What happened
On 4 February 1999, a McDonnell Douglas MD90, registration SE-DMG, was performing a public transport flight approaching London Heathrow Airport. During the approach phase, the flight crew noted that while the nose and right main landing gear showed green 'down and locked' indications, the left main landing gear failed to provide a corresponding indication.
After an initial attempt to re-cycle the gear proved unsuccessful, the crew performed a go-around to reassess the situation. The crew determined that the issue was likely an indication error rather than a mechanical failure of the gear itself. Following the procedures outlined in the Emergency Checklist, the crew utilized the emergency 'blow-down' system to extend the gear.
Because the emergency system operates without the hydraulic pressure required to close the gear doors, the doors remained in a fully open position. During the subsequent landing on Runway 27R, the extended edges of the landing gear doors made contact with the ground, resulting in the destruction of two runway centreline lights and a score mark on the tarmac. There were no injuries to the 3 crew members or 83 passengers on board.
The investigation
Investigators examined the aircraft's landing gear deployment sequence and the functionality of the emergency extension system. Following the arrival at the gate and the disembarkation of all passengers, maintenance personnel inspected the aircraft. The investigation established that the primary issue was an indication system fault which prevented the green lights for both left main gear sensors from activating.