Event Details
On 18 December 1998 at 1750 UTC, a Piper PA-23-250 (registration G-BATX) operated by a private pilot on a private flight experienced a landing gear failure during landing at Shoreham Airport, West Sussex. The pilot, aged 40 with 600 hours total flying experience (200 on type), had conducted a series of low approaches and go-arounds before a normal landing. After receiving landing clearance and confirming three green landing gear indicators, the aircraft touched down normally. As the aircraft slowed, the left main landing gear leg began to collapse slowly. The pilot shut down both Lycoming IO-540-C4B5 engines, reported the issue to air traffic control, switched off electrical services and fuel, and ordered the two passengers to evacuate. None of the three occupants were injured. The left propeller was damaged in the accident.
Salvage and Examination
During subsequent salvage operations by airport management and the fire service, an air bag was inflated under the left wing to lift the partly retracted leg clear of the ground. The other landing gears were found to be geometrically locked. An attempt to manually move the left leg into the fully extended position resulted in the right main leg retracting, causing that side of the aircraft to fall onto the ground. This caused additional damage to the right wing.
After the aircraft was moved to a hangar and jacked, an examination of the landing gear system found no mechanical failure and no external hydraulic fluid leakage. Functional testing using a hydraulic ground power unit, conducted in the presence of the AAIB, showed all operations functioning correctly.
A similar landing gear incident had occurred on the same aircraft in 1996, affecting the right main leg. No conclusive cause was established then. Documentation from that repair indicated replacement of worn pivot bolts and seals in the anti-retraction valve.
System Description and Testing
The PA-23-250 features a single fixed-displacement hydraulic pump driven by the No. 1 engine, supplying fluid to a power pack. Landing gear and flaps are controlled by levers operating spools within the power pack. During a gear-down selection, after extension is complete, pressure in the down line should remain at around 500 psi to maintain the down-lock engagement.
Under AAIB supervision, the power pack was tested on a hydraulic test rig. When landing gear down was selected, pressure rose to the correct threshold but then fell to zero after the selector returned to neutral, instead of retaining the specified residual 500 psi. When flap operation was subsequently selected, pressure rose in both landing gear lines, with the retract line pressure leading the extend line. This indicated significant internal leakage within the power pack. Additionally, the flap lever did not return to neutral due to insufficient pressure in the flap extend line.
Maintenance and Design Observations
The investigation noted that it is not normal practice for organizations maintaining PA-23 aircraft to use hydraulic pressure gauges during functional checks. If no problem is reported, systems are assumed serviceable. In this case, the malfunction might only become evident upon a gear collapse.
The PA-23 hydraulic system design does not align with modern Fault Mode Effects Analysis (FMEA) principles. A single unit containing multiple valves immersed in the same fluid performs both landing gear and flap functions. Contaminants could cause internal leaks, leading to various malfunctions.
Further analysis suggested that during salvage attempts on similarly disabled aircraft, moving an incorrectly extended landing gear leg is likely to cause retraction of another leg.
Subsequent Actions
The power pack was returned by the owners to an overhaul company in the USA for replacement under a service exchange scheme. The company, reportedly the only organization actively repairing PA-23 hydraulic power packs, was requested to provide a strip report. As of the publication of the AAIB bulletin, the report was still awaited.