Warrior suspected Carb ice incident at Mangalore

[Centaurus]

Recent ATSB short investigation report involving suspected carb icing in a Warrior at Mangalore. http://www.atsb.gov.au/media/5771543/ao-2016-059-final.pdf

The aircraft was involved in circuits and landings at Mangalore where the carb heat control was used at various parts of the circuit. The aircraft subsequently lost power and the instructor managed to successfully force land into a field adjacent to the airport.

It is common flying school teaching procedure for pilots to select carb hot air on before reducing power for the approach to land (or at anytime low power is used). Usually pilots then select carb hot air (cold) off on short final. Ostensibly in case a late go-around is needed

I don't know about the procedure in the Warrior but in the Cessna 172 and 152 manufacturer's POH, the hot air is kept on during final approach and landing and only selected back to off (cold) as part of the after landing drills. If on final a go-around is performed, the POH requires the carb hot air control is left on until after full throttle is attained. Only then is carb air off selected. It makes sense since carb ice can form surprisingly quickly at low power settings.

I suppose it may depend on engine design, but would the same principle apply to Warrior operations? i.e. Hot air on until the after landing checks and on a go-around full throttle first followed by hot air off?

[Walter Atkinson]

This is a copy of an article I wrote for an aviation magazine a while back:

Carburetor icing is one of those pesky problems that can ruin your day and remains a serious problem for light aircraft as evidenced by the continuing number of incidences reported. Charles Lindbergh experienced carb icing while crossing the continental divide in The Spirit of St. Louis so his mechanics equipped the Ryan with a carburetor air heater before he crossed the wet, cold North Atlantic.

A 1971 report by the National Research Council (NRC) of Canada revealed that carburetor ice could be reduced by coating the throttle plate and shaft with 0.00125 layer of Teflon that “produced a marked reduction in ice formation.” What is not clear is why in the ensuing years the manufacturers never utilized this data. One might guess that the regulatory Certification process for aircraft parts and accessories might be involved.

There are other types of induction icing besides carburetor ice that can reduce airflow. Throttle plate icing and induction filter icing are two common ones, but we’ll leave those for another discussion. Let’s concentrate on what happens inside the carburetor.

A carburetor basically consists of a “throat” or “barrel” through which the air passes into the induction manifold of the engine. The pipe is in the form of a venturi that narrows in cross-section and then widens again, causing the airflow to increase in speed in the narrowest part. This results in a drop of the air temperature across the venturi. Below the venturi is a butterfly valve. This valve controls the flow of air through the carburetor throat and thus the quantity of air/fuel mixture the system will deliver.

There are four operational regimes in which carburetor ice can be a problem—in order of frequency of occurrence, they are:

A) Startup
B) Take-off and climb
C) Cruise
D) Power reduction for landing

Yes, the least likely time to encounter carburetor ice is on the power reduction to land! Let's examine each of these scenarios and discuss why.

A) On the initial startup of the day you are dealing with a cold engine and cold carburetor body and all of a sudden it's sucking air through the venturi. When the OAT and humidity are right--BINGO, carburetor ice is formed. As a result, it is important to check that the carb heat is working BEFORE take-off (during the run up). The purpose of this carb heat application is two-fold... First to be sure the mechanism is working and second, to be certain there is no ice present in the carburetor. If there is no ice present, there will be a drop in power as noted in rpm drop as the heated air enters the carburetor, thereby reducing the number of air molecules available for combustion. If there is ice present from the startup, there will be an increase in rpm as the ice is cleared from the carburetor and proper power is restored.

B) During high power application during take-off, there is a maximum flow of air through the carburetor which creates the greatest temperature drop across the venturi. Carb Ice can develop as a result.

C) During cruise, as the aircraft changes climates, OATs change, humidity changes, etc. and the engine is often operating at a wide open throttle, the venturi again is having it's maximum temperature drop for the given situation.

D) During the power reduction to land, there is the least chance of developing carb ice! There is the lowest power being applied and, therefore, the lowest temperature drop across the venturi!

Now, that understood, why do we apply Carb heat as a matter of routine during the least-likely phase of flight?

The answer is simple. Before take-off we have checked the carb heat and know the condition of the carburetor before applying power. During take-off and climb we will know if ice begins to develop as we will see a power loss as the ice builds up. In cruise, we will also see a power loss as ice forms in the carburetor.
It is during the low power condition of preparing to land, that if ice builds up, we will have no way of knowing it. The power is already low and in some cases the engine is not producing any thrust at all. If the carburetor ices up and we need to go around, we will be unpleasantly surprised as we push the throttle forward and we continue to descend!

So, in the event of an urgent need for power, add throttle, the engine will come to your aid since there is Carb Heat ON and no ice present, THEN push the carb heat in (off) to let the engine attain maximum power. It is common to see pilots push the carb heat in, then add power. This may not be optimal. If the atmospheric condition is conducive to the formation of carb ice, turning the carb heat off, then adding power is a suboptimal idea... remember, the second most frequent time to encounter carb ice is during a high power application!

Add power first; then carb heat OFF. That way, if you encounter carb ice as you go around, at least you'll already be developing power and know it. That likely scenario is power IN, carb heat OFF, power loss as ice develops, then carb heat back ON--you'll make the go-around. If you push the carb heat in first and the conditions are right for ice formation, the likely scenario is Carb heat OFF, power in, CARB ICE, power loss before you even get any... and just when you don't need that headache! As in most aviation situations, there is a reasonable argument for doing it either way.

Be careful up there.

[olm8tyrone]

Thanks for that Walter. It's definitely changed my position on carb heat!

[DeRated]

BOLLOCKS

It's not the venturi which is the issue but the throttle butterfly (or plate).

At low power settings (as when on approach) the throttle plate is closed or nearly closed, restricting the air flow past the throttle body - which is when the maximum vacuum is being produced [or least indicated Manifold Pressure].

[30" Manifold Pressure = 0" Vacuum or 25" Vacuum = 5" MP]



A reduction of air pressure causes an adiabatic loss of temperature - remember your Meteorology lapse rate of 2 degrees Celsius per 1,000 feet of altitude for moist air, ie, at 10,000 feet you have lost 20 degrees of temperature.

Further to that, the carburettor design places the fuel idle jet and transition jet very near the closed throttle plate. To flow the fuel at these low throttle settings requires a reduction of pressure [increased vacuum] relative to the atmospheric pressure in the fuel bowl.

Fuel and/or water (moist air) being atomized has a refrigeration effect, further reducing the air temperature.

Dew Point - the temperature at which moist air become visible (Fog).

If you have a 20 degree day with a 10 degree Dew Point then you have visible moisture with a 5,000 feet reduction in pressure. A further 5,000 feet reduction in pressure equals freezing point.

At 10,000 feet, the air density is about half the sea level pressure - with a 20 degree loss of temperature.

Simple arithmetic will suggest that a increase of 15 inches of vacuum has reduced the air temperature by 20 degrees so you now have possible induction system icing. The throttle plate area will be even more subject to icing because the venturi effect is most marked in this area.

The modern pilot drives a fuel injected car - knows bugger all about carburettors - and when he/she climbs into the Warrior or 172 - has little knowledge about what's under the cowl. The younger instructor who teaches you everything you know - knows what!


Carburettor Icing can form at any OAT or any power setting - depending upon the humidity and temperature and throttle opening.

So the comment [I]D) During the power reduction to land, there is the least chance of developing carb ice! There is the lowest power being applied and, therefore, the lowest temperature drop across the venturi! is the most misleading statement. Historical data shows it is where most carburretor icing is observed.

Follow the POH - and understand why!

Enjoy your tit-bits, Centaurus. Always thought provoking if you have sufficient brains to be prodded.

[Lead Balloon]

Carburettor Icing can form at any OAT or any power setting - depending upon the humidity and temperature and throttle opening. Indeed.

I think you'll find that Mr Atkinson comes from a country in which the spectrum of weather conditions is a little wider than in Australia, resulting in his list of comparable probabilities being accurate. The relatively benign weather conditions in the 'lucky country' just mean the conditions for carby icing usually (and luckily) arise more probably at reduced power settings for landing, compared with other phases.

In any event, what are the risks of applying full power for a go around/touch and go first, before setting the carby heat back to cold?

[Lead Balloon]

Just means the average PPL has been very poorly trained and tested.

[Lead Balloon]

Then why did you clear them to hire and fly the club's aircraft?

[Lead Balloon]

You didn't say you didn't. ;)

Presumably, then, the problem is all the other PPLs who didn't have the benefit of your scrupulous standards?

[Lead Balloon]

I reckon the average PPL can be taught to leave the carby heat on during reduced power settings, and return it to cold after application of full power for a touch and go/go around (or after a normal landing). Understanding why that's a good idea is an important first step.

[triton140]

I don't know about the procedure in the Warrior

Unfortunately for the discussion, the Warrior POH specifically recommends against using carb heat during landing.

Under section 4.31 Approach and Landing it states:

Carburetor heat should not be applied unless there is an indication of carburetor icing, since the use of carburetor heat causes a reduction in power which may be critical in case of a go-around. Full throttle operation with carburetor heat on can cause detonation.

Another OWT perpetuated in the POH?

[Ex FSO GRIFFO]

And for those people 'fond' of using the carby heat at 'low level' because they may have been 'taught to'....

'Tis worth remembering that the use of carby heat delivers hot UNFILTERED air into the carby from the shroud around the engine exhaust.

Beware of 'overuse' in dusty conditions, as you may very well wind up with scored cylinders....I have seen a C150 used for mustering up near Carnarvon in WA where the pilot employed used the carby heat 'on' all day at low level, because that's what he was taught.....
This particular engine was replaced on the next 100 hourly because of.

No cheers there.....

[Squawk7700]

'Tis worth remembering that the use of carby heat delivers hot UNFILTERED air into the carby from the shroud around the engine exhaust.


Only on specific aircraft types. This is not the norm by any stretch.


Most concerning is that everyone is harping on about how GA in Australia is dead and yet at Mangalore two go-arounds is acceptable to be able to land! I've been there recently and have seen it myself! I waited 15 mins to get out on the runway and I'm usually happy to make a very quick departure, however I simply couldn't get on the runway safely to do so AND that was on a Sunday.

[Lead Balloon]

Carburetor heat should not be applied unless there is an indication of carburetor icing, since the use of carburetor heat causes a reduction in power which may be critical in case of a go-around. Full throttle operation with carburetor heat on can cause detonation.What dangerously silly stuff.

An "indication of carburetor icing" is that the engine stops on approach because the carby's iced up. You use carby heat to avoid that.

And no one's advocating leaving the carby heat on continuously during full power operation. It's just about the order in which the throttle is pushed to full and the carby heat is set back to cold.

[Captain Nomad]

It was a long time ago, but from memory, wasn't part of Piper's reasoning to do with the position of the carburettor on the engine? It is a warmer ambient location and less prone to icing than with the Cessna 152 engine.

[Lead Balloon]

Even if that's true, waiting for "an indication of carburetor icing" seems a riskier approach to take than applying carby heat when the propabilities of icing are higher and the application of carby heat can do no harm.

[ChrisJ800]

I did PPL training in both the UK and Oz in the 80's and carbie heat was rigorously trained in the UK as select briefly every 15min and also select briefly on downwind leg as part of checks. In Oz it seemed to be left alone unless an engine stutter so seemed to be reactive rather than proactive? I had 2 engine stutters I can recall, one when scud running through the blue mts in ordinary weather where carby heat selection seemed to work and one where it didnt help and turned out to be a fuel additive problem.

[oggers]

Lead Balloon

I think you'll find that Mr Atkinson comes from a country in which the spectrum of weather conditions is a little wider than in Australia, resulting in his list of comparable probabilities being accurate

Easily dealt with. Let's ask him: Walter, does your advice pertain only to the USA and did you post it on the oz forum for a laugh?

:rolleyes:

[27/09]

It was a long time ago, but from memory, wasn't part of Piper's reasoning to do with the position of the carburettor on the engine? It is a warmer ambient location and less prone to icing than with the Cessna 152 engine.

Both the Piper Warrior and the C152 have Lycomings fitted. Though different model engines I thought they were both configured the same so far as the location of the carburettor goes.

The difference may to more to do with differing operating philosophies and/or the climatic conditions at the location of either manufacturer influencing what procedures the respective Flight Manual writers thought important.

[Lead Balloon]

Your question is based on a false dichotomy, oggers. Please troll somewhere else. :ok:

[Squawk7700]

I'm a little skeptical on the diagnosis of carby ice for this flight. So they had ice, cleared it, applied power, headed home, did a couple of go-arounds and then ice builds up again on downwind at cruise power?

Sounds a bit strange, although I note that it says that no other fault was found.

Surely this must be a big issue up there in similar weather?

If icing was so likely that day, I'd have thought other aircraft in the circuit may have experienced the same issues.

[oggers]

Lead Balloon, seriously? Here is Walter Atkinson's list of flight regimes conducive to icing, "in order of frequency of occurence"...

A) Startup
B) Take-off and climb
C) Cruise
D) Power reduction for landing

...and you say it is accurate for the USA...

I think you'll find that Mr Atkinson comes from a country in which the spectrum of weather conditions is a little wider than in Australia, resulting in his list of comparable probabilities being accurate. The relatively benign weather conditions in the 'lucky country' just mean the conditions for carby icing usually (and luckily) arise more probably at reduced power settings for landing, compared with other phases.

...so please feel free to clarify whether you really think that list he pasted here in the Oz forum does or does not hold good for Oz? Because right there you said it didn't.

[Sunfish]

the warrior has the inlet manifold and carb built into the sump which perhaps provides some warmth, in a C172 the carb hangs out. I have only had carb ice once - when given clear for takeoff, I pushed the throttle and the c152 stalled, most embarrassing. I generally remember to apply carb heat, removing it not so much.

[Lead Balloon]

If you had any interest in aviation safety, oggers, you'd understand why it's important for pilots everywhere on the planet to understand the underlying reasons for the list of probabilities set out by Mr Atkinson (and the operational practice in the UK to which ChrisJ8900 referred). Then pilots would be more likely to use carby heat on the basis of the risks in the specific operational circumstances in which they find themselves, rather than on the basis of rote-learned, one-size-fits-all procedures.

But as you seem to be obsessed on other things, I ask again: Please troll elsewhere. :ok:

[27/09]

the warrior has the inlet manifold and carb built into the sump which perhaps provides some warmth, in a C172 the carb hangs out. I have only had carb ice once - when given clear for takeoff, I pushed the throttle and the c152 stalled, most embarrassing. I generally remember to apply carb heat, removing it not so much.

I don't believe there is any difference between either installation in the manner you suggest.

The C152 is very susceptible to carb icing while taxying when compared to other aircraft.

[27/09]

I'm a little skeptical on the diagnosis of carby ice for this flight. So they had ice, cleared it, applied power, headed home, did a couple of go-arounds and then ice builds up again on downwind at cruise power?

Sounds a bit strange, although I note that it says that no other fault was found.

Surely this must be a big issue up there in similar weather?

If icing was so likely that day, I'd have thought other aircraft in the circuit may have experienced the same issues.

Various factors could influence the development of carb icing. For example, what power setting was being used downwind, were they in fact removing all ice with the application of carb heat during the approach, in other words was the level of ice slowly building up with each circuit, how effective was the carb heat system, when was carb heat applied - before or after power reduction. The amount of heat in the exhaust reduces significantly when power is reduced.

[IFEZ]

I've experienced plenty of instances of carb icing in C150/C152's. Particularly prone to it on start-up on a cold morning, esp inland from the coast. Also had it whilst idling in the run-up bay doing checks etc. Also as 27/09 says, can also happen during taxying and be careful if you're stuck at the holding point too long on a cold morning or you may have a repeat of Sunfish's experience!


It's a good learning experience if a student is on board under the above circumstances, as it allows them to see 1st hand the symptoms and what happens when corrective measures are taken, engine instrument indications etc, rather than just reading about it.

[UnderneathTheRadar]

Investigation: AO-2015-079 - Collision with terrain involving a Glasair SH-2FT, VH-HRG near Wedderburn Airport, NSW on 19 July 2015 (http://www.atsb.gov.au/publications/investigation_reports/2015/aair/ao-2015-079/)

A timely report on the subject......

[oggers]

Lead Balloon

If you had any interest in aviation safety, oggers, you'd understand why it's important for pilots everywhere on the planet to understand the underlying reasons for the list of probabilities set out by Mr Atkinson

Well, if you could just explain why you believe his 'most' and 'least' likely phases of flight for carb icing would swap places in Oz, perhaps between you you would have contributed something new. But you are just evading with accusations of trolling (again), so I will not divert this thread further.

[OZBUSDRIVER]

. On the reality of which model is susceptible to icing. The most savage installation for carbie ice at any temp if the humidity is up is the 0-200 installation in a Cassutt III. The induction of this system just produces icing that would result in full loss of power on takeoff..and just as quickly restoring power the instant heat is applied. The exact same engine install in a C150 never gets icing at any power setting at the exact same time and aerodrome as the Cassutt.

.....basically, agree with everything IFEZ says(Edit to add) The instructor is best to impart some wise gen to anyone they train or check as a matter of course if a particular model they fly exhibits icing in specific conditions. By rote or by experience...which has the best educational outcome?