Incident Overview
On 23 July 2021, at 0933 UTC, a Piper PA-31 (registration G-UKCS) experienced the opening of its lower cabin door in flight near Doncaster Sheffield Airport. The aircraft was conducting an aerial survey with one pilot and two systems operators on board. The door opened with a loud bang, prompting the commander to declare a PAN call and return to the airport. The aircraft landed uneventfully at 1000 hrs. No injuries were reported among the crew or passengers.
Aircraft and Door Mechanism
The PA-31 features a clamshell cabin door with upper and lower outward-opening sections. The lower door is secured by two latch pins (forward and rear) that engage hook lock plates when the door handle is closed. A visual indicator on the forward latch displays "SAFE" when engaged. A microswitch on the upper door extinguishes a door ajar warning light when both doors are closed.
Examination Findings
After the incident, examination revealed the forward latch pin screw had fractured. The fracture surface showed fatigue beachmarks covering about 60% of the cross-section, with ratchet marks indicating multiple initiation sites. The remaining area showed overload failure. The screw was loose within the assembly; the self-locking nut was insufficiently engaged, with no thread projection beyond the nut. Thread crests near the fracture were worn and rounded, consistent with the screw being loose and rotating under load.
The rear cable-stay mounting was damaged, and the forward cable-stay had broken at its attachment point.
Operator Testing
The operator conducted post-incident testing. When closing the doors from inside, the rear latch pin sometimes failed to fully engage in its hook plate while the forward latch engaged. This condition could only be detected with a torch from inside the cabin. The door ajar warning extinguished and the lower door appeared flush, but the external door handle protruded outward in the unlocked condition. After replacing the forward latch pin screw, the forward latch locked reliably, but the rear latch misalignment could still be reproduced.
Analysis
The investigation determined that the rear latch was probably not locked when the cabin door was closed, allowing the door to open when the forward latch pin screw fractured. The screw had not been securely fastened and was loose, which contributed to a fatigue failure. The looseness was likely due to a self-locking nut not being tightened during a previous maintenance event. The fatigue crack propagated from multiple sites around the screw thread root, ultimately leading to overload failure. The upper door remained closed due to its separate sliding latch, contributing to the safe outcome.
Safety Actions
The operator plans to inspect all PA-31 aircraft for loose latch pin screws and ensure correct fastening. Amendments to maintenance and operations manuals will include checks for flush door handles, correct latch pin engagement, and proper rigging of door handles. The internal occurrence report will be disseminated to engineers and flight crews, and related safety information will be included in recurrent training.