Why now?
Carb ice at cruise power settings
By Glynn Dennis
From our first flight training lesson we are 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.
What happened
At cruise power setting, mostly clear sky, a few clouds scattered around, the outside air temperature at 58 degrees, and the humidity approximately 60 percent, yet we had carb ice.
Systems
Venturi effect
As 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 (the venturi) of the carburetor, its speed increases and its pressure drops. That increased speed and lower pressure reduces the temperature of the incoming air by 50 degrees Fahrenheit or more. When that massive reduction in air temperature is accompanied by moisture, ice can form.
The potential for carb ice even varies among aircraft types; Piper and Cessna are prime examples. The difference in the position of the carburetor on Pipers and Cessnas, 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.
On a recent Saturday, a friend and I attended a fly-in at Shafter-Minter Field in California’s Central Valley in my Cessna 172B. While the weather in Salinas, California, was spectacular, much of the San Joaquin Valley was shrouded in the usual winter Tule fog.
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 Shafter-Minter, we cruised at 5,500 feet. The Lemoore military operations area was cold, and Lemoore Approach cleared us through its airspace direct to our destination. As we began our descent, I added carb heat, reduced power, 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 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 Municipal Airport and Pinnacles National Park, 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—2,450 feet, 2,400, 2,370. I added full power. I added full carb heat. As the engine stumbled so did my heart! If it quits, we’re going to New Coalinga, I thought. Yet 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: At cruise power setting, mostly clear sky, a few clouds scattered around, the outside air temperature at 58 degrees, and the humidity approximately 60 percent, yet we had carb ice. 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.
We soon landed at the Salinas Municipal Airport, 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.
