What happened
On 16 May 2023, a Diamond DA 40 NG, registration G-LDGB, was conducting an instrument flight training lesson approximately 3 nm north-west of Oxford Airport. The flight involved an instructor, a student pilot, and a second student acting as a passenger.
During the climb out, while the student pilot was flying, the instructor and students were discussing how to interpret various Primary Flight Display (PFD) warnings. To demonstrate how to respond to a potential door warning, the instructor reached back to interact with the rear door handle. During this interaction, the primary locking mechanism was inadvertently disengaged. The door opened into the airflow and subsequently detached from the airframe. The instructor took control of the aircraft and performed a safe landing at the airfield. There were no injuries to the three occupants.
The investigation
The AAIB examined the aircraft and the detached door. The investigation focused on the functionality of the door' and the primary and secondary locking mechanisms. The door features a primary lock operated by a handle and a red safety latch designed to hold the door in a near-closed position when parked.
Technical analysis by the Continuing Airworthiness Management Organisation (CAMO) found no faults with the primary or secondary locking mechanisms. The hinges showed damage consistent with being torn away by aerodynamic forces, but no impact damage from the door hitting the airframe was found. While the instructor's specific intent—whether to demonstrate a warning or to ensure the door was secure—could not be definitively established, the physical action resulted in the door unlocking. The investigation also looked into why the secondary safety latch failed to prevent the door from opening once the primary lock was released.
Findings
- The instructor's interaction with the rear door handle inadvertently disengaged the primary locking mechanism.
- The door hinges were designed with a predetermined breaking point to prevent the door from remaining attached and compromising aircraft controllability if it were to open improperly.
- It could not be determined why the secondary safety latch failed to retain the door once the primary lock was disengaged.
- There was no evidence of a technical failure in the primary locking mechanism itself.