Carb heat

[bytheseatofmy]

A couple of days ago I was flying with my student in her 1960 C172.
We were doing some pattern work for 1.5 hours.
On each downwind leg preparing to decend I noticed when carb heat was applied we ran rough for a few seconds and I told the student we need to be aware of carb icing. It was a clear day,no visible moisture and temp of-3C with a dew point of -7C.
On the 7th take off (all landings to a stop) we got to 400' AGL and the engine started to splutter and lose power,instantly I applied carb heat and after 10 anxious seconds we had climb power back.
I am a little puzzled as to why this happened on the 7th take off and not before if temperture hadn't changed.
I might add that the carburetter was taken off the day before this happened by an A & P to install an oil warming device.
Any comments on this would be appreciated.

 

[Groucho]

Re: Carb Heat

Two things I note in this post, 1. A 1960 C-172. Is that a continental powered aircraft. I seem to recall that the older skyhawks had continental motors. They are more suspetable to carb icing than the Lycomings in later models.

2. The Carb was removed and the engine runs rough with the application of heat. Well this one needs to be looked at. If the engine is running rough with the application of carb heat at all times it would appear the mixture is getting a little rich or something. I would try, at altitude, to apply carb heat and lean the mixture and see if the roughness goes away. If it does the Mech. may have something more to work with.

I don't think that this is a nomal condition.

 

[bytheseatofmy]

Yes it is a continental engine......Thanks for your comment

 

[avbug]

There's a number of old wives tales about lycomings vs. continentals being more subject to icing...but it's not the engine that's icing, it's the carburetor...those myths hold no credence.

Your temperature and dew point spread were close. You were already getting carburetor ice, and experienced it every time you went around the pattern...you remember the roughness you got when you applied carb heat on the downwind?

What causes carb ice? A drop in pressure and temperature as air increases in velocity through the carb venturi, right? (or at idle between the carb barrel and throttle plate). Accordingly, when do you suppose the greatest temperature drop and greatest velocity (and pressure drop) occurs in that carburetor? When the throttle is wide open, right?

You experienced carburetor ice at it's greatest potential, at a time when carburetor ice was known to be forming. Don't let that surprise you, especially as you'd been experiencing it the entire time.

Are you making your approaches with the carb heat on, and then pushing it in (shutting it off) as you perform your touch and go? If so, and you have been preventing carburetor heat on the approach to landing, don't be surprised when a sudden build up occurs as you increase carb air velocity, decrease carb air temperature and pressure, and apply full power to go around or touch and go.

Are you adjusting your mixture when you apply carburetor heat, or are you accepting prolonged operation with a rich mixture? If you're doing idle descents and have an already rich idle mixture (something you can't control with your cockpit mixture control...but your mechanic can), you may be looking at fouling issues. Why do they occur on one trip and not the other? The answer may lie in several places, from power useage to the duration you were at idle, but never the less, it remains a possibility.

As you described a few moments of roughness as you applied carburetor heat on the downwind, we may determine that you had carburetor ice. You didn't indicate the roughness continued, but only that you had roughness "for a few seconds." If indeed that's the case, and the roughness went away, you were developing carburetor ice and were removing it. The roughness occured as the ice was being removed. You indicated this happened each you entered the downwind, for the duration of an hour and a half, including seven trips around the pattern.

You were developing ice the entire time. It finally just caught up with you.

One option is to apply power and then remove the carburetor heat after you are developing power. Carburetor heat may be required a little of the time, more of the time, or all of the time when conditions are conducive to the formation of carburetor ice. You needn't be in visibile moisture for induction icing, and it can happen at low temperatures through very, very warm temperatures.

In the radial engines we gained an intimate appreciation for carb ice and carb air temperture. Each carburetor was fitted with a carb air temp gauge, and we spent a lot of time ensuring that the carb air temperatures stayed in the appropriate range (not too hot, not too cold). Your aircraft doesn't have the luxury of that feature, but you still need to be aware that it can form any time, any place. Don't assume that because it's arid or cool that it won't form, nor that if it's hot out, it won't form. Such an assumption can be fatal.