On 23 July 2011, a Boeing 737-300 being operated by Jet2.com on a passenger flight from Leeds/Bradford to Paris CDG experienced violent vibration from the main landing gear at touch down in normal day visibility on runway 27R at a normal speed off a stabilised approach. This vibration was accompanied by lateral acceleration that made directional control difficult but the aircraft was kept on the runway and at a speed of 75 knots, the vibrations abruptly stopped. Once clear of the runway, the aircraft was stopped and the engines shutdown prior to a tow to the gate. None of the 133 occupants were injured.
An Investigation was carried out by the French Bureau d'Enquêtes et d'Analyses (BEA). It was found that the right main landing gear had suffered severe damage, especially to the lower torsion link. Flight Data Recorder (FDR) data showed that the recorded lateral load factor during the initial deceleration had reached 0.41g at 80 knots and the recorded amplitude had reached 0.8g at 90 knots. Deviation from the runway centreline was found to have reached a maximum of 10 metres.
The flight was the first one following overnight scheduled replacement of the main landing gear at the Operator’s own base maintenance facility. It was found that the hydraulic line to the shimmy damper on the right main landing gear had not been reconnected after the new gear assembly had been fitted and that the specified post replacement function checks carried out on jacks did not engage the shimmy dampers and so the anomaly was not detected prior to release to service. It was concluded that undamped vibrations during landing had quickly weakened the lower torsion link until it failed suddenly.
It was found that the Work Card used for the landing gear replacement had been exactly as issued by Boeing without any optional enhancements which maintenance organisations are permitted to add. The requirement to reconnect all hydraulic lines after installation of new landing gear was worded generically. The task required a supervisor sign off but did not require, either at the behest of the regulator or by decision of Jet2.com, a second independent supervisor sign off which is reserved for tasks classified ‘critical’.
It was noted that on the night of the gear replacement, the expected 2 aircraft in the Jet2.com hangar for maintenance had become 3 at short notice The additional aircraft required an unplanned repair to its fuselage and led to the overall workload exceeding the capacities of the personnel on duty.
The Supervisor for the landing gear change had frequently interrupted his work in order to deal with issues arising on the two other aircraft and “the mechanics were redirected from one aeroplane to another without any coordination between them being set up in accordance with the existing procedures”. It was concluded that “this had led to errors and approximations” including the failure to reconnect the right hand gear shimmy damper to the hydraulic system even though the mechanic’s and the supervisor’s endorsements for the corresponding task were stamped on the work card.
The Conclusion of the Investigation was that:
The Cause of the accident was “the failure, undetected by the maintenance personnel, to reconnect the right main landing gear shimmy damper hydraulic line to the aeroplane’s hydraulic system. The vibrations caused during landing rollout could not be damped down and weakened the torsion link until it failed suddenly”. The following Contributory Factors were identified:
- Poor resource planning caused overstretching of the personnel’s capacities when unexpected extra work turned up. This generated increased time pressure that led to things being forgotten and approximate execution of the work cards.
- The supervisor had worked for an excessive number of hours over a long period of time, which lowered his performance and his aptitude to carry out checks efficiently.
- Boeing work card n°32-011-00-02 in force in July 2011 did not specifically mention re-connecting the shimmy damper hydraulic line to the aeroplane’s hydraulic system.
Safety Action taken by Jet2.com to improve the standards at their Leeds/Bradford base maintenance facility was noted. It included:
- the introduction of a new technical procedure for managing and checking plugs to blank off hydraulic systems, pneumatic and gas systems which specified the attachment of visible identification tags on the plugs used and the making an entry in a log for every plug installed on a pipe or line. Removal of any plug then requires a log entry and upon completion of any task involving plug removal a check that the total number of plugs installed is the same as the number previously removed.
- the addition to the Jet2.com landing gear change Work Card of a specific requirement for a supervisor check of the hydraulic connection on the shimmy damper and a further independent check by a second supervisor.
- the introduction of a revised rostering system for maintenance personnel aimed at better management of fatigue risks
- a re-evaluation of maintenance task planning and modifications to avoid overloading available resources.
Safety Action taken by Boeing to revise the Boeing 737 -300/400/500 AMM in respect of the requirements for the removal/installation of the shimmy damper and the removal/ installation of the main landing gear was noted. These revisions introduce a specific requirement to reconnect the main landing gear shimmy damper hydraulic line as well as carry out a purge thereof. This action automatically triggers updates to the associated Work Cards.
The Final Report of the Investigation Accident Report: Vibrations, failure of the right main landing gear torsion link during landing roll was published in March 2012. No Safety Recommendations were made.