Some use of metric in this NTSB report.
Baron Carter
NTSB
Issues Icing Alert
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.
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