What happened
On 7 April 2005, a McDonnell Douglas MD-11, registration N413LT, was conducting a series of scheduled test flights at Singapore Changi Airport. These flights followed a passenger-to-freighter (PTF) conversion performed at a local maintenance facility. The flight program included several landings and a go-around to verify the aircraft's systems.
While the first two landings proceeded without issue, the third landing on Runway 02L resulted in a structural failure. Immediately after touchdown, the crew felt heavy vibrations. As the aircraft decelerated and the pilot manually disarmed the autobrake, a "Gear Unsafe" warning appeared on the flight displays. Upon inspection, it was discovered that the centre landing gear (CLG) was no longer in its normal position, having shifted from a 15-degree forward angle to approximately 45 degrees aft of vertical.
The investigation
Investigators examined the aircraft's mechanical state and recent operational history. The inspection revealed that the forward attachment points for the CLG lower drag brace link had broken, and the lugs on the gear retraction actuator had also failed. This structural failure caused the gear to swing back, resulting in dents to the fuselage and damage to the wheel well bulkhead.
Prior to the incident, the aircraft had undergone high-speed braking tests. Data from the Brake Temperature Monitoring/Tyre Pressure Indicator (BTM/TPI) showed that certain brakes reached temperatures exceeding the 550°C threshold. Additionally, investigators found aluminium shavings from a manifold shim contaminating an inlet filter in the brake control valve. The aircraft had also been out of service for an extended period during its conversion process.
Findings
- The failure of the CLG lower drag brace link and the retract actuator was caused by overloading, though the specific trigger for this overload could not be determined.
- The aircraft's brake system exhibited irregular behavior during post-incident testing, with certain pistons failing to function correctly.
- There is currently no specific maintenance requirement to inspect the attachment lug of the CLG lower drag brace link.
- The aircraft had been grounded for a significant duration for heavy maintenance and conversion, which can lead to sluggishness in mechanical and electrical components.
Safety action
Following the investigation, the aircraft manufacturer issued a Service Bulletin recommending the inspection or replacement of the Brake Check Valve Manifold Assembly and inlet filters to prevent contamination-induced anomalies. Furthermore, a Service Letter was released advising operators to perform thorough operational tests of the antiskid system and shock strut servicing when returning an aircraft to service after long-term storage.