Like it or not, colder weather is coming and airframe icing doesn’t just happen to IFR aircraft flying in cloud. It has happened to me in clear air with the cloud cover at least 3,000 feet above me, and it could happen to you.
I was flying a C-170 with a student in the practice area in late October when it started to rain. The rain froze all over our aircraft. We descended and turned back to the airport. The ice continued to build until we were about 4 miles from landing when the rain stopped.
We were flying under a winter warm front where the air above us was above freezing and the air we were flying in was below freezing, giving us freezing rain.
This ice was mixed ice (rime and clear) and added considerable weight to the aircraft as well as considerable drag. Also, our windshield was covered with ice reducing forward visibility to zero.
Airframe icing can occur any time there is an inversion and you are flying in cold air below warmer air aloft and there is cloud above you.
Rime ice is formed by small water droplets that freeze on contact without spreading, making it look opaque, milky and rough. As it occurs, it disrupts the airflow, causing an immediate loss of lift. Fortunately, it easy to see as it forms.
Clear ice is formed from very large super cooled droplets that spread out on impact making a glassy heavy coating over the leading edge of aircraft surfaces and a fair distance back over the upper surface of the wing. Clear ice is heavy and very hard to see forming. It eventually also changes the shape of the wing causing a loss of lift and an increase in drag.
When airframe icing is encountered, it is imperative to leave the area immediately and select pitot heat and cabin or windshield heat “on.” If it is possible to descend to warmer air, do so and the ice will melt off quickly. If it is not possible to descend, turn around to leave the area. With luck the icing will melt off. In my case, luck was not with us. We remained in cold air and the ice was too thick for the cabin heat to melt.
The Cessna Supplement for light Cessna aircraft suggests that if visibility is impaired, perform a forward slip to gain better visibility. It also states that the approach should be flapless at 70 mph. Piper has no recommendations for flight with airframe icing and other light aircraft manufacturers may not have any advice either.
I would be careful about slipping with airframe icing. Drag has already been increased by an unknown amount. A forward slip is normally performed to increase drag to lose altitude. Do we really want to increase drag any more at this point? Even with an increase in power to maintain airspeed we may possibly stall at the approach speed.
A 70 mph approach with the flaps up may not be enough. An experience in the Air Force taught me that an aircraft may stall at an airspeed 20 knots above the normal indicated stall speed. This would not leave much margin at 70 mph with the flaps up.
If it is possible to open a side window or slide the canopy back a little, you may be able to scrape enough ice from the windshield to see ahead. I was not able to scrape enough ice off the windshield to see ahead so I flew a little to one side of the approach path and leaned my head as far to the left as possible. This enabled me to see enough to perform an approach. As I approached the runway, I moved the aircraft left and used my peripheral vision to stay in the centre of the runway.
I flew the approach at 80 mph with the flaps up. I had 5,000 feet of runway and I did not want to even think about a stall.
Lowering the flaps changes the camber and angle of attack of the of the wing. This change in angle of attack with ice on the wing could precipitate an immediate stall.
I flew the aircraft onto the runway, raising the nose of the aircraft only enough to avoid landing nose wheel first. I reduced the power very slowly and just enough to get the aircraft to land and then I reduced the power to idle. Flaring the aircraft for landing may increase the angle of attack of the wings to beyond the critical angle of attack with the ice build-up. A rapid power reduction as the aircraft nears the ground may also precipitate a stall.
If you are landing on a short or icy runway, you may have to modify this procedure some, but carefully. It is better to use all of the runway or even go off the end rather than stall short of or over the runway.
Watch for and try to avoid flying under a winter warm front or inversion with a cloud cover. At the first sign of airframe icing, get away from it. Flying is supposed to be fun. Flying an ice cube is not.
Dale Nielsen is an ex-Armed Forces pilot and aerial photography pilot. He lives in Abbotsford, B.C., and currently flies medevacs from Victoria in a Lear 25. Nielsen is also the author of seven flight training manuals published by Canuck West Holdings.
Got an aviation safety story to tell? Dale Nielsen would like to hear from pilots who have educational aviation experiences to relate. Excerpts from these stories will be used in upcoming safety articles. Dale can be contacted via e-mail: firstname.lastname@example.org