Location: Texas

Friday, June 17, 2005


...on airplanes, not cakes.

Over at Freight Dog Tales John mentioned some of the characteristics of the Caravan in icing conditions. The airplanes reaction to ice does not inspire confidence and, indeed, one or two Caravans go down every year due to icing related conditions.

The problem seems to be the amount of ice the Caravan can carry vs. the amount required for the boots to work properly. In my experience most POH's recommend that the amount of ice needed to break properly when the boots inflate is a minimum of 3/4". Less than that and the boots just push it out away from the wing leaving an air pocket and an arch of ice which then accumulates more ice. The pocket of air in which the boots are now moving renders them useless.

My experience with icing involves several types of aircraft. In a Lear, for instance, icing concerns are almost non-existent; the airplane literally goes so fast that the wing heats up from friction. The wing is heated with bleed air from the engines (which is anti-ice not de-ice) but is only necessary in the most severe conditions. I have departed (I was the co-pilot) in conditions that I would have questioned the wisdom of doing in a King Air or other turboprop and never collected a speck of ice. This was after needing 3 hours in a heated hangar to de-ice the airplane before the flight. We never utilized the wing heat and were through the conditions so fast that they were not a concern.

This is not usually the case with turboprop airplanes. The sales brochures have always touted flying at FL250 "above the weather". Well it isn't practical for most turboprop aircraft to go to those kind of altitudes on the trips they fly. Also, the average King Air 90/100 is a 16,000' airplane which puts it smack in the middle of most icing conditions over most of the country. I imagine the Piper Cheyennes are the same and I know the Commanders are. They can get high if they need to but it takes some doing.

Icing conditions can be encountered at any time of the year at turboprop altitudes but sometimes it is better to seek a lower altitude that is ice free. Many times in Texas it may be freezing on the ground and up to six or seven thousand at which point you may encounter a temperature inversion that is two to three thousand feet thick and then freezing from ten up into the twenties. I have flown all over Texas in a King Air at 9000' and remained free from ice. I have also flown from Nebraska to Texas and was unable to be free of ice at any available altitude until south of Dallas. I had accumulations of ice on the inboard sections of wing (which has no boot) that exceeded three inches in thickness and the airplane was approaching its minimum icing penetration speed of 140 KIAS. These conditions are what I consider to be the maximum of what would be called moderate icing. I could keep the wing clean with the boots but eventually it was going to be a losing battle and was a relief to fly out of the icing on passing Dallas enroute to Houston. This was an E-90, btw.

The Commander has a laminar flow wing which is quite thin. It is easily affected by ice since anything that "dirties it up" degrades performance immediately. As I mentioned in the comments on John's blog the Commander POH spells out that a minimum of 3/4" of ice buildup is required before activating the boots. Activating them too soon results in ineffective and disappointing performance. In my opinion the Commanders will not carry much ice. Other pilots that have flown them disagree. YMMV

The Commanders also had an annoying habit of flaming out both engines on short final when exiting icing by descending on the ILS glideslope. What happened was that ice formed around the air inlet was ingested by the engine upon entering warmer air near the surface. The fix was an AD that required installation of a passive auto-ignition system. Passive in that it is always on and no pilot action is required. The annoying incidents of landing short of the runway stopped.

I have only minimally encountered any ice in a Baron. My impression is that it behaved pretty much like a King Air would. They do have the same wing so performance characteristics should be similar although I would not want to seek out any ice while flying a Baron in order to verify.


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