Flight History
On 11 December 1995, at approximately 1315 local time, a Piper Seneca PA-34-220T, registration HB-LRC, departed Zurich Airport for an instrument flight rules (IFR) training flight. The flight plan included two VOR/DME approaches with touch-and-go landings at Grenchen, followed by a return to Zurich. After receiving ATIS Golf from Grenchen, the student pilot discussed the approach with the flight instructor. The crew received approach clearance for runway 25 and departure instructions for the VOR Willisau after the first landing. According to both pilots, the approach was stable and correct. The final check was performed at 8 NM DME from Grenchen. The student and instructor verified that all three landing gear indicator lights were illuminated, showing the gear extended and locked.
At approximately 3500 ft, the aircraft entered the cloud layer. The descent rate was planned so that the minimum descent altitude (MDA) of 2040 ft was reached simultaneously with the visual descent point (VDP) at 1.9 NM DME GRE. After air traffic control cleared the aircraft for a touch-and-go landing, the student pilot saw the runway threshold at the VDP and fully extended the flaps. The runway centerline and glide path were maintained using runway lights and PAPI. In short final, the speed was 80 kt, decreasing to 75 kt over the threshold. As the instructor rechecked the landing gear indicators, the student pilot retarded the power levers and landed with both hands on the controls. When the main gear touched down, a brief stall warning sounded, and the nose then lowered and contacted the runway surface.
The Seneca came to a stop after 300 to 350 m on the runway. The master switch was turned off and fuel valves closed. The airport fire service arrived shortly, but no fire occurred. The uninjured occupants exited the aircraft unaided.
Findings
- Both pilots held valid licenses. No evidence of health impairment.
- The aircraft had undergone its Swiss technical acceptance check on 22 September 1995 at Grenchen, with a total flight time of 128:43 hours. At the time of the accident, the total flight time was 241:42 hours. The last 100-hour inspection was performed on 16 November 1995 at 192:9 hours.
- Weight and balance were within prescribed limits.
- A technical inspection on 12 December 1995 revealed that with the aircraft jacked up, the nose wheel extended but did not lock, yet all three landing gear indicator lights remained illuminated. The torque link, which ensures nose wheel locking, could not complete its travel to the over-center locked position.
- The angle bracket to which the hydraulic nose gear actuator is attached was loose. All rivets securing it to the bulkhead between the nose and cabin were sheared, allowing the bracket to shift up to 2 cm from its original position.
- The torque link arms on one side were worn due to repeated contact with the bolt heads of the hydraulic actuator transfer chain.
- A further examination on 12 January 1996 found that the upper nose gear attachment bolt was bent. This deformation shifted the working axis of the torque link and reduced its clearance to the actuator housing, causing contact between the torque link arms and adjacent bolt heads.
Technical Assessment
The bent upper nose gear attachment bolt was clearly the result of one or more impacts exceeding the system's strength, typical of hard nose wheel contacts with the runway. The deformation caused the torque link's working axis to shift closer to the hydraulic actuator, leading to contact between several bolt heads and the torque link arms during gear retraction and extension. The hydraulic system developed sufficient force to push the bolt heads past the arms, but this overstressed and sheared the attachment rivets. Without proper support, the bracket could not perform its holding function, preventing full extension of the torque link and thus locking of the nose wheel. Despite the malfunction, all three green landing gear indicator lights in the cockpit illuminated.
Operational Assessment
The approach procedures for Grenchen were correctly executed. The technical inspections confirmed that all three green landing gear indicator lights illuminated even though the nose gear was not locked. This malfunction was not apparent to the crew and could not have been anticipated. Because the aircraft was stopped on the runway, the damage remained limited. The weather conditions allowed landing within the airport's limits.