January 1, 1995
By Bruce Landsberg
Aviation is full of folklore offered to the fortunate few who may listen to grizzled old-timers. Pilots who've flown airplanes in ice for decades share some strong opinions about it.
Periodically there are accidents that surprise the authorities. The answers may have escaped the aircraft manuals and formal training curricula. However, there is usually some pilot folklore that recounts the hazard of ignoring history.
The National Transportation Safety Board is increasingly concerned about ice problems in general aviation, particularly with aircraft that have received "known ice certification." This concern developed before the recent Aerospatiale ATR-72 crash in Indiana, in which ice forming behind the leading edges of the wing is suspect. In the past nine years there were 285 accidents in which icing conditions were listed as a factor, according to the Air Safety Foundation database. There were 294 fatalities and 75 serious injuries.
An accident involving a Beech Duke in November 1992 raised some disturbing points that, while part of the folklore, should be brought into mainstream awareness. The Duke was equipped and approved for flight in known icing conditions. The IFR departure from Boeing Field in Seattle involved a climb to 17,000 feet. Pilot reports suggested moderate mixed icing from 3,000 feet to FL210 in the region. The aircraft was fully loaded with six people on board.
A performance study by NTSB based on recorded radar data, winds aloft, and other information showed that the Duke's calculated true airspeed varied from 82 to 123 knots. This was well below the minimum speed recommended for flight in icing conditions by Beech's Safety Information pamphlet.
After passing 13,500 feet, the airplane experienced two abrupt altitude excursions and then entered a steep descending left turn. There was an in-flight airframe failure. The aircraft was estimated to have exceeded 60 knots above V NE (the redline) and impacted the ground in a near-vertical attitude. There was no evidence of pre-accident problems with the airframe, engines, or the control system.
So what happened? The pilot was climbing as rapidly as possible to get through the icing conditions and was in an aircraft equipped with deicing boots to protect the wings and tail. Everything was as it should be. But the folklore says not to climb too steeply.
According to the NTSB investigator: "On booted aircraft, if the indicated airspeed during climb is too low, ice will accumulate on the under surface of the wing aft of the area protected by the boots.... As the speed decreases, the angle of attack increases, and the 'iceable' water droplets impact the lower surface of the wing behind the boots." This changes the shape of the wing, which will increase drag and can dramatically increase the stall speed. What you can't see definitely can hurt you. This ice buildup cannot be seen from the cockpit, so its thickness — and even its presence — may be unknown to the pilot.
This is not an isolated incident. Several other Dukes have had difficulty with ice behind the boots. So have King Airs, Cessnas, and Pipers. You had better not collect ice on parts of the airplane that don't shed it well. The flight manuals for many icing-approved aircraft do not specify a minimum indicated airspeed in icing conditions. Climb too steeply and presto! Ice under the wing.
Ice on the tail is another hazard to be reckoned with. It is also frequently unseen. The prestall shudder is commonly an indication that, aerodynamically speaking, things are not normal. Tail icing has brought down several regional airliners, and the FAA has issued airworthiness directives on an least five models of turboprop commuters. It could well be a problem on other general aviation aircraft.
The horizontal stabilizer is a great place to collect ice since its leading edge has a much smaller radius than a wing. Smaller, pointier objects will catch ice much faster than blunt or fat ones. There may be very little ice on the wings and a significant amount on the tail.
In normal flight the tail produces a downward force to balance the weight of the engine and other structure ahead of the center of gravity. It flies with a negative angle of attack (AOA). Without getting into a lot of aerodynamics, the negative AOA increases with the addition of flaps. When the tail stalls, the negative AOA has become too great. The down force that the tail normally provides is lost and the aircraft noses over sharply.
We also have aircraft that were certified under different rules and may have demonstrated only limited capability in icing conditions. Some of the shortcomings are not clearly explained in the manuals. There is much folklore from pilots who have survived in the ice. But for a newcomer or casual reader, the manuals are not always clear enough. Sometimes the information is there but is not presented in the normal operating section or critical airspeeds section. It may be buried in a supplement on options or special safety information that was published after the manuals were printed.
Years ago, when I flew light twins for Cessna, we were always admonished not to return to Cessna Aircraft Field in Wichita if there was any appreciable ice on the airplane. The runway was approximately 3,800 feet long and about as wide as your driveway. Instead, we were directed to Mid-Continent Airport and its 10,000 feet of concrete. The training gurus were very clear: leave the flaps up and don't let the speed drop below 120 knots until just over the runway. The old-timers knew the folklore about tailplane stalls in ice — and that flaps aggravated the problem. I should emphasize that this applies to those flying unapproved, unprotected airplanes as well, should you happen to inadvertently encounter the frosty stuff.
The NTSB has issued a safety recommendation to the FAA to "require all pilot operating handbooks/flight manuals applicable to aircraft certified for flight in icing conditions to contain precautionary operational information to help ensure that ice will not accumulate on the undersurface of the wing aft of the area protected by deicer boots or on other unprotected areas of the airplane. The information should include specification of a minimum indicated airspeed that should be maintained during sustained operations in icing conditions."
The AOPA Air Safety Foundation agrees. The information should be readily available on each aircraft, to prevent surprises. Folklore concerning ice needs to become mainstream information.
See also the index of "Safety Pilot" articles, organized by subject. Bruce Landsberg is executive director of the AOPA Air Safety Foundation.
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