Virgin Atlantic B744 at London on Dec 29th 2014, hydraulic problem, landed safely without right main gear

Last Update: October 8, 2015 / 16:29:17 GMT/Zulu time

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Photo of Virgin Atlantic G-VROM, Boeing 747-400 — Virgin Atlantic G-VROM, Boeing 747-400 (Photo credit: Jonathan Palombo Photography / Flickr / License: CC by)

Incident Facts

Date of incident
Dec 29, 2014

Classification
Incident

Airline
Virgin Atlantic

Flight number
VS-43

Departure
London Gatwick, United Kingdom

Destination
Las Vegas, United States

Aircraft Registration
G-VROM

Aircraft Type
Boeing 747-400

ICAO Type Designator
B744

A Virgin Atlantic Boeing 747-400, registration G-VROM performing flight VS-43 from London Gatwick,EN (UK) to Las Vegas,NV (USA) with 447 passengers and 15 crew, was enroute at FL320 about 190nm west of London Gatwick when the crew decided to return to Gatwick due to hydraulic problem. Upon approaching London Gatwick the right main gear did not fully extend. The crew went around, worked the related checklists without success, performed a low overflight of Gatwick Airport to have the landing gear inspected from the ground which confirmed the right main gear had not extended. The aircraft was holding to burn off fuel while external emergency services were called to Gatwick to assist for the landing. Preparations were made for the aircraft landing on left main gear, both body gear, nose gear, but without the right main gear. The aircraft touched down, rolled out safely on Gatwick's runway 26L and stopped wings level on the runway about 4 hours after departure from Gatwick.

Mobile stairs and busses are heading towards the aircraft, the passengers are going to disembark onto the runway before the aircraft is going to be towed off the runway.

The airline confirmed a problem with the right main landing gear and reports the crew is going to implement a "non-standard landing procedure".

Gatwick Airport reported the aircraft initially returned to Gatwick due to a minor technical problem.

A passenger reported the crew announced they were returning to Gatwick due to a hydraulic problem, then the issue with the landing gear unfolded. They were instructed to assume brace positions for the landing. The landing was smooth.

On Oct 8th 2015 the AAIB released their bulletin into the serious incident reporting that it turned out the right hand main gear hydraulic retract actuator had been incorrectly installed.

The aircraft had undergone maintenance over night, the crew was notified of the work. During the walk around they noticed the gear pins were still installed and requested them to be removed. The captain (47, ATPL, 12,279 hours total, 9,771 hours on type) was pilot monitoring, the first officer pilot flying. A second first officer was occupying the observer's seat.

The aircraft departed runway 26L, the gear was selected up and the aircraft was climbing through 1000 feet AGL when the crew received indication that the hydraulic quantity of system 4 was low. The quantity gauge was showing the quantity was rapidly decreasing. The flaps were retracted and the according checklists were carried out. The problem appeared to be contained and the climb was continued.

The crew contacted maintenance and was requested to return to London. The aircraft dumped fuel for about 40 minutes and a slow (alternate) flaps extension as well as an alternate gear extension was carried out, the right main gear however failed to lock down. The crew aborted the approach and levelled at 3000 feet, the crew requested a visual inspection from the ground which resulted in reports by ground observers that the right hand gear was not visible.

In consultation with onboard manuals, company maintenance and direct contact with an advisor of Boeing the crew attempted to resolve the issue including attempts to lock the gear down by G-maneouvering (climbs, descends and turns), however without success. The crew therefore prepared for an abnormal landing. The captain assumed role as pilot flying and landed on runway 26L, the aircraft came to a stop with a slightly right wing low attitude, right bank angle of 4 degrees. The situation was assessed, the captain decided that an evacuation was not needed.

The passenger remained on board while the aircraft was checked for stability, then disembarked onto the runway about 45 minutes after touchdown, the disembarkation was finished another 90 minutes later.

The AAIB wrote: "Once the passengers had been disembarked, an initial inspection of the landing gear was conducted. The right wing landing gear door was partially open, with the outboard rear wheel of the wing gear resting on the outboard section of the door. The outboard section of the door was significantly deformed, but the door itself was still firmly attached and the gear was securely held in a partially deployed position. ... The lower section of the strike board in the right wing landing gear bay was missing."

The AAIB wrote: "The right wing landing gear actuator was found installed 180° out of alignment. The hydraulic port boss fitting on the head end of the actuator was distorted and damaged."

About three months after the occurrence a farmer found an object in one of his fields and reported the piece to authorities. The piece, not showing any serial or otherwise identification numbers, was visually identified to be a strike board and was reported to the AAIB. The AAIB wrote: "Using data from radar and the aircraft FDR, a comparison of the flight path and timing of the original gear extension showed this coincided exactly with the location where the strike board was found, giving a high probability that the recovered item was from G-VROM." and explained the function of the board was: "The board is designed to guide the wheel past the door should contact occur during alternate system extension of the landing gear."

The AAIB described the maintenance action the previous night: "The installation procedure commenced at approximately 2145 hrs and began with the team positioning a set of steps and a lifter platform, carrying the replacement actuator, underneath the aircraft. In order to install the actuator it had to be passed through a section of structure in the wing. The team positioned spill bags to prevent damage from any contact between the actuator and the wing structure. The sling and hoist were not used by the team, who instead manhandled the actuator between the two technicians standing in the lifter and the engineer standing on the steps. The weight of the actuator was then supported by the two technicians, while the engineer attempted to install the pin which secured the actuator to the hanger. After 20 minutes of unsuccessful effort, the team’s positions were rotated and they tried again to locate the pin for a further 10 minutes. Eventually the actuator was successfully secured in place by one of the technicians. The team then continued to work through the night to reconnect the hydraulic hoses and leak check the hydraulic system. The AMM did not require a full operational test of the landing gear actuator following replacement, just a selection of the gear lever up with the gear locking pins in place, to check the gear leg began to move before being restrained by the locking pin and to check for leaks. The aircraft was then prepared and released for service that morning."

The AAIB analysed: "The successful outcome of this event hinged on good communication and co-operation in a number of areas. The additional pilot on the flight deck enhanced the task sharing and reduced the workload on the co-pilot and the commander. The crew were able to spend time working through all the possible options available to them and to be sure that everything had been considered before the landing. The consideration of available options was assisted by the input from the operator’s IOCC facility. In being external to the aircraft the IOCC personnel were able to contribute from additional resources, which included expertise from the aircraft manufacturer. However, communication with the IOCC was not straightforward because of the interruptions and interference from other stations on the shared frequency. Shared frequencies for company communications are a normal arrangement and in most cases interruptions constitute a nuisance and are not critical. The Very High Frequency (VHF) frequencies allocated to aviation are a resource with limited capacity but it would have been useful in circumstances such as these to have been able to switch to a dedicated frequency."

The AAIB continued analysis: "There are two separate aspects to this incident; the maintenance issues which led to the in-flight hydraulic leak, and the circumstances which resulted in the right wing landing gear becoming ‘hung-up’ on the gear door." and stated:

Replacement of landing gear actuators is not a common maintenance task on the 747‑400. As such there is limited opportunity for individual maintenance organisations to develop internal “best practice” techniques or to identify and rectify weaknesses or missing information within the manufacturer’s AMM instructions. The maintenance teams tasked with the replacement of the gear actuator on G-VROM faced a number of problems. They were not able to locate a number of the specialist tools required by the AMM, including the hoist which the manufacturer specified for safe lifting of the weight of the actuator whilst it was being manoeuvred into place. The operator’s internal investigation has made a recommendation within the company to address this issue. However, the team identified that even if the hoist had been available, the manual did not specify how to operate the sling, or how best to utilise it together with the hoist in the difficult task of manoeuvring the actuator through the wing structure surrounding the actuator location. The AMM is the main source of guidance for completing any maintenance task. If specific guidance is not found in the AMM, then engineers and technicians might develop improvised techniques to accomplish a task, particularly outside normal office support hours such as during night shifts.

Ultimately, the maintenance team working on G-VROM elected not to use any form of mechanical support, thus greatly increasing the difficulty and risk associated with installing the replacement actuator. The result of this decision was that the task became so physically demanding that the maintenance team became entirely focused on just attaching the actuator to the aircraft, in order to relieve themselves of the 85 kg weight they had manually supported for over 30 minutes. As such, they had no remaining capacity to ensure they installed the actuator in the correct orientation. It was subsequently determined that they had rotated it 180° about its long axis during installation, effectively installing it upside down.

The significance of this maintenance error was that the hydraulic fluid ports on the actuator were now transposed, with the port at the head end of actuator facing upwards. The AMM did not require the gear to be fully cycled following maintenance. Consequently, the insufficient clearance between the hydraulic port and the top of the landing gear bay, when the gear was in the retracted position, was not identified until the first time the gear was retracted fully during the incident flight the following morning. The force exerted on the hydraulic port as the gear retracted, caused it to distort and release hydraulic fluid at the full system pressure of 3,000 psi. This rapidly depleted the reserve of hydraulic fluid in system 4, generating a low quantity and then low pressure warning in the flight deck.

Whilst the manner in which the actuator was installed by the maintenance team significantly increased the likelihood of a maintenance error occurring, the design of the actuator itself increased the probability of the error remaining undetected. The actuator was virtually uniform in shape and colour, such that there was no obvious top or bottom to it. The structural connections could be installed in either orientation and the use of flexible hoses meant the hydraulic connections could be made to fit an incorrectly installed actuator. Finally, the hydraulic port on the bottom of the actuator was labelled ‘UP’, with the one on the top labelled ‘DN’, which was inherently open to misinterpretation.

With respect to the gear becoming hung on the gear door the AAIB analysed:

Due to the location of the leak on G-VROM, the right wing landing gear system was drained of hydraulic fluid. The landing gear alternate extension system is designed to work with hydraulic fluid present in the system. Most significantly for this event, the rate at which the gear leg descends, when deployed using the alternate system, is controlled by slowing the flow of hydraulic fluid around the system by means of a restrictor valve. When the hydraulic fluid is lost, the descent of the gear leg is undamped and accelerates under gravity. This has two potential implications for the wing landing gear, as demonstrated by the G-VROM event. Firstly, the gear door may not have fully opened prior to the arrival of the descending gear leg. Given the concertina design of the door, this will result in the gear leg becoming ‘hung up’ on the door, with no way of releasing it prior to landing. Secondly, as a result of the door being partially open, the strike board is mechanically held in the horizontal position when the tyre strikes it. The strike board attachment hinge was not designed to withstand the load imparted when the board is stuck in this orientation by an undamped gear leg. On G-VROM, this caused the hinge to fail and the board to be released from the aircraft. The aircraft was at an altitude of approximately 3,000 ft and travelling at 180 kt when this occurred. The 5 kg strike board would therefore have reached the ground with sufficient energy to cause significant damage or injury.

Four safety recommendations were issued as result of the investigation.