AS350 B3e course · Emergencies
Autorotation
No engine, still flying
The essentials
In autorotation, air flowing up through the disc keeps the rotor turning. The freewheel lets the stopped engine go, and the rotor drives the tail rotor and gearbox on its own.
You fly it in three phases: entry (collective down, rpm kept), glide (the RFM's airspeed, rpm in the power-off range) and flare and landing (trading speed and rotor energy for a slow touchdown).
Rotor rpm is the currency throughout.
No engine, still flying
When the engine stops, the freewheel lets go and the rotor keeps turning — but now the air does the driving. Lower the collective and descend, and air flowing up through the disc keeps the rotor turning.
Source 1
The disc in autorotation
The disc in autorotation: stall, driving and driven regions. This diagram needs JavaScript.
Tap a region to see which way its aerodynamic force leans.
Source 3
Three regions on every blade
Inboard, the blade is stalled. In the middle — the driving region — the aerodynamic force tilts ahead of the rotor axis and pulls the blade round. Outboard — the driven region — it tilts behind and drags.
Rotor rpm settles where the drive and the drag balance.
Source 3
Entry depends on where you were
From cruise, there is airspeed to work with. The FAA handbook notes the nose tends to pitch down as the collective goes down; holding the attitude keeps the disc loaded and rotor rpm up. Pedal takes out the yaw as torque disappears.
From the climb, the blades carry more pitch and the airspeed is lower, so rpm decays faster and there is less energy in hand. From low speed, airspeed has to be built, which costs height. The lower and slower the start, the less of the autorotation is left for mistakes.
Source 1
Airspeed in the glide
Two airspeeds matter. Minimum rate of descent keeps you in the air longest. Best glide takes you furthest. They are different speeds, and both are in the RFM.
The FAA handbook explains the curve behind them: descent is fastest at zero airspeed, falls to a minimum at a moderate forward speed, then rises again as speed increases. Flying slower than minimum-descent speed, or faster than best-glide speed, gains you nothing. The handbook adds that rotor rpm towards the low end of its range stretches the distance.
Source 1
Controlling rpm
In autorotation the collective controls rotor rpm: raise it and rpm falls; lower it and rpm rises. Turns and flares load the disc and raise rpm too.
The aim is to arrive at the flare with rpm in the power-off range and enough airspeed to flare with.
Source 1
Turning in autorotation
A turn loads the disc, which raises rotor rpm and increases the rate of descent. Rolling out unloads it, and rpm falls back. The collective follows both: up a little to hold rpm in the turn, down again on the roll-out.
Turns are how you reach the area you chose, but every degree of turn costs height. The FAA's autorotation guidance singles out 180-degree autorotations: at least six accidents in the eight years before AC 61-140A had them in the NTSB's probable cause.
The flare and touchdown
Near the ground there are two energy stores: the aircraft's forward speed and the rotor's rpm. Spend them in that order.
The flare — aft cyclic — tilts the disc back, trading forward speed for a lower rate of descent and a rise in rotor rpm. Flared too high, the aircraft runs out of speed with height still to lose. Too low or too steep, the tail meets the ground first. Then the aircraft is levelled and the collective is raised to cushion the touchdown on the rotor's stored energy, which is used once.
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Touching down
The skids want to meet the ground level and aligned with the direction of travel — sideways or nose-high contact is how autorotations end in rollovers and tail strikes.
Whether to touch down with some forward speed or as close to zero as possible depends on the surface: a run-on needs smooth, firm ground, and soft or rough ground punishes it. Heights, attitudes and techniques for your aircraft are taught in training and set out in the RFM.
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Hovering autorotation
Cut the power in a low hover and the aircraft settles on rotor energy alone. The FAA handbook warns that any lateral movement must be avoided to prevent dynamic rollover.
Two things happen at once in the Astar: the nose yaws right as torque disappears, and the tail rotor's sideways push goes with it — so the left drift you were holding with right cyclic reverses. For counter-clockwise rotors the handbook describes a drift to the left, held with slight right cyclic; on a clockwise rotor the mirror is a drift right. That is physics, not Astar test data.
Practice autorotations and power recovery
Most autorotations are flown in training, ending in a power recovery — the engine is brought back before touchdown. That relies on the engine accelerating in time.
AC 61-140A reports that the predominant cause in autorotation-training accidents is failing to keep rotor rpm and airspeed within the flight manual's range, ending in a descent too fast to recover. Its message: if rpm, airspeed, trim and alignment aren't right by the decision point, abandon the manoeuvre and go around — a far safer option than trying to recover lost rpm. A practice autorotation is planned so that a failed recovery still ends somewhere survivable.
Source 4
Mount Pearl: the recovery that didn't come
During recurrent training in an AS 350 BA (Arriel 1B), an engine failure was simulated from about 600 ft above ground and an autorotation flown. Near the end, the fuel flow control lever was advanced for an overshoot — and the engine did not spool up as expected.
The helicopter struck the ground at a high rate of descent. Both pilots were seriously injured and the helicopter was destroyed. The TSB found rotor rpm low in the descent — the horn sounding on and off — partly because collective was applied before engine power was confirmed.
A power recovery is a plan, not a guarantee. Fly every practice autorotation to a place where a full touchdown would be survivable.
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Autorotation with a load on the hook
An engine failure with a load attached is the case utility pilots train for in their heads. The load adds mass and drag the autorotation wasn't planned for; a long line can swing, foul the terrain or come back towards the aircraft as it decelerates in the flare.
That's why releasing the load is part of the response, why the release is checked before every load flight, and why where the load would fall is part of choosing the route.
In steady autorotation, which region keeps the rotor turning?
The driving region, roughly the middle of the blade, where the aerodynamic force tilts forward of the axis.
What lets the rotor keep turning when the engine stops?
The freewheel disconnects the stopped engine, so the rotor is free to be driven by the air.
In autorotation, rotor rpm is too high. What does raising the collective do?
Raising the collective increases blade pitch and drag, slowing the rotor.
Which glide airspeed keeps you in the air longest?
Minimum rate of descent. Best glide goes furthest; flying slower than either only increases the descent.
Why does rotor rpm rise in a turn during autorotation?
Load factor increases the airflow through the disc. Rolling out lets rpm fall back.
What does AC 61-140A name as the predominant cause of autorotation-training accidents?
Losing rpm and airspeed control. Its answer: if the parameters aren't met, abandon and go around.
Sources
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 11, Helicopter Emergencies and Hazards
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 2, Aerodynamics of Flight
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA
- AC 61-140A, Autorotation Training · FAA · 31 August 2016, paras 5.1, 5.4, 6.4 and 7.1.3
- Clockwise-rotating helicopter differences · Helicopter Ground
- Aviation Investigation Report A08A0007 (AS 350 BA C-FHHH, power-recovery autorotation, Mount Pearl) · Transportation Safety Board of Canada · Summary, History of the flight, Analysis, footnote 2
- AC 133-1B, Rotorcraft External-Load Operations · FAA
- 14 CFR Part 133, Rotorcraft External-Load Operations · U.S. eCFR
Educational only. Not approved training material, and never a substitute for the Rotorcraft Flight Manual, an instructor, or your operator's procedures. Figures marked as manufacturer data or test results are not limitations. This content is a draft and has not yet been reviewed by an instructor.