A Boeing 747-400 which made an emergency landing at Gatwick had an 85kg piston installed upside down, investigators have announced.
Virgin flight VS43 to Las Vegas made national news on December 29 last year while it spent nearly four hours circling over Sussex before returning to Gatwick.
A 5kg metal board, found in a field in Kent three months later, turned out to have fallen off the plane while it was at 3,000ft.
An Air Accident Investigation Branch (AAIB) report, published today, said the plane had begun losing fluid from hydraulic system four soon after its 11.43am takeoff.
The plane’s handbook told the crew that losing system four would make the plane harder to control for pitch and roll, and would require them to use backup and manual brakes on landing. It also said they would have to use an alternate system to extend the wing landing gear.
The crew informed Virgin’s maintenance control department, who requested they return to Gatwick.
Since the plane was too heavy to land, it was kept in a holding pattern for 40 minutes while fuel was jettisoned, and preparations for approach began at 1.25pm.
The descent was abandoned at 3,000ft when the crew realised that the right wing landing gear could not be locked down for landing.
After consulting onboard manuals and discussing the problem with an advisor from Boeing, the pilot tried unsuccessfully to lock the gear out by manoeuvring the aircraft in climbs, descents and turns.
Eventually, the crew resorted to landing with only three of the four sets of landing gear in place, beginning their approach at 3.40pm.
The plane landed safely at 3.45pm, though leaning four degrees to the right.
However, the aircraft had to be checked for stability to make sure it was safe to disembark - in the end it was 6pm before the last passengers were off the plane.
When the plane was examined, its right wing landing gear actuator (a hydraulic piston which extends and retracts the landing gear) turned out to have been installed upside down.
This meant that a hydraulic port fitted to the head end had made contact with the landing gear door when it closed, distorting the port and allowing hydraulic fluid to leak out.
Loss of hydraulic pressure meant that, when it was extended, the landing gear emerged much too quickly and it lodged against the partly-opened landing gear door.
The strain on the door dislodged the metal ‘strike board’, an object which would normally guide the wheels into position.
The actuator had been wrongly fitted by ground crew the previous evening, the AAIB said.
The AAIB report noted: “The port at the head end of the actuator is labelled ‘UP’ and is located on the bottom of the actuator, whilst the port labelled ‘DN’ is located on the top of the actuator at the opposite end. Apart from a bleed valve on the opposite side of the head of the actuator to the port, there are no other distinguishing labels or features on the actuator to assist with orientation.”
The report said the ground crew’s job was made harder because the specific sling and hoist designed for removing and replacing an actuator were not available.
In the end, they manhandled the 85kg piston into place and two of them held it there while a third attached it.
The report said: “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.”
The AAIB has recommended that the relevant section of the 747-400 manual should be re-written to explain the process more clearly, and that the actuator design should be changed to make it more obvious which way up it should go.
The Board has also recommended that strike boards should be made more secure, and that the manual should warn crews about the possibility that landing gear might not extend properly after a loss of hydraulic pressure.
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