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From: "NTSB Press Releases" <NTSBPressReleases@NTSB.GOV>
Sent: Wednesday, December 29, 2004 3:31
PM
Subject: ALERT TO PILOTS: WING UPPER
SURFACE ICE ACCUMULATION
NTSB ADVISORY ************************************************************ National Transportation Safety Board Washington, DC 20594 December 29, 2004 ************************************************************ ALERT TO PILOTS: WING UPPER SURFACE ICE ACCUMULATION ************************************************************ 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 604 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: (1) -- 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. (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 non-slatted, 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 (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. |
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