South Asian Airlines of Dhaka, Bangladesh

On my very first helicopter solo flight in an R22 at Cranfield airfield, the low Rotor RPM horn and light came on during the downwind leg. My initial reaction to this was “bother”, followed by a well rehearsed ‘opening the throttle, lowering the lever and aft cyclic’. Even on a first solo, when my stress levels were elevated, this was a ‘non-event’.

I just wonder how the R22 or R44 pilot of today would react if the low RRPM horn and light came on in-flight?

Times have changed. The R22 in which I completed my flight training was without the engine governor we have today; therefore, the pilot had no option but to learn the skill of manually operating the throttle. The pilot was the engine throttle governor.

There are two fundamental lessons to be considered from the above event:

  • Operation of a primary flight control – 'the throttle'
  • Pilot reactions to 'caution lights'

In “the good old days,” RPM control via throttle manipulation during flight was as instinctive as any other flight control input; an action that required little or no thought. You automatically corrected as required in response to an indication on the RPM gauge; change in sound (which was most often the case); increase in vibration level; or the low RRPM horn and light activating.

rev counter

The pilot was a ‘Rotor RPM aware pilot’. There is an old adage in the fixed-wing world: “If an aeroplane pilot looks after the airspeed, everything else looks after itself.” In the rotary wing world it is “If the helicopter pilot looks after the rotor RPM, everything else looks after itself.”

Rotor RPM is your life blood: ‘keep it in the green’

As a result of having to manually operate the throttle, the pilot became reasonably comfortable with the sound and sight of the low RRPM horn and light, just as I did, which produced a pilot who did not overreact to the horn and light but just calmly corrected the RPM. It was exactly this training that prevented me from overreacting on my first solo. The low RRPM light is an amber caution light; it is not an “I am going to die” light, as they tend to be red!

Consider the following:

  • In your training how many times did you hear the Low RPM horn and see the caution light?
  • How many times did you recover from Low RPM?
  • What are the insipient stages of Low RPM?

The answers should be:

  • Often
  • Often
  • Increase in aircraft vibration, change in engine/rotor sound, horn and light.

A few years ago I was conducting a European Robinson Pilots Flight Safety Course at a well established flight training school. Unusually, I had a number of flight instructors attending including their Chief Flying Instructor. During the course I discussed manual RPM control and low RRPM recovery techniques etc. When I was conducting flight training I would explain and demonstrate the remarkable engine corolator on the R22 by setting the aircraft in the cruise at say 80kts IAS at a suitable height, then enter and establish autorotation at 65kts IAS, followed by recovering back to the 80kts cruise, without having to make any correction to the throttle. I would then follow through with this and get my student Bloggs to do the same. The Chief Flying Instructor suggested that it was not possible but would try it and let me know the result. Sometime later I received an e-mail to say that he had completed it successfully and was amazed that the engine corollator was so effective and efficient over such a wide throttle range. Well of course I was pleased that he had seen how well the R22 will fly without the governor but in my day that was ‘old hat;’ we did it all the time. How times have changed!

Without doubt the governor is a remarkable addition to the engine throttle system and was designed to reduce the pilot's workload, therefore making the aircraft easier to fly. It has enhanced the safety of the product but this positive effect is being overshadowed as it has led the industry/pilot to believe and rely on it as the primary RPM governor and I consider that to be a dangerous misconception and in my view is consequently having a negative effect on flight safety.

Prior to the engine governor the pilot was the primary governor and the governor was designed to be an aid to assist the pilot in maintaining RPM. Now, due to the way we teach ab initio PPL flight training with the governor on all the time, it has become ‘the primary governor.’ Thereby producing a pilot that has little or no ability to operate the throttle manually and by default has effectively become no more than a ‘passenger’ when it come to this primary flight control.

The pilot is and has to be the primary RPM governor because the engine governor is by design overrideable.

The Robinson R22/R44 piston engine governor is not like a turbine engine governor where the pilot manually opens the throttle to the flight RPM stop; then an engine Power Turbine Governor (PTG) takes control based on power demands via a collective lever position, changing fuel flow to the engine to achieve constant Power Turbine/Rotor RPM. The throttle remains in the flight stop position during flight i.e. fully open. It is not overrideable by the pilot (unless they close the throttle).

Throttle governor assembly
MI 6

By contrast, the R22/R44 governor physically opens and closes the throttle as it reacts to maintain constant engine RPM via a ‘slipping clutch’ which requires a differential of friction between the clutch friction and the inherent friction within the throttle system (the clutch friction being greater). The clutch is there so that the pilot is able to override the governor to prevent possible governor runaways (increases or decreases in ERPM) by gripping the throttle; thus increasing the throttle friction above the clutch friction causing the clutch to slip; thus" preventing further deviation from 104% (R22) or 102% (R44).

Governor control unit
MI 6

"So what?" I hear you say. The governor maintains the RPM and the pilot is able to correct any malfunction by overriding it: Great!

Under normal circumstances the governor does a very good job of maintaining your RPM; however, when things start to go wrong the pilot’s stress levels will inevitably increase.

What happens when the pilot gets stressed? Simple tasks that the pilot was able to perform at lower stress levels become difficult if not impossible to carry out. The pilot loses the ability to think clearly, coupled with a physical reaction which is to tighten their grip on the controls: you will be familiar with this during your training when ‘sir’ had to peel your hand from the cyclic stick post early flights. The pilot has just unwittingly, but very effectively, removed the governor i.e. overriding it, by increasing the throttle friction above the clutch friction and will be preventing the governor from maintaining their RPM. How good are your manual RPM control skills?

It is a sad reflection of the training environment today when I find that the majority of the pilots attending the European Robinson Pilots Flight Safety Course have very little awareness of:

  • manual throttle manipulation
  • how the governor works
  • governor/pilot interface
  • the simple fact that the governor was designed as an aid and by design cannot be the primary governor

Therefore, it is imperative that the pilot has to be taught manual throttle skills because without those skills the pilot who finds themselves in a low RPM situation will over react to the horn and caution light, turning a benign indication into a life threating indication, but most importantly will not have basic skills to recover.

My advice is that if you already have your pilots licence endorsed with the Robinson type then during your next and subsequent LPCs then ensure that you get the instructor to give you some serious manual throttle manipulation time and ‘Low RPM recovery’ work. This vital primary flight control operation must become instinctive. If you have to think about it then you will become a passenger for the time it takes to hit the ground.

If your flight instructor tells you that you cannot fly the R22/R44 without the governor switched on, which I hear all the time, then they are wrong. The Pilots Operating Handbook (POH) states "Flight is prohibited with governor selected off, with the exceptions for in-flight system malfunction or emergency procedures training" and if Low RPM recovery is not emergency training, then I do not know what is.

Get your flight instructor to demonstrate the insipient stages of Low RPM so that you are able to recognise the symptoms of this fatal situation i.e. learn to recognise the change in sound and the increase in vibration level that happens before the Low RPM horn and light activate: these are very early indications.

When I was with the French Accident Investigators (BEA) in Paris a few years ago assisting in an R22 fatal accident, I was shown a video of an R44 fatal accident that had been filmed from inside the subject aircraft. The film was taken from one of the backseat passengers as the R44 was slowly climbing within a mountainous area. After watching the film I was asked for my comments. There were several striking aspects about this film:

The Low RPM warning system was inactive during the event: there was no audible horn and I could not see if the light had activated.
The governor was not maintaining the RPM and this could be for a number of reasons; pilot was at full throttle (over pitching), the pilot was gripping the throttle too tightly (overriding the governor) etc.

However, for me the most striking element was the fact that the pilot was blissfully unaware that the helicopter was doing its best to warn him that the RPM was getting dangerously low by the very obvious and dramatic increase in its vibration levels: 'the aircraft was shaking.' The onset of this increased vibration level was gradual and should have easily been identified as a symptom of Low RPM and it also allowed plenty of time for the pilot to recover the situation. However, unfortunately for him and his passengers, this particular pilot failed to recognise and correctly react to these obvious symptoms of Low RPM. Without knowing the full history of this pilot and the accident it is very difficult to make any valid judgment; however, I would make a reasonable assumption based on what I saw on the film, and it is purely an assumption, that the pilot had very little throttle manipulation skills and therefore very little awareness of the insipient stages of Low RPM, for he clearly ignored them for some time, to his ultimate peril.

As far as I am concerned, based on what I see from conducting the European Robinson Pilots Flight Safety Courses around the World, the reason we have accidents such as the example I have given above, is that there this a total lack of RPM awareness. This situation has been bought about by the training industry’s misconception that the Robinson R22/R44 ‘engine governor’ will always maintain the RPM automatically for the pilot. Therefore the view seems to be that there is no need for the pilot to be taught manual throttle skills; just as there is no need to teach the Turbine helicopter pilot manual throttle skills. That given, there is no need to teach manual throttle skills; therefore there is no reason to teach the recognition of the insipient stages of Low RPM and the subsequent recovery skills.

‘You are seriously mistaken.’

Keep your RPM in the green and I will keep harping on.

back to articles page →