Maintenance personnel installed a Boeing 747-400 main landing-gear actuator upside-down before the aircraft suffered an undercarriage jam while returning to London Gatwick.
The Virgin Atlantic aircraft (G-VROM) had departed the airport for Las Vegas on 29 December 2014, but the crew was alerted to a depletion of hydraulic fluid shortly after gear retraction.
Although the situation was contained by the crew, the carrier requested that the aircraft return to Gatwick.
The wing main landing-gear was one of the systems affected by the hydraulic failure and deployment required use of the alternate gear-extension procedure.
But as the undercarriage lowered, the right wing main gear – falling quickly as a result of being undamped by the loss of hydraulic fluid – jammed against its door and did not lock in place.
The crew, unaware of the reason for the jam, attempted to dislodge it with a series of manoeuvres. But these were unsuccessful and the aircraft was forced to land at Gatwick on its three remaining main-gear bogies.
In its analysis of the event the Air Accidents Investigation Branch found that the aircraft had undergone replacement of the actuator on the right wing main gear the night before the flight.
The maintenance team did not use a sling to hold the 85kg actuator in place as it was fitted. Two technicians instead manually supported it – holding the heavy component for over 30min, owing to delays in locating an attachment pin.
Opting not to use the sling “greatly” increased the difficulty of installation, says the inquiry.
“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 [weight],” it states.
“As such, they had no remaining capacity to ensure they installed the actuator in the correct orientation.”
The inquiry says the actuator was inadvertently installed upside-down. No full operational test was required and the error remained undetected until the 747’s landing-gear was retracted after take-off for Las Vegas.
Incorrect installation resulted in forces that distorted a hydraulic port during retraction, causing the fluid leak and subsequent gear jam.
Virgin Atlantic produced its own “comprehensive” report following the incident, which included 28 recommendations mainly relating to internal process improvements, says the inquiry.
But the investigators also point out that the actuator was “virtually uniform” in shape and colour, with no “obvious” indicators regarding orientation.
Labels on the hydraulic ports were “inherently open to misinterpretation”, and the design meant that hydraulic connections could be made to fit an incorrectly-installed actuator, says the inquiry, which has recommended that Boeing modify the component to reduce risks of a recurrence.