Carb icing
Joined: Aug 2003
Posts: 1,579
Likes: 0
From: Not a million miles from EGTF
I concur with those who say don't use carb heat while taxying. Use it on the ground while stationary by all means.
As for the rest of it, don't follow what Bob in the cafe told you he does. His aircraft is probably different from yours. Follow what your instructor told you and (hopefully) the POH also says.
I've flown instructors in mine just to demonstrate things they may not have come across in PA28s or C172s. Even the handbook may not address some of these areas adequately.
Other owners of your type might be a better bet.
Joined: May 2002
Posts: 1,704
Likes: 1
From: Who can say?
One other thing.
If you apply carb heat and get all the symptoms of ice melting, such as an initial drop in RPM and then a gradual increase, leave the carb heat on, for at least 30 seconds, and possibly much more - a couple of minutes or so.
You've just melted a load of ice in your carb and turned it to water. You don't want it freezing again.
Mention has been made of the temperature range.
Adiabatic cooling of the air in the carb (the cooling effect of coming through the venturi) is generally about 2-3 °C., but can be up to 5 °C. Most of the cooling effect is from the latent heat of evaporation of the fuel, and this is about 15 °C. Therefore, the total temperature drop in the carb will be about 20 °C.
At (for the UK) medium-to-high OATs such as 20 °C, anything above low humidity will reach saturation at 0 °C. Air with a relative humidity of 25% at 20 °C, or 50% at 10 °C, will reach saturation at 0 °C.
Check the graph worrab posted on page 1 of this thread.
At this time of year, carb icing is much less likely to be encountered. However, it is a good habit to behave as if it is, and always check for carb icing in FREDA checks, always check the carb heat in your power checks and downwind/prelanding checks, and always apply carb heat on final approach, cancelling as you cross the fence committing to land.
If you apply carb heat and get all the symptoms of ice melting, such as an initial drop in RPM and then a gradual increase, leave the carb heat on, for at least 30 seconds, and possibly much more - a couple of minutes or so.
You've just melted a load of ice in your carb and turned it to water. You don't want it freezing again.
Mention has been made of the temperature range.
Adiabatic cooling of the air in the carb (the cooling effect of coming through the venturi) is generally about 2-3 °C., but can be up to 5 °C. Most of the cooling effect is from the latent heat of evaporation of the fuel, and this is about 15 °C. Therefore, the total temperature drop in the carb will be about 20 °C.
At (for the UK) medium-to-high OATs such as 20 °C, anything above low humidity will reach saturation at 0 °C. Air with a relative humidity of 25% at 20 °C, or 50% at 10 °C, will reach saturation at 0 °C.
Check the graph worrab posted on page 1 of this thread.
At this time of year, carb icing is much less likely to be encountered. However, it is a good habit to behave as if it is, and always check for carb icing in FREDA checks, always check the carb heat in your power checks and downwind/prelanding checks, and always apply carb heat on final approach, cancelling as you cross the fence committing to land.
Joined: Jun 2002
Posts: 258
Likes: 0
From: Bradford
Icing . . . .
Re the taxiing in hot air argument.
Having regularly flown numerous VW powered aircraft I can't think of one which had an air filter on the cold air supply. Most took the cold air from inside the cowlings, so it wasn't true cold air in every case. The hot air comes from a muff on the exhaust(s) which are generally outside the cowlings. So taxiing in hot air means you may pick up slightly more debris than by using cold air ! Applying hot air well before closing the throttle is good advice. In humid conditions it's a good idea to use slightly higher power settings to generate more heat in the exhaust system, also, icing is more likely to occur at small throttle openings.
I've never had a VW carb-ice problem, but other bits of "excitement", yes !
Having regularly flown numerous VW powered aircraft I can't think of one which had an air filter on the cold air supply. Most took the cold air from inside the cowlings, so it wasn't true cold air in every case. The hot air comes from a muff on the exhaust(s) which are generally outside the cowlings. So taxiing in hot air means you may pick up slightly more debris than by using cold air ! Applying hot air well before closing the throttle is good advice. In humid conditions it's a good idea to use slightly higher power settings to generate more heat in the exhaust system, also, icing is more likely to occur at small throttle openings.
I've never had a VW carb-ice problem, but other bits of "excitement", yes !
