carby heat

[sako]

have always been told carby heat either full on or full off , you never set it at an intermediate setting, WHY ?

[scrambler]

The purpose of the carby heat is to clear ice using hot air. If the heat is only partialy on it may melt only some of the ice and cause icing problems further down the carby (Cold water re-icing)

With only partial carby heat you cannot tell the temperature of inlet air you are supplying.

Some aircraft have a carb air guage (forget the correct name for it, but i do recall it being fitted to a later model C182 I flew). Only in this case partial carby heat may be used as you can ensure inlet air is above the required temperature to melt all the ice.

[Monkey Magic]

Does anyone remember a pilot flying from Camden a few years back who had an icing problem- applied full carby heat and had an engine failure? The ice broke of and was ingested into the engine.

My understanding was that this was investigated (and featured in one of the CASA flight safety mags) and since then the rules have been changed.

[cjam]

How about, the mixture is already pretty rich because of the ice restricting the air's passage past the butterfly, you introduce hot air, does the ice vanish immediately? nup, the hot air is less dense, the restriction remains( for a short while), therefore the mixture is enrichened further, cough cough sputter, The hot air melts the restrictive ice, air/ fuel ratio gets reasonable and you are laughing. Not laughing as much as I am when people tell me the coughing is chunks of ice going through the carby though.

[farqueue]

Several possible reasons, some, all or none may apply to a specific AC. The hot air is sourced from a baffle around part of the exhaust. This MAY be vented to free air with heat off so as to cool the manifold. Selecting part heat could block the venting, but not feed the hot air to the carby resulting in over temped ducting and hoses.

Also it could restrict the flow into the carb, thus cutting max power, or the swirl from the flap can result in very strange performance and mixture problems.

This is what test pilots are payed for!

[dogcharlietree]

Please read the airframe and engine manufactures manual for the aircraft YOU operate. Do not listen to crew room myths.

"It requires far less heat to prevent the formation of ice than it does to remove ice once it has formed."

De-Icing Procedures.
In the event of ice actually forming in the carburettor, use the following de-icing procedures, in sequence.
1. Place mixture control in Auto-Rich.
2. Apply full caburettor heat. Hold fully on for 30 seconds or until carburettor air temperature rises to 38 deg C.
3. Adjust heat controls to maintain 32 deg C carb air temperature.

In the event of ice taking such a hold that 38 deg C air temp cannot be maintained, then -
4. Carburettor heat full cold.
5. Alcohol ON for one minute.
6 Alcohol Off.
7. Carburettor heat ON to maintain 32 deg C.

If the alcohol has failed to remove the ice, it can be cleared, as a last resort, by causing the engine to backfire, as follows -
8. Place carburettor heat control full cold.
9. Throttle to fully open.
10. Move the mixture control slowly towards Idle Cut-Off until engine backfires.
11. When ice has cleared return engine controls to normal settings and maintain 32 deg C air temperature.

The drastic procedure of leaning the mixture to the point of engine back firing, in an effort to dislodge throttle ice already formed, is only employed in extreme emergency.
N.B. When using this procedure carburettor heat controls must be in full cold position, otherwise damage to the control shutters will result.

The old girl would actually backfire itself if the carby got too choked with ice. That, combined with the ice flinging off the props and hitting the side of the fuselage was enough to sometimes arouse the F/O
These are laid down procedures for (some) radial engines. DO NOT try them if they are not authorised for your aircraft.