robert_windsor
04.01.05, 21:51
ALERT TO PILOTS: WING UPPER SURFACE ICE ACCUMULATION
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As a result of a recent takeoff accident that has generated much discussion
about the effects of wing upper surface ice accumulations, the National
Transportation Safety Board is issuing the following alert letter to pilots:
Wing Upper Surface Ice Accumulation Alert
The National Transportation Safety Board has long been concerned about the
insidious nature of the effects of small amounts of ice accumulated on an
airplane's upper wing surface. The Safety Board's preliminary investigation
of the November 28, 2004 accident involving a Bombardier Challenger 601-1A in
Montrose, Colorado,(1) has revealed that atmospheric conditions conducive to
upper wing surface ice accumulation existed at the time of the accident
(airplane performance issues, including the possibility of upper wing ice
contamination, are being investigated).
For years most pilots have understood that visible ice contamination on a
wing can cause severe aerodynamic and control penalties; however, it has
become apparent that many pilots do not recognize that minute amounts of ice
adhering to a wing can result in similar penalties. Research results have
shown that fine particles of frost or ice, the size of a grain of table salt
and distributed as sparsely as one per square centimeter over an airplane
wing's upper surface can destroy enough lift to prevent that airplane from
taking off. The Safety Board has commented on the hazards of upper wing ice
accumulation in several previous aircraft accident reports; some excerpts
from these reports follow:
According to.wind tunnel data, a wing upper surface roughness caused by
particles of only 1-2 mm [millimeter] diameter [the size of a grain of table
salt], at a density of about one particle per square centimeter, can cause
lift losses.of about 22 and 33 percent, in ground effect and free air,
respectively.(2)
Research has shown that almost imperceptible amounts of ice on an airplane's
wing upper surface during takeoff can result in significant performance
degradation. Therefore, the Safety Board has urged pilots to conduct visual
and tactile inspections of airplane wing upper surfaces in past safety
recommendations (including Safety Recommendation A-04-66, which was issued to
the FAA on December 15, 2004).(3)
Ice accumulation on the wing upper surface is very difficult to detect..It
may not be seen from the cabin because it is clear/white.and it is very
difficult to see from the front or back of the wing..The Safety Board
believes strongly that the only way to ensure that the.wing is free from
critical contamination is to touch it.(4)
Accident history shows that nonslatted, turbojet, transport-category
airplanes have been involved in a disproportionate number of takeoff
accidents where undetected upper wing ice contamination has been cited as the
probable cause or sole contributing factor.(5)
The industry acknowledges that it is nearly impossible to determine by
observation whether a wing is wet or has a thin film of ice..a very thin film
of ice or frost will degrade the aerodynamic performance of any airplane.(6)
The Safety Board believes that even with the wing inspection light, the
observation of a wing from a 30- to 40-foot distance, through a window that
was probably wet from precipitation, does not constitute a careful
examination..the Safety Board acknowledges that the detection of minimal
amounts of contamination, sufficient to cause aerodynamic performance
problems, is difficult and may not be possible without a tactile inspection.
(7)
The Federal Aviation Administration's (FAA) Environmental Icing National
Resource Specialist (NRS) indicated that he was concerned that most pilots
were not aware that a slight amount of frost or ice accumulation could result
in a significant degradation of airplane performance. The Icing NRS
stated, 'pilots may observe what they perceive to be an insignificant amount
of ice on the airplane's surface and be unaware that they may still be at
risk because of reduced stall margins resulting from icing-related degraded
airplane performance.'(8)
From an aerodynamic viewpoint, there is no such thing as "a little ice."
Strict attention should be focused on ensuring that critical aircraft
surfaces are free of ice contamination at the initiation of takeoff.(9)
Strange as it may seem, a very light coating of snow or ice, light enough to
be hardly visible, will have a tremendous effect on reducing the performance
of a modern airplane. (Jerome Lederer, M.E., 1939) (10)
Despite the accident and research evidence indicating that small, almost
visually imperceptible amounts of ice accumulation on the upper surface of a
wing can cause the same aerodynamic penalties as much larger (and more
visible) ice accumulations, recent accidents indicate that the pilot
community still may not appreciate the potential consequences of small
amounts of ice. For example, see the final report on the October 10, 2001,
accident involving the Cessna 208, N9530F that occurred in Dillingham,
Alaska; (11) also see the final report on the January 4, 2002, accident
involving the Bombardier Challenger 604, N90AG, which occurred in Birmingham,
England.(12)
It appears that some pilots believe that if they cannot see ice or frost on
the wing from a distance, or maybe through a cockpit or cabin window, it must
not be there - or if it is there and they cannot see it under those
circumstances, then the accumulation must be too minute to be of any
consequence. Despite evidence to the contrary, these beliefs may still exist
because many pilots have seen their aircraft operate with large amounts of
ice adhering to the leading edges (including the dramatic double horn
accretion) and consider a thin layer of ice or frost on the wing upper
surface to be more benign. However, as noted, research has shown that small
amounts of ice accumulation on the upper surface of a wing can result in
aerodynamic degradation as severe as that caused by much larger (and more
visible) ice accumulations.
It is also possible that many pilots believe that if they have sufficient
engine power available, they can simply "power through" any performance
degradation that might result from almost imperceptible amounts of upper wing
surface ice accumulation. However, engine power will not prevent a stall and
loss of control at lift off, where the highest angles of attack are normally
achieved. Further, small patches of almost imperceptible ice or frost can
result in localized, asymmetrical stalls on the wing, which can result in
roll control problems during lift off.
The Safety Board notes that there are circumstances in which upper wing
surface ice accumulation can be difficult to perceive visually. For example,
depending on the airplane's design (size, high wing, low wing, etc.) and the
environmental and lighting conditions (wet wings, dark night, dim lights,
etc.) it may be difficult for a pilot to see ice on the upper wing surface
from the ground or through the cockpit or other windows. Further, frost,
snow, and rime ice can be very difficult to detect on a white upper wing
surface and clear ice can be difficult to detect on an upper wing surface of
any color. However, it is critically important to ensure, by any means
necessary, that the upper wing surface is clear of contamination before
takeoff. That is why the Safety Board recently issued Safety Recommendation A-
04-66, urging pilots to conduct visual and tactile inspections of airplane
wing upper surfaces.
The bottom line is that pilots should be aware that no amount of snow, ice or
frost accumulation on the wing upper surface can be considered safe for
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