Torqued: Preventing icing-related accidents

 - December 11, 2007, 6:20 AM

As I prepared to write this column the television and radio news programs were reporting on the recent spate of business aviation accidents. One of the widely reported accidents that caused considerable concern at the NTSB was the November 28 crash of the Challenger 601 in Montrose, Colo. In this accident the NTSB is investigating airplane performance issues, including the possibility of upper-surface wing ice contamination.

The NTSB has included aircraft icing, both on the ground and in flight, on its list of most wanted safety improvements since 1997. The number of fatal accidents in the U.S. and around the world has continued at the same pace since then without any real indication of change.

While working at the NTSB on icing issues I would think back to my involvement with aircraft ground de-icing and recall how many flight crewmembers would not take seriously the dangers associated with wing contamination. I often believed this attitude was the result of a misunderstanding of the different wing designs and the positive effects of leading-edge devices.

A hard wing (a wing without leading-edge devices) is much more sensitive to contamination than a wing with leading-edge devices. I have experience with a twinjet that has a hard wing whose entire surface, including the leading edge, is painted. In service this paint would wear and chip off the leading edge, requiring touch-up that resulted in an uneven surface. Roughness of more than 0.007 of an inch (seven one-thousandths of an inch) would cause performance penalties, and a special tool was required for measuring this roughness. If the entire leading edge or a considerable length of it was in this degraded condition, the aircraft would be extremely difficult to control.

Upper wing ice contamination causes the same problems. Wind-tunnel testing has shown that a surface roughness of one to two millimeters at a density of about one particle per square centimeter can cut lift by about 22 percent in ground effect and by 33 percent in free air.

Most of us will look at a wing to determine if there is any ice accumulating, but this is not always the most accurate indication. Many readers will remember the problems the MD-80 series of aircraft encountered with clear ice on the wings after spending a few hours aloft in temperatures well below zero.

The fuel remaining in the fuel tank retained this temperature for a long time, maintaining a wing surface temperature well below freezing. Because of the temperature difference condensation would collect on both the upper and lower wing surfaces. It has long been common to notice ice on the lower surface but uncommon on the upper surface. After some icing-related problems with this aircraft we mechanics were required to inspect the wings from the cabin window or climb a ladder to look for ice accumulation before pushback. This procedure did not banish the problems, so we were then required to get onto the wing and make sure there was no ice accumulation.

The NTSB has said that some pilots seem to believe that if they cannot see ice or frost on the wing from a distance or through a cockpit or cabin window, it must not be there. But this is not the case. Unlike snow or freezing rain, ice from condensation can be very difficult to identify by looking from even a short distance. Most of us have acknowledged that it is nearly impossible to determine by observation whether a wing is wet or has a thin film of ice. Remember, even a thin film of ice or frost will compromise the aerodynamics of any airplane.

The NTSB asserts that the only way to ensure that a wing is free from critical contamination is to touch it. Please keep this in mind the next time you look out
the cockpit or cabin window to determine whether ice is present on the wing or
control surfaces.

The NTSB is also concerned that some pilots seem to think they can power their way out of any performance degradation that might result from small amounts of upper wing surface ice accumulation. This simply is not true. Engine power on transport aircraft cannot replace lift lost to wing contamination.

The NTSB has also cited the accident history that shows nonslatted transport-category jets have been involved in a disproportionate number of takeoff accidents in which undetected upper wing ice contamination has been cited as the probable cause or sole contributing factor.

Winter operations are demanding, and it is easy to allow the effects of long days, cold temperatures and tasks that take more time to accomplish to diminish our defenses, possibly leading to mistakes in judgment. We must not let this happen. One way to prevent this is to understand fully our tasks and remain vigilant about our actions, including the consequences of a mistake.

Icing on the Web

There is plenty of information on icing at the following Web sites: (Look for the following accident reports): DEN05MA028; NTSB/AAR-93/02; NTSB/AAR-91/09