A known defect in a BAE Systems-made flight-control system component is blamed for causing the crash of a $133 million US Air Force Lockheed Martin F/A-22 Raptor late last year.
The F/A-22 crashed 11s after take-off on 20 December at Nellis AFB, Nevada, the first production Raptor to be lost. The pilot ejected with the aircraft in a near-inverted attitude. The aircraft struck the end of the Nellis runway going backward. The USAF Accident Investigation Board (AIB) has traced the crash to the failure of all three rate sensor assemblies (RSA), which provide feedback on yaw, roll and pitch status to the flight-control system.
The pilot inadvertently triggered the failures of the RSA during pre-flight operations, according to the AIB report released on 8 June. The pilot shut down the engines during a maintenance check, believing the flight control systems were continuously powered by the auxiliary power unit (APU). The AIB attributed the pilot’s mistake to “ambiguous” language in the aircraft’s technical orders.
In fact, the flight-control system momentarily loses power during an engine shutdown. This interruption in the power supply then is linked to a known quirk in the RSA unit, which is programmed so that it could interpret a momentary power loss as an instruction to enter test mode, which freezes or “latches” the unit, according to the AIB report.
In the F/A-22 programme’s history, Lockheed has returned about 20 RSAs to BAE for suspected latching events, the report says. Before the crash, latching events froze only one or two RSA units simultaneously, which triggered a warning light in the cockpit. However, the aircraft’s flight-control system has no way to warn the pilot if all three RSA units are inoperative, so was unaware of the problem. An upgraded version of the RSA unit has been designed and is being installed on the fleet.
STEPHEN TRIMBLE / WASHINGTON DC
Source: Flight International
