What happened
On April 11, 2011, a PZL W-3AS helicopter, registration SP-SUH, was conducting a mandatory post-maintenance test flight after completing several scheduled inspections. During the cruise phase of the flight near Mijares, Spain, the two crew members heard an unusual noise originating from the engine area, followed by smoke filling the cockpit. The crew immediately executed an emergency landing on a nearby road. Upon landing, the crew observed smoke and fire emanating from the engine's air intake system and utilized onboard extinguishers to suppress the flames.
The investigation
An investigation conducted by the Polish State Commission on Aircraft Accident Investigation (SCAAI) focused on the failure of the accessory cooling system's fan. Technical examinations of the damaged components revealed that the fan drive shaft and its casing had suffered significant thermal damage, with temperatures reaching approximately 600 °C. The investigation found that the rotor shaft had fractured due to the combination of extreme heat and torsional stress.
Testing revealed that during recent maintenance, the lubrication of the aft bearing had been performed with excessive pressure. While the standard pressure was 20 bars, internal pressures within the assembly reached approximately 50 bars during dynamic lubrication. This high pressure caused denting of the outer case and damaged the bearing seals, leading to internal friction and subsequent overheating.
Findings
- The failure was caused by excessive grease pressure applied during the lubrication of the fan's aft bearing.
- The high pressure caused internal damage to the bearing, resulting in high friction and temperatures that reached levels sufficient to ignite the lubricating grease and the clutch grease.
- The heat caused the aluminum alloy drive shaft to reach its melting point, significantly reducing its structural strength and leading to the fracture.
- The design of the fan makes it impossible for technicians to verify the lubrication of the aft bearing because the telltale orifice is inaccessible during the process.
- There was a lack of consensus and specific instructions in the maintenance documentation regarding the exact amount of grease to be applied and the required lubrication pressure.