Helicopter Clutch and Freewheeling Unit: A&P Oral Answer
A helicopter clutch lets the engine start before gradually taking up rotor load; a freewheeling unit automatically disconnects the engine when its revolutions per minute (rpm) are below main-rotor rpm. For reciprocating helicopters, the Federal Aviation Administration (FAA) handbook names centrifugal and belt-drive clutches as the two main types.
FAA-H-8083-31B, Airframe, printed 2-35, PDF 106
For Airframe and Powerplant (A&P) oral and practical (O&P) preparation, separate three ideas in a helicopter centrifugal clutch/freewheeling unit answer: starting the engine, engaging rotor load, and allowing the rotor to overrun the engine. Use the handbook’s name, freewheeling unit, when explaining that overrun function. The handbook also calls the freewheeling unit the autorotative clutch.
FAA-H-8083-31B, Airframe, printed 2-34, PDF 105
On this page
A brief spoken O&P answer
The clutch allows an engine start followed by gradual pickup of the rotor load. A centrifugal clutch engages through outward-moving shoes; a belt-drive clutch engages as tension is applied to the belts. The freewheeling unit automatically disengages the engine from the main rotor when engine rpm is less than main-rotor rpm, so the main and tail rotors can keep turning if the engine fails. Use that as a short explanation to a Designated Mechanic Examiner (DME); if you are shown a drawing or a part, point to each component as you name it.
FAA-H-8083-31B, Airframe, printed 2-35, PDF 106
Why is a clutch used during starting?
The handbook says the rotor must be disconnected when the starter is engaged because rotor weight is large relative to engine power compared with an airplane’s propeller and engine. It separately says free-turbine engines require no clutch because their gas-producer turbine is essentially disconnected from the power turbine. Keep that engine-type distinction instead of claiming all helicopters use the same engagement arrangement.
FAA-H-8083-31B, Airframe, printed 2-34, PDF 105 · FAA-H-8083-31B, Airframe, printed 2-35, PDF 106
How does the centrifugal clutch engage?
The centrifugal clutch has an inner assembly and an outer drum: the inner assembly, connected to the engine driveshaft, consists of shoes lined with material similar to automotive brake linings, and the outer drum is attached to the transmission input shaft. At low engine speed, springs hold the shoes inward so they do not contact the drum. As engine speed increases, centrifugal force moves the shoes outward and they begin sliding against the drum; the transmission input shaft begins to rotate, and the rotor turns slowly at first, then faster as friction between the shoes and drum increases. In the described indication, superimposed engine and rotor tachometer needles show synchronization, full engagement and no further shoe slippage.
FAA-H-8083-31B, Airframe, printed 2-35, PDF 106
How does the belt-drive arrangement differ?
The belt drive has an engine pulley, a transmission-input pulley, a belt or V-belts, and a means of applying belt tension. Untensioned belts fit loosely so the engine can start without transmission load. After the engine starts, belt tension is gradually increased until engine and rotor synchronization indicates full engagement. The handbook lists vibration isolation, simple maintenance and engine warm-up without rotor engagement among the arrangement’s advantages.
FAA-H-8083-31B, Airframe, printed 2-35, PDF 106
What does the freewheeling unit do?
The most common assembly described is a one-way sprag clutch between the engine and main-rotor transmission. It is usually in the upper pulley on a piston helicopter or on the accessory gearbox on a turbine helicopter. When the engine is driving the rotor, inclined surfaces in the sprag clutch force rollers against an outer drum, which prevents the engine from exceeding transmission rpm. If the engine fails, the rollers move inward and allow the transmission to exceed engine speed. The transmission-system description says the freewheeling unit permits the main-rotor transmission to drive the tail-rotor shaft during autorotation. In your answer, keep the handbook’s words “most common” and “usually”; they describe typical arrangements, not every installation.
FAA-H-8083-31B, Airframe, printed 2-35, PDF 106 · FAA-H-8083-31B, Airframe, printed 2-36, PDF 107 · FAA-H-8083-31B, Airframe, printed 2-34, PDF 105
Connect the mechanisms to the Airman Certification Standards (ACS)
AM.II.N.K3 — Transmissions. AM.II.N.K5 — Design, type, and operation of rotor systems. These ACS knowledge elements frame the explanation; this article is not a clutch adjustment, engine run or autorotation procedure.
FAA-S-ACS-1, printed 42, PDF 47
Practice the distinction as a spoken explanation
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Start your free drillFrequently asked questions
Can I say every helicopter needs a starting clutch in an O&P answer?
No, the handbook says no clutch is required on the free-turbine engines it describes.
FAA-H-8083-31B, Airframe, printed 2-35, PDF 106
Does a belt-drive clutch engage by throwing shoes against a drum?
No, its described engagement method is gradually increasing belt tension after the engine is running.
FAA-H-8083-31B, Airframe, printed 2-35, PDF 106
Is the upper pulley the freewheeling-unit location in every installation?
No, the handbook says it is usually there in a piston helicopter and describes an accessory-gearbox location for a turbine helicopter.
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Related guides
- Rotorcraft Fundamentals Oral Exam Questions and Answers
- Coriolis Effect in Helicopter Rotors: A&P Oral Answer
Sources
Primary sources checked October 25, 2026.
About this guide: drafted with AI from the FAA sources listed on this page and checked claim by claim against them. Not reviewed by an A&P mechanic or DME. How we make our guides
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