What happened
On 23 July 2021, a Piper PA-31, registration G-UKCS, was conducting an aerial survey flight from Doncaster Sheffield Airport. The aircraft was carrying one pilot and two systems operators. While flying at 2,000 ft in instrument meteorological conditions (IMC) near Hull, the crew heard a loud bang as the lower cabin door opened.
The pilot immediately declared a PAN call to Humbersont Radar and requested vectors for an emergency return. To manage the flight, the commander opted for a faster approach using full flaps to reduce the aircraft's pitch attitude. The aircraft returned to Doncaster Sheffield Airport and landed without further incident.
The investigation
Investigators examined the aircraft's clamshell-design cabin doors and the latching mechanism. While the upper door remained closed due to its separate sliding latch, the lower door had sustained damage, including a broken forward cable-stay and damage to the rear mounting structure.
Upon disassembling the latch mechanism, the AAIB found that the screw for the forward latch pin had fractured at the thread root. The examination of the fracture surface revealed fatigue beachmarks and ratchet marks, indicating the screw had been rotating under load. Furthermore, the self-locking nut was found to be insufficiently engaged on the screw threads.
Subsequent testing by the operator revealed a secondary issue: the rear latch pin did not always fully engage in its hook plate when closing the doors from the inside. This specific failure was difficult to detect as it was not visible from the cockpit and required a visual inspection with a torch from within the cabin.
Findings
- The primary cause of the door opening was the fatigue failure of the forward latch pin screw.
- The screw had become loose because its self-locking nut was not properly tightened during a previous, unidentified maintenance event.
- This looseness allowed the screw to rotate and experience bending loads, leading to the propagation of fatigue cracks.
- It is probable that the rear latch pin had failed to fully engage in its hook plate during pre-flight checks, even though the cockpit door-ajar warning light was extinguished and the forward latch indicator showed 'SAFE'.
- The upper door's independent sliding latch prevented the entire door assembly from opening completely, which assisted the safe landing.
Safety action
Following the incident, the operator initiated a fleet-wide check of its PA-31 aircraft to ensure all latch pin screws are securely fastened. The operator is also updating its maintenance programmes and pre-flight checklists to include specific visual inspections of the door handles and latch pin engagement.