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.