Comair B734 at Johannesburg on Oct 26th 2015, left main gear collapse on landing
Last Update: March 14, 2017 / 22:32:45 GMT/Zulu time
Incident Facts
Date of incident
Oct 26, 2015
Classification
Accident
Airline
Comair
Flight number
MN-6234
Departure
Port Elizabeth, South Africa
Destination
Johannesburg, South Africa
Aircraft Registration
ZS-OAA
Aircraft Type
Boeing 737-400
ICAO Type Designator
B734
A passenger reported the captain announced prior to landing that they would need to land without left main gear. The aircraft subsequently touched down at an angle and skidded along the runway with sparks spraying off the left hand side.
The airline confirmed a landing incident just after 12:00L, all passengers disembarked safely without injuries. The airline later reported that shortly after touch down the crew noticed an unusual vibration which was shortly followed by the collapse of the left main landing gear.
A ground witness reported it appeared the left main tyres burst on touchdown, the aircraft fell onto the wheel bogies, the left main gear strut was ripped off and left behind, the aircraft skidded on its left engine to a halt.
South Africa's Civil Aviation Authority (SACAA) reported in their October bulletin that the crew reported they had a normal landing, but a few seconds after touchdown they felt vibrations from the undercarriage.
In an interim statement of December 2016 the SACAA reported that the gear collapse was due to a shimmy damper event. The aircraft, with the first officer being pilot flying, flare at 65 feet instead of recommended 20 feet resulting in a long float. Following touch down excessive vibrations built up which resulted in the upper torsion link failure. During disassembly of the shimmy damper oil was found in the thermal relief valve which might have impaired the operation of the shimmy damper. Significant wear was found at the upper torsion link bushes which may have contributed to the collapse. "The landing gear following the sequence of events, failed in accordance with the failsafe design purpose." The investigation is ongoing.
In March 2017 the SACAA released their final report concluding the probable cause of the accident was:
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.
The SACAA reported the flight data recorder showed the aircraft touched down at 139 KIAS and 167 knots over ground at 2-3 degrees left bank angle and 1 degree drift angle to the right, the vertical rate of descent was 120 feet per minute and the aircraft experienced +1.1G. Wheel brakes were immeditely applied after touch down, the left main gear collapsed upon de-rotation of the aircraft.
The first officer (40, CPL, 5,817 hours total, 480 hours on type) was pilot flying, the captain (49, ATPL, 9,186 hours total, 2,899 hours on type) was pilot monitoring.
The SACAA analysed:
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.
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.
With respect to the landing the SACAA analysed:
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.
Metars:
FAOR 261200Z VRB04KT 9999 SCT048TCU 28/03 Q1027 NOSIG
FAOR 261100Z 12006KT 060V170 CAVOK 27/04 Q1028 NOSIG
FAOR 261000Z VRB05KT 9999 FEW045 26/04 Q1029 NOSIG
FAOR 260900Z VRB07KT CAVOK 26/05 Q1029 NOSIG
FAOR 260800Z VRB03KT CAVOK 25/06 Q1030 NOSIG
FAOR 260700Z VRB05G18KT CAVOK 23/09 Q1030 NOSIG
Incident Facts
Date of incident
Oct 26, 2015
Classification
Accident
Airline
Comair
Flight number
MN-6234
Departure
Port Elizabeth, South Africa
Destination
Johannesburg, South Africa
Aircraft Registration
ZS-OAA
Aircraft Type
Boeing 737-400
ICAO Type Designator
B734
This article is published under license from Avherald.com. © of text by Avherald.com.
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