Strange behaviour doing carb heat check in cruise

[davecessna]

I flew last night around Sherbrooke, QC and noticed some strange behaviour. Once I was at cruise at 3500, I would do my periodical carb heat checks. The engine was leaned out. I applied full carb heat left it for about 20 seconds then went carb heat cold. The RPM dropped enough to make my heart leap into my throat. As there aren't a load of forced landing options in that area at night, I turned it around and cut my trip short. I flew back full rich as well.

Does anybody have an idea what I did wrong if anything?

[Colonel Sanders]

What was the OAT at the time?

[davecessna]

-20 c

[davecessna]

Altimeter setting was at 30.40

[Colonel Sanders]

Under those conditions, I doubt it was
carb ice - there's virtually zero moisture
in the air, when it's that cold.

I suspect the mixture went a bit lean when
you put the carb heat cold, and one cylinder
probably was the culprit.

Richen the mixture before you put the carb
heat cold again. Remember that carb heat
often has the effect of improving fuel distribution,
so when you get rid of it, likely one cylinder is
going to go lean.

[davecessna]

That makes sense. The drop felt identical to the mixture leaning check you do in the run up.

[+RA]

Colonel Sanders, I'm curious as to why there would be a difference if the mixture wasn't changed? Surely the application then deactivation of the carb heat wouldn't effect the mixture from beforehand to after?

[CpnCrunch]

If you have a look in the maintenance thread you'll see more discussion on this:

viewtopic.php?f=27&t=86519

It was happening to me in a 150. When I checked under the cowl I noticed a pinched hot air intake hose. It's now been fixed but I haven't flown since, so I can't confirm whether the issue is still happening or not.

I don't understand how the mixture could go lean if the engine was running perfectly on cold air before carb heat was applied (assuming no leaning after applying carb heat). The only possible explanation I can think of is an air pressure change, i.e. blockage in the hot air intake. When you turn off carb heat the air pressure increases, resulting in a lean cut. Richening the mixture also cured the problem for me, but it shouldn't be necessary.

I'm certainly open to the idea that it's just a design quirk in the 150, but so far nobody has said this is normal. I certainly haven't seen it in any other planes before.

I think I might have had the cabin heater on every time it happened, which combined with the pinched hose could have robbed even more hot air from the intake. It would be an interesting experiment to try turning off the cabin heat when it happens, to see if that cures the problem.

[davecessna]

I should have mentioned I was barely getting any heat in the cabin either.

[CpnCrunch]

I should have mentioned I was barely getting any heat in the cabin either.

I also noticed in the 150 that when you turn on carb heat you don't hardly get any cabin heat. Again I'm not sure if this is due to the pinched hose or not. The hot air from the exhaust shroud enters a T connector and is split between the cabin hot air intake and the carb heat box, so that could explain it.

(According to the internet, the cabin hot air and carb heat should be taken from two different exhaust shrouds. Also according to the internet, the hot air is just "dumped" out of the cabin heat and carb heat boxes when not being used. In my experience both of those seem to be false, at least in the 150 I'm flying).

[iSight]

According to the internet, the cabin hot air and carb heat should be taken from two different exhaust shrouds. Also according to the internet, the hot air is just "dumped" out of the cabin heat and carb heat boxes when not being used...

This is the case in the 172 I fly.

[FenderManDan]

We just had recent discussion here about this. Check it out under the cowls but it imost likely what CS said. This happened on my 172 recently with similar conditions. Yesterday i flew it leaned just a tad (lot less then Summer time) and carb heat check behaved better. I have 160 hp engine and was burning 6.5 gl/hr.

[davecessna]

Sorry Ill admit I skipped the forum search before posting this. Thanks for the answers. Cold dense air, leaner mixture, what CS said was almost definitely the case. My school's aircraft don't have EGT gauges so we lean it by watching the tachometer. I may have been running a bit too lean.

[CpnCrunch]

According to the internet, the cabin hot air and carb heat should be taken from two different exhaust shrouds. Also according to the internet, the hot air is just "dumped" out of the cabin heat and carb heat boxes when not being used...

This is the case in the 172 I fly.

Can you clarify? You mean two different exhaust shrouds for cabin heat and carb heat?

To be honest you never seem to get much cabin heat out of any spam can in -10C or lower in my experience.

[pdw]

Friday night/ Feb 28 shows clear in the Sherbrooke area at 30.40 and -18/-20C ... where the relative humidity is recorded as 67-68%.

[FenderManDan]

CpnCrunch. On 172 there is a cable that opens and closes the flap that routes the hot air. This cable gets some dirt or the assembly gets loose and flap would not open completely even though you pulled the knob all the way. Some cleaning helped on mine it was -24 oat and heat worked much better. I mean older cessnas have limited number of vents so my hands were getting cold but feet and head were toasty.

I also don't have EGT so it is rpm monitoring when leaning. Now when it is this cold air sucked in it leans the engine more than on reg temps. Without touching the mixture control, cold air leans hot air enriches the mixture, right?

Anywho done talking about the cold air. Fendermandan is going to Cuba in a couple of hours to heat up the bones.

[CpnCrunch]

Now when it is this cold air sucked in it leans the engine more than on reg temps. Without touching the mixture control, cold air leans hot air enriches the mixture, right?

Yes, that's correct. However I don't understand how the engine can be running perfectly normally, then after applying and removing carb heat (without touching mixture control) it will suddenly become incredibly rough as if one cylinder isn't firing.

Anywho done talking about the cold air. Fendermandan is going to Cuba in a couple of hours to heat up the bones.

[/quote]

I'm going to Winnipeg in a few days, and it's going to warm up to -10C for my arrival.

[Colonel Sanders]

Heh. Bring your shorts. It was -30C this morning.

[2R]

When you are using warmer unfiltered air the fuel to air mixture is going to change.
If it was at night you might have swallowed a bat, although I have never seen a bat flying below freezing temps. It might have been the engine getting rid of snow/dust that can accumulate in the scat hoses if the aircraft is parked without cowl plugs you will get all kinds of critters moving into the dark recesses. Mice and birds will build nests in the scat air intake at the rear of the baffling.
Make sure you check that the scat hose has no obstructions in it before the next flight as the sputtering may have been a warning and the opportunity to have someone check the scat hose and replace it if it in poor condition. Very cheap fix compared to an off airport excursion.

One other trick is not to move your head when you apply the carb heat as the Doppler effect can cause concern as the engine note changes. Fun thing to do to nervous people is to look at them and ask if they heard a change in the engine ,as they turn to answer the engine note will appear to change as their head will be in a different position to when they first heard the question, and they will agree that the engine noise is changing. Evil, I know but great fun heheheheh......