A too high flare in an attempt of a smooth landing led to unstable approach, some maintenance issues found as contributing factors to the main landing gear shimmy. Final report released 27th February 2017
Photo: South African Civil Aviation Authority Final Report
South African Civil Aviation Authority Accident and Incident Investigations Division
AIRCRAFT ACCIDENT REPORT
Name of Owner: Comair Limited
Name of Operator: Comair Limited
Manufacturer: Boeing Aircraft Company
Nationality: South African
Registration Marks: ZS-OAA
Place: O.R. Tambo International Aerodrome
Date: 26 October 2015
All times given in this report are Co-ordinated Universal Time (UTC) and will be denoted by (Z). South African Standard Time is UTC plus 2 hours.
1. FACTUAL INFORMATION
History of flight
The aircraft Boeing 737-400, operated by Comair, flight number BA6234, was on a scheduled domestic flight operated under the provisions of Part 121 of the Civil Aviation Regulations (CARs). The aircraft was on the third leg for the day, after it had performed two uneventful legs. According to their recorded flight plan, the first leg departed from King Shaka International Airport (FALE) to O.R. Tambo International Airport (FAOR), the second leg was from FAOR to Port Elizabeth International Airport (FAPE) on the same day, during which the Captain was flying. During this third leg, the aircraft departed from FAPE at 0820Z on an instrument flight plan rule for FAOR. On board were six (6) crew members, ninety-four (94) passengers and two (2) live animals.
The departure from FAPE was uneventful, whereby the first officer (FO) was the flying pilot (FP) for this leg. During the approach to FAOR, the aircraft was cleared for landing on runway 03R. The accident occurred at approximately 1 km past the threshold. The crew stated that a few seconds after a successful touchdown, they felt the aircraft vibrating, during which they applied brakes and deployed the reverse thrust. The vibration was followed by the aircraft rolling slightly low to the left. It later came to a full stop slightly left of the runway centre line, resting on its right main landing gear and the number one engine, with the nose landing gear in the air.
The crash alarm was activated by the FAOR Air Traffic Controller (ATC). The Airport Rescue and Fire Fighting (ARFF) personnel responded swiftly to the scene of the accident. The accident site was then secured with all relevant procedures put in place. The aircraft sustained substantial damage as the number one engine scraped along the runway surface when the landing gear detached from the fuselage. ARFF personnel had to prevent an engine fire in which they saw smoke as a result of runway contact. The occupants were allowed to disembark from the aircraft via the left aft door due to the attitude in which the aircraft came to rest.
The accident occurred during daylight meteorological conditions on Runway 03R at O.R. Tambo International Airport (FAOR)
Injuries to persons
None of the aircraft occupants sustained any injuries. Two live animals were also accounted for and no injuries were noticed on either of the animals.
Damage to aircraft
The aircraft sustained substantial damage to the No. 1 engine as the engine scraped along the runway surface after the landing gear detached from the aircraft structure.
At the time the landing gear was detaching from the aircraft mountings, more damage was caused to the landing gear mounting points and mechanisms, including flaps fairings, wing root attachment fairings and the landing gear extension/retraction link (lower torsion links and shimmy damper assembly).
Runway surface damage was caused by the landing gear during the accident sequence. There was also fuel and hydraulic fluid spillage. The runway was closed until the aircraft was recovered and all the runway surface repairs were completed.
First Officer (F/O)
Meteorological information as obtained from the South African weather service website:
The aircraft landed on runway 03R at O.R. Tambo International Aerodrome.
The aircraft is equipped with a flight data recorder (FDR) and a cockpit voice recorder (CVR). Both these units were removed from the aircraft for further investigation.
The aircraft was also equipped with a quick access recorder (QAR). This device records the same data as the FDR.
FDR Data Analysis
Time history plots of the pertinent longitudinal and lateral-directional parameters are attached as Figures 1 through 4 on Annex B in the full final report. The FDR data show the airplane on a flaps 30 manual approach at approximately 1000 feet radio altitude with the speed brake handle in the armed detent. Tambo International Airport is situated 5558 feet above sea level, and the pressure altitude of the airport at the time of landing was approximately 5100 feet. A combination of latitude/longitude data, Instrument Landing System (ILS) frequency and magnetic heading data indicate the airplane was positioned to land on Runway 03R. The winds were out of the south-southwest at approximately 5 knots, increasing to close to 15 knots prior to touchdown. This resulted in a predominant tailwind component (runway magnetic heading = 34 degrees).
The landing reference speed (VREF) for the airplane’s configuration was 133 knots. Below 1000 feet radio altitude, computed airspeed was 148 knots (VREF+15), on average, and reached as high as 154 knots (VREF+21) prior to flare. High rate column and control wheel inputs were observed during the approach, resulting in a fluctuating pitch attitude and bank angle, respectively. The fluctuations in pitch attitude contributed to variations in calculated vertical speed (relative to the center of gravity [CG]) between -500 and -1500 feet/minute. Furthermore, the airplane was consistently above the glide path, as evidenced by glideslope deviation. The autopilot pitch mode and roll mode were engaged in Glideslope and Very High Frequency (VHF) Omnidirectional Range (VOR)/Localizer mode, respectively, indicating Flight Director (FD) guidance (FD switches were), relative to glideslope and localizer deviation, was available to the crew.
Flare was initiated with a nose-up column input at approximately time 4767 seconds at a radio altitude of around 65 feet. The throttles reached forward idle (0 degrees throttle lever angle [TLA]) by time 4771 seconds. The calculated vertical speed became positive momentarily around time 4772 seconds, as the airplane floated prior to touchdown. The airplane was experiencing an approximate 10-knot tailwind below 100 feet radio altitude. Touchdown occurred at time 4776 seconds at a computed airspeed of 139 knots (VREF+6) and a ground speed of 167 knots. The airplane had an approximate 2-3 degree left bank angle and a 1-degree right drift angle (ground track right of heading) at touchdown. The calculated sink rate (negative vertical speed) at touchdown was around 1.8 feet/second with a normal load factor of approximately 1.1 Gs.
The closure rate at touchdown was approximately 3 feet/second. Closure rate is the rate of change of distance between the landing gear and the local terrain and is generated by calculating the sink rate of the main landing gear and accounting for any runway slope near the point of touchdown (based on Jeppesen runway information). The effect of an upsloping (positive) runway would increase the closure rate of the main landing gear with the runway compared to the airplane sink rate at the CG. Conversely, the effect of a downsloping (negative) runway would decrease the closure rate of the main landing gear with the runway compared to the airplane sink rate at the CG. In addition, the effect of a roll rate at touchdown would increase the main landing gear closure rate on the down-wing side. The runway slope for Runway 03R is 0.52%.
Following touchdown, the speed brakes automatically deployed and wheel braking was applied, although it was unclear whether it was due to manual or autobrake application. Thrust reversers were commanded approximately 4 seconds after touchdown. Around this same time, as de-rotation completed, the left main gear collapsed, as evidenced by a rapid increase in bank angle to the left and a momentary increase in pitch attitude. Spikes were observed in both longitudinal acceleration and normal load factor, and lateral acceleration shifted significantly to the right. The left wing trailing edge flap measurement also indicated a lower flap setting than the right after time 4782 seconds. As the landing rollout continued, the crew commanded right control wheel and right rudder (note: significant bias exists in rudder deflection data). The thrust reversers remained deployed throughout the rollout, but the left engine began spooling down after time 4795 seconds. The airplane came to a stop approximately 40 seconds after touchdown, left of the runway centreline.
Summarizing, the FDR (P/N: 980-4120-DXUN; S/N: 7197) data analysis indicated the following readings as assisted by the state of manufacture, which was the same as the data analysis at the local data analysis facility downloading lap at SAT:
- Airplane touched down at 139 knots computed airspeed (VREF+6) and at 167 knots ground speed
- The sink rate at touchdown was approximately 2 feet/second, and the touchdown normal load factor was around 1.1 g. These are not characteristics of a hard landing
- The airplane had an approximate 2-3 degree left bank angle and a 1 degree right drift angle at touchdown
- The left main gear collapsed as the airplane completed de-rotation
- Wheel brakes were applied immediately following touchdown, although it was unclear whether it was manual or autobrake.
The CVR (Pat N: 980-6022-001; S/N: 1732) data recordings revealed that all landing checklist procedures were followed in accordance with the pilot operating procedure handbook manual for the aircraft type.
Wreckage and impact information
The accident occurred during landing on a cleared runway 03R. The on-site observation was that, during landing, the aircraft was subjected to a slight left roll attitude which might have enabled the aircraft to make contact with the runway with the left main landing gear first. It is not clear where the aircraft initially touched down, but at approximately 1 km past the threshold, the left main landing gear tyre marks indicated a form of shuddering manoeuvre beginning slightly and increasing gradually. This was inconsistent with the pilot’s statement that during touchdown the aircraft started vibrating.
The aircraft’s left-hand main landing gear shuddering tyre marks occurred for a distance of approximately 200 m, whereby the runway surface damage was observed at the same place where the No.1 engine began scraping along the runway surface. At this time the left main landing gear had collapsed and was dragged along, causing the left wing to drop. There was evidence of runway surface damage at three intervals of approximately 1.7 m apart, appearing together with the hard tyre contact marks. The damage was consistent with damage caused by the outer wheel tyre assembly as it was dragged along. The damage on both wheel tyres was consistent with damage caused by an object that punched through at the same level with the upper broken torsion link. This may also indicate that both tyres were damaged after the shimmy damper bolt failed and the oscillations were occurring.
There was also evidence of some runway damage which was consistent with the damage caused by the shimmy damper component prior to detaching from the upper torque-link as the landing gear was dragged along. The shimmy damper was found on the left side lying a few metres from the landing gear extension/retraction mechanical actuator, just after the main landing gear doors debris. About several metres away there was an upper torsion link piece of a broken torsion link which connects directly to the shimmy damper. The damage on the shimmy damper is consistent with a component which was subjected to excessive tensile force.
The damage to the trailing edge of the left wing, towards the wing root, was caused by the landing gear as it detached from the main assembly points. The inner flaps were also damaged by the landing gear. The trunnion forward-bearing bolt is designed to fail if the landing gear receives a severe impact. The observations revealed that the fuse lugs on the trunnion link gave a positive reaction characteristic with regard to design failure as it prevented damage to the left wing tanks during the accident sequence.
Photo: South African Civil Aviation Authority Final Report
The wreckage debris was scattered along the runway length during the accident sequence. There was further visible runway surface damage caused by the left landing gear assembly as it moved towards the left side of the runway. The left landing gear was violently detached from the airframe assembly points and came to a complete stop approximately 1.7 km from the threshold during the accident sequence
Photo: South African Civil Aviation Authority Final Report
During the scraping of the No.1 engine (left side) an engine warning initiated. The pilot in command pulled the No.1 engine fire handle, once the aircraft had come to a stop, to cut fuel and hydraulic supply to the engine. On arrival the aerodrome rescue and fire-fighting (ARFF) personnel responding to the accident immediately observed the fire smoke and proceeded to extinguish it.
Tests and research
The failed upper torsion link and the shimmy damper components were shipped to the National Transport Safety Board Laboratory at Washington DC in the United States of America for failure analysis test. Damage assessment inspection was carried out on the components prior to preparing for laboratory tests. (1) The hydraulic tests for both the shimmy damper and its compensator were carried out. (2) The components were later disassembled for inspection. For the full component test report please refer to Annex B in the full final report.
It was reported that on October 26, 2015, during the landing rollout, the left main gear collapsed and departed the aircraft.
Examination and testing of the subject shimmy damper and associated parts were performed in the Boeing Equipment Quality Analysis (EQA) lab in Seattle on May 10, 2016. During the testing, the damper failed a step of the hydraulic functional tests. This failure was indicative of a condition that would have impaired damper function and thus may have been a contributing factor to the shimmy event. An attempt was made to isolate the faulty component within the damper. Testing showed that the thermal relief valve within the damper had intermittent internal leakage, which was causing the test failure. In addition to the hydraulic failure, significant wear was found on the upper torsion link bushings. Wear in these areas can also be a contributing factor to shimmy since the wear allows undamped torsional free play of the landing gear.
Based on the information provided regarding the accident site wreckage mapping and the flight data recording (FDR) information, the following discussion was established by the National Transport Safety Board (NTSB):
Shimmy Event Discussion
At the time of writing, there has been no confirmation that the main landing gear collapsed due to a shimmy oscillation. However, the characteristics of the landing are consistent with past landing gear shimmy events. The airplane touched down at a high ground speed and low sink/closure rate. The air/ground discrete transition to GROUND occurred approximately one second after touchdown, indicating that the struts were extended for that period of time. As a result, the torsion links of the shimmy damper remained in an extended, vertical position, where the damper has less mechanical advantage for longer periods of time. Despite the presence of shimmy damper hardware, which is designed to reduce the torsional vibration energy generated during landing, airplanes occasionally experience main landing gear shimmy.
The reference (a) Boeing Aero Magazine article and reference (b) Fleet Team Digest article provide information on the causes of shimmy and how Boeing has addressed the issue. Due to the geometry of the torsion links, the shimmy damper is most effective when the landing gear strut is compressed in the ground mode. Lower touchdown descent rates increase the likelihood of a shimmy damper failure. It is important to note, however, that proper maintenance of the gear components is the best way to prevent shimmy damper failures. The possibility of landing gear shimmy events is greater at high altitude airports. The Aero Magazine article concludes with the following:
Boeing also recommends that pilots strive for a landing with normal sink rates with particular emphasis on ensuring that the auto speedbrakes are armed and deploy promptly at touchdown. An overly soft landing, or a landing in which the speedbrakes do not promptly deploy, allows the landing gears to remain in the air mode longer, which makes them more vulnerable to shimmy. This is especially true when landing at airports located at higher elevations, where the touchdown speed is increased.
A manual approach was performed by the crew that was determined to be unstable, based on the stabilized approach criteria. A combination of a tailwind and high approach speed, while landing at a high altitude airport, resulted in excessive ground speed (167 knots) at touchdown. An extended flare that was the result of an early flare initiation led to touchdown at a low sink rate (1.8 feet/second). The left main gear collapsed approximately 5 seconds after touchdown, and the airplane came to a stop around 35 seconds later. Although the official investigation has not determined the cause of the landing gear collapse, prior service experience on the 737 has shown that touchdown at high ground speeds and low sink rates increases the likelihood of the initiation of main gear shimmy. Prior service history on the 737 has shown that gear collapse is a possible outcome of shimmy.
Note: The full test results are provided as Annex B attachment in the full final report reference.
Stabilized Approach Assessment
The Flight Crew Training Manual, with regards to the Flight Safety Foundation’s published criteria for flying a stabilized approach, recommends that a go-around should be initiated if the approach becomes unstabilized under 1000 feet above the ground for instrument meteorological conditions and under 500 feet for visual meteorological conditions.
According to the findings, below 1000 feet radio altitude, the accident airplane did not adhere to three of the recommended stabilized approach criteria. These criteria are summarized below:
- the airplane is on the correct flight path. The recorded glideslope deviation indicate the airplane was above the intended glide path.
- the airplane should be at approach speed. For a tailwind approach, the recommended approach speed (VAPP) is VREF+5, which in this case was 138 knots. On average, computed airspeed was 148 knots (VAPP+10) but reached as high as 154 knots (VAPP+16) prior to flare, which exceeded the allowable deviation above approach speed by 6 knots (10 knots is allowed).
- sink rate is no greater than 1000 fpm. Throughout the approach, there were several sink rate exceedances of 1000 fpm.
In this event were also found some failures to adhere to the Flight Crew Training Manual flare guidance recommendations: During final approach, flare was initiated early at a radio altitude of approximately 65 feet, which is higher than the recommended 20 feet. The early flare initiation contributed to the airplane float that led to touchdown at a low sink rate.
1. The flight crew was licensed, well equipped and qualified for the flight in accordance with existing regulations.
2. The actions and statement of the First Officer, who was flying the aircraft, indicate that his knowledge and understanding of the aircraft system were adequate. No negative operational factor which could have contributed to this accident was noticed during investigation.
1. The aircraft was certified, equipped and maintained in accordance with existing regulations and approved procedures. The aircraft had a valid Certificate of Airworthiness at the time of the accident, and it was attained in compliance with the existing regulations.
2. During post-investigation, the following were revealed from the Flight Data Recorder analysis and the components failure tests/analysis:
- According to the recordings of the FDR, the aircraft had an early flare initiated at 65 ft AGL, as compared to the recommended 20ft AGL. This resulted in the aircraft floating and caused a low rate of descent during landing touchdown. The forward touchdown speed was also high, at 167 kt.
This condition was induced with the good intention of achieving a smooth landing touchdown, but it had a negative impact on the landing gear shimmy effectiveness. According to Boeing the low sink rate during landing touchdown increases the likelihood of shimmy damper failure. Due to the geometry of the torsion links, the shimmy damper was less effective during a prolonged touchdown roll with the main gear strut in an extended position. This might have allowed the torsional forces to effect damage to the upper torsion link. The upper torsion link had a remaining lifespan of approximately 26091 landings, of its total expected lifespan of 75000 landings. It is also possible that the torsion link had already lost its maximum strength during the cause of the life it had already spent in operation. At the time of landing, due to the excessive vibration which was not damped at the time, the strut was still extended, the torsion link failed at its weakest design material strength.
- The shimmy damper also failed a step during tests in which oil was found in the thermal relieve valve. The presence of the oil could have hampered the effectiveness of the shimmy damper. This shows that there had been an internal leak over a long period. This could have been due to the inner seals damage, which was noticed during disassembling of the components following test failure.
During the wheel brakes application, the shimmy damper might have also been less effective, due to the impaired damper failure. The shimmy damper works most during initial touchdown and during brake application.
- Also, according to the test results, significant wear was found on the upper torsion link’s bushing and the flanges. Although the wear was not far beyond limits it could also play a role due to undamped vibrations continuing to increase shimmy events.
The three above-mentioned findings merged together can play a significant role in inducing the shimmy events that led to this accident. From the wreckage distribution it is evident that the upper torsion link failed first, allowing the shimmy damper attached to the remaining parts that link to the bottom torsion link to drop during wheel oscillation. This played a significant role in a complete landing gear failure. The shimmy damper detached as it sustained damage during the impact sequence, as the main landing gear became detached from the main attachment points. The landing gear detached as per the design fail-safe system, which prevents damage on the wings main spar. Should it be that the main spar damaged and affected the fuel tank, there was a high chance of fire erupting during the accident sequence, due to the running engine and the possible heat generated from impact friction with the runway surface. The results could have been catastrophic.
3. According to the maintenance details prescribed for the aircraft maintenance organisation (AMO) on the shimmy damper, it does not involve overhaul. It is in most cases based on condition; however, the AMO do not have overhauling capabilities. Should it be that during the main landing-gear overhaul, the shimmy damper‘s test and condition remain serviceable, the shimmy damper is reinstalled and it continues in operation. The shimmy damper component has an unlimited life span, unless it is certified unserviceable due to its ineffectiveness during operational tests. Also, in most inspections carried out, AMO personnel only look out for external fluid leaks and wear bushing. The shimmy damper was found with an internal leak during the laboratory tests following the accident.
On the basis of the service history, the investigation concludes that the damage to the shimmy damper seal was sustained during assembly on the last overhaul.
1. R. Tambo International Airport is at an altitude of 5558 ft above sea level and the pressure altitude of the airport at the time of landing was approximately 5100 ft. The wind was from of a south-southwest direction at approximately 5 kt, increasing to 15 kt prior to touchdown. There was a predetermined tailwind component heading at 34 degrees at 10 kt during touchdown. The effect of the tailwind prolonged the landing roll, but the runway length was sufficient to execute a safe landing in the existing wind conditions.
2. It is every pilot’s desire to execute a safe smooth landing for passenger comfort and aircraft sustainability. These skills are acquired with flying experience. They also are regulated by the environmental conditions at the time of the flight. The technique differs from one operating aerodrome altitude to another, due to pressure altitude and latitude conditions. The aircraft flaring was initiated at an earlier stage at approximately 65 ft AGL as compared to the recommended 20 ft (AGL).
According to the FDR, the pitch angle was correct, but the pilot allowed the airplane to float or attempted to hold it off. When prolonged flare is attempted to achieve a perfectly smooth touchdown, the aircraft is at risk of landing gear torsion link failure, due to ineffectiveness of the shimmy damper as the landing gear remains extended for a long period of time.
The flight was conducted in accordance with approved procedure in the company’s Operational Manual. The flight crew carried out a normal radio communication with the relevant ATC at the time of approach and during landing execution.
1. The aircraft crew was qualified, licensed and equipped for the operation in accordance with existing regulatory procedures.
2. The flying pilot at the time was medically fit to conduct the flight with sufficient experience on the aircraft type.
3. The aircraft had a valid Certificate of Airworthiness, which was attained in accordance with approved procedure.
4. The aircraft weight and balance were within limits.
5. According to the FDR recordings, the aircraft flare was initiated earlier at 65ft than at 20ft as recommended by aircraft manufacturer, which contributed to the low sink rate.
6. The shimmy damper failed the post-accident lab-test and fluid was found in the thermal relief valve, which could have contributed to the shimmy damper failure.
7. According to the lab results, significant wear was found on the upper torsion link bushing and flange, which could have contributed to undamped vibration continuation.
8. The aircraft had a tailwind component during landing, which could have prolonged the landing distance.
9. The aircraft touchdown was at a high forward speed.
Unstable approach whereby the aircraft was flared too high with high forward speed resulting with a low sink rate in which during touch down the left landing gear experienced excessive vibration and failed due to shimmy events.
4. SAFETY RECOMMENDATIONS
1. The aircraft manufacturer (Boeing) has taken a safety action by publishing The Safety Issued Magazine AERO QTR_03, 13 The Boeing Edge: It is recommended that the Boeing consider the review of Aircraft Operations Manual and / or issue a safety bulletin in light of the findings contained in the Safety Issued Magazine.
2. The safety action taken by the operator:
(a) Pilot’s notice was issued informing pilots of the potential gear shimmy and failure problem during high-speed and soft landings.
(b) Ordered that all operator’s fleet of B737-400 be inspected in order to establish further potential excessive wear and more regular inspections have been scheduled (over-and-above Manufacturer’s requirements) in order to monitor the fleet going forward.
By Laura Duque-Arrubla, a medical doctor with postgraduate studies in Aviation Medicine, Human Factors and Aviation Safety. In the aviation field since 1988, Human Factors instructor since 1994. Follow me on facebook Living Safely with Human Error and twitter@dralaurita. Human Factors information almost every day