What happened
On 2 December 2009, a Fokker F28–MK100, registration PK-MJD, was operating a scheduled passenger flight from Makassar to Kupang, Indonesia. The aircraft was carrying 94 people, including 88 passengers, the flight crew, cabin crew, and a maintenance engineer.
During the approach to El Tari Airport, the crew selected the landing gear to the down position. However, the left main landing gear indicator light remained red, signaling that the gear was not locked and the aircraft was unsafe for landing. The crew attempted to resolve the issue by cycling the gear and using the alternate selector, but the left gear remained in the up position. After several unsuccessful attempts and a discussion involving the onboard engineer, the crew decided to proceed with an emergency landing with the left gear retracted.
During the landing, the pilot maintained the aircraft on the centerline while the engines were shut down. The aircraft came to a stop on the left shoulder of the runway. Despite the high-stress nature of the arrival, there were no injuries to the passengers or crew.
The investigation
NTSC investigators examined the hydraulic systems and the landing gear assembly to determine why the gear failed to extend. The investigation focused on the restrictor check valve within the hydraulic lines.
Upon dismantling the component, investigators discovered six pieces of debris, which appeared to be fragments of an O-ring. The investigation established that during previous gear-up cycles, high hydraulic pressure (3000 psi) had pushed these fragments through the check valve, damaging a screen filter and trapping the debris in the chamber between the orifice and the stopper. When the crew selected the gear-down position, the lower hydraulic pressure (500 psi) was insufficient to move the debris, which subsequently blocked the orifice and prevented the actuator from extending the gear.
Findings
- The primary cause of the incident was debris trapped in the restrictor check valve chamber, which blocked the hydraulic flow necessary to extend the left main landing gear.
- The debris, consisting of O-ring fragments, was hidden from both the maintenance and flight crews.
- The left main landing gear was eventually able to move to the down position only after the hydraulic pressure was released by loosening a flexible hose fitting during the post-landing evacuation.
Safety action
Following the investigation, the NTSC issued recommendations to the aircraft manufacturer to review the design of the landing gear hydraulic system. Additionally, the Directorate General Civil Aviation issued a safety circular requiring operators of Fokker 28 MK 0070 and MK 0100 aircraft to perform detailed inspections of the landing gear hydraulic restrictor check valves.