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Advanced Autorotations

Issue No 56, 26 February 2024

By: Anthony O. Ives

Autorotation is the emergency procedure to land a helicopter safely after an engine failure, the basic procedure was covered in an earlier article [1]. This article will discuss some more advanced procedures that will allow the pilot to vary his chosen touchdown point depending where the engine failure occurs.

The basic autorotation procedure was covered previously [1], it is also covered in a number of textbooks [2,3]. The basic autorotation usually involves lowering the collective to maintain rotor RPM, while simultaneously applying back cyclic stick and applying the correct anti-torque pedal. Once the autorotation has been established the pilot may need to increase the collective lever to stop rotor RPM becoming too high. The pilot selects the correct autorotation airspeed, it is by varying the airspeed the pilot can vary to some extent his final touchdown point.

Advanced autorotation procedures are covered in more detail in reference [4]. However, a summary of different types of autorotations is as follows:

Range Autorotation - This autorotation is used to maximise the distance the helicopter can travel during autorotation. Pilot follows the standard autrotation procedure except they select the range autorotation airspeed which is usually higher than standard autorotation airspeed. Using the R22 helicopter as an example the standard autorotation airspeed is about 65 knots while the airspeed for range autorotation is about 75 knots. The pilot usually select this autorotation type when their only available landing site is far away from the point an engine occurs.

Max Range Autorotation - This autorotation is similar to the previous range autorotation with the exception it lets you travel a little more in autorotation. The max range autorotation is usually performed by following the same procedure as the range autorotation and also reducing the rotor RPM to its lower allowable limit by carefully raising collective lever slightly.

Zero/Low Speed Autorotation - If the engine failure occurs just above or very close to the only available landing site then the pilot can reduce the airspeed to zero or a very low value after entering autorotation. This type of autorotation will keep the helicopter over the landing site. This autorotation is best performed with the helicopter into wind and the airspeed needs to be increased at a safe altitude back to the standard autorotation in order to be safely able to flare on reaching the ground. The pilot may need to also increase the collective to contain the rotor RPM and then reduce the collective again on returning to the standard autorotation airspeed.

180/360 S Turn Autorotations - If there is no or little wind and the engine failure occurs very close or above the only available landing then the pilot could perform a 360 autorotation. A 360 autorotation involves standard enter to autrotation then reducing airspeed to about 40 knots and entering a steep turn and basically just spiralling down to the landing site levelling up into wind and then again increasing the airspeed before touchdown to allow a safe flare. Of course the pilot could use this same technique if they are downwind of the landing site instead during just doing 180 turn rather than a 360 turn. If the engine failure occurs at very high altitude more than 360 turns could be performed to keep the helicopter close to or over the landing site. The pilot may need to also increase the collective to contain the rotor RPM in the turn and then reduce the collective again on returning to straight flight.

Constant Altitude/Speed Autorotation - For engine failure at night or when flying under IFR (Instrumental Flight Rules) the constant attitude autorotation is the best choice as does not require a flare. Visual references close to the ground are usually required to safely flare, these are usually not available at night or in IFR. After the usual standard entry to autorotation the pilot reduces airspeed to about 30/40 knots, probably needing to increasing collective to contain the rotor RPM. Then the pilot continues at that constant airspeed of 30/40 knots increasing collective just before touchpoint. This type of aitorotation is not recommended apart from at night or in IFR. If the helicopter is equiped with flares or autorotation lights and can allow the pilot to see visual references clearly at night it is safer to perform autorotation ending in flare.

Minimum Rate of Descent Autorotation - Minimum rate of descent autorotation is when the pilot sets the airspeed to the minmum power airspeed after the usual enter to autorotation. Minmum power airspeed is also known as the best rate of climb airspeed. I cannot really think of a practical scenario for this autorotation technique unless your waiting for something? This autorotation will last the longest. As with zero/low speed autorotation the pilot may need to also increase the collective to contain the rotor RPM and then reduce the collective again on returning to the standard autorotation airspeed.

Reference [4] was written by an experienced helicopter test pilot and goes into great detail about autorotation also covering all the advanced autorotations as well as giving lots of advice on how to do precision autorotations and reach any available landing site in the event of an engine failure. The picture below shows typical glide paths for each autorotation:

Advanced Autorotations

The height-velocity graph is another important performance aspect about autorotation that a pilot needs to be aware of. The height-velocity chart shows which combinations of airspeed and altitude are not practically possible. The height-velocity chart is given for specific conditions such as environmental temperature and the helicopter's weight. The height-velocity is sometimes referred to as dead man's as an engine failure at the wrong airspeed and altitude can have fatal consequences. However, generally hovering or low speed flight (less than 50 knots) between 10 ft and 500 ft is risky for most helicopters if an engine failure occurs. Reference [4] goes into more detail on height-velocity curves, an example of one is given below:

Height-Velocity Chart

Helicopter autorotation speeds are only used for example in this article please always refer to the RFM (Rotorcraft Flight Manual) or POH (Pilot's Operating Handbook) for the specific helicopter's correct airspeeds to use in autorotation as well as the correct autorotation procedures.

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References:

[1] http://www.eiteog.com/EiteogBLOG/No20EiteogBlogAutorotation.html

[2] Learning to Fly Helicopters, R. Randall Padfield, 1992, McGraw Hill

[3] The Helicopter Flying Handbook, FAA-H-8083-21B, 2019, United States Department of Transportation, Federal Aviation Administration, https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook

[4] The Little Book of Autorotations, Shawn Coyle, 2013, Eagle Eye Solutions

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