Structural ice is a known flight hazard and there are plenty of forecasting products to help a pilot avoid it. Failing that, there are volumes of text and hours of videos to advise the unlucky or the unwise on how to deliver themselves from its grip.
Curiously, there is another type of icing that has sent its share of airplanes to the salvage yard, and pilots to the graveyard. Because it is mainly an affliction of low-performance aircraft, it doesn’t receive as much attention.
Many of us (I’d guess most) had our initial training in carburetor-equipped airplanes where the drill of applying carburetor heat before landing was strictly enforced. I did it mechanically with no conception of what would, or wouldn’t, happen if I ever stopped. We’d purchased a Cessna 150 before I began instruction and would be wedded to it for some time. The 150, being what it is, does not confer any claim to expertise on most aviation subjects. One exception is carburetor ice.
Conventional wisdom holds that certain engine designs are predisposed to carburetor ice. Notably the Continental O-200/300s and the older O-470 series, while Lycoming engines are resistant to icing because the induction piping efficiently transfers engine heat to the carburetor. I can’t dispute that but icing susceptibility also depends on the engine installation in a particular airframe and it can vary considerably between airplanes, even of the same model.
Sooner or Later
Our 150 had a Continental O-200-A and its Marvel carburetor would pick up ice under a cold stare. My first encounter with it was while practicing a hold on a clear, cool morning over a large body of water. I’d pulled the throttle back to 2200 RPM while circling but every couple of minutes the RPMs would slowly drop off and I’d need to add a touch of throttle. I tightened the throttle tensioner until my hand hurt but it still seemed to be creeping back. Then, and with no warning, the engine went silent. “This must be carb ice,” I thought and reached for the carburetor heat knob.
Sure enough, the engine came back for one second and then quit outright. In a panic, I tried to undo everything but before I could remove the heat, the slug of melted ice passed through the engine and full power was restored. I learned two things.
First, if there is a substantial amount of ice present, the engine will go into distress when carb heat is applied. Just wait it out, power is coming. Second, there is a fine emotional line between the smug pride someone feels handling a situation which is well understood, and the panic that sets in when it isn’t. The first lesson never needed reinforcing. The second seems to require periodic re-exposures – like a tetanus booster.
Fool Me Twice
Working toward a private certificate, I cannot remember anyone telling me that carburetor heat might not always resolve an icing problem. That it might have limitations. During my instrument training, one instructor mentioned in passing that he had an issue flying a Piper Apache (not a stellar performer in ideal conditions) where he’d been unable to remove, or even arrest, the ice building in both carburetors. He’d applied full heat, adjusted mixtures, manifold pressures, RPMs. No combination of anything helped. I asked what he did then. He answered, “I got scared.”
I took what he said to heart, but in the succeeding years I logged a fair amount of time flying the 150 in visible moisture without any ice control issues. We had a routine in IMC. Every one or two minutes I would pull the heat control, whereupon the RPMs would momentarily drop and then recover. Engine roughness and RPM decrease would determine how often the heat needed to be applied. Sometimes I just had to leave it on, lean the mixture, and live with the power deficit – undesirable in such a low-powered airplane.
Much of that flying was done at low altitude in the stratus layer which often forms in California’s central valley. Those conditions were fairly benign but, eventually, my flights extended into more varied geography and weather. Two of those stand out. The first caught me by surprise while crossing the Sierras. The flight originated in Truckee, California, with temperatures in the mid-30s (F) and melting snow on the taxiway. I kept the carburetor heat on while taxiing until advancing the throttle for takeoff.
Climbing east-bound at full throttle over Interstate 80, the engine wouldn’t maintain power. Full heat, and jockeying the mixture, preserved what power remained but it would not clear out the accumulated ice. Over the next 15 minutes, the highway relentlessly out-climbed the airplane, raising the specter of a forced landing on that steep, twisting concrete ribbon. A few miles before reaching Donner Summit, the ice began to clear.
Entering drier air from the valley rescued my situation with a scant 500 feet between the landing gear and the pavement. Using mental sleight of hand, I convinced myself the experience was an aberration unique to mountain operations. I clung to that conceit for a couple of years until it was stripped away during an instrument flight across the Sacramento Valley.
This hop was at lower altitude in somewhat warmer temperatures but it was heading toward a deep low pressure system moving in from the Pacific. The FSS briefer assured me structural icing would not be a factor but gusting winds would.
With a clearance in hand, I launched from Auburn and contacted the approach facility at Stockton. As soon as we leveled off at 4000 feet, there was a perceptible decay in power. So the heat came on and the throttle went forward to the stop. It may have been the higher moisture content in that warmer air, or convergence around the low, or something else, but power continued to drop (although more slowly).
I asked Stockton for lower, but sector MVAs and conflicting traffic wouldn’t allow it. Intense wind held down the ground speed, further delaying any relief. Finally, negotiating 3000 got the 150 below the clouds and gained a little manifold pressure. But the engine was just holding its own. Meanwhile, twilight fell – along with the dew point spread and visibility – as a one-hour flight morphed into two. The next controller expedited a VOR approach into Tracy.
After leaving the approach fix, I suffered for a couple of minutes in genuine anxiety knowing that the missed approach procedure called for an initial climb which was, by now, beyond the capability of the airplane, and conditions for maintaining power were rapidly deteriorating.
That night the low deepened and passed directly over the airport, ripping an airplane from its moorings behind our 150 and flipping it on its back.
In order to gain the full utility from a low-performance airplane, you will need to explore some outlying regions of the operating envelope. These exercises can be a source of gratification but they also demand judgment. That was my last instrument flight in the C-150.
Most carb ice encounters don’t induce sweaty palms. In fact, some deliver a dollop of humor.
Heading down the northern California coast, just inland of the coastal mountains on a pleasant spring afternoon, the RPMs began decreasing. I was a little surprised to see it at this temperature, but I realized an onshore flow was bringing moist Pacific air up over the hills. Giving the carburetor heat knob a good tug, I watched in dismay as the knob, the actuating wire and part of the sheath pulled out of the panel and fell into my lap. Santa Rosa airport was 10 miles ahead. I told the tower I’d like to make a precautionary landing straight in. The controller asked the nature of my problem and I did my best do describe what happened. Then he asked if I’d like them to dispatch the emergency equipment (I could hear laughter in the background). I demurred.
With the cowling removed, you could see where the actuator wire had pulled out of the B-nut securing it to the air box flapper. That was easy to fix. I also saw that the air box flapper automatically fell to the “full heat” position when it was disconnected – a smart fail-safe design. If I’d understood the system better, I could have proceeded to my destination and spared us all the trouble.
All you need
Even on a beautiful warm day, in dry air, it is possible to build up ice in a carburetor. All you need is a willfully negligent pilot.
The airplane had been sitting in the Bay Area for several rainy days when I picked it up at San Carlos. A faulty gas cap gasket had admitted an impressive amount of water into the fuel tanks and I spent half an hour rocking the wings and draining the sumps until I was positively convinced no water remained.
Yet the carb heat check showed a steep drop on the tachometer before returning to its normal drop of 80 RPM. It couldn’t be carb ice. The dew point spread was too wide. Since my ride had departed, and planes were stacking up behind me, I decided to figure this out back home and accepted a takeoff clearance.
The east departure from San Carlos keeps you down around a thousand feet while transiting the Bay under San Francisco’s Class B airspace. It’s a poor place to deal with an engine problem – even an impossible one – so I kept heat on the carburetor. Clearing the Sunol Pass, with more altitude, and in very dry air, I removed the heat and promptly experienced the same ice-like symptoms.
On the ground, the sumps were drained again with no hint of contamination. The air box, carburetor and linkages all seemed to be in order. I quit in frustration and went home. A couple of days later, during a routine preflight, I discovered the gascolator was nearly full of water. In my preoccupation with the fuel tanks, I’d forgotten to drain it at San Carlos. In response, the gascolator had been sending a steady trickle of water into the carburetor. Like a companion quietly reminding me to pay attention.
During initial training, one of my excellent CFIs, Mike Walling, was giving me tailwheel instruction in a Super Cub. Mike suggested I forgo the constant pulling and pushing of the carb heat control since, “Lycoming engines pretty much don’t ice up under power.” He was right. But pretty much, when used as a tool for aeronautical decision making, has its limitations. Twenty years later, I found a notable exception to Mike’s rule. By then I was flying behind a Lycoming 0-360 with a constant speed propeller and a carburetor air temperature gauge in the panel, which pretty much eliminated the possibility of icing. It was a routine IMC flight into Santa Rosa (a hard luck airport for my carburetors).
The assigned altitude was right at the freezing level, but the airframe was clean and there were no PIREPs. During cruise, the carb heat control was set to keep the temperature needle just out of the yellow arc (5 degrees C). A substantial portion of the traffic at STS consists of corporate jets and airliners so ATC expected my approach to be flown at speed with power on.
Descending out of 800 feet, the runway came into sight. I closed the throttle and decelerated through the final mile to touchdown – at which point the propeller stopped cold. Coasting onto a turnout I notified the Tower I’d experienced an engine stoppage – there was no way to tell when it actually occurred. It took less than a minute for the ice to melt out of the carburetor. The Lycoming started without complaint.
So what happened on this flight? Five degrees worked fine on countless similar approaches. There is no way to know for certain. The temperature indication in the carburetor throat was probably accurate, but there was a severe ice accumulation in another, unmonitored part of the carburetor.
So what is a safe carburetor air temperature on approach? I don’t know. While a carb temperature gauge is an invaluable tool for setting partial heat during the cruise phase of flight, applying full carb heat may be a better bet during the approach phase. One thing I do know: If the throttle had been closed a little earlier, I might’ve been writing this to the NTSB.
Finally, yes, a fuel-injected engine would put an end to these difficulties, but it would not satisfy the cost/performance relationship that governs my flying.