Carb Heat in Engine Run up Checks

[smith]

Did my PPL in USA during the engine run-up the school policy was 2,000rpm, L/R mag checks, carb heat check rpm drop, then with carb heat still on power to idle, carb heat off check rpm increase, then power to 1000rpm.

When I have had check outs at the other schools, I have been asked "Why do you leave the carb heat on when reducing power to idle?" Its seems uncommon everywhere else, most people just seem to test for rpm drop using carb heat at 1,700/2,000rpm.

The rational for idling the engine with carbheat on in the run up is that this will produce the absolute minimum rpm in the engine and therefore the engine is unlikely to cut out when you reduce the throttle to idle with carb heat say when practising stalls or on approach.

What method do other people use at their clubs?

[Mike Cross]

I do the carb heat first, I figure there's no point in looking at the mag drop if you have an unstable system that will vary the RPM due to ice accretion or shedding. You need to ensure ice is not present before doing the mag check,

[Irv]

Smith: Don't forget you should be interested in any rise in RPM! Most pilots I meet have only be told to look for an RPM drop and that's it. I try and point out the difference by calling one method a carb-ice check, and the other a carb-heat check. (I'm really only interested in a student doing a carb-ice check!).

If you apply carb heat, the rpm drop is basically telling you the level or knob you used is connected and the system is working as designed - that is what I call a carb-heat check. What you really want to know is - do you have any ice? and if you did, have you cleared it all? That is a carb-ice check.

So to do a carb-ice check, whether at power checks or in the cruise - stabilse RPM to a known value, apply carb-heat. RPM instantly drops - 99.9% of time. (If it rises on application rather than drops, check the mixture control, you might be running 'weak'.) Assuming it does drop, you are now interested in whether it subsequently rises, as that would indicate ice had been removed and the engine is performing better. When you restore the cold air, is the RPM above your previous RPM? If so, you burned off ice, but don't know if you burned it all off, so need to repeat the loop.

If you're talking pre-flight power checks, do a 'carb-ice' check at power rpm as above, then close the throttle from 'power levels' with carb heat cold and check RPM. If it's too high (over recommended value) then you may have trouble landing. Assuming it's ok, apply carb heat again to see if the engine is happy to continue running in that configuration, as that would be the configuration in a glide approach.

[omcaree]

Please forgive my ignorance but is there much point in reducing the engine to its absolute minimum RPM (that is idle with carb heat applied) during the power checks? Any inflight situation with the engine idling and carb heat applied would result in a higher RPM value because of the forward motion of the prop through the air, even at the stall you're still going faster than at the power checks.

I always thought that the application of carb heat during the power checks is to clear the engine of any ice before checking the mags. Then the throttle is fully closed to ensure that the plunger/level can move freely all the way back and is not restricted in anyway. That way, presuming nothing major changes in flight, the throttle can be closed when required for landing. Any restriction to the application of full power should be seen at the start of the take off roll and the take off aborted, so a full-power check during the power checks isn't required (nor is it desiralbe).

These have been my beliefs up to now but i'm willing to be educated if incorrect.

[fireflybob]

The carb heat check on the Power Check prior to take off is simply a function check - select hot air and the rpm should decrease slightly (circa 50 spm), select cold air again the rpm should restore to the original value.

This assumes no ice is present. If the rpm exceeds the original after you have selected cold air then the most likely cause is ice - worth noting prior to take off since in the event of a significant delay it might be prudent to clear any ice immediately prior to take off. As has also been said its useful to clear any ice prior to the mag check.

However remember also that when you select hot air the air is unfiltered and most pilot operating handbooks/flight manuals say to keep use of carb heat on the ground to a minimum to avoid getting "bits" into the system.

Cannot see the point of leaving the carb heat on whilst you throttle back to idle and the unfiltered air aspect point against doling this. Also we should be conducting the power check in accordance with the POH/Flight Manual.

[BEagle]

I've heard of other American FIs having this weird idea about engines stalling because of low idle rpm - seems odd to me!

We teach (PA28):

Start engine and set 1200 rpm. Complete remainder of after start checks whilst the engine warms up, then do a 'drop, not stop' magneto dead cut check before starting taxying.

Never use power against brake during taxying (apart from necessary manoeuvring in tight spaces) !

Carry out the power check by stopping into wind, then setting 1200 rpm and switching to the other fuel tank.

Check Ts and Ps and clear behind, then gently up to 2000 rpm checking the a/c doesn't move. Stabilise at 2000 rpm, then operate carb heat for 5-10 sec, note rpm reduction on application and also that it doesn't increase when cold air is reselected. Then switch to left mag, check drop, back to both and check rpm restored, then right magand back to both. Finally check vacuum and ammeter, then power gently back to idle - take hand off throttle at idle so that you're not forcing it to an artificial minimum position. Check idle rpm, then reset 1200 rpm and continue.

I see no point whatever in checking carb heat at idle on the ground.

[smith]

BEagle

I see no point in it either, but as I was trained this way and at the time it seemed quite rational i.e. idle + carb heat=absolute minimum revs in that configuration, I still use this method. As such I have to explain to my instructor why.

[nimbuscumulus]

I say Irv is right. The checking of an increase in rpm after your carb-heat check is at least as important as the checking of the meganism itself.
I like the carb-heat/carb-ice check too. nice one.

[Kolibear]

Bear in in mind how effective a propellor is at stirring up dust, grit etc and an air induction system is really just a very good Hoover.

So sitting at high revs with carb. heat applied and unfiltered air passing into the engine for any length of time really isn't a very good method of prolonging engine life.

I'd be interested to know how much leakage past the carb heat flap when its in the closed position is deemed acceptable.

[DubTrub]

unfiltered air passing into the engine for any length of time
My air intake has no filter. At all. Well, not since 1945, anyway.
Never had a problem with it. Since I don't live in the Sahara, I don't think a few insects inside the cylinders is doing too much harm.

[Irv]

We get the odd day at Popham (usually 'November' type weather - not too cold, but moist, etc) where there is plenty of carb ice built up just during a long taxi - very noticeable on power checks and needs clearing.

I've seen/heard of enough engine problems which with hindsight people have said 'now you mention it, I'd noticed ...' and they would recount some minor detail which, looking back, was not a direct indication, but a clue to the later major problem developing.

The point of the 'try the carb heat' at minimum revs is merely to see what happens - if the rpm drops and engine keeps running, great -that's what I'd expect. If anything else happens, why would it? Having had engine failures on landing (Staverton), take off (Popham) and in flight (Berkshire), perhaps I'm should be allowed to be a bit sensitive to pre-take off engine oddities!

On the subject of checks during take-off run, I've been in an aircraft where the standard "chant" of various rolling checks was forthcoming, but that's all it was - a chant! I had to point out that we only had 1900rpm withg throttle fully open. We would have gone into the hedge if I hadn't.