What happened
On 28 June 2005, an Enstrom F-28A helicopter, registration G-BBHE, was conducting a private sightseeing flight near Fethard, Co. Tipperary. While cruising at an altitude between 900 and 1,100 feet, the pilot and passenger heard a loud impact from the rear of the aircraft. Simultaneously, the helicopter experienced a sudden yaw to the right, followed by a yaw to the left, as the pilot discovered that the foot pedals had become ineffective for directional control.
In response to the loss of tail rotor control, the pilot immediately reduced power and initiated an autorotational descent. The pilot navigated the aircraft toward a suitable field approximately 0.5 miles away, avoiding obstacles such as power lines and hedges. The landing was heavy, resulting in damage to the undercarriage and the main rotor blades striking the tail boom. Despite the significant impact, both the pilot and the passenger escaped the aircraft without injury.
The investigation
Investigators examined the wreckage and recovered several sections of the right-hand stabilizer from a quarry located 0.5 miles from the landing site. Analysis of the recovered components revealed that the stabilizer had struck the tail rotor during flight. Further inspection of the tail rotor assembly showed that the control cables had become wrapped around the gearbox output shaft, causing the mechanism to seize and the drive shaft to break away.
Metallurgical examination of the stabilizer spar was a key component of the inquiry. The investigation focused on the structural integrity of the mounting points where the spar connected to the fuselage. Additionally, the aircraft's maintenance logs were reviewed to determine the timing of required inspections mandated by previous service bulletins and airworthiness directives.
Findings
- The primary cause of the incident was the failure of the stabilizer spar due to high cycle fatigue cracking.
- The fatigue cracks originated at the inner edges of the bolt holes within the spar.
- The structural failure caused the right-hand stabilizer to detach from the aircraft.
- The departing stabilizer struck the tail rotor, which subsequently led to the loss of tail rotor control cables and the loss of directional stability.
- While the aircraft was within its inspection interval, the investigation noted that the current inspection schedule was based on flight hours rather than annual intervals.