Carburettor Icing

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JohnHubb
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Carburettor Icing

Post by JohnHubb »

I'd not flown the Cherokee 180 before - but I do know about engine icing. This time I was caught out and got very wet and cold....

Flight was daylight VFR from Belfast Aldergrove (EGAA) to Islay in the Inner Hebrides (EGPI), distance of 66nm. This involved a short hop (20 nm or so) over the sea. Weather was acceptable at Belfast, cloud base 3000 ft and the METAR at EGPI was CAVOK with a 6 knot runway crosswind. Ground temp 4C.

All was good until about 4 nm N of Rathlin Island when dense sea mist appeared out of nowhere. I mistook it for cloud and planned to maintain heading and altitude (2500 ft, evens +) on the basis of the METAR at Islay. I had been using carb heat every 10 min or so as a routine. Then I noticed that the RPM were dropping. Applying carb heat caused no further drop, but after 10 secs or so the RPM picked up ever so slightly, but not enough to maintain altitude. I switched tanks, fuel pump on, and left the carb heat ON. The engine did not respond and eventually I ditched.

I should have recognised sea mist as a hazard at low altitude in winter. At the first sign of it, I should have done a 180 and gone home. In the real world I would definitely have done so.....I hope!

Apart from that, having got myself into a very nasty situation, I wonder if I could have managed it better. Is the carb heat effect in the 180 weak, or maybe it is very strong? Should I have left it on permanently earlier? Or should I have turned it off when the engine temporarily picked up? I suspect if I'd done that, the carb would have frozen up again pretty quickly.

It was an interesting learning experience even for a lapsed PPL!
Last edited by JohnHubb on 05 Feb 2021, 06:54, edited 1 time in total.

flapman
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Re: Carburettor Icing

Post by flapman »

the standard temperature "lapse rate" is 2°C decrease for every 1,000ft increase in altitude. So if I take your altitude of 2.5 and double it, I get 5. I subtract 5 from the reported OAT on the ground of 4, and I get -1. You report you entered sea mist, which is a form of visible moisture. So you are flying through visible moisture, with an outside temperature likely at or just below freezing.

Thus, I estimate you were in icing conditions at that point. Carburetor heat probably didn't work because the icing your were experiencing was likely accumulating outside of the carburetor. I'm not exactly sure how structure icing would result in a steady decrease in RPM. My guesses would be a blockage of the air intake, but carburetor heat should bypass the cowling intake. My other guess is ice buildup on the propeller blades, increasing their weight which may reduce RPM slightly, but will result in a substantial loss of thrust. 4°C may not be hot enough to shed the ice quickly enough to avoid you ditching the aircraft.

Granted, I don't own the 180, and this judgement comes from other aircraft like the C-172 and the Piper Arrow, but the carb heat systems should be identical.

Jarek
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Re: Carburettor Icing

Post by Jarek »

It's bit tricky with this plane, as there are no instruments that would allow to measure this phenomenon and counteract. The first indication is a drop in manifold pressure. But there is no such gauge.
For best performance and to minimize risk, carb temp should be somewhere in range between 4 and 10 degrees C (if possible), but again - no carb temp gauge here and it's not recommended to operate this airplane with partial heat application.
So everything needs to be judged "by ear".

It is possible to experience icing even if it is warm outside - esp. when engine is cold and runs on idle speed and I would be careful on approach in instrument weather as there are two key contributing factors.

First - relative humidity (~ above 50 percent). Second one is part throttle operation - it's extremely conductive to the formation of ice.

So in case when engine roughness is experienced - Carb Heat needs to be set to Full On and throttle to Full Open (but with care as there is a increasing danger of detonations in this configuration)
Then if engine is still rough, adjust mixture for best smoothness. Usually something like 60 seconds is enough to clear the ice. Reaction needs to be quick, because if ice blocks the carb completely, there is nothing you can do.

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AKar
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Re: Carburettor Icing

Post by AKar »

I recall never having had a case of carburetor icing in A2A Pipers where the icing did not clear fairly soon (within 30 seconds or so) after application of carburetor heat.

-Esa

JohnHubb
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Re: Carburettor Icing

Post by JohnHubb »

Thanks guys! That's all very interesting.

There's no doubt that I got severe icing as evidenced by the dropping RPM and gradual loss oi engine power. I was too slow to deal with it. There is no doubt that I was flying in icing conditions, probably the OAP was -2C or less.

From what jarek wrote, I did exactly the wrong thing by reducing throttle - my idea was to get down to warmer air - but that encouraged even more icing. There was a small response to carb heat initially. I should have kept carb heat on continuously as soon as I saw the mist, and I might have been better flying a powered descent to 500 ft on the most recent QNH available.

The instruments in the 180 don't help much. The RPM is the only indication of power output. Prop icing may have been a factor too but there is no prop heat facility in this plane.

All in all, I was flying to the wrong destination in the wrong aircraft on the wrong day.

I might try and replicate the flight and see if I can do better next time!

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ClipperLuna
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Re: Carburettor Icing

Post by ClipperLuna »

Hey JohnHubb, was this real-world weather? What day/time? You've got me curious what this flight was like so I may give it a whirl myself (I've never flown around Northern Ireland or Scotland but I've always wanted to).

R

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Oracle427
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Re: Carburettor Icing

Post by Oracle427 »

The worst carb icing conditions tend to occur at around 10C. Freezing conditions are not as conducive to carb icing because the air doesn't have much moisture in it to begin with.

Carb icing will be worst when the air is saturated with moisture. When the temp/dewpoint spread is low watch out!
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N28021 1979 Super Viking 17-30A

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ratty
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Re: Carburettor Icing

Post by ratty »

The times I've experienced carb icing in the 180 it has appeared as a slow reduction in RPM and I've been quick to deal with it, but if you wait too long, in the real world the induction system can accumulate so much ice that there isn't enough heat to clear it, and I suspect that's the case with A2A aircraft. Also if, for example, you experience carb icing on approach, apply the heat and by all means increase throttle, but do not go to full throttle as that's where the risk of detonation occurs.
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JohnHubb
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Re: Carburettor Icing

Post by JohnHubb »

I tried to repeat my flight again using historic weather. I couldn’t do it. Why?

I was using ActiveSky for P3Dv4 but it is not clear what elements of historic weather are stored from the original. I suspect it might not be everything. In particular, the position of banks of sea mist may not be replicated exactly, or maybe not at all.
The second time, because I was flying VFR using dead reckoning/pilotage, I may have flown a slightly more westerly route, passing over Rathlin Island’s western coast, instead of the centre of the island.

VFR flight plan from Belfast Aldergove (EGAA) to Islay (say I-lah) (EGPI) was filed on 24th January 2021.
Departure, EGAA, 15:45 Z. Flight ended 16:25 Z. Duration 40 min. Flight plan distance, 66 nm.

If you wish to replicate my flight it might be better to use customised weather.

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Bixby
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Re: Carburettor Icing

Post by Bixby »

Thanks for responding to my PM John. I will check it out.

JohnHubb
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Re: Carburettor Icing

Post by JohnHubb »

I'd be interested to know how it goes. Check your survival kit. Best of luck!

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Oracle427
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Re: Carburettor Icing

Post by Oracle427 »

I wouldn't worry about giving full throttle under the circumstances of carb icing. The heat from the carb heat will reduce the density of the intake air further increasing the richness of the mixture reducing the risk of detonation.

The engine is also producing less power due to carb icing, so detonation should remain unlikely until nearly full power is restored and the mixture is left leaned.

This is not simulated, but when carb heat is initially applied it is common for the engine to cough and sputter quite a bit and lose more power as the ice starts to melt away and get ingested. One needs to be patient, wait for the engine to smooth out, add power as required while all the ice melts away. It can be a little scary the first time it happens!
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ClipperLuna
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Re: Carburettor Icing

Post by ClipperLuna »

Just finished flying the route. Didn't get carb icing, but the pitot iced up--pretty quickly too--as I descended through a cloud layer to into Islay.

Icing or no, it was a neat, neat flight! Perfect for the Cherokee.

-Ryan

JohnHubb
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Re: Carburettor Icing

Post by JohnHubb »

In the old days, that route could be used for 'night VFR' as part of a night rating. Pretty hairy stuff those 20 nm over sea at night for a 50 hour PPL. 8)

That leg was the first of my winter Hebridean tour. I guess I will have to fly it again. Practically every small island has an airfield and if there isn't one you can land on the beach.
Loganair runs the only scheduled air service in the world (AFAIK) which uses a beach runway - at low tide, naturally :roll: , at Barra, EGPR.

Keep warm!

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