Airframe icing usually occurs when flying in cloud above the freezing level. Weather forecasters are getting better at predicting when and where possible icing conditions will occur so that we can avoid this problem. As weather forecasting is still not an exact science, some of us still get caught in icing conditions in cloud.
It is also possible for an aircraft to get iced up in clear air. You may be flying in cold air under an isothermal layer. If it starts to rain, the rain will freeze all over your aircraft.
A Diamond DA40 aircraft, en route from Halifax (CYHZ) to Quebec City (CYQB) lost communication with Air Traffic Control. The aircraft’s last reported position was in northern Maine, near the Quebec Border. The aircraft had encountered icing conditions and the pilot declared an emergency, eventually descending below Minimum Vector Altitude. The aircraft was eventually located by Search and Rescue in Maine, two miles from the Quebec border. One person was deceased and the other was injured and was transported to Quebec hospital.
An instructor and instructor candidate in a C-170 aircraft were in transit to a training area near Castlegar, B.C. when it started to rain. The rain immediately began to freeze on the aircraft.
An instructor and student encountered freezing rain during a cross country flight in southern B.C.
Any time an aircraft in flight begins to ice up, a turn away from the icing conditions or a descent must be initiated immediately. A turn away may not help if the sudden rain is widespread. A descent into warmer air may be the best choice. A descent may not be a good choice if the aircraft is in cloud and it is at the minimum descent altitude (MDA) or it is at the ATC minimum vectoring altitude. A 180 degree turn may be the best hope of leaving the icing area.
You will note that the choices listed are turn or descend. Most of us are not IFR rated. All the IFR training we have is what we took during our Private, VFR Over The Top or Commercial Pilot training. This training is to teach us that in the event of inadvertent cloud entry to make a level 180 degree turn or a wings level descent.
A combination manoeuvre, turning while descending, will likely be beyond the skills of most of us. We may have been able to complete such a manoeuvre under the hood in training, but with anxiety high and an aircraft made sluggish by airframe ice, our chances of pulling off such a combined manoeuvre is slim.
The DA40 crashed. We don’t yet know why as the investigation is not complete. It is possible that the aircraft could not sustain level flight with the amount of ice on the airframe and above freezing temperatures may not have been encountered.
The C170 was able to return to the airport. The C-172 was able to divert to a nearby airport. Both crews were able to turn away from the freezing rain, but were not able to descend into warmer air.
The Supplement to the Cessna 150 and 172 Pilot Operating Handbooks give us some good advice on how to handle this situation. First keep the airspeed up. If you are lucky and the pitot tube has not frozen up, turn on the pitot heat to prevent it from doing so. The aircraft windshield is likely iced up so turn up the aircraft heat and use the defrost mode if available, as soon as possible. This will not likely be enough to melt the ice.
Try opening a side window and scraping some of the ice off. If this doesn’t work, you may have to fly slightly to one side of the approach path to the runway until over the approach end and then the aircraft can be positioned over the runway and peripheral vision used to make the landing.
The Cessna Supplement suggests slipping slightly so that you can better look out the side window. I would avoid doing so. Slipping adds drag and with the added drag you already have, you may increase your rate of descent beyond what is safe or you may stall.
The aircraft is heavy and has a lot more drag so a lot more power is likely to be required to keep it flying. As you approach the runway for landing, do not lower any flaps. The sudden increase in the angle of attack of the wing caused by lowering the flaps may precipitate a stall.
The Cessna advice is to fly a normal flapless approach speed. I disagree. I have flown in a lot of icing conditions all over North America. I have had iced up aircraft give indications of an approaching stall as much as 30 knots above the normal stall speed. With ice on the wings, you may not get any warning of an approaching stall. I would add 10 to 20 knots to the normal flapless approach speed.
Do not reduce power and attempt to flare the aircraft over the runway for landing, there is a good chance the aircraft will stall. Reduce power only enough to be able to raise the aircraft nose enough so that the nose wheel does not touch down first. As soon as the main wheels are on the ground, reduce the power to idle and brake as necessary.
A good weather briefing is the best way to avoid airframe icing. But since weather forecasts are not always accurate, be prepared by knowing what to do and by keeping your instrument training current.
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.