Air Contractors B734 at East Midlands on Apr 29th 2014, parts of left main gear failed

Last Update: April 9, 2015 / 13:24:29 GMT/Zulu time

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Incident Facts

Date of incident
Apr 29, 2014

Classification
Accident

Airline
Air Contractors

Departure
Paris Charles de Gaulle, France

Destination
East Midland, United Kingdom

Aircraft Registration
EI-STD

Aircraft Type
Boeing 737-400

ICAO Type Designator
B734

An Air Contractors Boeing 737-400 freighter, registration EI-STD performing freight flight QY-1748 from Paris Charles de Gaulle (France) to East Midland,EN (UK) with 2 crew and 10 tonnes of freight, had safely landed on East Midland's runway 27 at about 02:26L (01:26Z) and slowed to taxi speed. While attempting to turn off the runway parts the left main gear collapsed disabling the aircraft on the runway. There were no injuries.

The runway needed to be closed for most of the day, current estimate is the runway re-opens by 19:00L (18:00Z).

The airline reported the aircraft had landed normally and had slowed safely. While turning off the runway parts of the left main gear failed. The aircraft was carrying 10 tonnes of freight (maximum capacity 17 tonnes). The captain (39) had 4,500 flight hours, the first officer (38) 3,900 hours. The aircraft had undergone its last A-maintenance check the last weekend (Apr 26th and 27th) and its last heavy maintenance check in February 2013. The Irish AAIU and the British AAIB have been informed.

The AAIB have opened an investigation into the occurrence rated an accident and dispatched investigators on site.

On Apr 9th 2015 the AAIB released their final bulletin concluding the probable causes of the accident were:

The damage to the flap system, fuselage, and MLG equipment was attributable to the detachment of the left MLG axle, wheel and brake assembly. The damage to the MLG outer cylinder, engine and nacelle was as result of the aircraft settling and sliding along the runway.

The left MLG axle assembly detached from the inner cylinder due to the momentary increase in bending load during the transition from auto to manual braking. The failure was as a result of stress corrosion cracking and fatigue weakening the high strength steel substrate at a point approximately 75 mm above the axle.

It is likely that some degree of heat damage was sustained by the inner cylinder during the overhaul process, as indicated by the presence of chicken wire cracking within the chrome plating over the majority of its surface. However, this was not severe enough to have damaged the steel substrate and therefore may have been coincidental. Although the risk of heat damage occurring during complex landing gear plating and refinishing processes is well understood and therefore mitigated by the manufacturers and overhaul agencies, damage during the most recent refinishing process cannot be discounted.

The origin of the failure was an area of intense, but very localised heating, which damaged the chrome protection and changed the metallurgy; ie the formation of martensite within the steel substrate. This resulted in a surface corrosion pit, which, along with the metallurgical change, led to stress corrosion cracking, fatigue propagation and the eventual failure of the inner cylinder under normal loading.

The AAIB reported the first officer was pilot flying, due to low visibility procedures in progress at East Midland the crew decided to conduct a CATIII autoland. During roll out, at about 60 KIAS, the first officer handed controls to the captain (38, ATPL, 4,279 hours total, 377 hours on type), who pushed the brakes pedals to disengage autobrakes, the system remained connected however. The captain pushed the brake pedals harder, autobrakes disconnected, the aircraft shuddered and rolled slightly left. The captain used the steering tiller to keep the aircraft on the runway. The first officer saw smoke drift by the aircraft, the crew of another aircraft reported smoke from the landing gear, the captain concluded one of the main gear legs had failed and due to the other crew report was concerned the aircraft might be on fire. He moved both engine start levers to the cut off position to shut off the engines. Three fire engines had reached the aircraft but took positions rather far from the aircraft prompting the commander to conclude that the aircraft was not on fire (he reasoned that if there had been any fire the engines would move closer and start to apply fire agent).

Runway marks indicated that the aircraft had travelled 115 meters/380 feet from the point of its left main gear failure to its final resting position.

The flight data recorder indicated a spike in longitudinal acceleration s the aircraft slowed through 52 KIAS and another spike 2.25 seconds later. The aircraft rolled to a bank angle of 7 degrees left afterwards and remained in that attitude until end of recording.

The left main landing gear inner cylinder was found fractured across the full diameter through the chrome portion about 75mm above the axle, the upper torque link arm failed, anti-skid wiring harness, conduit and brake pipes were parted. The left main wheels, brakes unit and axle assembly came to rest 27 meters behind the aircraft.

Examination of the left main assembly did not reveal any anomalies except for the chrome plating at the inner cylinder, which showed extensive crazing over the majority of the surface with small flakes of the plating having separated leaving an imprint of the crazing in the form of ferrous oxide tracks. The fracture surface showed stress corrosion at the origin of the fracture, further into the material there was evidence of fatigue and the remainder of the fracture surface exhibited evidence of ductile overload.

The AAIB continued: "As the investigation progressed it was also found that the ferrous oxide tracks on the substrate were present beneath the chrome which was exposed to the elements, but not on the upper area normally surrounded by oil in the outer cylinder. The inner cylinder area above the fracture face on the portion of the cylinder which had
been forced up into the outer cylinder during the runway abrasion exhibited circumferential helical bands in the chrome plate. When the chrome was removed, these marks were also present on the steel substrate. Metallurgical analysis revealed localised heating damage correlating to the bands" and stated: "Metallurgical examination of the microsection through the fracture face showed an area in the substrate steel of over and undertempered martensite, consistent with localised heating."

The AAIB reported that the left main gear assembly had undergone overhaul between December 2012 and January 2013, during which the plating of the inner cylinder had been re-chromed and re-finished.

The AAIB analysed that the brakes application by the captain was within usual range and did not contribute to the failure of the landing gear stating: "The brake pedal application to deselect the autobrake is likely to have imparted a short duration increased drag load to both MLG. This load was not excessive, but was enough to overload the already weakened structure of the left MLG inner cylinder."

The AAIB analysed: "It is possible that damage was as a result of very high temperature localised heating of the chrome plated surface which also affected the substrate beneath. This senario is supported by the presence of the martensitic area in the substrate steel as shown by the examination after microsection through the fracture surface. However, this is the only area of localised heat damage other than the helical banding. Therefore, it can be concluded that the chicken wire cracking is likely to have been caused by a grinding anomaly during the finishing process, but that it was not severe enough to impart heat damage into the steel substrate."

Related NOTAM:
C2206/14 - RWY 09/27 CLOSED. 29 APR 02:53 2014 UNTIL 29 APR 18:00 2014 ESTIMATED. CREATED: 29 APR 02:55 2014

Metars:
EGNX 290250Z 08005KT 3000 BR BKN006 10/09 Q1013
EGNX 290150Z VRB03KT 3000 BR BKN006 10/09 Q1013
EGNX 290120Z 10004KT 070V140 3000 BR BKN006 10/09 Q1013
EGNX 290050Z 12006KT 1500 BR BKN004 10/09 Q1013
EGNX 290020Z 13006KT 100V180 1100 R27/P1500 BR FEW002 BKN003 10/10 Q1013
EGNX 282350Z 11006KT 0900 R27/P1500 FG FEW002 BKN003 10/10 Q1014
EGNX 282320Z 13007KT 0900 R27/P1500 FG FEW002 BKN003 10/09 Q1014