Carb Icing PA28

[Centaurus]

A DC3 crashed in Canberra many years ago because the crew put the heat controls in full hot and both engines detonated themselves to death. Followed by the airplane and crew.

Have you got any details on that accident? I flew Daks for many years in that area and don't recall that specific accident. There certainly were two Dakota accidents that I knew of; of which one was at Canberra airfield around 1953-4 . That was a runway port propeller shortly after lift off and the aircraft crashed inverted killing all aboard. Another was an engine failure in cruise over the mountains north of East Sale and tracking for Canberra and despite full power on the live engine it was unable to maintain height and eventually crashed into a hill killing a few on board. Whether carb icing was involved or not I don't know.

[moonburn]

Hi Genghis,
Yes it is, but what I am trying to find out is why a flight training organisation has made the statement that use of carb heat on the approach can cause carb icing. For a certainty Piper do say fly the approach with cold selected but the POH then seems to say that this is in order to obtain full power on a go around and avoid detonation. There is no mention of carb heat causing icing and so I repeat, Is there any official publication, bulletin, AD, AIC etc: that has caused this FTO to make such a claim with specific reference to the PA28 ?
Thanks all for the input and happy new year.

[763Nick]

My own GA experience while training had Carb heat on for the downwind checks then select cold when complete, while turning base carb heat on again before throttle idle or near idle revs and continue to fly approach with carb heat on until on final down to 300ft where carb heat is selected cold, I once questionned this in the context of a go-around situation to which my then instructor said, "its the way we have been told to teach it and besides if a go-around is required you have time to select cold and commence go-around from 300 ft for full power"

Well, I agree with what he said, but didnt feel particularly confident with the "its the way we have been told to teach it"..........got to love those CFI's on a power trip.

[boofhead]

Centaurus. Yes it was the second one. I was told the engine went out because of detonation caused by the crew selecting the carb heat to full. I don't know why the first engine failed but was led to believe it also failed due to improper use of carb heat.
I was flying over the bush in WA one summer in a Dakota with an OAT of 45C and both engines were detonating. Every 30 minutes one or the other would cough and recover. Too high a temp is probably just as bad as too cold.

[SNS3Guppy]

Just be careful that the reverse policy applies if landing a Cessna single (150 and 172). The POH requires full carb heat on until after landing when it is selected in to Cold before taxiing. Same with go-around procedure in that it includes first full throttle and after that carb heat in to cold. The short time the throttle is full open with carb heat hot will presumably not cause detonation but will clear any ice that may have gradually formed on final.

This certainly is NOT a requirement. The proper use of carburetor heat is entirely dependent on the existing conditions, and not on a blanket practice or procedure. Carburetor heat is a tool to be used where necessary and appropriate. Cessna, Piper, Lycoming, Continental, Bendix, Precision Airmotive, etc...don't stipulate otherwise.

The fact is that the ONLY consideration in when to apply and remove carburetor heat is the carb air temperature. Unfortunately, most light aircraft don't come equipped with this feature, leaving largely judgement in when to apply it, and when not to apply it or when to shut it off.

[fullyspooled]

This is particularly the case with respect to light aircraft and horizontally opposed piston powerplants.

I'm not sure I understand what it is you are trying to make clear here Guppy: Light aircraft - yes they usually have piston engines, but does it make any difference to the carb whether thay are of the hoizontally opposed variety? Suerly the carb doesn't know what piston line up the manifold leads to?

As an aside, many here will know there are some older variety of powerplants that have contiuous carb heat applied by design, and without any control linkage to the cockpit.

[ali1986]

so if this is true then why do flying schools teach u to put it on for the downwind leg and approach and only take it off at say 300ft if it can lead to detonation and/or icing?

[tigermagicjohn]

I think with regards to this it depends what climate you are flying in. If flying in the UK there is a big chance for carb. icing.
I was very surprised first time I flew here in the UK, as we I did not use Carb. heat as much when I was in Scandinavia flying.

But I believe my instructors words here in the UK are quite logical, it is better to have 80 - 90 % power for a go around, then nothing!
I have been thought here, Carb Heat checked on for Downwind, then off, after turning base and Carb Heat on until landing, Carb Heat on during all descent/ low power settings.
In the case of a go around, how long time will it take to move your hand from throttle to the carb heat switch?
And the RPM drop with Carb Heat on, is very little - specially with the climate in the UK, the go around might end with engine failure an forced landing - I personally would rather prefer to have 10 % less power when starting go around procedure.
Of course if you are flying in a dry hot/cold climate - you don't need to use this procedure.

[ali1986]

yeh but what about it actually contributing towards carb ice like the source that the thread starter states though? though i agree 'it is better to have 80 - 90 % power for a go around, than nothing!'

[Antunes]

Moonburn,

You might try the engine's manufacturer (Lycoming) manual. If I'm not mistaken, it's a tiny pink and white binder (size of a pocket book). There you have some general guidelines regarding the use of carburetor heat. Right now I don't remember if there's something on PA28 POH about use of carb heat.

Operationally speaking, when I was working as a CFI, the school SOP during power-off descents and approach was the following:

Power-off descents (including emergency simulation)
*Select carb heat on if OAT in the current altitude is below 20C;
*Idle power;
*At every 300ft (you may choose the interval), revs up to +- 2000 RPM and throttle to idle

Power-off approaches
*Select carb heat on if OAT < 20C when you're abeam touchdown point and;
*Retard the throttle to idle;
*On base leg (altitude loss of +- 300/400ft since beginning), revs up to +- 2000 rpm and then back again to idle;
*Land
*In case of go-around - full power > ensure proper flight path (the most important item, of course ) > check aircraft configuration > carb heat off

Power-on approaches
*Abeam touchdown point, RPM around 1500-1700 - no need to use carb heat
*Short final - throttle to idle (no carb heat, because it's a really short period of time)

OAT range there was from 0C up to 35-40C. Typical PA28 cruise levels with temperatures around -5C to 10C.

Hope this helps a little

[SNS3Guppy]

I'm not sure I understand what it is you are trying to make clear here Guppy: Light aircraft - yes they usually have piston engines, but does it make any difference to the carb whether thay are of the hoizontally opposed variety? Suerly the carb doesn't know what piston line up the manifold leads to?

Yes, it really does make a difference. Most light airplanes using horizontally opposed powerplants today don't come with carb air temp gauges, for one thing. Most use updraft float carburetors. Light singles using various radial engines often use pressure carburetion of a downdraft variety through a supercharger, and often do have carb air temp instrumentation, and different requirements and application procedures and techniques. A 1340 or R985 isn't operated the same as an O-360.

The techniques and practices for a given airplane aren't universal, either. One uses carburetor heat where needed, as a tool for engine operation. To blindly apply full carb heat at a given phase of flight is doing little more than taking an ignorant, wild shot in the dark...and doing little if anything to address the issue.

Should carb heat be applied blindly on the downwind then shut off? It's appropriate some times, and not others. Should it be left on until power is applied at the end of an approach to landing? Sometimes appropriate, sometimes not. It's a tool to be used, and it's use varies according to the conditions, installation, appplication, and even operator experience, to say nothing of instrumentation available.

[moonburn]

Antunes,
Thanks, good call.

[Genghis the Engineer]

Originally Posted by moonburn

Hi Genghis,
Yes it is, but what I am trying to find out is why a flight training organisation has made the statement that use of carb heat on the approach can cause carb icing. For a certainty Piper do say fly the approach with cold selected but the POH then seems to say that this is in order to obtain full power on a go around and avoid detonation. There is no mention of carb heat causing icing and so I repeat, Is there any official publication, bulletin, AD, AIC etc: that has caused this FTO to make such a claim with specific reference to the PA28 ?
Thanks all for the input and happy new year.

Happy new year to you too.

By best guess, if you'd like it, is that this training organisation made it up on the basis of some FI's half-baked understanding, and then put it in writing.

Why not ask them for their source of reference - I'll bet a fiver they haven't got one that holds water.

G

[fullyspooled]

some FACTS

I expect that this may clear up a few of the misunderstandings and put a few old wives tales to bed as well....

AIC Pink 161 of 1997 should, if I have done this correctly, be found here:

http://www.nats-uk.ead-it.com/aip/cu...7_P_145_en.pdf

[Genghis the Engineer]

Originally Posted by fullyspooled

I expect that this may clear up a few of the misunderstandings and put a few old wives tales to bed as well....

AIC Pink 161 of 1997 should, if I have done this correctly, be found here:

http://www.nats-uk.ead-it.com/aip/cu...7_P_145_en.pdf

Para 5.11.1 pretty much covers it I think.

G

[Semu]

I have instructed fairly widely in both Cessnas and Pipers in a variety of seasons and locations. It is very difficult to make a generalized statement about the use of carb heat for either aircraft due to massive number of different models, many of which have since been modified. It is an exceptionally good idea to do whatever the aircraft handbook says. I personally nearly killed myself and an unsuspecting student because of being too lazy to read the handbook early in my instructing career, on a different issue in a new to me aircraft.
If you must have a single procedure for all piston aircraft, then by all means use carb heat for a full minute before your power reduction, and remember to lean appropriately (which may mean not at all). But it is better to fly each aircraft as it was designed to be flown.