What happened
On March 3, 2001, at approximately 09:25, a Schweizer 269C helicopter, registration OH-HHS, was performing a moose-counting flight near Petäjäskoski, Finland. The pilot was flying at an altitude of approximately 400 meters when a sudden, loud noise was heard from the engine, followed by an immediate and near-total loss of engine power.
At the time of the failure, the aircraft was at an altitude of 100–15 and traveling at approximately 60 knots. The pilot immediately initiated an autorotation and attempted to reach a clearing located to the right, following a riverbed. However, due to a strong headwind, the aircraft could not reach the intended clearing. The pilot subsequently decided to aim for the riverbed itself, but the altitude was insufficient to clear the treeline.
The helicopter struck the tops of several spruce trees at an altitude of approximately 18 meters. The rotor blades struck trees as low as 5 meters above the ground. The aircraft then impacted the ground 44 meters from the initial tree contact, striking the earth on its left side. The pilot, who was the sole occupant, managed to crawl from the wreckage and sustained only minor injuries.
The investigation
The investigation focused on the mechanical failure of the engine and the flight path during the emergency. Investigators examined the engine components, specifically the intake valve assembly, and conducted metallurgical testing on the broken parts. The investigation also analyzed the aircraft's flight path using GPS data and evaluated the pilot's decision-making regarding the emergency landing site.
Technical examinations revealed that the upper valve spring retainers in the third cylinder had fractured. This fracture allowed the intake valve to become unseated and be sucked into the cylinder, where it was crushed. Fragments of the valve were then distributed through the intake manifold into the other three cylinders, causing widespread internal damage.
Findings
- The primary cause of the engine failure was the fracture of the upper valve spring retainer in the third cylinder.
- This mechanical failure caused the intake valve to lose its locking mechanism, leading to the destruction of the valve and subsequent damage to all cylinders.
- While the exact force or mechanism that caused the initial fracture could not be definitively determined, the engine manufacturer noted that similar failures of this specific component had been reported previously.
- The pilot's decision to perform an autorotation immediately was correct, and the attempt to flare the aircraft before impact helped mitigate the severity of the crash.
- Strong headwinds prevented the aircraft from reaching the safer, pre-selected clearing, forcing the landing into the forested area.