By GLYNN DENNIS
I know, the headline confused me at first too but, if you take a moment to think about what it really says, it makes sense.
From our first flight training lesson we were taught that carbureted engines are susceptible to the formation of “carb ice” and it can occur at any time and at any power setting — even at high-cruise power settings, but not always. The potential for carb ice even varies among aircraft types — Piper and Cessna are prime examples. That’s part of the dilemma: The “unknown.”
First, let’s review the basic differences with the installation of the popular Lycoming O-320 engine used in many Piper models and the Continental O-300 found in many early Cessna models. The difference in the position of the carburetor on the Piper and the Cessna, and the specific methods used to get intake air to the carburetor, can be dramatic. And that difference has a major influence on the potential for carb ice to form.
The general rule is to add carb heat any time there is a power reduction below the green arc on the aircraft’s tachometer. (You must check your specific Pilot Operating Handbook (POH) to know what is correct for your model and type).
But, here are two examples of the instructions in the POH for a Piper and a Cessna that employ completely different instructions to handle the same potential for carb ice:
Excerpt from a Piper POH: “Apply carb heat only as required.”
The tachometer on the Piper has the green arc (normal operating range) that includes the full scale, 500 rpm to red line (usually 2,700 rpm). With the Lycoming O-320 installed on the Piper, carb heat would only be applied if carb ice was “suspected.”
Excerpt from a Cessna POH: “Apply carb heat any time the throttle is reduced below 2,100 rpm.”
The tachometer on the Cessna has the green arc that includes 2,100 or 2,200 RPM to redline (usually 2,700 rpm). Therefore, carb heat should be applied anytime there is a power reduction that reduces the RPM below RPM limit. Additionally, the application of carb heat should be applied “before” the power reduction.
That’s a lot of numbers, I know, but they are important ones.
While you’re digesting those differences in operating procedures, let’s bring some new numbers into the discussion.
Conditions That Produce Carb Ice
What are the conditions that can potentially produce carb ice — cold and wet, of course! Warm and humid? Maybe, but not always. How warm, how humid?
It’s impossible to have this discussion without bringing our old friend Bernoulli and its Venturi effect on air temperature into the conversation. As incoming air enters the open throat of the carburetor, it has a relatively low speed, high pressure, and is the same temperature as the outside air. As that incoming air passes through the narrow portion of the carburetor (the venturi), its speed increases and its pressure drops. That exemplifies the Venturi effect.
That increased speed and lower pressure reduces the temperature of the incoming air by 50° Fahrenheit or more.
When that massive reduction in air temperature is accompanied by moisture ice can form.
Look at the accompanying chart for details of the wide range of temperatures and moisture content that can lead to carb ice.
The part that surprised me the most was the blue portion. The temperature range is from 10° to 80°, with a moisture content as low as 20%. The potential for carb ice is at cruise power, as well as reduced power settings.
Don’t ask me how I know about carb ice at cruise power settings…OK, go ahead ask me, as I’m going to tell you anyway.
On a recent Saturday, a friend and I attended a fly-in at the Shafter-Minter Airport (KMIT), near Bakersfield, California. While the weather at our home base in Salinas was spectacular, much of the San Joaquin Valley was shrouded in the usual winter time tule fog. (Tule fog is a radiation fog that condenses when there is a high relative humidity (typically after a heavy rain), calm winds, and rapid cooling during the night.)
Fortunately, our destination was VFR with visibility of three to five miles in haze, sky clear. At least it was clear if you looked straight up!
On the flight to KMIT, we cruised at 5,500 feet. The Lemoore MOAs were cold and Lemoore Approach cleared us through their airspace direct to our destination. As we began our descent, carb heat was added, along with a power reduction, and we landed without any surprises.
The fly-in was a huge success with a massive turnout. There were custom hot rods alongside modern Super Cars and vendors displayed a variety of items for sale. Many antique aircraft were on display as well, some airworthy, some not. Several food booths offered a variety of choices and catered to long lines. We wandered around the airport for a few hours doing our part to help out the food booth operators and taking pictures.
At approximately 3 p.m. we departed the fly-in and began our flight home. We chose a cruising altitude of 6,500 feet and were soon on a heading that would take us over New Coalinga, The Pinnacles, and into Salinas.
Just moments after passing New Coalinga, I felt a vague shudder and noticed a slight drop in the RPM setting I had established. At first, I thought it was just a minor throttle slip, so I increased the RPM back to the prior setting and made sure the throttle lock was set.
Just a minute or so later the RPM began to decrease again from 2,450 to 2,400 to 2,370. Add full power! Add full carb heat! As the engine stumbled, so did my heart!
A few seconds later the engine began to run smoother and the RPM began to rise. Soon, the engine was running normally — and so was my heart.
So there you have it: Cruise power setting, mostly clear sky, a few clouds scattered around, the outside air temperature was 58°, and the humidity was approximately 60% — leading to carb ice.
Look back at the chart again. Those were perfect conditions for the formation of carb ice, at any power setting.
For the remainder of the flight I added carb heat every few minutes, just to be sure, and there were no additional issues with carb ice.
But why? The same temperature and dew point spread existed as we continued our flight. That’s the dilemma. The known conditions for carb ice… and the unknown… and if or when carb ice will occur.
We soon landed back at Salinas Municipal Airport (KSNS), safe and sound, but with a new story to tell.
The lesson here seems clear to me: Stay proficient, remain alert, and be prepared for the unexpected!
I do have one unanswered question, though. Why did the carb ice and the partial loss of power happen just as we began to cross the mountains of the southern end of the Diablo Range?
I would have preferred for it to have occurred while flying over the flat terrain of the San Joaquin Valley with runways and airports in every direction. Just sayin’!
Fly often and be safe.