Carb heat/icing

[N1277D]

The carb heat cable on my 170A also would tend to slide out during cruise. The cause was a worn ident on the side of the carb airbox, after some careful rerouting of the cable it would hold for a while and then start slipping again.

An A&P fixed the problem by using a flexible piece of steel wire wrapped around the outer cable shealth to tighten up the inside cable. He made a loop with the steel wire, wrapped it around the cable and then bent the steel wire to add some friction to the carb heat cable. Since then it has worked great with no need to hold the carb heat in on takeoff and during cruise. It was a simple fix with no need to replace the carb air box.

[GAHorn]

From the Operators Handbook, C-Series, 6-cylinder, Continental Red Seal Aircraft Engines (This includes the C125/145/O-300 series engines)
"Item 8. Carburetor Heat Control
The engine should be operated on COLD AIR at all times, except when operating under conditions where icing is likely, in which case the carburetor air control should be placed in the FULL HOT position. " (My Suggestion: That instruction is in error. Disregard it, as it will unnecessarily cost you performance and penalize you. Besides, how do you determine that "conditions" are "likely"? I've had carb ice in desert, hot conditions, and I've never had ice along the humid, temperate Gulf Coast except in O-200 engines. Leave the carb heat COLD during takeoff/climb/cruise unless a perceptible rpm loss or manifold pressure loss occurs, or unless you have a carb temp gauge by which to judge potential icing conditions.)
cont'd: "During the warm-up period, landing approach and during long glides, carburetor heat control should be in the FULL HOT position. To obtain the maximum rpm for take-off and climbe, and atmospheric conditions permitting, the carburetor heat control should be in the FULL OFF position. There is a drop from 100 to 20 engine rpm when the carburetor heat is full on."

It's important to take note and monitor the power setting/rpm you've last selected. The classic carb ice surprise occurs to the pilot who absently adds throttle to an engine that isn't maintaining the desired power setting. A loss of 25 rpm leads him to add throttle to recover the loss. A few minutes later, it happens again and he adds throttle again.
He may not realize that the power loss is due to carb ice until he either cannot add more power because the throttle is already wide-open, or the engine coughs and sputters due to the accumulated ice. Now it may be too late because if the engine dies there may be insufficient residual muffler heat to melt the ice out of the carb throat.
Therefore, monitor your selected power setting/rpm and if you detect a loss , FIRST check for carb ice by fully applying carb heat. A loss of rpm due to the heat application is normal. After a few seconds, if an rpm recovery occurs,...that is a sign that carb ice was present, and is now melted/melting out. Returning the carb heat to COLD/OFF should return the rpm to that originally selected. Keep a close eye on it for repeated ice build up.
If however, the application of carb heat causes a normal drop in rpm that does NOT increase again on it's own....you have no indication of carb ice, and you can remove the carb heat by pushing it back off. IF your rpm remains at the lost (25 rpm low, etc) position, then you may assume some other cause and correct it. (Mixture, pitch attitude, throttle position drift, magneto, etc.)

[GAHorn]

An excellent "predictability" chart for carb ice can be found at:
http://www.aopa.org/images/asf/ePilot/nyc02fa025_1.jpg

I don't know how to use this intelligently in-flight any more effectively than has already been discussed. While I found it interesting to look at, I expect to continue to operate as I've already been doing for years and so mentioned. It's pretty difficult to expect anyone to carry this chart around, constantly consulting it while flying along. The real indicator is that RPM gauge. Just, FYI.

[cessna170bdriver]

Neat chart, George, and I'm going to print in out and stick it in the book in my airplane with the Density Altitude and Crosswind Component charts. I agree that it would be difficult to use in flight, but interesting for reference nonetheless. For example, how would you determine the dewpoint at cruise altitude, other than the mental gymnastics of estimating lapse rates and cloud base heights?

I have had carb ice in cruise when operating relatively close to cloud bases (more than 500 ft below in controlled airspace, of course ). I installed a 2-1/4 inch manifold pressure gauge to go with my stand-by manifold vacuum system years ago, and found that it is much more sensitive to carb ice onset than the tach, at least in level flight.

I normally pay little enough attention to engine gauges in cruise that my first indication of carb ice is usually an audible drop in RPM. After clearing the first load of ice, I keep a closer watch on the gauges and can see the reappearance of ice on the MP gauge well before I notice a drop in RPM. When the MP drops by a quarter to half inch, I give it another shot of carb heat.

As far as carb ice at glide power, I learned to fly behind an O-200 in a Cessna 150 in Florida, and have never gotten out of the habit of full carb heat on all approaches. (I did have a brain fa-- one day in '98C in the spring in the Southeast and forgot to apply carb heat on the approach glide. Good thing I stayed within gliding distance of the airport, as on roll-out the prop got slow enough that I could almost track the tip as it went by. Believe it or not, I yanked the heat knob and there was just enough inertia in the prop and heat in the muffler that it caught and restarted. That experience stuck with me enough that I don't take chances, even on the single-digit humidity days we have here in the desert.

Miles