You live in the South, so icing isn’t a problem, right?
Wrong, as this winter’s Arctic blasts demonstrate.
Wing icing has been in the news a lot, lately, and pilots should be paying attention regardless of where they live.
On Dec. 29, the National Transportation Safety Board issued an Alert to Pilots concerning ice accumulation on wing upper surfaces. It advises pilots not only to look for ice or frost, but to feel for it, as well.
“The only way to ensure that the wing is free from critical contamination is to touch it,” the report states.
The unusual attention being paid to a problem almost as old as aviation is due to a noticeable incidence of recent crashes where icing was the cause.
“For years, most pilots have understood that visible ice contamination on a wing can cause severe aerodynamic and control penalties,” the letter says. “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.” It points out 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.”
Pretty convincing stuff, and based on serious research ranging from wind tunnel data to in-flight icing tests to accident studies.
The airframe industry acknowledges the difficulty of determining, strictly by observation, whether a wing is wet or is carrying a thin film of ice. The NTSB says that even with a wing inspection light, looking through a window which may, itself, be wet, “does not constitute careful examination.” It reiterates that detection of contamination that is minimal, but enough to degrade performance, may be impossible without actually feeling the wing.
An environmental icing specialist at the FAA adds credence to the NTSB letter. The FAA and the Air Force have done numerous in-flight icing tests, most by spraying fine water droplets from a KC-135 boom similar to those used for aerial refueling. Test airplanes flying through the droplet clouds developed structural icing which could be analyzed in real time. The FAA specialist believes that most pilots are unaware of what small ice accumulations can do to aircraft performance. “Pilots may observe what they perceive to be an insignificant amount of ice and still be at risk because of reduced stall margins from icing-related degraded performance,” he told the NTSB.
Judging from all the accident and research evidence, nearly imperceptible ice accumulation can cause the same aerodynamic problems as much larger and more visible accumulations. “A very light coating of ice or snow will have a tremendous effect on reducing the performance of a modern airplane,” wrote the late safety expert Jerome Lederer – in 1939.
Despite all the evidence, the NTSB believes, pilots generally do not appreciate the potential consequences of small ice accumulations. The Alert to Pilots letter is an unprecedented attempt to get our attention.
There are pilots – probably a lot of them – who have flown with quite visible icing on the leading edges of wings, and so believe that a thin layer of the stuff can do no harm. Others are convinced that sufficient horsepower will simply power them through an icing situation, not realizing that power won’t necessarily prevent a stall, especially at the high angles of attack during takeoff.
Frost, snow, rime ice and clear ice all can be difficult to detect, but history has shown that, with a careful and thorough preflight inspection – don’t forget to feel the wing surface – and proper de-icing, if necessary, airplanes can fly safely during icing conditions.
No matter how far south you live, read the Alert to Pilots, available on the NTSB website at NTSB.gov. There you will find that aircraft icing has been on the board’s “most wanted” list of safety improvements since 1997.