ERA25LA204

The helicopter was returning to its home base following a patient transport mission when it became uncontrollable and began shuddering violently. The pilot experienced a total loss of tail rotor control with no warning. He performed an autorotation, and the helicopter came to rest on a residential lawn. Postaccident examination of the helicopter revealed that the air conditioning louver assembly, tail rotor gearbox, and tail rotor assembly had separated in flight. A recovered louver fragment exhibited contact damage from a tail rotor blade, and both tail rotor blades were destroyed. Examination of the louver’s forward attachment points revealed stripped rivet nuts and evidence of a prior repair; bolts for fastening the louver assembly to the rivet nuts were found within the airframe. Maintenance records indicated that the louver assembly had been removed and reinstalled about 16 flight hours before the accident. Based on the evidence, the rivet nuts for the air conditioning louver assembly were likely stripped during maintenance, resulting in the louver assembly separating from the helicopter in flight and impacting the tail rotor. The tail rotor assembly then separated from the helicopter, which led to a loss of control.

On May 19, 2025, at 1941 eastern daylight time, a Eurocopter MBB-BK117 C-1 helicopter, N117NC, was substantially damaged when it was involved in an accident near Fort Washington, Pennsylvania. The commercial pilot and two passengers sustained minor injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 135 air medical flight. According to the pilot, the flight originated from the Penn Presbyterian Medical Center helipad (PA39), Philadelphia, Pennsylvania, and was en route to Wings Field Airport (LOM), Philadelphia, Pennsylvania, the helicopter’s home base, following the completion of a patient transport mission. While cruising at an altitude of about 1,300 ft mean sea level and 120 kts indicated airspeed, about 3 to 4 nautical miles south of LOM, the helicopter became uncontrollable and began shuddering violently. The pilot reported there were no visual or aural warnings before the event. He experienced a total loss of tail rotor control, including a loss of pedal authority and tail rotor thrust. The pilot lowered the collective and initiated an autorotation descent. During the descent, he identified a grass field adjacent to a driveway as a suitable forced landing area. The helicopter touched down following a rotation of about 180°. After landing, the pilot performed an emergency engine shutdown and applied the rotor brake. Examination of the airframe at the accident site revealed the top portion of the vertical stabilizer was missing, consistent with the in-flight separation of the tail rotor gearbox and tail rotor assembly. The skid gear exhibited splaying, and the fuselage structure was displaced about 12 to 18 inches to the left of centerline, consistent with impact in soft terrain. One main rotor blade (the “A” blade) exhibited minor damage about 3 ft inboard from the blade tip. The left vertical stabilizer and tail stinger exhibited damage consistent with ground contact. Further examination revealed the air conditioning louver assembly was missing from the helicopter, with only the aft portion of the louver frame remaining attached to the air conditioner coil assembly. The aft portion exhibited tearing and aft-bending deformation adjacent to the left and right attachment tabs. The forward portion of the louver assembly had separated in flight. About half of the right side of the louver was recovered at the accident site. The recovered louver fragment exhibited bending consistent with contact from a tail rotor blade. Both tail rotor blades were destroyed; one blade exhibited broomstrawing consistent with overstress separation. The 90° tail rotor gearbox, along with the tail rotor assembly, had separated from the helicopter. The severed gearbox and tail rotor were located along the helicopter’s flightpath. Inspection of the forward attachment points for the louver assembly revealed that both rivet nuts were stripped and exhibited damage. An AN3-4A bolt (17/32-inch length; 1/8 inch grip) was recovered from theright side of the engine deck, and an AN3-6A bolt (25/32-inch length; 3/8-inch grip) was located within the driveshaft plenum area on the left side of the helicopter. The forward right corner of the louver assembly featured a doubler at the attachment interface, consistent with a prior structural repair or reinforcement. A review of maintenance records indicated that work was performed on the air conditioning coil assembly on May 7 and 9, 2025, at an airframe total time of 9715.9 hours (15.7 flight hours prior to the accident). According to the aircraft manufacturer, this would have required removal and reinstallation of the louver panel.