Dragging sprag clutch detected during an autorotation
An Airbus H125 (AS350 B3) is used for charter. During a practice autorotation at 3,000 ft AGL and 65 KIAS, the pilot lowers collective. Nr, which should stabilise near 100%, decays to 92% and only recovers to about 98% when he raises collective and re-engages the engine. He hears a rhythmic clicking from behind the cabin and feels a small yaw. The engine Ng rises from idle to about 70% even though the throttle is at idle. On the ground, the main gearbox chip detector carries a fine grey metallic paste but no discrete flakes. The freewheeling unit is a sprag clutch inside the main gearbox. Maintenance has 4 hours before the aircraft is due for a scheduled charter. The pilot reports that the engine was clearly being driven by the rotor during the autorotation.
1. Explain the operating principle of a sprag-type freewheeling unit and why a dragging sprag would make Ng rise during an autorotation.
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A sprag clutch is a one-way drive. The inner and outer races are separated by sprags - slender steel elements that tilt and jam when one race tries to overrun the other in the drive direction, transmitting torque. When the rotor overruns the engine, the sprags lie down and the races freewheel. In a helicopter installation the engine drives one race and the main gearbox input the other, depending on the design. During an autorotation the rotor must be able to overrun the engine so the engine does not drag the rotor. If a sprag does not release cleanly because of wear, brinelling, debris or inadequate lubrication, the engine is driven by the rotor. At 100% Nr the corresponding engine speed is well above idle, so the pilot sees Ng rise from idle toward 70% even with the throttle at idle, and Nr decays because energy is being used to motor the engine. The rhythmic clicking and grey paste are consistent with sprag chatter and wear. In the worst case, a dragging unit can prevent the rotor from reaching and holding autorotative RPM.
2. Distinguish between a sprag that slips under drive and one that drags in overrun. What would the pilot see in each case, and what are the consequences?
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Slipping under drive means the sprags fail to jam when the engine tries to drive the rotor. The engine accelerates but the rotor does not follow; Nr droops or does not reach normal, and the engine may overspeed because it is unloaded. In flight this is potentially catastrophic because the rotor loses drive. Dragging in overrun means the sprags fail to release, so during autorotation the rotor drives the engine. The pilot sees Ng rise above idle and Nr decay faster than expected; the aircraft may yaw, and re-engagement produces a bang as the already-loaded sprags grab. The reported symptoms - Nr decaying to 92%, Ng rising from idle to about 70%, clicking and a small yaw - fit dragging or chattering rather than a clean slip. A slipping sprag would not make Ng rise; it would let the engine run free while the rotor slowed. The distinction matters because the two failures have different causes and different in-flight consequences, and both are airworthiness defects.
3. Recommend the maintenance and airworthiness action before the charter, including what evidence would allow release.
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Do not release for the charter. A dragging freewheeling unit compromises autorotation and can lead to loss of rotor RPM. The gearbox must be drained and the freewheeling unit inspected per the maintenance manual: remove and examine the sprag clutch for wear, brinelling, sprag tilt, cage damage and debris; check the oil and chip detector for flakes; inspect the gearbox oil filter; and check lubrication supply to the unit. Grey paste alone may be normal wear, but combined with the flight symptoms it is not acceptable evidence of serviceability. After repair or replacement, carry out the specified freewheeling and autorotation check on the ground and a functional flight test, and make an entry in the maintenance release. Release only when the unit passes the check, the oil system is clean, and a rated engineer signs the release. If parts are serviceable and the unit passes, the charter can be considered, but the decision must rest on the inspection and test, not on the chip detector alone.