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Issue No 57, 11 March 2024
By: Anthony O. Ives
The two most common hazards of engine failure and vortex ring were covered separately and in more detail previously [1],[2] and [3]. The less common but possibly just as serious and dangerous hazardous flight conditions are covered in this article. Some hazardous flight conditions such as ground resonance can only experienced in certain helicopter designs so the type of helicopter the hazard is most relevant to will be described as will be the most suitable recovery procedure.
Most recovery procedures from hazardous flight conditions are fairly simple involving one action however some may require autorotation. Autorotation [1] is a more complicated recovery procedure from a hazardous flight condition and is more commonly used for the three inflight emergencies (particularly in the UK at the time of writing):
Inflight Engine Failure
Inflight Tail Rotor Failure
Engine Fire (The recommended engine fire procedure is possibly different in different countries but at least in the UK at time of writing entry to autorotation is recommeded)
Most other inflight emergencies if they become serious require an immediate landing at most. For some situations a precautionary landing at next available landing site is only required. This article is only giving a general overview of emergencies, for your specific helicopter always refer to the RFM (Rotorcraft Flight Manual) or POH (Pilot's Operating Handbook) for the correct emergency procedures for the helicopter you are flying.
Low Rotor RPM is extremely dangerous particularly in helicopters with low rotor inertias such Robinson type helicopters and has been the cause of a number of fatal accidents. Modern Robinson helicopters have an electronic grovenor which keeps the rotor RPM within a safe operating range and reduces the possibility of low rotor RPM. However, in helicopters without an electronic grovenor too high a collective and too low an engine power can result in a low RPM scenario, environmental condtions can also be a factor such as high density altitude (combination of high elevation/altitude and high temperature). The recovery procedure is as follows:
Recovery Procedure: Reduce the collective lever while simultaneously increasing the engine power without overspeeding the rotor.
This hazardous flight condition occurs during lift into hover particularly with a skid equipped helicopter but could happen with a wheels equipped helicopter. The skids or wheels on oneside of the helicopter get stuck which causes the helicopter to roll and particularly if the pilot keeps applying collective lever to eventually flip over on it side doing a lot damage and possibly causing a death or an injury. A common mistake is to try to correct with opposite cyclic stick which will not stop roll the correct action is as follows.
Recovery Procedure: Reduce the collective lever to put the skids or wheels on both sides of the helicopter on the ground again.
A phenomenal known as 'mast bumping' can occur in 2 bladed helicopters as a result of a negative g manevrers. A negative g manevrer is usually the result of a forward push on the cyclic stick, a negative g manevrer will give a feeling of weightlessness. The negative g manevrer removes the weight force which causes a force imbalance which induces a roll, of course the pilot's natural reaction is to apply opposite cyclic stick to correct the rolling motion. However, this is a fatal mistake because no amount of cyclic will correct the roll instead it just causes the rotor contact the mast at the pivot point and detach from the helicopter hence its name 'mast bumping'. This phenomenon is particular to two bladed helicopters which was first encountered during the Vietnam war when UH-1 'Huey' Helicopters starting mysteriously falling out of sky [4].
Recovery Procedure: For this hazardous flight condition prevention is better than cure generally avoid negative g manevrers or situations where they are likely to be encountered such as gusty conditions. However, if a negative g manevrer is inadvertently encountered the best response is to restore positive g forces usually by applying aft cyclic stick.
Ground resonance is encountered in helicopters that have drag (lead-lag) hinges where a hard landing misaligns the blades. If ground resonance is not corrected it can result in violent vibrations which shake the helicopter apart [5]. The picture below shows a helicopter with drag hinges and how they can become misaligned.
Recovery Procedure: If the helicopter starts to vibrate violently on landing and there is sufficient rotor RPM simply lift off again and allow the blades to realign. If there is insufficient rotor RPM to lift off then apply the rotor brake to stop the rotor spinning as quickly as possible.
Retreating blade stall is the result of dissymetry of lift typical of helicopters in forward flight [6]. The blade angle of attack is higher on the retreating side of rotor which at high speeds can cause it to stall. A retreating blade stall causes the helicopter to pitch up and roll to retreating side of the rotor. The natural response to apply forward cyclic causes the helicopter to speed up making the situation worse. Aft cyclic causes the blade pitch to increase on the retreating side of the rotor which causes the angle of attack to increase again making the situation worse. So the correct response is as follows:
Recovery Procedure: Lower the collective lever reducing the rotor blade pitch hence angle of attack, then reduce the helicopter speed using aft cyclic stick.
Emergencies are covered in more detail in other references [7],[8] and [9] including some not covered in this article. Please take this article as general guidance and always refer to the RFM (Rotorcraft Flight Manual) or POH (Pilot's Operating Handbook) for the correct emergency procedures for the helicopter you are flying.
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] http://www.eiteog.com/EiteogBLOG/No20EiteogBlogAutorotation.html
[2] http://www.eiteog.com/EiteogBLOG/No26EiteogBlogVortex.html
[3] http://www.eiteog.com/EiteogBLOG/No26EiteogBlogVortex.html
[4] https://case.hks.harvard.edu/mast-bumping-in-huey-and-cobra-helicopters/
[5] Chinook CH-47 Ground Resonance Test
[6] http://www.eiteog.com/EiteogBLOG/No26EiteogBlogVortex.html
[7] Learning to Fly Helicopters, R. Randall Padfield, 1992, McGraw Hill
[8] 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
[9] Principles of Helicopter Flight, 2nd Edition, W. J. Wagtendonk, 2006, Aviation Supplies & Academics
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