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:

• 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 takeoff.  However, history has shown that with a careful and thorough preflight inspection, including tactile inspections and proper and liberal use of deicing processes and techniques, airplanes can be operated safely in spite of the adversities encountered during winter months.    

(1)  Additional information regarding this accident can be found on the Safety Board's Web site at http://www.ntsb.gov, accident number DEN05MA028. 
(2)  This information is from the Safety Board's final report on the March 22, 1992, accident involving USAir flight 405, at Flushing, New York.  For additional information, see National Transportation Safety Board. 
1993.  Takeoff Stall in Icing Conditions, USAir flight 405, Fokker F-28, N485US, LaGuardia Airport, Flushing, New York, March 22, 1992.  Aircraft Accident Report NTSB/AAR-93/02. Washington, D.C.
(3)  For additional information, see http://www.ntsb.gov/recs/letters/2004/A04_64_67.pdf.
(4)  This information is from the Safety Board's final report on the February 17, 1991, accident involving Ryan International Airlines, at Cleveland, Ohio. For additional information, see National Transportation Safety Board. 1991. Ryan International Airlines, DC-9-15, N565PC, Loss of Control on Takeoff, Cleveland-Hopkins International Airport, Cleveland, Ohio, February 17, 1991. Aircraft Accident Report NTSB/AAR-91/09.  Washington, D.C.
(5)  See Aircraft Accident Report NTSB/AAR-93/02. Washington, D.C., cited above. 
(6)  See Aircraft Accident Report NTSB/AAR-93/02. Washington, D.C., cited above.
(7)  See Aircraft Accident Report NTSB/AAR-93/02. Washington, D.C., cited above.
(8)  This is information contained in the Safety Board's final report on the January 9, 1997, accident involving Comair flight 3272 at Monroe, Michigan. For additional information, see National Transportation Safety Board. 1998. In-flight Icing Encounter and Uncontrolled Collision with Terrain, Comair flight 3272, Embraer EMB-120RT, N265CA, Monroe, Michigan, January 9, 1997.  Aircraft Accident Report NTSB/AAR-98/04. Washington, D.C.  
(9)  This statement is a quote from a technical paper, titled, The Effect of Wing Ice Contamination on Essential Flight Characteristics, by Douglas Aircraft Company's deputy chief design engineer for the MD-80/DC-9 program (presented in 1988 and again in 1991). See appendix E of the previously cited Aircraft Accident Report NTSB/AAR-91/09.
(10) This quote is from Safety in the Operation of Air Transportation, a lecture presented by Jerome Lederer, M.E., at Norwich University, in 1939, and cited in the Safety Board's final report on the March 22, 1992, accident involving USAir flight 405 at Flushing, New York. See Aircraft Accident Report NTSB/AAR-93/02.  Washington, D.C., cited above.  
(11)  As a result of this and other icing-related accidents involving Cessna 208 series airplanes, on December 15, 2004, the Safety Board issued Safety Recommendations A-04-64 through -67. Additional information on the Dillingham, Alaska accident (DCA02MA003) and on Safety Recommendations A-04-64 through -67 can be found on the Safety Board's Web site at http://www.ntsb.gov.
(12)  This accident was investigated by the Air Accidents Investigation Branch (AAIB), Department for Transport, Great Britain. Additional information on this accident can be found at www.dft.gov.uk/stellent/groups.dft_avsafety/documents/page/dft_avsafety_030576.hcsp.

FMI: Although broader than the issue of wing upper surface ice accumulation discussed in this alert notice, aircraft icing has been an issue on the NTSB's Most Wanted List of Safety Improvements since 1997. A summary of the Board's actions and recommendations in this area may be found on its website, at www.ntsb.gov/Recs/mostwanted/air_ice.htm.