What happened
On 19 June 1999, a Piper PA-3/4-200T, registration G-BOSD, was conducting a private instrument training flight from Bristol to Exeter. The aircraft was operated with a crew of two and one passenger. After completing two instrument approaches and go-arounds, the aircraft performed a visual circuit and landed on Runway 26. During the final approach, the crew noted that all three green landing gear lights were illuminated, indicating the gear was down and locked.
Following a normal landing, the aircraft vacated the runway and proceeded via Runway 13/31 onto the southern taxiway. While taxiing at a slow walking pace with both engines set to 1000 RPM, the nose landing gear suddenly collapsed. The impact caused both propellers to strike the ground, resulting in the engines stopping and causing damage to the nose and propellers. There were no fatalities and no injuries to the three occupants.
The investigation
Investigators examined the nose landing gear and identified a failure in the downlock spring link, which is responsible for maintaining the over-centre downlock position of the drag brace. The examination revealed that the lower eye end of the link had suffered a fracture due to low cycle reverse bending fatigue. Furthermore, the telescopic section of the link was found to be jammed.
Detailed metallurgical analysis showed that the downlock link had been subjected to heavy loads in both tension and compression, leading to distortion around the transverse pin. This distortion caused an interference jam between the sleeve and the plunger. While the jammed condition did not prevent the gear from appearing operational, it prevented the spring from applying the necessary load to the rear drag brace. This meant the over-centre condition was not adequately protected against inertial loads, such as those encountered while taxiing over bumps.
Findings
- The primary cause of the collapse was the failure of the downlock spring link due to fatigue and a jammed mechanism.
- The downlock link had become jammed due to distortion around the transverse pin, which investigators attributed to incorrect rigging of the gear.
- The lack of sufficient spring load meant the drag brace was vulnerable to unlocking under the stresses of taxiing.
- The maintenance manual provided insufficient guidance regarding the precise adjustment and rigging of the downlock link.
Safety action
Following the investigation, a safety recommendation was issued (2000-45) for The New Piper Aircraft Company to review and expand the instructions for rigging the nose landing gear downlock mechanism within the PA-34 Maintenance Manual.