What happened
On October 15, 2017, a Cessna 177RG, registration F-BSHB, departed Ambérieu aerodrome for a local flight near the Mont Blanc massif accompanied by three passengers. During the return to the airfield, the pilot joined the downwind leg for runway 19R. While performing the pre-landing checklist during the base leg, the pilot believed the green light indicating the landing gear was extended and locked was illuminated. This perception may have been caused by a sun reflection on the indicator.
As the aircraft transitioned to final approach, the pilot reduced power and deployed the flaps to the landing configuration. The aircraft subsequently struck the runway on its belly, sliding approximately 100 meters before coming to a halt. The aircraft sustained heavy damage.
The investigation
The investigation focused on why the landing gear remained retracted and why the cockpit warning system failed to alert the pilot. On this specific aircraft model, a landing gear warning alarm is designed to trigger when manifold pressure drops below approximately 12 inches of mercury, provided the throttle is not at idle.
Video footage captured by a passenger during the final approach confirmed that the amber landing gear indicator was illuminated and that the flaps were in the landing position. The footage also confirmed that the landing gear warning alarm was not audible, despite the manifold pressure being below 10 inches of mercury.
Technical examination of the aircraft's throttle assembly revealed that the electrical contact responsible for triggering the warning alarm was improperly adjusted. The contact only activated when the throttle was moved to the full idle position, rather than at the threshold specified in the flight manual.