PA24-250 carb ice

[nick14]

Hello all,

I wanted to draw on the experience of you fine upstanding ladies and gents on the PA24-250 single commanche. I have a few hours on the aircraft now but I am still struggling with carb ice on the ground. I am reluctant to use hefty amounts of Carb heat due to the amount of crud we might suck up from the ground.

Are there any little tips tricks we could use to try and keep the carb clean? Would leaning and hence less fuel vapourising in the carb make it less prone to ice buildup?

Cheers all

[Spadhampton]

Don't think carb heat has anything to do with your FOD sucking factor. You do have an airfilter...right?

[SNS3Guppy]

Carb heat bypasses the air filter, hence the original poster's dilemma.

induction air when using is drawn from within the nacelle area. Not from outside, and not from debris near the ground. Perhaps student manuals make carburetor heat sound as though one is sucking up rocks and debris from a hose by the ground, but this is definitely not the case. Unfiltered air is the least of your concerns.

Leaning on the ground should be done for efficient engine operation, not to change the carb air temperature. Carb air temperature is largely a matter of mass airflow and airflow velocity. Don't forget that two types of carb icing can occur; venturi icing, and idle icing along the throttle plate. Throttle closed, idle icing occurs differently than open throttle icing, produces different results, and does different things.

When operating at idle power, the carburetor is operating on the idle jet. The mixture control in the cockpit does not adjust the idle jet; it's adjusted using a screw on the side of the carburetor, and is a maintenance function. This is known as idle mixture adjustment, and should be performed several times a year as the seasons change, or if the aircraft is based from a different elevation airport. Generally the idle mixture isn't properly set on light airplanes; I seldom see one that is...you will generally end up aggressively leaning on the ground to compensate for an improperly adjusted idle mixture. (again, you're not actually leaning the mixture, so much as partially cutting it off).

If operating carburetor heat on the ground for an extended time (such as in ideal carb ice conditions), then you will need to adjust the mixture accordingly. You'll need to enrich the mixture when you're done with the carburetor heat.

[nick14]

Guppy,

Many thanks for the lesson. I have once again picked up a few more bits of knowledge from the aviation world :ok:

Cheers all

[Big Pistons Forever]

Are you sure you are getting carb ice ? The Piper Lycoming engine installations are generally not particularly prone to icing except in extreme conditions. I have never experienced a situation where I was regularly getting carb ice on the ground. If the symptom is rough running on the ground at low power settings than another possible cause is an overly lean mixture. The carb heat you are applying on the ground will have the effect of enrichening the mixture and the engine will then smooth out as it is now operating at a more favorable mixture range. A usefull check of the correct idle mixture setting is to set 1000 RPM and slowly lean the engine until it stops. Just before the engine dies the RPM should increase about 50 RPM. No increase indicates an excesively lean idle mixture, and a large increase indicates an excessively rich idle mixture.

[flyingfemme]

The big, old Lycoming doesn't generally suffer carb ice too badly. Leaning for taxi is pretty good practice and also keeps the engine warmer - less need for carb heat.

Rough running at low rpm can also be due to a very rich mixture.........try leaning when the carb heat is on and see if that improves it.

How old is your carb - they can be very old on an aircraft of that age and don't last forever. Have your engineer check it out if you have doubts.

My PA24 is a 1960 model and was burning a lot of avgas as well as running less than smoothly at low rpm. Some fairly vicious leaning made it run more smoothly and a recon/exchange carb made a whole world of difference.

[nick14]

Thanks for the reply guys,

The main issue we have is that on two occasions now we have done the power checks and subsequently when applying power on the runway the engine doesnt give full power. After abandoning the takeoff we have tried the power checks again and the engine has stopped when applying high power. After it has been left to stand for a few minutes it starts and there are no signs of the issue.

The engine is fairly new I believe.