Carb Heat causing Detonation?

[FlyingKiwi_73]

Can somebody abit more familiar with A/C engines explain to me how Carb Heat 'ON' in a normally operating engine (lets say in the climb 2500rpm) can cause detonation.

Is it something to do with Higher CHT due to the "hotter" intake air or unfiltered air?

I always thought Detonation was caused by bad leaning?

[Pilot DAR]

In a certified type, being run on appropriate fuel, it would not be possible to cause detonation in any meaningful way, by the application of carb heat. It is a design requirement that the engine be free from detonation in all operating conditions, The requirment wording is:

Sec. 33.47

Detonation test.

Each engine must be tested to establish that the engine can function without detonation throughout its range of intended conditions of operation.

During detonation testing, I have caused and observed detonation in a large Continental engine. I was surprisingly hard to do, but then very hard to stop once started. To cause detonation, it was necessary to run the engine at maximum operating temperatures, and on 80 octane gasoline, and well over square. If all of these adverse factors were not combined, I could not cause detonation. Though different engines behave a little differently.

The reason for the concern about detonation in aircraft engines (unlike cars) is that you will not know detonation is occurring, until the engine is damaged - probably significantly. And, because the engine might operate at full power for an extended time (which in a car it hardly ever would), you would operaionally have less margin to detonation. Hence the need for caution at the design stage. When the design testing and approval has been properly done, the pilot has no reason for concern, as long as you follow the operation requirements for the engine.

Similarly, bad leaning will not directly cause detonaton, it simply makes the engine run very hot. When the engine is running very hot, your detonation margins are reduced. Thus, if you make another operating error, (over square power setting/wrong fuel) you could be very close to detonation - and you'll only find out when it begins to shake like hell, and piston parts go where they should not.

I hope that helps... Operate in accordance with the flight manual, and don't worry...

[Dan Winterland]

It's mentioned in some training notes and the question does pop up in some syllabi. However, I've never come across it in real life and I suspect it happened on older engines or is urban myth.

[Ryan5252]

Operate in accordance with the flight manual, and don't worry...

It's stated in the POH/flight manual that full throttle operation with carburetor heat on can cause detonation. This may be more of a 6 covering excercise than anything else, based on PilotDARs observations.

Ryan

[Pilot DAR]

POH/flight manual that full throttle operation with carburetor heat on can cause detonation

Can you tell me the aircraft type for which this is written? I am quite surprised! I could never approve that!

[Morrisman1]

the o-320 in the r2160/a160a says that, I think the o-360 in the pa-28-181 does too

[englishal]

I'd have thought that with carb heat applied, the mixture is effectively richer due to the less dense air and so would prevent detonation?

[Morrisman1]

yes englishal, but you increase the intake temperatures a considerable amount, add to this the heating from compression and you are getting a lot closer to the temperature where detonation is possible than without carb heat applied.

[Jan Olieslagers]

I was surprisingly hard to do, but then very hard to stop once started.

Myself too can be very hard to stop, once started. Just have to wait till it's over.
Sorry, couldn't resist.

[englishal]

Does intake temp increase that much, and do the CHTs show redline with carb heat applied.

Ok, then you could go the other way, lean right back to LOP which will have the effect of cooling the engine ;)

[SkyHawk-N]

I seem to remember this subject being done to death recently, either on here or another forum.

If you were to use carb heat at full power, at sea level while taking off, on a VERY hot day then you MAY get within or near the detonation range as the detonation margins are reduced, but the chances of detonation occurring are very slim especially when running on AVGAS.

Having said that most aircraft POHs state 'Carb Heat.....COLD' for take-off and taking off with carb heat on during a VERY hot day is not a good idea in most circumstances.

Detonation is usually associated with leaning the mixture not enrichening.

[Ryan5252]

Can you tell me the aircraft type for which this is written? I am quite surprised! I could never approve that!

Sorry PilotDAR, was slightly vague in my post - I'll put that down to the time of the morning it was made ;)

Aircraft is our PA28-181, I have found a link to one online here (http://www.google.com/url?sa=t&source=web&cd=1&ved=0CBQQFjAA&url=http%3A%2F%2Fwww.glasscockpitaviation.com%2FMainPages%2F documents%2FPA-28-181-POH.pdf&rct=j&q=pa28-181%20poh&ei=1CRJTferJcO3hAfRoOWDDw&usg=AFQjCNFsVMWJQNfeHICms6jG-11MD5AmDw&sig2=Fi38z_FarL1bHhjqbuXDpA&cad=rja), page 29 refers.

Cheers
Ryan

[SkyHawk-N]

Page 29. says...

The mixture should be set in the full “RICH” position.

The mixture control should be kept in full “RICH” position to ensure maximum acceleration if it should be necessary to open the throttle again.

Normally, the best technique for short and slow landings is to use full flap and enough power to maintain the desired airspeed and approach flight path. Mixture should be full “RICH,” fuel on the fullest tank, and electric fuel pump “ON.”

[Off Topic]My pet POH peeve, "Mixture - RICH".[/Off Topic]

[Pilot DAR]

Well Kiwi, and Ryan, I agree that the POH says that - I had never noticed, though it's been a long time since I've flown a PA-28. I have a flight test coming up on a PA-28-161 in March, so I'll review that POH (and FM, if there is one) carefully!

Several things are worthy of note in the POH Ryan provided:

It does say that it is not an "actual POH or AFM". I would be very surprized to read an FAA approved Flight Manual which referred to the use of carb heat causing detonation. Perhaps someone who has access to the FAA approved version of the Flight Manual would review it to see of any similar reference is made to carb heat use. You are only legally bound to operate any aircraft in accordance with the "Approved" FLight Manual, and any published limitations and placards. Everything else is just a "nice to have".

There is no limitation, in that section, referring the use of carb heat. If a particular action is a "do not do", I would expect it to be a stated limitation - in which case it would either say do not exceed value "X", or just do not do. Nothing.

And, throughout the emergency procedures section, there are numerous references to the use of carb heat (entirely what I expect to see) associated with attempts at engine restart. There is no reference there to detonation once the engine restarts, and the pilot will probably have full power on before too long!

So I agree the "POH" (for whatever authority it has) says this, but I still don't believe it! Kiwi, keep flying, and keep this in the back of your mind, but don't loose sleep over it. I have never heard of detonation damage in O-360 series Lycomings, and I have flown a lot of them....

[BackPacker]

Just out of curiosity I checked the PA-28-161 Cadet POH I have here.

This is again not an actual POH taken from an aircraft, but rather a copy from somewhere, apparently set, bound and sold by Piper. More specifically, it says "Published by PUBLICATIONS DEPARTMENT, Piper Aircraft Corporation, Issued September 9. 1988" Report number is VB-1360, handbook part no 761 826. It's not tied to any specific airframe, even though the first page says "Application of this handbook is limited to the specific Piper PA-28-161 model airplane designated by serial number and registration number on the face of the title page of this handbook."

The exact same line "Full throttle operation with carb heat on can cause detonation" is there, again at the end of section 4.33 (Normal Operations; Approach and Landing). Which is, in my book, page 4-26.

I can't find any specific reference to this in either the Limitations section, or in any of the other normal operations subsections. Other than that "carb heat off" is in the take-off checklist, with no further explanation.

[ShyTorque]

Detonation caused by high intake air temperature is a well known phenomenon in the motor industry and the same rules of physics apply to aviation too.

Supercharged/turbocharged car engines normally have an air/air intercooler to reduce intake temperatures for exactly this reason, i.e. to prevent detonation caused by compression heating.

We were required to select intake air to "cold" on our Lycoming engined RAF Bulldogs (fuel injected engine with constant speed prop) prior to selecting full power to climb or go around (Max/rich/cold).

Intake icing problems are unlikely where the throttle plate is open wide, the depression across it is less and if icing does occur, it won't cause such a problem because it's likely to be ingested by the engine.

I once hired a 4x4 in NZ to get us up to the ski slopes. It pinked like mad going up the mountain road until I realised it had its air inlet "spout" pointing at the exhaust manifold. I swivelled it to the summer position and it transformed it; no further detonation trouble and much better power.

[rgsaero]

Shytorque - "I once hired a 4x4 in NZ to get us up to the ski slopes. It pinked like mad going up the mountain road until I realised it had its air inlet "spout" pointing at the exhaust manifold. I swivelled it to the summer position and it transformed it; no further detonation trouble and much better power."

I once - in '83 - used a Nissan Sunny in New Zealand during mid June to go over the pass from Thames to Whitianga. All the roads were gravel in those days and it was a typical bright, showery winter day. The Sunny kept running out of power, would only run on full throttle, but after being left for ten minutes was fine - for another ten minutes, than failed again. Carburettor icing - the air filter spout was NOT pointed at the manifold. My passengers didn't believe it until I changed it and the problem was solved. They'd never heard of carb icing!

My experience of detonation is with V6 twin turbo Nissan US built race car engines - producing up to 1000 bhp from 3 litres. Get the mixture / inlet temperatures wrong with those and they detonate themselves to destruction if a few seconds.

Never had any with a normally aspirated Lycoming though....

[ShyTorque]

rgsaero,

Yes, I know what you mean about the problem with the Nissan. I brought my Asian version Mitsubishi (carburettor equipped) back to UK when I came home some years ago. As it was made for warmer climates, It had no intake hot air facility fitted from new. I got to the stage where I could sense the weather conditions where it was going to suffer it's common bouts of carb. icing. It caused me some embarrassment by it's misbehaviour a few times until it was renedered beyond economic repair by having it's tailgate ripped off by a skidding HGV on a snowy back road.

[Big Pistons Forever]

I think the note in the Piper POH was put into address the issue of flight school
"one size fits all" training. Students are told to always put the carb heat full on when landing. So on it goes every time even if the temperature is 30 deg C. A prolonged application of full power with carb heat on at very high OAT's could possibly result in detonation, but my question is why was the carb heat on in the first place ?

Certification requirements ensure that if you are flying within the temp/dew point range that results in possible/probable carb icing conditions, carb heat can be fully applied at any power setting without harming the engine.

Personally I think the UK practice of selecting carb heat off at 200 feet on final is silly

[BackPacker]

I agree. And actually, every POH I've read so far only says "apply carb heat as and when required" or something along those lines. There's no POH that says "apply carb heat below 2000 rpm" or similar hard rules. (Have to admit I haven't been exposed to that many POHs though. And I've never flown an aircraft that had a carb temperature probe.)

I was flying aerobatics with a non-aerobatics instructor in the carbureted R2160. He got nervous when I closed the throttle fully for a spin, without applying carb heat. At 5000 feet or thereabouts. In CAVOK conditions. Come on, do you really think I'm going to let go of either the throttle or the stick during a spin, to select and deselect carb heat?

Yes, there is an ingrained fear somehow about carb ice during low power operations. It's even on the club-issued checklists: "Final approach / carb heat on". On the other hand, nobody has ever told me that you could get carb ice while taxiing. And I've had that happen twice now. Both times just before a rainshower would be passing over the field.

[stevef]

A company I used to work for abroad lost a large piston-engined aircraft and its two crew when it suffered an engine malfunction on take off and crashed into an airport building. The failure was attributed to detonation caused by carb heat selection (as well as poor checklist procedure).

[Big Pistons Forever]

A company I used to work for abroad lost a large piston-engined aircraft and its two crew when it suffered an engine malfunction on take off and crashed into an airport building. The failure was attributed to detonation caused by carb heat selection (as well as poor checklist procedure).

"two crew" and "piston engine" implies a large radial engined airliner. I think it is very unlikely the average reader will ever fly such an aircraft and have to worry about the special procedures they demand. I think one has to be careful when extrapolating large aircraft operating procedures across to simple part 23 certified airplanes.

In particular the fact that mishandled carb heat can detonate a large radial engine does not automatically mean that the same threat exists for the simple carburated 4/6 cylinder engine found in GA trainers/tourers. I would strongly argue that there is no threat of detonation at any power setting unless carb heat is used at OAT's far far above any that would indicate the possibility of carb ice forming.

As my handle suggests I have considerable experience operating big round engines. I am curious as to what accident you are talking about. My bet is the engine was operated with extreme carb air temps (ie over 40 deg C) for a considerable period ( ie not just a momentary carb temp spike) in order to cause severe enough detonation to fail both engines.

[Pilot DAR]

Yes, I agree. Of the three engine failures I have had which put me on the ground, my selection of carb heat after takeoff was the direct reason for one. I do have a carb air temperature indicator, and aside from a preflight function check, I never use the carb heat, unless conditions warrant it.

I have noticed that pilots tend to like to move controls, whether the controll really needs moving or not (increases the sense of importance of the pilot role). This can be aggravated by training which teaches to "do because you are told to, not because you necessarily understand why". So carb heat controls get moved a lot, and I bet that 95% of this use is not even effectively preventative, much less restorative.

Do pilots realize that if you really need the carb heat to be effective, it is likely necessary to turn off cabin heat, and to peak lean the engine after carb heat selection?

[stevef]

Big Pistons Forever:
It was a C47A, the crash happened in Palma de Mallorca in March 1992 and it's not particularly warm there at that time of year, especially early evening when the crash occurred. I'd left the company well before then but the information I received from someone closely involved in the DCA investigation was that as mentioned previously. It was only one engine that failed by the way.
There may have been more to it than that of course, but my source was very experienced in piston-engined operation.

[FlyingKiwi_73]

Thanks all, this helps. The question comes from rushing the downwind/final checks and forgeting Carb Heat OFF, did my touch and go with Carb Heat ON and had a bit more of a climb into the air ... figured out why pretty quick (flaps up,.. no thats not it...aha!). Chatting to instructor mates and they both waggled figures and warned of detonation, when i asked them to explain... they couldn't. I guess its one of those "we were told not to do and we'll tell you not to do it"

I don't as a rule run the Carb Heat close to the ground as where i fly can be dusty or grassy so unfiltered air can be full of nasties (i do apply Carb Heat on run up checks of course, and run it longer on cold mornings).

I try to gentle on the engines, no fast throttle movements and i hardly ever lean (don't get high enough for long enough) but i really only know as much about them as i learnt in the Air Tech exams so issues like this can confuse me :-)

Thanks All Again