History of the Flight
The aircraft, a Piper PA-34-200T (registration G-BOSD), was conducting an instrument training flight from Bristol to Exeter on 19 June 1999 at 1030 UTC. On board were a commander (holding a Basic Commercial Pilot's Licence with Instrument and Instructor's Rating), a second pilot (a CPL holder), and a passenger (also a pilot). After two instrument approaches and go-rounds at Exeter, the crew flew a visual circuit and landed on Runway 26. During final approach, all three occupants observed that the three green landing gear lights were illuminated. The landing was normal, and the aircraft vacated the runway, taxiing via Runway 13/31 onto the southern taxiway. While taxiing at a fast walking pace with both engines set to 1000 RPM, the nose landing gear collapsed. Both propellers struck the ground, stopping both engines. The aircraft was secured and vacated.
Examination of the Nose Landing Gear
Examination revealed that the downlock spring link, which maintains the over-centre downlock position of the drag brace, was fractured at the lower eye end, and its telescopic section was jammed. Metallurgical examination showed that the lower eye end had fractured in very low cycle reverse bending fatigue, with final separation occurring due to stresses well above the elastic limit from compression loads. The downlock link consists of a plunger and a sleeve retained by a transverse pin, spring-loaded in the extended position. Evidence indicated the link had compressed over most of its travel at some point, but lacked internal wear marks from relative movement between plunger and sleeve, suggesting it became jammed after only brief operation. The link had been overloaded in both tension and compression, causing distortion around the transverse pin, which led to the jam. No other damage explained the collapse. The aircraft had accumulated approximately 10,000 hours at the time; the last maintenance or rigging of the nose landing gear drag brace and downlock link could not be determined but was not recent.
The jammed condition would not have prevented normal gear operation; gravity would maintain the drag brace in the locked over-centre position. However, the spring load intended to maintain the over-centre condition against inertial loads (e.g., taxiing over bumps) could not be applied. There was no assurance the nose gear would not collapse at any time. The low cycle fatigue in the eye end suggested the gear may have unlocked on previous occasions.
Probable Cause
The distortion leading to the jam could only occur if the gear collapsed or was incorrectly rigged. Given evidence that the link jammed early in its life, mis-rigging was identified as the probable cause of the jam. The Maintenance Manual provides limited adjustment information for the downlock link; if rigged over-length, damage can occur during retraction testing, causing a jam and inadequate over-centre loading of the drag links.
Safety Recommendation
The AAIB issued Safety Recommendation 2000-45, recommending that The New Piper Aircraft Company review and amplify the instructions for rigging the nose landing gear downlock mechanism in the Piper PA-34 Maintenance Manual. The company responded that they consider the collapse was caused by a worn downlock spring and had revised the manual in February 1997, with no plans to redesign the drag brace.
Similar Occurrence
A similar nose gear collapse of a Piper Seneca (G-BOCG) on 31 October 1997 was reported in AAIB Bulletin 3/98, also attributed to jamming of the downlock link due to mis-rigging.