R V College of Enginnering Department of Aerospace Engineering FAILURE OF NOSE LANDING GEAR – SELF STUDY REPORT Failure analysis of an aircraft Nose Landing Gear Nithin S – 1RV17AS031 H R Manasa – 1RV17AS017 Sagar Hedge E K – 1RV17AS043 Department of Aerospace Engineering R V College of Engineering Supervisor Benjamin Rohit Assistant Professor Department of Aerospace Engineering R V College of Enginnering This report is presented as part of the requirements for the Internal Evaluation of the subject Fatigue and Fracture Mechanics for the degree

R V College of Enginnering
Department of Aerospace Engineering
FAILURE OF NOSE LANDING GEAR – SELF STUDY REPORT
Failure analysis of an aircraft Nose Landing Gear
Nithin S – 1RV17AS031
H R Manasa – 1RV17AS017
Sagar Hedge E K – 1RV17AS043
Department of Aerospace Engineering
R V College of Engineering
Supervisor
Benjamin Rohit
Assistant Professor
Department of Aerospace Engineering
R V College of Enginnering
This report is presented as part of the requirements for the Internal Evaluation of the subject Fatigue and Fracture Mechanics
for the degree:
Bachelor of Engineering
Vishveshwaraya Technological University
RV College of Engineering
October 15, 2018
Declaration
We,
Nithin S – 1RV17AS031
H R Manasa – 1RV17AS017
Sagar Hedge E K 1RV17AS043
Department of Aerospace Engineering
RV College of Engineering
Declare that this report submitted in partial ful llment of the requirements for the Self Study course work for the completion of internal evaluation of Failure analysis of nose landing gear at the Department of Aerospace Engineering,is wholly our own work unless or otherwise referenced or acknowledged. This document has not been submitted for quali cations at any other academic institution.

Nithin S – 1RV17AS031
H R Manasa – 1RV17AS017
Sagar Hedge E K – 1RV17AS043
Department of Aerospace Engineering
RV College of Engineering
1
Abstract
This assignment presents a detailed analysis of failure of aircraft nose landing gears.In the cases discussed below, failure of nose landing gears is either due to structural de-fects or inexperience of the pilot.Nose gear failures are a high concern in the aviation industry.When a failure is caused due to fractured materials, an analysis was per-formed and was concluded that the referred area was subjected to stresses which originated and propagated through cracks inside the material.

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Contents
1 Introduction 2
1.1 Cause of Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Literature Review 3
2.1 Nose landing gear failure caused by fatigue . . . . . . . . . . . . . . . 3
2.1.1 Laboratory Examination . . . . . . . . . . . . . . . . . . . . . 3
2.2 Failures in Nigerian Aviation Industry . . . . . . . . . . . . . . . . . 4
2.2.1 Failure Investigation . . . . . . . . . . . . . . . . . . . . . . . 4
2.2.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . 4
2.3 Failure in aircraft-Saab 2000 . . . . . . . . . . . . . . . . . . . . . . . 8
2.3.1 History of ight . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3.2 Examination of the aircraft . . . . . . . . . . . . . . . . . . . 9
2.3.3 Flight records . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4 Review Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3 Future work and Research gap 10
4 References 11
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Chapter 1
Introduction
People remember the aircraft accidents because of the unusual high loss of life and the extent of damage caused by it. Compared to others ying is a safer form of trans-portation,but accidents rarely happen due to human error or mechanical problems, or sometimes due to criminal activity.

Cause of Failure
Landing gears,are a major component in an aircraft. Usually they are subjected to very harsh environmental conditions,like severe temperatures, bad weather and other operational situations like as runway conditions. Many experimental studies showed that fracture had occurred in di erent parts of the landing gear like cylinder attachment lugs which are made of aluminium alloy.

Failure and defect investigations on the aircraft structural components have a very important role in improving the aircraft safety. Therefore, identifying the primary cause of failure and then analyzing the failure enables us to recommendation for the corrective actions to be made that will prevent failures like them from occurring in the future .

Failures happen due to various reasons like poor design, use of inferior material or following poor fabrication techniques, or due to a phenomenon called as fatigue.The Fatigue design criterion of an aircraft structure is usually one of the following: in nite-life, safe-life, fail-safe or damage-tolerant design. Since the landing gear structure does not have a redundancy in their means of support, we generally use the safe-life criterion. The safe-life criterion include margins for scattering of fatigue results. The fatigue life consists two stages, crack initiation and crack propagation. Landing gear materials usually show the initiation stage, which consists of 90 to 95 percent of the total life, and a propagation stage which is of 5 to 10 percent of the total life.

Because of the safe life criterion, landing gears must have well de ned inspection techniques and good frequency of replacement so that the probability of failure due to fatigue cracking becomes very remote .

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Chapter 2
Literature Review
Nose landing gear failure caused by fatigue
During the landing process the nose landing gear of a turboprop airplane failed. After the disaster in an examination it was found that a fracture in the trunnion arm of the oleo strut housing was the main cause. In the aircraft the landing gear was installed after the overhaul. The laboratory investigation found out that the initial ductile overload fracture was because of a pre-existing fatigue crack. The fatigue fracture surface consisted of two distinct zones di erent in macroscopic and microscopic appearances as well as chemical composition. After the tests, results were obtained, from those results it was found that the older region of the fatigue crack pre-dated the overhaul, but the fresh fracture region grew during the post-overhaul.

The airplane experienced no problem before landing. As landing was on course part of the nose landing gear structure fell o which resulted in a brief disturbance o the runway before the airplane was somehow brought back into the runway. after the incident there were no injuries but still the aircraft was damaged. The nose landing gear in the aircraft was installed 20 months before this dreadful incident occurred which shook many lives.Because it was such a long time the gear was in the overhauled condition, and had performed about 1900 take-o /landing cycles after the installation. An inspection after the incident found a fracture in the trunnion arm of the oleo strut housing and out of suspicion this part was sent for laboratory examination.

Laboratory Examination
The crack propagated from the inner surface of the strut housing trunnion arm to the outside. A ngernail-shaped fatigue crack occurred after which a ductile overload fracture occurred. But there was an interesting development which was a bright rim surrounding the dark fatigue region. Its quite interesting because the shiny band is not like the dull and brous outlook of the surrounding overload fracture surface. The higher magni cation helped in distinguishing the characteristic of fatigue such
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as crack propagation marks in this zone. After this , this region of the fatigue crack is called the fresh fatigue zone,and the larger dark area is called the old fatigue crack.

Failures in Nigerian Aviation Industry
In the year 1994,for the training of pilots, sets of aircraft were introduced into the Nigerian Aviation Industry. The nose wheels of some of the aircraft had collapsed particularly during hard landings. The failure modes were either due to complete fracture of the landing gear from the rewall or the buckling. This led to the study which was the reason for failure mechanism experimentally with a view to preventing future occurrence. Fractography of the failed samples showed the fractograph with high energy fracture | beach marks, initiation sites and the propagation area on the failure surface which indicates fatigue failure. Experimental analysis detailed the characterization of fractured parts of the undamaged and failed nose wheel struts by determining the mechanical properties and examining the structural morphology of test samples. In the fatigue tests, it was revealed that high cycle low stress fatigue had failed the material. Micro-structural examination showed inter-metallic inclusions within the microstructure of the material which acted as stress raisers causing crack initiation and eventually fatigue fracture.

Failure Investigation
The failed landing gear was inspected visually rst, then later macroscopically. In order to identify the type of fracture, the fracture surface of the gear strut was ul-trasonically cleaned and examined under a Leica M400 electron microscope. The material in the vicinity of the fracture of the failed gear was later taken as samples for Brinell hardness measurement, fatigue tests and impact tests. After this met-allographic specimens were prepared for optical microscopic examination. In order to identify the type of steel used chemical analysis of the landing gear material was performed.

Using a universal testing machine and a Brinell reading microscope hardness mea-surement was taken. Impact strength of the gear material was determined with Izod testing machine and fatigue data was obtained by applying reversed loads on the sample with the Avery Denison 7305 fatigue testing machine. Samples for these tests were taken from an undamaged nose landing gear of the aircraft which was obtained from the aviation industry.

Results and Discussion
Visual Examination
Examining the gear visually revealed a fractured strut surface with beach marks which propagated from a point of crack initiation as indicated by the arrow. This
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is the common characteristic of fatigue fractures and particularly high cycle fatigue. Propagation of the crack can be seen by the at plateaus that are joined by narrow regions of tensile tearing.

Figure 2.1: Fracture surface of gear strut
Hardness Pro les
The hardness readings of the gear struts are given in g 2.5

Figure 2.2: HBR data for failed and undamaged gear struts
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It shows that the failed gears hardness di ered from the undamaged by 92HBr. One of the factor for the marked di erence may be due to loss of strength in the material as a result of fatigue. In every case, the hardness distribution across the gear surface is seen consistent. According to 6, the stronger the steel, the less likely it is that striations as observable on the fracture surface. Hence, it can know that the gears hardness was probably not su cient to prevent fatigue damage.

Composition Analysis
A spectrometer was used to analyze the Chemical composition of the gear strut materials. The average values are shown in Fig 2.5. Contradicting the expectations, the compositions indicated that the material was made using medium carbon steel of the tough grade to SAE 0050 standard instead of using spring steel. The strut should have been made using spring steel because the aircraft design did not have a shock absorbing mechanism. To have a spring e ect,Silicon in both samples should be in the percentage range of 1.90 to 2.4, with carbon content less than 0.65.

Figure 2.3: Chemical compositions and undamaged and failed gears
Fatigue Data
In failure mechanism, fatigue results in the initiation and gradual growth of cracks until the remaining section of material cannot support the load that is applied. The gure below shows the result of fatigue tests on undamaged and failed gear struts.

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Figure 2.4: Endurance curves for failed and undamaged strut samples
The fatigue strength (Se) of a sample which is undamaged is 3790N/mm2 while the endurance limit (Se) of a sample which has failed is 3067N/mm2 . According to the results obtained it was found thatthat below the endurance limit obtained for either of the samples, the material will endure and there won’t be any damage. this means that the structure will have an in nite life. The curve shows high cycle low stress fatigue strength of the materials.

Adding on to the above, the beach marks (chevrons) characteristic of fatigue which moves from the site of initiation and the zone of fracture, being o -centre, shows that crack initiation originated from a particular area on the strut. The main causes for the fracture were low fatigue resistance of the samples and high stress to which the strut was subjected. This was con rmed by visual examination in which chevrons were seen on the surface initiating from a point which could be the initiation site.

Conclusions
Based on experiments and comprehensive analysis, the following conclusions can be drawn: Visual examination with the unaided eye and fractography helped in knowing that chevrons which act as a indicator for a brittle fracture associated with high cycle fatigue failures.

After the compositional analysis, it was found that the as-received material was found to be medium carbon steel of the tough grade. It did not meet the standard requirements for strong spring steel. The failure mode was impact fatigue failure initiated at the inclusions present in the microstructure resulting in crack propagation through the matrix and eventual fracture. By Controlling the average composition of the inclusions which are already present in the microstructure can drastically improve the fatigue properties of the steel.

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Failure in aircraft-Saab 2000
History of ight
At 7.05 pm, after travelling for an hour, the plane began to land in Paris Charles-de-Gaulle airport. The pilot made a thorough check and decided to land the airplane. At an altitude of 2,300 ft the con guration chosen for the landing ( aps 20 ) and the target approach speed (123 kt) were selected. The PF called for the nal check. The pilot con rmed that everything was right and they were cleared to land and it was announced that the wind was coming from the left. At a height of 1,000 ft, the everything was stable and the runway was within vision. At a height of about 400 ft, the PF disengaged the autopilot and at 200 ft, the decision height, the crew decided to continue the approach. Descending through 50 ft, the PF started to move the power levers towards the ight idle position, decrabbed and ared; the pitch trim increased from 0 to 3 . Before the main landing gear was down, control column movement was recorded at its maximum pitch-up value (11 ) and the pitch trim then reached a value of 5 . The main landing gear wheels touched down very rough on the ground, the aeroplane was then at an indicated speed of about 120 kt. The automatic ap retraction system (AFR) modi ed the ap de ection (from 20 to 15 ). This resulted in the airplane to bounce two times. During these bounces, the crew alternated between pitch-up and pitch-down input.During the nal touch down, the airplane came at very high velocity and the nose landing gear hit the runway, the resulted in the breaking of the nose landing gear. The aeroplane came to a stop on the runway.

Figure 2.5: Positions of aircraft touchdown
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Examination of the aircraft
After the examination of the aircraft it was clear that all of the damage that was done to the aircraft was solely because of the failure of the landing gear. The reason for the breaking of the landing gear was overload. The examinations performed on the parts from the nose landing gear showed that it carried a load much greater than the ultimate speci ed load it could carry.

Flight records
Analysis of the ight data made by the manufacturer based on the aeroplane model showed that the aeroplane touched the ground three times in the space of ve seconds: 1st touchdown: Only the two main landing gear units were in contact with the ground 2nd touchdown:Only the two main landing gear units were in contact with the ground 3rd touchdown: Only the nose landing gear was in contact with the ground. The failure occurred during this
Last touchdown:The vertical speed of nose landing gear was estimated to be 3.5 m/s.

Conclusion
During the emergency one pilot decided that the landing was going to be hard. Due to the urgency of the situation, he made quick pitch-up inputs on the control column without announcing his intentions to the copilot. This absence of crew coordination led to dual input piloting and successive opposing inputs on the ight controls during management of the bounces.

Review Table
Aircraft Date Place Type of Persons on Consequences
and Flight board and Damage
Time Nigerian Dec Nigeria Commercial Captain,copilot,1 Aircraft severely
aviation 1994 transport cabincrew,0 pas- damaged
aircraft sengers Turboprop 1 Canada Commercial Captain,copilot,1 Aircraft severely
airplane June transport cabincrew,25 damaged
2011 passengers Saab 28 Charles- Commercial Captain,copilot,1 Aircraft severely
2000 Jan de- transport cabincrew,16 damaged
registered 2014 Gaulle passenegrs HB-IZG Air- port,Paris 9
Chapter 3
Future work and Research gap
More research is going on the eld of nose landing gears as airlines drive for reduced maintenance costs and improved operational e ciency, reliability and performance expectations for gear manufacturers increase.

The outward appearance of landing gear has not changed signi cantly over the past 50 years. The changes are with the only signi cant di erence being the number of wheels on the main gear bogies. To cope with higher weights, wide body aircraft have more wheels than narrow bodies do. Landing gear design are also a ected by the alteration of aircraft weights and the center of gravity. One example is the Airbus A350-1000, which features a six-wheel main gear rather than the four on smaller A350 types.

In the modi cation of materials used, application of composite and titanium land-ing gear structures and actuation are in development to further reduce weight, cost and corrosion potential . New steel and titanium alloys contribute to stronger, lighter landing gear structures, while composites also are used where appropriate. In the upcoming years hydraulics may be left behind altogether and moved to electrically actuated landing gear systems. The 787 already features electric brakes, while elec-trically powered nose wheel systems are in use with some airlines. Landing gear make use of the increased Wi-Fi connectivity, allowing equipments to transmit health mon-itoring data through the aircraft maintenance reporting system to reduce the number of electrical hardness.

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Chapter 4
References
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