Incident Overview
On 30 December 1997 at 1350 hours UTC, a Europa aircraft, registration G-BWYD, was engaged in a private training flight at Kemble Airfield, Gloucestershire. The aircraft was powered by one Rotax 912-UL piston engine and had been manufactured in 1997. The commander held a Basic Commercial Pilot's Licence with an Instructor Rating, aged 70, with 12,267 total flying hours, including 73 hours on type. The co-owner was receiving re-familiarisation training following a previous landing accident at Biggin Hill in August 1997.
Sequence of Events
The sortie included circuits from grass Runway 22, with wind reported as 220° at 12 to 15 knots. During downwind checks on the first circuit, the pilot selected the landing gear DOWN, and the instructor confirmed the selection via the green gear-down indication light and engagement of the safety latch on the gear selector lever. The subsequent landing touchdown was normal, though an initial 'skip' occurred due to an undulation in the grass surface. Near the end of the landing roll, the instructor noticed the right wing dropping, suspecting a failure of the right outrigger. Before any reaction, a loud bang was heard in the cockpit, and the aircraft sank onto its underside. After a short ground slide, one propeller blade broke off at the hub, and the aircraft came to a halt. The pilots selected all switches to OFF. It was then observed that the landing gear selector lever was in the forward (UP) position, although not in its gate.
Subsequent Collapse
Following the incident, the aircraft was raised by a mobile crane, and the landing gear was lowered. The aircraft was pushed onto a taxiway for recovery to a hangar. While on the tarmac, a cockpit occupant applied brakes as the aircraft was manoeuvred down a slope. After a short distance, the landing gear collapsed again, this time with the selector lever remaining in the DOWN position.
Aircraft Landing Gear System
The Europa's landing gear consists of a single retractable mainwheel, a tailwheel, and two outrigger wheels under the outboard wing sections. The retraction lever is directly attached to the gear operating linkage, which incorporates an over-centre mechanism. Flaps and outrigger wheels share the same linkage. The safety latch prevents inadvertent gear retraction on the ground; it rotates under its own weight into a guide slot, requiring a positive action to disengage. This aircraft had a modification: a green light illuminated when the gear was selected DOWN, but the switch was mounted on the mainwheel mechanism, not the safety latch, so it could not confirm the gear was 'locked'.
Investigation Findings
Consultation with the repair organisation suggested a possible explanation: the safety latch may not have been fully engaged prior to landing, as the retraction lever was found out of the DOWN gate after the incident. However, the instructor recalled the latch position appeared normal. On undulating surfaces, flexing of the landing gear system can cause the linkage to rebound from its over-centre stop, moving the retraction lever unless the safety latch is in place. Three previous landing gear collapse incidents on this type had been attributed to incomplete latch engagement. It was considered unlikely that the gear could collapse with the latch correctly engaged without substantial damage to the retraction lever gate structure.
Examination of Retraction Lever
The landing gear retraction lever was examined by the AAIB. Two of three bolt holes used to attach the lever to the main gear mechanism showed elongation of about 1.5 mm. While some elongation resulted from the incident, file marks in the bores indicated earlier 'opening out' of the holes, possibly to facilitate fitting. One round hole suggested the lever was assembled with a bolt through it, and alignment of the other two holes was achieved by filing. This created a poor joint relying on flank friction, evidenced by indentations around the holes where washers had dug into the lever. Deep smears around the elongated holes indicated the joint slackened in service and pivoted about the non-elongated hole. The resultant free play, though small, could have allowed additional movement of the main gear mechanism away from its over-centre stop under ground undulations.
Possible Cause of Second Collapse
It is possible that distortion to the landing gear linkage occurred during the first incident, so that after raising and lowering the gear, the mechanism did not achieve its over-centre condition. When placed on the ground, the gear may have been initially prevented from collapsing only by the safety latch holding the selector lever in its DOWN gate. The subsequent collapse may have been due to additional loads from the occupant's weight inducing distortion of the selector lever while retained in the gate by the safety latch.