What happened
On 11 May 2011, a Schweizer 269D helicopter, registration PH-HHF, was performing a commercial flight from Kempen Airport to Lelystad Airport. While cruising at approximately 1,000 feet near Veghel, the pilot experienced an unexpected noise followed by a sudden pitch-up and a left roll. Suspecting a tail rotor drive failure, the pilot initiated an emergency landing in a nearby field.
During the descent, the pilot transitioned into an autorotation, but the aircraft made a hard landing with significant vertical velocity. Although the pilot successfully exited the aircraft, a post-crash fire broke out, which completely destroyed the helicopter. The pilot sustained no injuries.
The investigation
The investigation, conducted with support from the NLR and NTSB, focused on the mechanical failure of the engine-to-main-gearbox drive train. Investigators examined the engine drive shaft and discovered that the power transmission had been interrupted. The physical evidence showed that the tail boom had been severed from the airframe due to a strike from a main rotor blade during the impact.
Technical analysis of the engine components revealed that the engine itself had no pre-impact mechanical failures. However, the investigation found that the battery had been displaced during the impact, which likely caused an electrical failure or linkage damage, preventing the pilot from using the electrical throttle detent to shut down the engine. The post-crash fire was traced to fuel leaking from a fractured housing and igniting on hot engine components.
Findings
- The primary cause of the flight instability was a metal fatigue fracture in the KaFlex forward coupling drive lug.
- This failure caused a sudden disruption in the drive to the main rotor, leading to an immediate drop in rotor RPM.
- The rapid decay in rotor RPM induced a retreating blade stall, manifesting as the observed pitch-up and left roll.
- The severity of the impact caused the landing gear to collapse and led to the structural separation of the tail boom.
- The post-crash fire was fueled by released fuel dripping onto hot engine parts and ignited by electrical shorts or hot exhaust.
Safety action
Following the investigation, the helicopter manufacturer issued an Alert Service Bulletin (ASB5 DB-043). This directive requires operators to perform engine alignment checks using a specific tool within 90 days or 100 flight hours of the bulletin's release, with subsequent inspections required every 25 flight hours to prevent misalignment-induced fatigue.