Originally Posted by
A and C
After this incident the carb heat system was checked and a very well hidden crack found in the carb heat valve, the result of this crack was that the engine was only getting partial heat and so the systems ability to clear ice much impaired.
The efficiency of the carb heat system will be indicated by the magnitude of the RPM drop during the runup. I have grounded several airplanes when they did not IMO appear to be getting sufficient heated air during the runup carb heat check as indicated by a non existent or very small drop in RPM when the carb heat was selected full on. In all cases various problems were found with cracked carb air boxes, deteriorated scat hoses or improperly adjusted carb heat controls.
In my opinion the "carb heat check" is one of the most poorly understood and performed of the runup checks.
To do a proper check I recommend the following. (for your typical C 172 Pa 28 aircraft)
Have runup RPM RPM set for at least 30 seconds to allow time for the exhaust to heat up
1) Apply full carb heat. You should observe a reduction of RPM of at least 100 RPM. This indicates that the carb is getting hot air
2) Wait at least 15 seconds with carb heat full on. If there is no rise in the RPM the engine has no carb ice. If a rise in RPM is noted than take the appropriate cautions.
Originally Posted by
Maoraigh1
I often lean if a carb heat check indicates ice. I was taught to leave carb heat on until touchdown, but later was told to go to cold air on very short final. I went back to hot air until on the ground after the engine almost stopped when I opened the throttle to turn 180 and backtrack. O200 in Jodel DR1050 in Scotland is an excellent ice-making combination.
This switching off of Carb heat on short final seems to be a unique to British flight training practice that seems to have been mindlessly passed down through the generations.
It is IMO a stupid practice and directly contrary to the direction on what every carburated Cessna SEP POH provides.
Specifically in the POH Before landing Checklist the following direction is given
Quote
Carburetor Heat: ---ON (apply full heat before closing throttle)
Unquote
In the event of a Go Around apply full power and select carb heat cold, simples.