What happened
On January 2, 2004, at approximately 20:48 UTC, an Air Luxor Airbus A320, registration CS-TQE, was preparing for a flight from Madeira Airport to Oporto. Following a successful push-back from stand A06 and the start of the number two engine, the flight crew began starting the number one engine.
During the start sequence, specifically when the engine reached 15% N1, the nose landing gear underwent an uncommanded retraction. As the gear moved forward, the aircraft nose struck the ground on taxi lane A. The impact caused severe damage to the nose gear doors, the nose gear leg, the forward fuselage, and the engine cowls. While the 6 crew and 172 passengers on board escaped without injury, a ramp official assisting the aircraft sustained minor injuries after being struck by the fuselage.
The investigation
The GPIAAF investigation focused on determining how the landing gear could retract without any input from the gear selector lever, which remained in the "down" position. Investigators examined the aircraft's hydraulic and electrical systems, including the flight data recorder (DFDR) and cockpit voice recorder (CVR).
Technical testing of the landing gear selector valve, conducted by the manufacturer, revealed a manufacturing defect involving a faulty sealing ring. Furthermore, Airbus technicians identified an erratic failure in a proximity sensor on the left main gear strut. The investigation also analyzed the aircraft's electrical power transfer sequence from the APU generator to the engine-driven generator during the engine start process.
Findings
The investigation established that the incident was caused by a rare combination of two independent technical failures:
- A defective sealing ring in the landing gear selector valve allowed hydraulic oil to leak into the return line, creating abnormal pressure in the gear retraction line.
- A momentary power loss occurred during the electrical bus transfer from the APU to the number one engine generator. This transient period de-energized the landing gear door selector valve "A" solenoid, causing a momentary drop in pressure in the door closing line.
This simultaneous loss of pressure in the door closing line allowed the downlocks to be withdrawn, which, when combined with the abnormal pressure in the retraction line, permitted the uncommanded nose gear retraction.