What happened
On April 11, 2011, a PZL W-3AS helicopter, registration SP-SUH, departed from La Iglesuela aerodrome in Toledo, Spain, to perform a mandatory test flight following recent maintenance work. During the cruise phase of the flight, the crew heard unusual noises originating from the engine area and observed smoke filling the cockpit.
The crew promptly executed an emergency landing on a nearby roadway, which was identified as the most suitable area for a safe descent. Upon landing, the crew observed smoke and flames emanating from the engine air intake system and used onboard fire extinguishers to suppress the fire. There were no injuries to the two crew members, and the aircraft sustained only minor damage.
The investigation
An investigation involving the CIAIAC and the Polish State Commission on Aircraft Accident Investigation (SCAAI) examined the damaged components of the accessory cooling fan. The investigation focused on the failure of the fan's drive shaft and the subsequent thermal damage.
Technical tests revealed that the components had been exposed to extreme temperatures, ranging between 320°C and 766°C. The investigation found that the drive shaft had actually fractured due to the intense heat and torsional stress. Furthermore, testing on the lubrication process demonstrated that applying grease at high pressures (approximately 50 bar) caused deformations in the external casing and seals of the bearing. The investigation also noted that the rear bearing's lubrication inspection port is inaccessible during standard maintenance, making it impossible for technicians to verify if the rear bearing had been properly lubricated.
Findings
- The primary cause of the failure was the malfunction of the rear bearing of the fan rotor shaft, triggered by excessive grease pressure applied during a recent lubrication task.
- High internal pressure during lubrication caused deformations in the bearing seals and the external casing.
- The resulting friction within the bearing led to a progressive temperature increase, reaching approximately 600°C, which ignited the lubricating grease and the clutch grease.
- The extreme heat caused the aluminum alloy drive shaft to reach its melting point, significantly reducing its structural integrity and leading to its eventual fracture under torsional load.
- The fan's design prevents the verification of lubrication at the rear bearing during routine maintenance, creating a risk of uncontrolled grease accumulation if the front inspection port becomes blocked.
Safety action
Following the investigation, safety recommendations were issued to the Polish Civil Aviation Authority, the aircraft manufacturer (PZL Swidnik, S.A.), and the fan manufacturer (WSK Kraków Sp. Z o.o.). Proposed actions include:
- Modifying the fan construction to allow for the inspection of the rear lubrication port.
- Replacing graphite seals with more resistant alternatives.
- Standardizing the use of Grease 6 for civil helicopters.
- Updating maintenance documentation to specify lubrication pressure limits, required grease quantities, and specific procedures for aircraft that have undergone prolonged storage.