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In all
honesty I'm more frightened about the theme of the item.
Is
this happening to passenger-carrying aircraft?
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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