ enables a determination of the higher
velocity peak and therefore the instant of touchdown.
- Time_td is the presumed instant of touchdown which allows to deduce the others parameters needed.
The following figures show graphs of :
- The landing gear wheel altitude of the first critical point (in red) and the touchdown point (in green) on Figure 4.15 .
- The right and left earth vertical speed of the first critical point (in red) and the touchdown point (in green) on Figure 4.16.
Figure 4.15 Landing gear altitude of the first critical point/instant (in red) and
Figure 4.16 Right and left landing gear vertical speed of the first critical point/
instant (in red) and touchdown point/instant (in green) for Landing_Wed120709
0 50 100 -1600 -1400 -1200 -1000 -800 -600 -400 -200 0 200 Time [s] R ig ht G ea r E ar th V er ti ca l S pe ed [ ft /m in ]
Right Gear Earth Vertical Speed
Vz-gear-rt 0 50 100 -1600 -1400 -1200 -1000 -800 -600 -400 -200 0 200 Time [s] L ef t G ea r E ar th V er ti ca l S pe ed [ ft /m in ]
Left Gear Earth Vertical Speed
Vz-gear-lt 85 90 95 -200 -150 -100 -50 0 50 Time [s] R ig ht G ea r E ar th V er ti ca l S pe ed [ ft /m in ]
Right Gear Earth Vertical Speed
85 90 95 -200 -150 -100 -50 0 50 Time [s] L ef t G ea r E ar th V er ti ca l S pe ed [ ft /m in ]
Left Gear Earth Vertical Speed
Analysis of ‘’Landing_Wed120709’’
In the landing case shown by the graphs above, the touchdown (in green) occurs before the critical point(in red).
It can be also viewed how that point corresponds to the highest peak around the first critical point, but in particular how after reaching it, the vertical speed logically oscillate toward zero(idle) without no other higher peak registered .
However, the highest peak is reached by the right main landing gear, with a vertical speed of nearly -193 ft/min and 43 ft/min for the left one, because at the presumed moment of touchdown the aircraft is subject to a roll rate of -0.2 rad/s (anti-clockwise oriented), that increases the right landing gear speed and decreases the left one.
4.5 Implementation on board
The final objective of the project was to use the algorithm directly aboard the aircraft. At this stage two options were possible:
Implementation Solution
Advantages Drawbacks
1. To encode the Matlab algorithm in Labview, and implement it directly in the computer aboard the aircraft
Embedded solution, the landing severity monitor, is totally integrated into the existing system and Labview provides a visual interface for displaying results.
Not sure Labview can deal with the Matlab algorithm; especially as it utilizes loops that might not be easily converted into Labview language.
2. To install Matlab on the computer aboard the aircraft, and run it just after landing with data recorded by the IRS
Simple, fast and integrated solution as the Matlab algorithm is already available.
The computer installed aboard the Jetstream31 might not be powerful enough.
Table 4.2 Implementation solutions
Labview is a graphical programming platform that helps engineers to design, integrate and perform tests on systems. One of the most interesting functions of Labview is to provide tools that allow the visual display of data and solutions to various problems in a simple and practical way.
The software was already installed aboard the Jetstream31 which meant it was preferable to integrate the LSM and utilize its capacity of its visual display to design an interface for a visual interpretation of results. Figure 4.17 shows an example of how the display function might be used to design a system and Figure
4.18 is an example of a graphical interface that might be used to display the results of the LSM.
Figure 4.17 Graphical and structural interfaces of Labview
However, because it was not easy to encode the landing severity monitor code in Labview, solution 2 was finally adopted.
Consequently, it was necessary to install Matlab on the computer aboard the Jestream31 in order to run the Matlab code at the end of each landing.
4.6 Matlab implementation
The Matlab solution required two steps to be implemented on board: The first step was to Create a path in the LSM code that will allow
Matlab to load the newest landing file recorded
Figure 4.19 Identification of the newest landing recorded file
To achieve this , three commands were used:
‘’ Dir ’’ indicates in Matlab the directory of the landing records
with a comma separated value (csv) file extension, to take account of only the landing files in csv format within the folder using ‘‘/*csv.’’
Landing_records_folder =
dir('C:/Users/Ericbergson/Desktop/TESI/CA/Wamba/LSM/Landing
Records & Severity Monitor Code/*.csv')
‘’ (end).name ’’ allows selection of the last file in a folder Newest_file = Landing_records_folder(end).name
‘’csvread’’ allow reading and loading in Matlab of the file selected
above
The second step consisted in creating a dynamic file of results ,
containing the landing severity parameters computed at touchdown.
Dynamic means that, at each landing, the results of the current landing will be added to the previous results’ file. To achieve that, the followings steps were performed:
Creation of a row vector of results containing the different relevant parameters at the touchdown instant as shown below:
Output = [Gear_alt_co_td , az_cg_td , Vz_CG_td , Vz_gear_lt_td , Vz_gear_rt_td ]
Then the command ’’ csvwrite’’ was used to generate a csv file in the landing record folder that contained a row vector with the five outputs named above
csvwrite('results',Output)
Once the csv file was generated, it was necessary to delete that command and write a new one to add a new row of output in the csv results file previously generated.
However, writing data to the previous results file required first that it must be read before the addition of a new row of results.
Output_i=csvread('C:/User/Ericbergson/Desktop/TESI/CA/Wamba /L
SM/Landing Records & Severity Monitor Code/Results')
Results_i=[Output_i;Output]
4.7 Matlab code outputs
The code was finally run for 20 landing recordings and the results are presented in the next table.
Landing Cases
[Name & Date]
Touch down Altitude [ft] CG Earth Vertical Acceleration [ft/ ] CG Earth Vertical Speed [ft/min] Right wheel Earth Vertical Speed [ft/min] Left wheel Earth Vertical Speed ft/min] Landing_Fri120511 339.52 1.25 -48.70 -28.70 -59.01 Landing_Fri130544 339.69 0.5 -101.26 -110.80 -82.02 Landing_Fri145404 347.10 0.06 -47.84 -54.79 -35.50 Landing_Mon091444 43.15 0.26 -80.13 -90.85 -69.39 Landing_Mon133350 40.79 0.64 -66.81 -79.22 -51.38 Landing_Mon152444 35.86 1.57 -92.59 -94.43 -83.50 Landing_Mon161414 34.42 -0.16 -89.49 -88.23 -86.23 Landing_Thu101256 40.19 -2.13 -31.7390 -41.57 -17.72 Landing_Thu105941 19.97 -0.93 -51.37 -59.43 -41.72 Landing_Thu121229 37.60 0.92 -85.47 -89.93 -72.70 Landing_Thu151325 38.30 0.92 -84.1600 -72.66 -85.97 Landing_Thu160707 36.89 -1.96 -41.61 -34.09 -47.40 Landing_Tue115932 32.45 -0.91 -170.75 -157.73 -174.80 Landing_Tue124157 53.64 1.99 -98.94 -111.74 -83.49 Landing_Tue154149 351.22 1.38 -118.33 -96.82 -134.64 Landing_Tue154848 44.22 -0.30 -134.27 -114.32 -140.81 Landing_Tue170916 29.16 -1.00 -89.21 -85.57 -91.63 Landing_Wed120709 39.90 -0.05 -69.77 43.54 -193.06 Landing_Wed130034 36.21 -0.07 -134.73 -111.03 -150.11 Landing_Wed150907 37.75 0.28 -226.82 -223.15 -223.06
5. CONCLUSION
5.1 SummaryThe landing severity monitor is a device that enables an assessment of the severity of landings to be made. As stated in the introduction, the assessment of landing severity is a vast, new subject within aerospace research.
The LSM is generally designed with sensors that determine the instant of touchdown and then communicate that information to others parts of the device in order to compute the values of relevant parameters needed to characterize the landing severity.
A smaller aircraft (the Jetstream31) was selected for the project as continuous landing severity monitoring is beneficial for preventative maintenance as it reduces long-term replacement costs. Indeed, the project consisted of finding out a way of assessing landing severity without using any sensors.
After a brief description and explanation of the complexity of a landing maneuver, accidents that can occur at the landing phase - and the dynamics behind these accidents, the next step was to inquire about relevant parameters that will allow landing severity to be evaluated: earth vertical speed and acceleration.
The next phase consisted of determining the presumed touchdown instant which was the most important objective of the project, as it enabled deduction of the values of parameters needed to assess landing severity.
The last phase consisted of implementing the device aboard the aircraft to ensure that, after every landing, a new landing record file would be run and results registered within a file originally created in the landing recording folder updated.
5.2 Conclusion
According to the landing cases analyzed on table 4.3, the earth vertical acceleration of the CG is very low at touchdown as the maximum absolute value is 2.13 ft/ . This is probably due to environmental conditions or others reasons that should be the subject of further researches.
In this regard, the touchdown determination was based on aircraft altitude instead of the earth vertical acceleration to avoid the possibility of compromising acceleration data due to atmospheric disturbances and others factors.
The manufacturer’s specifications for the earth vertical speed threshold at touchdown are:
- 6 ft/s at Maximum take off mass (7059 kg) - 10 ft/s at Maximum landing mass (6759 kg).
In summary, no landing case analyzed exceeded the value of - 600 ft/min , as the highest earth vertical speed registered at touchdown is -223 ft/min, which means no hard landing case could be suspected.
5.3 Recommendations for further work
For further work on this project, some recommendations can be highlighted: The main concern of this project was the touchdown instant determination which relies completely on the quality of data recorded.
The accuracy of the data measurement by the IRS was fairly good. However the update frequency of the GPS installed on the aircraft is 1 Hz which is too small to generate a GPS altitude data of good quality (a higher update frequency would have permitted better quality GPS altitude data and better approximation of touchdown instant).
Another approach to solving this problem might consist of generating a different landing gear altitude data by studying the dynamics of the aircraft at the landing phase and using some of the data recorded by the IRS to determine new landing gear altitude data.
More accurate determinations of touchdown instants are therefore theoretically possible.
