What happened
On 29 January 2018, a Sikorsky S-92A, registration G-CHHF, was performing a commercial passenger flight from the Magnus offshore platform to Scatsta Airport in the Shetland Islands. During the final approach to Runway 24, the flight crew attempted to extend the landing gear according to standard checklists. While the main landing gear indicators showed green, the nose landing gear (NLG) remained in an unlinked state.
The crew initially entered a visual holding pattern and attempted to resolve the issue by recycling the gear and utilizing the emergency nitrogen blowdown system. When these efforts failed, the crew declared a PAN. To facilitate a safe landing, the crew decided to attempt a manual release of the gear. The helicopter was hover-taxied to the apron under the supervision of emergency services. By stabilizing the aircraft on its main wheels and keeping the nose elevated, the crew allowed an engineer to access the gear. The engineer used a lever to re-align the nosewheels, which allowed the leg to extend and lock. There were no injuries and no damage to the aircraft.
The investigation
AAIB investigators examined the aircraft on jacks and found that while the nosewheels did not initially centre during retraction tests, the issue became intermittent during repeated cycles. The NLG assembly was replaced and sent to the manufacturer for analysis.
Examination of the unit revealed it was in good condition but showed signs of being well-used. Investigators noted a lower fluid level, with 3,250 ml of hydraulic fluid present compared to the required 3,890 ml. There were also oily deposits and residue on the lower piston tube. However, the investigation could not definitively prove that the fluid loss caused the malfunction, as the reduction could have occurred during the discharge process after the incident.
Findings
- The failure of the automatic nosewheel self-centring mechanism prevented the nose gear from extending.
- The exact mechanical reason for the centring failure could not be determined.
- The operator's emergency operating procedures (EOPs) lacked a clear link between the procedure for gear failure and the procedure for landing with gear retracted.
- The manufacturer had previously identified that improper servicing, specifically low oil or nitrogen charges, could prevent the self-centring cams from interlocking correctly.