Updated: Dec 2, 2021
With 88 passengers, 4 cabin attendants and 2 flight crew Merpati Nusantara Airline Fokker 100 (PK-MJD serial number 11474) was descending towards El Tari Airport, Kupang in the Republic of Indonesia on this day in aviation history in 2009.
When 10 miles from the runway, at an altitude of approximately 2000 feet, the Pilot Flying (PF) asked the Pilot Monitoring to lower the landing gear. Only two of the three green down and locked lights came on, the lefthand main landing gear indication showed red. This showed that the left main landing gear was not down and locked.
While the crew continued the approach the landing gear lever was selected to up, and a short while later all gear indications were showing that the gear was up and locked, without other warnings or alerts showing.
The landing gear was subsequently selected down again and again the left-hand main landing gear showed unsafe, the red left main landing gear light was on.
FDR Readout of the accident, please note the green line indicating the left main landing gear remained not down all the time. (© NTSC)
A go-around was initiated and air traffic was informed about the reason for it. A holding was requested to troubleshoot the problem. ATC instructed the crew to enter a holding pattern over Kupang Bay, ~15 miles from the airport. The crew was aware that one of the passengers was a company aircraft engineer, he was asked to come to the flight deck to assist the crew and give his advice. The purser was also informed of the situation. While in the holding the crew performed the Emergency Check List (ECL) procedure for a gear unsafe situation. This included the alternate gear extension procedure, Although performing the procedure several times the situation remained unchanged. A low pass over the airport was subsequently flown and ground observers reported that the left main landing gear door was open, but the gear had not extended. The aircraft entered the holding again and further attempts were made to lower the gear;
Increasing the G-loading by pulling the aircraft up sharply
De-pressurising the #1 hydraulic system followed by an alternate gear down selection
Both attempts were unsuccessful, The crew had no other option than to land the aircraft with only two gears down and locked. ATC was informed about their intention and the cabin crew was instructed to prepare the passengers and the cabin. ATC arranged emergency services and informed the crew the airport was ready for them. The aircraft left the holding and was vectored on the approach, on short finals the command "Brace for impact' was given from the flight deck which was repeated by the cabin crew. The touchdown was in the touchdown zone. The left-wing was kept up as long as possible while the aircraft remained on the centre line. With speed decreasing the leftwing dropped on the runway and the aircraft started to swing to the left. 1200 meters down the runway the aircraft came to stop on the left shoulder of the runway. The cockpit crew completed their evacuation checklist while the cabin crew instructed the passengers to remain calm and evacuate the aircraft. The aircraft was safely evacuated with no injuries reported.
Aircraft recovery in progress (Source https://www.baaa-acro.com/)
The accident was investigated by the Republic of Indonesia National Transport Safety Committee (NTSC), they identified the following causes:
"Debris trapped in the chamber between the orifice and the stopper of the restrictor check valve in the left main landing gear hydraulic system.
This caused the orifice to be blocked. this resulted in the hydraulic oil in the landing gear actuator being blocked (effectively a hydraulic lock), causing the left main landing gear to jam in the up position"
The full report by the NTSC on the accident can be found by clicking here.
The O-ring debris found on the left restrictor check valve (© NTSC)