To Readers' Queries for Fellow Pilots.

[Tomo]

I came across this in this months "Flight Safety" Mag ~

 

I have been flying Cessna 172 aircraft for over twenty years, When I was doing my training on the Central Coast of NSW it was drummed into me to always use carby heat when lowering revs prior to landing. I was instructed that it is good airmanship to use carby heat no matter where you fly. I now fly out of the Gold Coast, and have done so for twelve years. After moving here, and doing check flights - when it's time to apply carby heat, some instructors tell me not to. They say it is not necessary in warmer climates. One even pushed the carby heat control in, saying it takes away power. I would be interested to hear FSA and your readers' attitude on this.

 

 

[turboplanner]

If I was a tightrope walker I'd use the safety net.

 

I've read reports of carb ice in the most unlikely situations.

 

 

[Guest Decca]

This may have been discussed before, but it's good to review a pilot killer like this.

 

"not necessary" ain't necessarily so. Conditions in & around the carb venturi are very different to how the weather outside can make one feel safe.

 

Tomo there's a carb icing probability chart on p.103 of the vfr guide. Do a couple of exercises on that to find that humidity doesn't have to be 100%, and temp doesn't have to be freezing. You'll need a magnifying glass to read it, & there may be charts easier to read if you google them, but it will give you some idea that it's not worth the risk not to suffer a slight decrement in power by using carb heat.

 

Regards, Decca.

 

 

[facthunter]

Carb. heat in warm (& moist) climates.

 

Carburettor ice is formed when there is enough moisture in the atmosphere, and the temperature in THE CARB. is below freezing. The temp is LOWERED in the carb by pressure drop (due to venturi and throttling) and the evaporation of fuel. ie the carb acts like a refrigerator. (put some avgas on your hand and shake it in the breeze, and it will get quite cold).

 

Because of the variability of these factors, it is hard to put down ABSOLUTE temps where you will/won't get icing. for example in the polar regions where the air is the driest on the planet, you would not get icing, (unless there is moisture in the fuel). In tropical areas in the rainy season where the R.L. is high you might get it at 20 degres C. Nev..

 

 

[GraemeK]

The chart is here. I'm always extra careful when the air temp and dew point are close together (ie close to 100% RH) no matter what the temperature - you can get temp and DP from METARS for an airfield near you. And as the chart shows, it doesn't need to be cold - you can still be at risk of serious icing on descent power into the 30's.

 

And there's an ATSB article here.

 

 

[Yenn]

In central Qld coast in summer I used to fly a C150 and it would get carby ice if there was a hint of a cloud in the sky. I heard that later it was put down in the sea off Redcliffe, with ice being the probable cause. C172's never seemed to be so fussy but it was good advice to apply heat on descent. You are not looking for power then, but heat off immediately if there was any thought of doing a go around. With the Jab in my Corby I have never needed carby heat.

 

 

[Guest Qwerty]

One even pushed the carby heat control in, saying it takes away power.

If he did this to me, I would cross my arms, sit back and say "your aircraft". My next action would be to find an instructor who;

 

1. New what he was talking about, and

 

2. Didn't interfere with "my" aircraft.

 

It has been drilled into me that it is necessary to use carby heat when the throtle is closed (below cruise setting). The reasons for this are adequately canvassed above.

 

 

[Guest ozzie]

apply carby heat BEFORE reducing power.

 

remove carby heat AFTER adding power.

 

many a cessna jump pilot has been caught out from carb ice.

 

it is wise to apply power while you still have the height in the circuit just to check you still have it and can make the strip if you don't, rather than finding yourself a little short on final and adding power and finding that you don't have any. power or height that is.