Incident Overview
On March 6, 2016, a four-person crew consisting of two pilots, a paramedic, and a doctor was on duty at the EPBK (Białystok Krywlany) aerodrome. The pilot under supervision (PICUS) was conducting line flying under supervision (LIFUS) as commander of an EC 135 P2+ helicopter. The supervising pilot occupied the left seat. At 07:42, the duty paramedic received a call to perform a flight to Pieczyski, in the Perleje commune. The helicopter departed at 07:45 without any issues. The flight was conducted in visual meteorological conditions (VMC) and proceeded as planned. Meteorological conditions were good and did not hinder the mission.
Discovery of Damage
After arriving at the destination, the PICUS landed at the agreed location. After the patient was placed on board, before engine start, the supervising pilot noticed a crack approximately 20 cm long in the front windshield on the left side. After assessing the risks associated with the identified defect, the supervising pilot decided to continue the mission but changed the patient delivery location from the hospital helipad in Białystok to the EPBK aerodrome. Due to the cracked windshield, the flight was conducted at a reduced speed of 100–110 knots. During the transit, the crack did not enlarge, and the landing was uneventful.
Post-Flight Actions
The helicopter was grounded, and the occurrence was reported in accordance with the company's reporting system. The manufacturer, Airbus Helicopters (AH), was also notified. In response, AH indicated that with such a crack the helicopter could be ferried to a maintenance facility with a speed limitation of 80 knots, avoiding abrupt maneuvers, and without the need to drill stop holes to prevent crack propagation.
Service Bulletin and Repair
AH had issued Service Bulletin No. SB EC135-56-007 dated December 16, 2015, which prescribes installing a windshield reinforcement near the upper door hinge attachment. However, during correspondence with AH, it turned out that the bulletin could not be applied to the left hinge on the series of helicopters operated by the operator. AH was seeking an alternative solution for that series.
To repair the windshield, the operator applied to EASA for permission to ferry the helicopter to SOT Warsaw-Babice. On March 7, 2016, approval (No. 60050004) was granted with specified flight conditions. Based on that approval, the Polish Civil Aviation Authority issued a Permit to Fly (No. LTT/011/2016) for the route EPBK–EPBC.
After preparation by technical personnel according to the maintenance task card, the helicopter was ferried on March 8, 2016. The windshield was replaced under a maintenance order. After completion of all tasks and a test flight, the helicopter was returned to service.
Probable Cause
The probable cause of the windshield crack was an improper design of the door hinge attachment, which did not account for forces that could act on the front windshield when the door was fully opened during patient loading. Additionally, imprecise grinding of the windshield, leaving a notch, may have contributed to weakening the hinge attachment area.