20240721012319A

Mechanical failure root cause

Due to multiple damaged parts when the nose gear collapsed on landing, it is difficult to identify exactly the failed part that caused the nose gear to collapse. Based on the evidence of the damage and aircraft ground path on ground roll, we can only speculate that the likely cause of the nose gear collapse started with a broken arm assembly p/n 330038 (refer picture a(2)) that attaches the right rod assembly p/n 900009-25-14.10 (refer picture a(3)). With the arm assembly failed (break) at the right rod attachment point, the top end of the right rod fell on the side of the nose landing gear body and jammed the nose gear in a partial down position during extension. Evidence that points to the right rod jamming inside the nosewheel restricting the nose gear from fully extending: (I) the right rod assembly (refer picture a(3)) is bent near the right aft door attachment bracket, caused by the outward force of the right aft door on ground impact while the rod was jammed inside the nosewheel body; (ii) nose tire rub marks (refer picture a(4)) on the left side of the wheel well wall is an indication that force (from the right rod) was applied to the right side of the nosewheel during landing gear extension pushing the nose gear to the left inside the nose wheel well; (iii) aircraft veering right after touchdown (refer picture d) is evidence that nosewheel was pushed left inside the wheel well displacing the nosewheel slightly off center to the left causing the aircraft to veer right.

Human factor

The twin commander 690B model landing gear extension incorporates an emergency extension system for emergency when the normal hydraulic system fails. If the normal hydraulic system fails:

1.

The main landing gear is extended by a nitrogen-charged air bottle when the landing gear lever is selected down.

2.

The nose landing gear emergency extension spring that acts against the normal hydraulic pressure extends the nose landing gear down to its down and lock position.

With the landing gears fully extended, each landing gear safe down and locked position is indicated by its respective green light. A transit (unsafe amber) light is also incorporated to indicate that all or any one of the 3 landing gears are not in their down and locked position, and the green light for the landing gear not fully extended and locked will not come on. After the incident with the aircraft sitting on its nose, the nose landing gear green light was off (not illuminating) and the unsafe amber light was illuminating.

Right after the incident with the aircraft still on the runway, the aircraft nose was lifted up to check if the emergency extension spring had failed to extend the nose gear. When the nose came off the ground the nose gear dropped to its down and locked position (by the extension spring), the nose down and lock position green light illuminated, and the unsafe amber light turned off. Aircraft was ferried with gear down to homebase for further investigation and inspection. A landing gear swing (extension/retraction) was carried out using the normal hydraulic pressure and the emergency extension system, and the down and locked indication systems were all satisfactory.

The pilot reported carrying out the landing checklist and verifying the landing gear 3 green lights illuminated. The landing gear retract/extension indication system was checked, and no fault was found in the extension/retract indication system that would have given the pilot a false warning during the landing gear extension. The pilot was stood down from flying for 1 week. Remedial training and route checks were carried out before the pilot was released back to line flying.