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Many helicopter pilots are completely unaware of how close they have come to sudden engine silence in flight. 


If you read the ATSB reports on helicopter accidents, you will see that there are a number of reported unexplained AVGAS piston engine failures that have a common thread – they ran perfectly on the test bench AFTER the accident. 

Why?  There is an answer which is quite often overlooked because it is so simple.


Let’s take a step back to basics and revise our AVGAS piston engine knowledge. 


Have you ever thought why the Bell 47, Hiller UH12E, R22 and similar helicopter engines all have a recommended idle speed of approx. 75% max Engine RPM  - 75% of max engine RPM in comparison to fixed wing aviation engines of the same type which idle around the 50% range.


Among the reasons are:


     Helicopter six cylinder engines have NO flywheel attached to the crankshaft to keep them turning over smoothly during each cylinder’s compression and power pulse at low revs.


Therefore they idle very fast  so that the inertia of the rotating crankshaft and heavy pistons is enough to provide energy for acceptable engine smooth running in the airframe.


    The four cylinder R22 engine has a minimal aluminum starter ring gear flywheel, engine cooling fan (really a big brake in disguise!) drive belts, alternator, drive pulley bearings etc - which all combine together to cause a strong drag (brake) effect on the engine.


Fine, you may say, but what has this to do with engine stoppage?


 Lets look at a couple of cases:

It was a lovely day for flying in Sydney – warm and a bit of shade from some low clouds blown inland from the balmy sea breeze. 

The B47 helicopter engine was just purring along when the pilot decided to plan ahead and let down from 2000 to 500 ft by maintaining his IAS and knocking the power back from 19” to 16”. 

Then at 800 feet – the engine suddenly stopped without any warning!


The R22 muster pilot in the gulf was trying to get some cattle in before the wet season. 

It was already hot (35oC) and the humidity was high.

The pilot had light fuel load and was by himself. 

The mooies (cattle) were moving well (not too fast!) and he was cruising at about 300 feet and 17” MAP. 

The engine then suddenly stopped without any warning!


A puzzle? Well this is what happened:


    As the pistons moved down in the engine cylinders – they created a low pressure area which was open to the atmosphere through the carburetor.


    Air flowed from a higher pressure source (atmosphere) to the low pressure area (cylinders).


    The flow speed (velocity) was regulated by how far the disc shaped carburetor throttle valve was held open or closed by the pilot.


    The low pressure (16 inches of MAP) on the engine side of the throttle valve was measured by the MAP gauge.


    When the pilot reduced the power to below 18” MAP, the throttle valve was in a nearly closed position – leaving only two  small gaps (one at each end) for the higher pressure atmospheric air to force its way through to the low pressure in the cylinder(s).


   The engine was still turning at normal flight RPM and the cylinders were creating a really low pressure on their side of the throttle valve.

Remembering our basics again, you will recall that when air velocity increases (going from a high to low pressure) it’s temperature drops considerably


If conditions are right, any water (moisture) suspended in the air turns to ice and builds up very quickly to block the 2 air gaps at the throttle valve and the engine immediately stops.


This is what happened to the two pilots above.

The Bell 47 pilot didn’t monitor his carburetor heat gauge.

The R22 pilot didn’t realise that the carburetor on his engine was bolted direct to the hot engine oil sump.

Therefore the R22 carb heat gauge sensor was picking up engine oil sump heat and was giving an erroneous reading below 18” MAP



This pilot hadn’t understood that this is the reason why the R22 is placarded that carb heat should be used below 18” MAP in conditions conductive to icing (high humidity, visible moisture etc).


Have another read your flight manual! 

Understand how and WHEN carb ice forms and how to prevent it! It can happen at over 30 degrees C. - read this link






Many Helicopter pilots initially trained on fixed wing light aircraft and have experienced or heard stories about fixed wing engines running rough, through carburetor icing, and then being cleared by use of carburetor heat. 

The news I have for those pilots is that the engine kept running, even though it was starving for air and wanted to quit, due to the flywheel inertia effect of the large metal propeller.


What helicopter pilots have to remember is:


Carb icing can occur at over 30 oC


their engine has no propeller to act as a flywheel


therefore their engine usually gives NO notice in flight by rough running before it stops.


Carburetor Heat is useless - after the engine quits.


Use of commercial vehicle air filters can also cause unexpected carb icing due the thicker filter medium.


Understand how to use Carb Heat and use it often!





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