Engine Failure & Autorotation

The most-trained helicopter emergency, and the one that defines the airframe. When the engine quits, the freewheeling unit lets the rotor disengage from the engine and spin freely on the airflow rising through the disc. The pilot's job is to protect rotor RPM, find a landing spot, and convert that stored rotational energy into a soft touchdown.

Why autorotation works

A helicopter's main rotor is connected to the engine through a freewheeling unit (sprag clutch). When the engine drives the rotor, the clutch transmits torque. When the engine quits — or even just runs slower than the rotor — the clutch disengages and the rotor is free to spin under whatever forces act on it.

In a descent with the collective lowered, air flows up through the rotor disc. That upward flow drives the rotor in the same way wind drives a windmill, sustaining rotor RPM as long as the helicopter keeps descending. The rotor disc divides into three regions during autorotation — a driven (outer) region, a driving (middle) region, and a stalled (inner) region — discussed in detail on the autorotation aerodynamics page.

The bottom line: as long as the collective is down and the helicopter is descending through clean air, you have a working flight control system. The whole emergency comes down to using that working system to get safely to the ground.

Immediate action: the first three seconds

The actions that protect rotor RPM happen in the first second or two after the engine quits. Hesitation is the killer — rotor RPM decays fast under load, and a rotor that drops below the green arc is hard or impossible to recover.

Lower collective IMMEDIATELY to flat pitch. This is the single most critical action — every degree of pitch you leave in is rotor RPM bleeding off.
Adjust cyclic to achieve best-glide airspeed (~65 KIAS on most piston helicopters — confirm in your POH).
Apply right pedal as engine torque disappears (counter-clockwise main rotor; reverse for clockwise).
Throttle to idle if the governor hasn't done it for you.
Declare emergency if time permits: squawk 7700, call Mayday on whatever frequency you're on or 121.5.

Lower-collective discipline is trained as a reflex for a reason. In an actual engine failure at low altitude — say, climbing out at 200 ft AGL — you have roughly two seconds to get the collective down before rotor RPM decay closes off your options.

Establishing the glide

Once collective is down and airspeed is established, the priorities are managing rotor RPM and choosing where to land.

Maintain Nr (rotor RPM) in the green arc. If RPM is sagging, raise collective slightly to load the rotor (which slows it) only if RPM is excess; if RPM is low, lower collective further or accept a steeper descent. Trade altitude for RPM if needed.
Pick a landing area early. Into the wind if possible. Flat ground beats sloped ground. Avoid wires (the deadliest hazard in an off-airport landing) and obstacles. A short, flat pasture beats a long, sloping field nine times out of ten.
Plan your flare point. Typically 50–75 feet AGL depending on airspeed and aircraft type — your POH and your instructor's training profile are the references. The flare needs to start with enough altitude that the helicopter can decelerate before ground contact, but not so high that you run out of energy in the cushion.

Your eye should be on the spot you intend to touch down at, not the spot directly below you. Helicopter glide angles are steep — a piston helicopter in autorotation drops at roughly 1,500–2,000 fpm — so you have less ground to work with than a fixed-wing pilot expects.

Flare and touchdown

The flare converts forward airspeed and descent rate back into rotor RPM. Done right, you arrive at the surface with low groundspeed, low descent rate, and enough rotor energy left to cushion the landing.

Aft cyclic to flare — this is the deceleration. The flare also re-loads the rotor and accelerates Nr, giving you energy to spend on the cushion.
Level the aircraft with forward cyclic just before touchdown. Skids should be roughly parallel to the surface; nose-high contact is a tail-rotor strike.
Raise collective to cushion — use the stored rotor RPM to soften the touchdown. By the time you contact the ground, collective should be at or near full up.
Touch down on skids — accept some forward run-on if needed. A smooth slide along the ground is far better than a hover-then-drop landing.

Once on the ground, the rotor is going to rapidly slow down — that's expected. Hold cyclic neutral, keep collective full up to bleed energy through pitch instead of RPM, and ride out the rotor stop.

Why this is the most-trained emergency

You will practice power-recovery autorotations dozens of times before solo, and probably a hundred or more times before the practical test. The reason is simple: in a real engine failure, you don't have time to think. You have time to react. The only way to make the entry reflexive is to train it until your hand goes to the collective without conscious thought.

Common errors in training and on the practical test:

Slow on the collective at entry — RPM decay shows up immediately on the gauge.
Wrong airspeed in the glide — too slow steepens the descent and reduces flare effectiveness; too fast trades RPM for ground speed.
Late or aggressive flare — over-flaring trades altitude for RPM you don't have time to use; under-flaring leaves descent rate at the cushion.
Touchdown not level — the dynamic-rollover-adjacent failure mode of a real autorotation. Skids parallel to ground at contact.

Full touchdown autorotations to a ground spot — as opposed to power-recovery autorotations where you fly out at the bottom — are practiced separately and are part of the full autorotation maneuver standard.