Cello B462 at London and Guernsey on Feb 23rd 2015, gear shock absorber separated
Last Update: March 10, 2016 / 17:22:42 GMT/Zulu time
Incident Facts
Date of incident
Feb 23, 2015
Classification
Report
Airline
Cello Aviation
Flight number
GR-1607
Departure
London Gatwick, United Kingdom
Destination
Guernsey, United Kingdom
Aircraft Registration
G-RAJJ
Aircraft Type
BRITISH AEROSPACE BAe-146-200
ICAO Type Designator
B462
The UK AAIB released their bulletin reporting that in the meantime another aircraft had landed on the runway possibly contaminated with debris and before a runway inspection had been performed.
Following the request by the crew to have the left main gear inspected emergency services responded. After the aircraft arrived at the remote stand it was discovered that the outboard left main tyre had burst and the left main shock absorber had separated into two parts.
A runway inspection did not detect any debris on the runway however later a piece of rubber was found on the parallel taxiway. The next morning a ring seal and some more rubber were found at the southern edge of runway 27 and upon further inspection a bolt head, still attached to a piece of locking wire, was discovered at the southern edge of the runway.
The AAIB stated: "An internal investigation by the operator determined that a pan call should have been made by the flight crew to alert ATC at Gatwick to the landing gear malfunction. Also, as the crew did not know the cause of the abnormal gear indications, a more suitable decision was to return to Gatwick (which has a longer runway) than to continue to Guernsey with the landing gear in the down position."
The AAIB continued: "From the downloaded data it was evident that there was a fault with the FDR in that several of the tape tracks, including the track containing the accident flight, contained a significant amount of corrupted data, while the remaining tracks were good. When the operator was asked to provide evidence to show that the FDR was being checked annually for correct operation, in accordance with the regulations, it was evident that the QAR data recordings were being used to demonstrate this instead, and that the last overhaul of the FDR had been conducted 25 months previously, in January 2013. A functional check of the FDR had been made in July 2014; however, this did not involve checking the quality or consistency of the data and so did not flag up any faults. This oversight by the operator has since been rectified and the correct acceptable means of compliance to the requirements of the regulations is now being followed."
The shock absorber had accumulated 33,394 flight cycles since new (60,000 cycles maximum) and 3,882 flight cycles since last overhaul (15,000 cycles maximum) when it was disassembled from another airframe and put into storage in April 2013. During a C-Check completed on Jan 8th 2015 the shock absorber was fitted to G-RAJJ, the aircraft flew another 27 cycles until the accident flight.
The AAIB analysed: "The component manufacturer reported that they had never previously experienced a BAe 146 shock absorber separating, nor were they aware of other occasions when the diaphragm locking bolts had sheared or fatigue damage had been found on the threads on the diaphragm. The number of score marks on the bore of the outer cylinder indicated that the head of the locking bolts had sheared off at least two flight cycles before the shock absorber came apart. Shards from the sheared thread on the diaphragm were found throughout the shock absorber indicating that the oil had moved them as the shock absorber cycled. From the physical damage it was assessed by the component manufacturer that the initiating factor was most likely to have been the threads on the diaphragm jumping across the threads on the inner cylinder. This jumping movement of the threads would have applied a shear load on the diaphragm locking bolts causing them to fracture. Thread jumping could have occurred one thread pitch at a time, or several at a time, thereby reducing the number of threads in engagement. During the subsequent landing at Gatwick, the compression damping load on the diaphragm exceeded the shear fracture load of the reduced number of engaged threads causing them to shear and thereafter shearing the non-engaged threads. As the aircraft departed Gatwick the shock absorber would have extended during recoil; however, because the threads on the diaphragm had sheared, there was no longer a mechanical stop to prevent the shock absorber from separating. At some time prior to the fracture of the locking bolts, the torque between the diaphragm and the inner cylinder would have reduced, allowing fatigue cracks to develop on the first full thread on the diaphragm. These fatigue cracks did not cause, or contribute, to the separation of the shock absorber. As a consequence of the separation, during the retraction sequence the left main landing gear would not be able to ‘shorten’ and fit into the bay, which resulted in the No 2 mainwheel fouling on the frame. It is likely that the right main landing gear, No 3 mainwheel, then fouled on the No 2 mainwheel preventing it from fully retracting. This resulted in the illumination of both main landing gear position red indicators. Examination of the shock absorber indicated that it had been correctly manufactured and assembled. Therefore, for the threads on the diaphragm to jump, and the locking bolts to shear, the shock absorber must have experienced recoil loads beyond its design load. This load could have resulted from a hard landing, with a subsequent bounce, or incorrect gas pressures and oil level in the shock absorber. The aircraft operator reported that the shock absorber had not had any maintenance since it had been fitted to the aircraft and there had been no reports of it having experienced a heavy landing. The shock absorber had not been disassembled since its last overhaul in 2003 and the physical damage to the diaphragm thread and locking bolts may have occurred whilst it was fitted to another aircraft."
The AAIB analysed the loss of the green hydraulic system and loss of (normal) braking could not be explained. About one liter of hydraulic fluid had been lost as result of the separation of the shock absorber, this however would not have rendered the hydraulic system inoperative, the AAIB reasoning: "This indicates that the hydraulic fuse in the Green No 1 brake supply line operated after the union became disconnected."
With respect to crew response the AAIB analysed: "The pilots followed the emergency checklist correctly and resolved the abnormal landing gear than at indication, but they did not appear to have considered the possibility that this indication was caused by a mechanical malfunction. This was because the takeoff run felt smooth and the commander’s thinking was influenced by his previous experience of abnormal gear indications. There was a delay before ATC was informed of a problem but no pan call was made and no details were shared other than there was an “issue” with the gear. The crew believed they properly assessed the situation before deciding to continue to their passengers’ destination; it was a short sector, the only abnormal indication was that they were cruising with the gear down and there was plenty of fuel to cope with the extra drag. The operator’s assessment was that a more suitable decision would have been to return to Gatwick. The landing roll was well-controlled by the crew in response to a burst tyre and the failure of one brake system. However, the operator and ATC expect an aircraft with a suspected burst tyre to be brought to a halt and not to continue taxiing until an external check has been completed. In this case the aircraft was not stopped and a protracted taxi routing was followed, even though the crew suspected a burst tyre and knew the Green brake system had failed and the aircraft was listing to the left. Their decision-making was influenced by not wanting to block the runway or taxiway and to continue to a parking stand."
Incident Facts
Date of incident
Feb 23, 2015
Classification
Report
Airline
Cello Aviation
Flight number
GR-1607
Departure
London Gatwick, United Kingdom
Destination
Guernsey, United Kingdom
Aircraft Registration
G-RAJJ
Aircraft Type
BRITISH AEROSPACE BAe-146-200
ICAO Type Designator
B462
This article is published under license from Avherald.com. © of text by Avherald.com.
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