Learning Experiences

The Heat Is On

It was an early January morning in Teterboro, New Jersey. As usual, I arrived at the airport at 6:30 a.m. to give instruction before going to my full-time, pay-the-bills job. My student had already completed a preflight inspection and preheated the engine of the Cessna 172. ATIS gave a temperature of 32 degrees Fahrenheit with only a 1-degree dewpoint spread. Scattered ground fog was visible, though the sun peeking over the horizon promised another wonderful winter morning. I pointed out the possibility that the air could become saturated with only a small change in temperature and the dangers of deciding to make a VFR flight under those conditions.

As we taxied out, I reminded my student of the potential for carburetor ice during taxi because of the low engine temperature and high moisture content of the air. During the runup, I explained what to look for, and we actually did detect carburetor ice. During the carb heat check, we heard the engine make some unhappy noises as it digested a bit of water, followed by smooth running a few hundred rpm above the original power setting. After verifying that we were clear of all carb ice and all systems were go, we were cleared for takeoff.

During our departure, I instructed the student to climb to 2,500 feet once we were clear of the Class B airspace. After about 10 to 12 minutes of instruction, I took the controls to demonstrate a transition to cruise flight. As I increased the throttle and retracted the flaps, I noticed no significant increase in rpm. I verified that the carb heat was on full, the mixture was rich, and the gauges were in the green. My student asked why the engine didn't sound right for cruise. I moved the throttle from low to high settings, and it felt as if the cable were slipping after the first inch of travel. In a concerned voice, my student asked for the plan. I confidently told him that we should be directly over Ramapo Airport, a recently closed hard-surfaced strip. We spotted the strip, but obstacles used to keep cars off of the runway were blocking our path. Now in a slow descent, I was sure that we wouldn't make Westchester Airport, so the highway below started to look pretty good. I looked everything over and determined that the problem was some sort of throttle restriction. I still felt that it might be ice. I tried turning off the carburetor heat, which, as normal, increased the rpm by about 100. I turned the carb heat back on and saw a slight decrease in rpm followed by an increase. Power came back to normal, and I concluded that we had experienced a severe carburetor ice condition.

We had managed to avoid an off-airport landing. When we were safely back on the ground, I asked many instructors and mechanics about the incident, but none had heard of this low-power, high-moisture condition where carburetor heat is ineffective. I finally spoke to a seasoned pilot who had been instructing in Cessna 172s for many years.

After hearing my story, his face broke into a smile. He explained the aircraft's carburetor heat design and discussed how the small carburetor bafflle (actually a small box around the exhaust manifold) will not always heat the air sufficiently to melt ice under very low power settings. Higher power settings produce higher exhaust temperatures and thus hotter carb heat. He explained that, with the carb heat on at low power settings, it is possible to develop carburetor ice under certain atmospheric conditions. As we were flying, we were building carburetor ice with no indication because the ice did not restrict airflow at the low power setting. As I increased the throttle, the carb ice limited the airflow, thus limiting the power. Since the power was extremely limited, so was the exhaust temperature; thus only an increase in power would create the heat necessary to melt the accumulated ice. When I turned off the carb heat, the denser air increased the amount of power being developed and slightly increased the exhaust temperature. When I turned the carb heat back on, the now-hotter manifold conducted enough heat into the carburetor to melt the ice. This severe icing condition can turn into a catch-22. You need to develop heat to melt the ice, but you must melt the ice to develop heat. I was told that an increase in rpm caused by reducing the pitch attitude might have increased the heat enough to melt the ice, though this did not seem like a wise option at low altitude with an ailing engine.

I learned two important lessons from that flight. First, I now clear the engine with a throttle increase whenever the power setting is low for more than a few minutes. The second lesson is to never expect a closed airport to be usable for a forced landing.

My student learned a lesson that few experience with an instructor in the right seat-how to evaluate a real problem and methodically narrow it down to the possible source while preparing for a forced landing. During an emergency we must use the short time we have as efficiently as possible. Methodical analysis increases the odds of solving a problem and allows more time to set up for a safe off-airport landing if it is needed.

The Cessna 172 pilot's operating handbook now says that carburetor heat under low power conditions "may further reduce engine rpm to the point where insufficient heat is produced to melt the ice." This reinforces my conclusion that clearing the engine during low-power, low-altitude work is a good idea. Refer to your aircraft's POH for exact carburetor heat recommendations.