Recovering from low RPM in a R22/R44 - Aviation Pilot Training
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Recovering from low RPM in a R22/R44

Recovering from low RPM in a R22/R44

This seems like something quite simple to sort out to most helicopter pilots, but I have done an experiment over the last few years during flight instruction and testing. I quietly reduced RPM by overriding the governor to activate the low RPM light and horn; a substantial number of pilots lowered collective instantaneously to correct.

This is surely the right thing to do, except that if you don’t reduce collective enough for the rotor to start auto-rotating there will be almost no change. This is because of the correlator. Its function is to operate the throttle linkage when the collective is raised or lowered, to reduce movement on the throttle while we make power changes. So lowering collective does not solve the problem immediately and could even make things worse since lowering collective is like closing throttle.

As the RPM is low already for whatever reason, lowering of the collective can exacerbate the issue, especially as the alerted pilot now grips the throttle tighter which effectively prevents the governor from working. Then the attitude changes to nose down as a result of the pilot’s eyes now focused inside and not noticing this change. Fixation becomes a real issue at this point as you expected to see the RPM increase with a collective down movement – your fool-proof plan is failing. This keeps the pilot busy all the way down and prevents him/her from looking for a suitable LZ, wind direction etc.

We should remember that the primary control for controlling RPM is the throttle. Although normally looked after and operated totally by the governor, the correct control for us to revert to when the low RPM horn sounds is the throttle. That is of course if this is not caused by a severe power loss like an engine or drive train failure.

We should note that at sea level there is almost no throttle movement during normal flying on the R22 and R44 due to the perfection of the correlator. Only at higher altitudes does the governor start to work more, making throttle adjustments to maintain RPM due to the now “imperfections” of the correlator. The result is that because of this, governor failures hardly gets practiced at the coast and the young pilot has no experience of manual RPM control. Put him on the spot in an inconvenient time and a young pilot will panic.

How should we then recover from a low RPM condition: We should lower collective and open throttle simultaneously. That will assure that the pitch (drag) is decreased while engine power is increased. This will improve the situation whatever the cause of the low RPM condition is. Also note that the low RPM hooter is just a device to get your attention on the condition and not the sound of certain death

– Philip Merrick

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