What happened
On 19 April 2010, a Diamond DA 42, registration HB-LUO, was conducting a multi-engine flight instructor training course. While performing flight maneuvers in the Villeneuve-Aigle region, the crew attempted to extend the landing gear. Although the nosewheel and right main gear deployed correctly, the left main landing gear remained retracted, indicated by a red unsafe light in the cockpit.
The crew performed various troubleshooting steps, including flying maneuvers with positive and negative acceleration and even temporarily switching off the aircraft's electrical power. During the electrical power loss, the crew observed that the gear appeared to move, but a visual inspection by a second aircraft confirmed the left gear remained stuck. Unable to rectify the situation, the crew decided to perform a gear-up landing at Zurich Airport. To mitigate fire risks, the crew shut down both engines and the fuel supply shortly before touchdown. The aircraft came to a stop on runway 34 with no injuries to the three occupants, though the aircraft sustained damage to the engine cowling, exhaust pipes, propeller tips, and entrance steps.
The investigation
The investigation focused on why the left main landing gear failed to extend despite the use of emergency extension procedures. Investigators examined the mechanical state of the wheel well and the physical characteristics of the tires used on the aircraft. The investigation also reviewed the manufacturer's procedures for verifying clearance between the tire and the wheel well, as well as the emergency checklists provided in the Airplane Flight Manual (AFM).
Findings
Technical analysis revealed that the left main landing gear was not properly centered within the wheel well, causing the tire to become lodged in the wheel well. Two primary contributing factors were identified:
- The procedure established by the manufacturer for checking the clearance between the tire and the wheel well was insufficient.
- The left main landing gear was fitted with a tire that had a slightly different outer diameter and cross-section than the manufacturer's specifications.
Safety action
Following the accident, several corrective measures were implemented:
- The aircraft manufacturer updated the maintenance manual to require a minimum 4 mm clearance check between the tire and the wheel well during tire or wheel replacements.
- The manufacturer revised the emergency checklist to advise against switching off the electrical master switch during a gear-up landing, as this could lead to an unintended and asymmetric extension of the gear.
- The tire manufacturer, Goodyear, updated its documentation to clarify that the Flight Custom III tire was not approved for use on the DA 42 aircraft.