What happened
On March 6, 2016, an EC1s35 P2+ helicopter, operated by a medical service unit, was performing a flight from Białystok Krywlany (EPBK) to Pieczyski for a medical transport mission. The crew consisted of two pilots, a paramedic, and a physician. The pilot in command was undergoing line flying under supervision (LIFUS) training, with a supervising pilot occupying the left seat.
After successfully transporting a patient to the landing site, the crew prepared for the return leg. Before starting the engines, the supervising pilot noticed a crack approximately 20 cm in length on the left side of the front windshield. Following a risk assessment, the pilot decided to continue the mission but altered the destination, diverting from the planned hospital landing in Białystok to return directly to the base at EPBK. To mitigate risks, the flight was conducted at a reduced speed of 100–110 knots. The crack did not expand during the flight, and the aircraft landed safely.
The investigation
The investigation, conducted by the operator's internal commission, focused on the structural integrity of the windshield and the potential for further damage. The aircraft was suspended from service, and the manufacturer, Airbus Helicopters, was notified. The manufacturer indicated that the aircraft could be ferried to a service station at a speed not exceeding 80 knots, avoiding abrupt maneuvers, and without the need for safety drilling.
It was noted that a previously issued service bulletin (SB EC135-56-007) intended to reinforce the windshield near the upper door hinge could not be applied to the left hinge of this specific aircraft series. Consequently, the operator obtained authorization from EASA and the Civil Aviation Authority to ferry the helicopter to Warsaw-Babice (EPBC) for repairs. The windshield was subsequently replaced, and the aircraft was returned to service following a successful test flight.
Findings
- The structural design of the door hinge mounting may be a contributing factor, as it may not adequately account for the forces acting on the windshield when the doors are fully opened during patient loading.
- Imprecise grinding of the windshield, which may have left a notch or burr, could have weakened the area near the hinge mounting.
- The crack was identified during pre-flight checks following the completion of a medical mission.