History of the Flight
The aircraft, a Piper PA-34-200T registered G-BOSD, had completed an instrument rating examination flight without incident. While taxiing from the apron towards the runway holding point for a second detail, the nose landing gear collapsed. The commander stated that this occurred just after the nosewheel had bumped over a taxiway light. Damage was confined to the propellers and engines (which were shock loaded), and to the nose landing gear, gear doors, and nose cone.
Previous Occurrence
G-BOSD had suffered a similar incident on 19 June 1999 at the same airport while taxiing. In that case, the nose gear down-lock spring link was found to have been seized for some time before the accident, and this was considered the most likely cause.
Replacement of the Drag Brace Assembly
During repair after the first accident, the nose landing gear mechanism was thoroughly inspected by the maintenance organisation, and an AAIB Inspector viewed the aircraft and took the down-lock spring link parts for metallurgical examination. The drag brace assembly was replaced with new parts from Piper. The over-centre measurement of the new drag brace was 0.31 inches. The Maintenance Manual specifies that a measurement of .300 inch or greater is acceptable; between .300 and .245 inches requires notification; and less than .240 inch requires not returning the aircraft to service and notifying Vero Beach Customer Service.
During the investigation of the June 1999 incident, another Piper Seneca, G-BPON, experienced a nose landing gear collapse after landing. Extensive investigation found no reason for the collapse. Other unexplained collapses of Piper Seneca nose gears have been reported, with conjecture that normal loads could cause a serviceable system to unlock.
Following the investigation, a further examination of G-BOSD was made. The new drag brace assembly was undamaged, with no contamination or defects. The down-lock spring link was damaged at the eye end due to movement during collapse, but otherwise in good condition, not seized, and had normal spring action. The drag links weighed around 0.6-0.7 lb, and a new down-lock spring link exerted a force of around 2-3 lb. Thus, a force of about 2 lb applied to the centre bolt of the drag brace could move it towards the unlocked condition.
Possibilities for Unlocking the Drag Brace
Tests using a Piper Seneca III (similar nose gear retraction system) showed that a load of 2-3 lb was necessary to move the drag brace when on jacks, but it could not be moved to the unlocked condition by hand alone. Shaking the nose gear vigorously fore and aft caused the drag brace to move up to 1/8 inch without external force. With normal ground static loads (no hydraulic pressure), shaking the nose gear allowed the drag brace to be moved into the unlocked position with light hand pressure. It was considered that hydraulic pressure would be insufficient to prevent collapse if the drag brace was unlocked. The shaking simulated striking a taxiway light.
These tests were not definitive. It is considered that certain combinations of vertical acceleration on the drag brace, vertical and fore/aft loading on the nose wheel, and particular time histories could cause a serviceable drag brace to unlock. This could allow the natural tendency of the nose gear to retract to overcome hydraulic extension loads. The effect might occur most during taxiing, possibly from a small sharp step followed by a step down. Increasing the over-centre may not help, as it could cause the drag brace to straighten when the nose gear is rotated aft, and inertia might carry it through centre, leading to unlocking when load reverses. The down lock spring link exerts relatively little force.
Safety Recommendation
There is concern that the drag brace moves during taxi, as it is the only effective down lock. It should be just over centre and positively locked; if not achievable, a suitably large force should be exerted by the spring link. Recommendation 2000-46: The FAA and CAA, with New Piper Aircraft Company, should investigate causes of reported Piper Seneca nose gear collapses and consider design modifications to minimise drag brace movement and ensure sufficient locking force. New Piper, Inc. opined that the collapse resulted from possible improper rigging and installation and have no plans to redesign the drag brace.