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Frost - Pretty But Dangerous

Reprinted with permission from FAA Aviation News
by H. Dean Chamberlain

Frost - it makes interesting patterns on windows, looks great on pumpkins, is hard to scrape off your car's windshield, and is potentially dangerous on your airplane. This is also the time of the year when you can expect to find frost or ice on your airplane.

I went to Truckee, elevation 5,900 feet mean sea level, hoping to take some ice or snow photographs to illustrate a winter safety article I wanted to include in this issue. As I drove west from Reno, Nevada, along Interstate 80 into the mountains, I realized the weather was not cooperating. I had hoped for snow like last year, but I quickly realized I was not going to find any. Frankly, the weather was just too nice. It was cold, but there was no chance of snow. But since I have yet to find an airport I didn't like, I drove to Truckee to see what I could find.

Imagine my surprise when I parked my car and saw a pilot, Clint Bazzill from El Granada, California, using the sun to remove frost from his white Kitfox airplane. As he worked on the aircraft, he would move the aircraft around to expose particular areas of it to the most direct rays of the sun. In talking to him, he was aware of the danger frost posed to his safe departure. He wanted a clean, dry aircraft.

Then to my surprise, I saw a large business jet, a Cessna 680, parked down the ramp. From the sounds I heard, either an engine or an auxiliary power unit was running on the jet. Seeing people walking around the aircraft, it was obvious the crew was preparing for a departure. As I watched, I saw an airport pickup truck drive up to the jet. A man took a tall stepladder off the truck and set it up near the tail of the aircraft. Then in a scene that would make the grumpiest FAA safety inspector happy, one of the men near the aircraft climbed the ladder to physically inspect the T-tailed horizontal stabilizer for frost.

I thought that crew was truly showing its professionalism as one of its members inspected the aircraft for frost. Frankly, it was not easy to do. The airport office had to be contacted. Someone had to go and get a ladder tall enough to reach the tail, and finally a crewmember had to climb the ladder to check for frost and possibly ice.

If it had been your aircraft, would you have done it? Even if it was your typical low-wing Mark I family flyer, would you have walked around the aircraft to physically inspect and touch the aircraft's surfaces for contamination?

The sad thing is some crews have not physically checked their aircraft's surfaces, and that failure has contributed to accidents. Surprisingly, according to one report I read, jet aircraft seem to be more susceptible to frost and wing contamination than some other types of aircraft. When I photographed the aircraft, it was my intention just to remind everyone of the need to check your aircraft for frost, ice, or snow. But as I started talking to people here at Headquarters about the article, several pointed out important safety material that I should review and consider adding to what at one point was going to be a short photo essay on the dangers of frost.

The first source is a September 2006 Safety Alert from the National Transportation Safety Board (NTSB) warning pilots of the dangers of aircraft icing. The Alert (SA-06) titled 'Aircraft Icing,' includes a subtitle that says, 'Pilots urged to beware of aircraft upper wing surface ice accumulation before takeoff.' The sidebar with this article is the complete text of the Alert.

Another good source of information I was told about is a NASA Internet Web site that has a video training course on it. The URL for the site is The NASA site has two video courses of benefit to general aviation pilots. The first is titled 'A Pilot's Guide to Ground Icing.' The second one is titled 'A Pilot's Guide to In-Flight Icing.' The NASA site also provides detailed resources for the courses as well as review material, media files, accident reports, and other icing related materials. NASA has done a lot of research on aircraft icing.

FAA has published many documents and advisory circulars (AC) on winter operations, such as frost, icing, freezing rain, and snow. Although much of that material addresses air carrier and commuter operations, the material provides important information for other types of flight operations as well. My initial FAA search returned 27 AC titles on the word 'frost.' In reviewing several of the ACs, one AC noted, 'Most pilots are aware of the hazards of ice on the wings of an aircraft. The effects of a hard frost are much more subtle. This is due to an increased roughness of the surface texture of the upper wing and may cause up to a 10 percent increase in the airplane stall speed. It may also require additional speed to produce the lift necessary to become airborne.' The AC, AC 61-84B, also said, 'Once airborne, the airplane could have an insufficient margin of airspeed above stall such that gusts or turning of the aircraft could result in a stall.'

AC 135-16, titled 'Ground Deicing and Anti-Icing Training and Checking,' was written for the Title 14 Code of Federal Regulations (14 CFR) part 135 communities. However, the AC made several important points that all GA pilots can benefit from. The first is the 'Clean Aircraft Concept.' This summarizes regulatory guidance in 14 CFR parts 121 and 135 that says no person may takeoff an airplane when frost, ice, or snow is adhering to the wings, control surfaces, or propellers of the airplane. The AC states, 'The rationale behind this concept is that the presence of even minute amounts of frost, ice, or snow (referred to as 'contamination') on particular airplane surfaces can cause a potentially dangerous degradation of airplane performance and unexpected changes in the airplane flight characteristics.' The AC also defined ground icing conditions as any time conditions are such that frost, ice, or snow may reasonably be expected to adhere to an airplane. The AC defined frost, including Hoarfrost, as a deposit of interlocking ice crystals formed by direct sublimation of water vapor on an object or aircraft surface, which are at or below 32 degrees Fahrenheit or 0 degrees Celsius.

What I found interesting in the AC was the statement that frost or ice could form on an aircraft's wing surfaces when the temperature is above freezing. Although we normally think of frost forming as a result of freezing temperatures at ground level, the AC said frost or ice can form on a wing when the wing is 'cold-soaked' and encounters high humidity, rain, drizzle, or fog, even though ambient temperatures are above freezing. A good example of this is a cold aircraft landing from the sub-freezing temperature of high altitude down through the above weather conditions. The aircraft could experience frost or ice forming on its surfaces. The AC also said cold-soaked fuel can also cause frost to form over the fuel tank areas in the above conditions.

AC 135-16 highlighted an important safety point. In its detailed section dealing with deicing and anti-icing fluids and the proper way to apply and use them, the AC said, 'Some fluids may not be compatible with aircraft materials and finishes and, some may have characteristics that impair aircraft performance and flight characteristics or cause control surface instabilities. Use of automotive antifreeze for deicing is not approved. Its holdover time and its effects on aircraft aerodynamic performance are generally unknown.' The AC lists several ways to deice an aircraft including several good references regarding winter operations. For your smaller GA type aircraft, one of the best ways is a heated hangar. Regardless of how you remove frost, ice, or snow from your aircraft, be it heat from the sun, deicing fluid, scrapping, or another method that works for you, a critical element is to ensure that the contamination does not refreeze back on your aircraft. This is especially true on a critical control surface or operating area such as a control hinge that might prevent the operation of that control surface.

Failure to properly check for any surface contamination such as frost, snow, or ice and to properly remove that contamination could result in you becoming a test pilot in an aircraft that might not be able to fly.

Have a safe winter.

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