\[ \]

Helicopter Autorotation

Issue No 20, 13 March 2023

By: Anthony O. Ives

There is a belief or maybe a myth that an engine failure in a helicopter means the helicopter falls out of the sky with certain death for its occupants. However, this is completely wrong engine failures in a helicopter are just as survivable as those in fixed wing aircraft, helicopter pilots and ethusiasts like me will tell you they are actually more survivable than one in a fixed wing aircraft. In the case of an engine failure in a helicopter, the helicopter is in theory capable of landing in a reasonably small area with very little forward motion, if the landing is done correctly the helicopter will not even get damaged. One the advantages of a helicopter is it can land anywhere the same applies in the case of an engine failure, a helicopter has a much better chance of finding a suitable landing area in an emergency. An engine failure or any emergency in a fixed wing aircraft presents a big problem because it has to essentially find a airstrip of some sort so usually most engine failures in fixed wing aircraft result in some damage to aircraft mainly due to need of fixed wing aircraft to maintain some forward motion and crashing into fences or rough ground. Engine failures in any aircraft are something that should be avoided that is why aircraft engines are required to be well maintained and pilots perform detailed checks all to reduce the chance of engine failures. The pilot should also monitor the engine instruments and plan the flight that they can get to a suitable landing site. Generally speaking engine failures in most modern aircraft are fairly rare, Ref [1]. Helicopters land safely after an engine failure by entering autorotation which involves the pilot carrying out a series of actions to maintain rotor RPM. The golden rule to staying alive in helicopters is to maintain rotor RPM this applies always and not just in autorotation, see Ref [2].

In the case of an engine failure in a helicopter there is a number of signs that will tell you that your engine has stopped here is list of some them starting with the most obivous:

Lack of engine noise, the engine might make funny noises just before it stops.
Low rotor RPM horn and light if your helicopter is equipped, most helicopters will have light but I have heard some do not have a horn.
Large yawing motion, due to loss of engine torque and the pilot still holding the same pedal position.
Rotor and engine RPM reading reducing on the rotor and engine RPM gauge.

It is important for the pilot to recognise an engine failure as soon as possible to avoid the rotor RPM becoming dangerously low. Different helicopters respond differently to engine failures depending on whether they are a high or low inertia rotor system. High inertia rotor system takes longer for the rotor RPM to decay due the rotor blade being heavy hence having a high inertia. I remember reading about a US Air Force helicopter that had such high inertia rotor system that in the case of an engine failure the pilot did not need to do anything it just floated to ground unfortunately I cannot remember which helicopter it was I will keep searching for it and talk about it a future article. The Robinson R22 and most of the Robinson type helicopter are low inertia rotor systems meaning their rotor RPM decays quite quickly so the pilot has to respond almost immediately hence why Robinson helicopters are equipped with a warning horn as well as a light for low rotor RPM. Once the rotor RPM decays below a certain value it becomes impossible to recover this is due to the rotor RPM providing a centrifugal force which keeps the rotor blades stiff and within a certain flap angle which is almost perpendicular to helicopter fuselage. Without this centrifugal force the blades flap up to an excessive flap angle which will give a very terrifying senario. Loss of rotor RPM also effects the reponsiveness of the cyclic control, the helicopter will respond slower to pitch and roll commands with loss of rotor RPM.

So it can be said that rotor RPM is very important for helicopters in order to keep them flying safely. In the event of an engine failure the pilot must take 3 immediate actions and 2 additional actions to enter autorotation and then hold a stable flight condition:

Reduce Collective Lever.
Apply Back Cyclic Stick.
Apply Right Pedal for counter-clockwise rotating rotors (Left for clockwise rotating rotors).
Check up slightly on collective lever to prevent rotor overspeed.
Set autorotation airspeed (e.g. In the R22 its 65 KIAS - Knots of Indicated AirSpeed).

The first 3 actions should be performed immediately and almost simultaneously. The purpose of the first action is to reduce the rotor blade pitch angle, the collective lever controls the rotor blade pitch angle. Reducing the rotor blade pitch angle reduces the drag on the rotor blade hence preventing a reduction in rotor RPM now that the engine cannot turn the blades, see Ref [3], [4] and [5]. The second action maintains airflow through the rotor to keep it spinning, with the reduction in the collective lever the helicopter will tend to pitch forward which would reduce the airflow through the rotor. The third action is mainly to reduce the yawing of the helicopter. However, it is quite common for novice pilots to attempt to use the wrong pedal which will be very disorientating and I think could potentional reduce the rotor RPM. The pedal that is used for high power settings when the engine is running is using some power to overcome the torque produced by the engine. In autorotation the main rotor is turning the tail rotor to maintain yaw control so using the wrong pedal which would be the pedal used for high power settings could potentionally reduce rotor RPM, I have never found this documented anywhere so it just something I have thought about. However, it is better to select the correct pedal and not cause any unnecessary confusion in an already stressful day. Action number 4 is to prevent a rotor overspeed, when the pilot reduces the collective lever the rotor RPM will eventually increase so a slight check up on the collective lever will be necessary to keep the rotor RPM within its limits. The rotor RPM gauge is usually given as a percentage with a 100% being the design rotor RPM. However, the specific Rotorcraft Flight Manual (RFM) will give a range of safe operating rotor RPMs for engine off and engine on. A typical range for engine off rotor RPM might be 90% to 110%, the gauge is usually colour coded for example green for safe range, red for unsafe. For engine operating the rotor RPM range could be smaller something like 97% to 103%, in the R22 the low RPM horn comes on at 97% but the rotor RPM is still safe down to 90% for engine off. The last action is the airspeed which depends on the type of autorotation but generally the pilot does not want to let the airspeed get too low for the R22 the design autorotation airspeed is 65 KIAS (Knots of Indicated AirSpeed), the specific Rotorcraft Flight Manual (RFM) will give the autorotation airspeed for the specific helicopter which is what should be generally be used. To perform different types of autorotation in order to increase or reduce range to land in a certain area the pilot can change the airspeed, these are advanced autorotations which will be discussed in future articles however, also see Ref [6] for further information.

The pilot should maitain airspeed and rotor RPM until between 150ft and 40ft above the ground where they will commence the flare. Judging when to flare can be the most difficult part of autorotation generally Ref [7] suggests the best time to is when low enough that ground appears to be going past you at a slow run or a brisk walk. To initiate the flare pull back gently and graduately on the cyclic stick. The flare reduces airspeed but as Ref [6] emphasizes the main purpose is to reduce the rate of descent. After the flare has reduced the rate of descent most references, including Ref [2] and [7] suggest pushing forward on the cyclic stick to level the helicopter and prevent the tail rotor striking the ground. Then just before the helicopter contacts the ground pull up on the collective lever to cushion the landing. Ref [6] suggests that pulling on the collective lever immediately at the end of flare will level helicopter more effectively as there not much authority in the cyclic stick at this point in the autorotation.

The entire autorotation sequence of events is described graphically in this diagram:

Helicopter Autorotation Procedure

Ref [2] points out if there one thing to remember about autorotation is when the engine stops push the collective lever down which I agree is true. Maintaining rotor RPM in an engine failure is the most important part of staying alive even if you do not get the flare correct you will probably still survive. The big advantage with helicopter is of course as soon as there is any problems you should be able to find a landing site quite quickly. Helicopter pilots usually plan there flight so they are not over built up areas and where is large number landing options available. A typical way to determine if a potential landing site is safe is to 5S it, in engine failure you may have time to do it but you could use it to plan your flight, the 5S is:

Size, make sure the site is big enough you typically want something the size of a football pitch.
Shape, is it the correct shape that you can perform in to wind landings and take offs
Surrounds, is there anything that you can run into like power lines
Slope, is the surface level, slopes greater that 5 degrees are not recommended
Surface, is there loose debris on the ground that could blow about and damage the helicopter or other things nearby

When doing practice autorotations always bring an flight instructor with you. Pilots should practice quite frequently a common mnemonic to help remember the checks before entering a practice autorotation is HASEL, each of the letters mean the following:

Height, are you at a height to perform the practice autorotation safely
Area, is the area around and particularly beneath the helicopter clear of other aircraft
Security, there are no loose objects in the cockpit and doors are securely closed
Engine, is the engine functioning normally, gauges are in the green, prevent a practice autorotation becoming a real one
Look out, watch out for other aircraft particularly beneath the helicopter

Ref [6] is probably the most detailed book I have found on autorotations, the author was a helicopter test pilot which means he probably carried out quite a lot more autorotations in his career than most other pilots. So therefore Ref [6] contains a lot of practical advice on performing autorotations and well worth reading for any helicopter pilot. Also remember to consult the Rotorcraft Flight Manual (RFM) for the specific helicopter you are flying for the correct autorotation procedure, airspeed and rotor RPM settings, etc. I hope you found this article helpful, I hope to do a future article looking at more advanced autorotations.

Please leave a comment on my facebook page or via email and let me know if you found this blog article useful and if you would like to see more on this topic. Most of my blog articles are on:

Mathematics
Helicopters
VTOL UAVs (RC Helicopters)
Sailing and Sailboat Design

If there is one or more of these topics that you are specifically interested in please also let me know in your comments this will help me to write blog articles that are more helpful.

References:

[1] https://www.aviationsafetymagazine.com/features/minimizing-the-risk-of-engine-failure/

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

[3] http://www.eiteog.com/EiteogBLOG/No4EiteogBlogThrust.html

[4] http://www.eiteog.com/EiteogBLOG/No11EiteogBlogThrust.html

[5] http://www.eiteog.com/EiteogBLOG/No16EiteogBlogThrust.html

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

[7] 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

Disclaimer: Eiteog makes every effort to provide information which is as accurate as possible. Eiteog will not be responsible for any liability, loss or risk incurred as a result of the use and application of information on its website or in its products. None of the information on Eiteog's website or in its products supersedes any information contained in documents or procedures issued by relevant aviation authorities, manufacturers, flight schools or the operators of aircraft, UAVs.