Ice and wings don't mix

Flying Safety, Oct, 2003 by Burton P. Chesterfield, Jim C. Bean, Tony Dawson

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Within the Air Force's flight operations community, there exist differences in the understanding and interpretation of published holdover times after deicing/anti-icing. Some tech orders specify a holdover time, while others say takeoff should be ASAP (a vague, unspecified timeframe), and still others give the freedom of an unlimited, unspecified holdover time. The following information and alternatives to existing ground deicing and anti-icing procedures are offered to the flight operations and ground-servicing communities to provide greater flight safety.

Wind tunnel and flight tests indicate that ice, frost, or snow formations on the leading edges, upper and lower surfaces of the wing and horizontal stabilizer, having a thickness and surface roughness similar to medium or coarse sandpaper, can reduce lift by as much as 30 percent and increase drag by 40 percent. These changes in lift and drag will significantly increase stall speed, reduce controllability, and alter aircraft flight characteristics. Thicker or rougher ice accumulations in the form of frost, snow, or ice deposits can have increasing effects on lift, drag, stall speed, stability, and control, but the primary influence is surface roughness critical to lift generation.

Improved deicing/anti-icing procedures, better fluids, and an increased awareness of the problems concerning ground and flight operations during periods of frozen precipitation will help us to avoid serious problems this winter season.

Frozen Contaminants--Their Causes and Effects

Frozen contaminants can form and accumulate on exterior aircraft surfaces on the ground during inclement weather. This accumulation can also occur during ground operational conditions conducive to icing. In either case, atmospheric conditions vary the type of accumulation, the amount and ice protection systems or procedures should be activated when the outside air temperature (OAT) is below 50 degrees F (10 degrees C) and visible moisture is present or when there is standing water, ice, or snow on runways or taxiways.

Aircraft in flight experience a variety of atmospheric conditions which alone or together can produce ice formations on the aircraft and its components. These conditions include:

* Supercooled Clouds. These are clouds containing water droplets that have remained in the liquid state even though the ambient temperature may be below 32 degrees F. These droplets are very small (5 to 100 microns), and they freeze on impact with another object. Water droplets have remained liquid even at temperatures as low as -40 degrees F. Cloud liquid water content, ambient temperature, droplet size, and the aircraft's size, shape, and velocity all contribute to the rate of accretion and the shape of the ice formed. (One micrometer or micron is one millionth of one meter or .00003937 inches.)

* Ice Crystal Clouds. These clouds exist at very cold temperatures where their moisture has frozen to the solid or crystal state.

* Mixed Conditions. These clouds have an ambient temperature below 32 degrees F and contain a mixture of ice crystals and supercooled water droplets.

* Freezing Rain and Drizzle. These are precipitation that exist within or below clouds at ambient temperatures below 32 degrees F. Rain droplets remain in a supercooled liquid state. Freezing rain is different from freezing drizzle only by virtue of droplet size. (Rain droplets range in size from 500 to 2,000 microns, and freezing drizzle droplets are less than 500 microns.)

Aircraft on the ground are susceptible to many of the same conditions as in flight even when they are parked or when they are operating on the ground. There are also conditions specific to ground operations. On the ground, the aircraft are exposed to:

1. Frozen precipitation--snow or sleet.

2. Residual ice from a previous flight--usually on the leading edges of wings, the empennage, training edge flaps, etc.

3. Moisture, slush, or snow on ramps, taxiways, and runways--which can remain in place on the air-craft if the temperature is low enough; particularly susceptible to this kind of frozen contamination are wheel wells, landing gear components, flaps, under surfaces of wings, horizontal stabilizers, etc.

4. Supercooled ground fog and ice fog--much like supercooled clouds and caused by advection or nighttime cooling.

5. Snow blown by ambient winds, other aircraft, or ground support equipment--the source can be snowdrifts, other aircraft, buildings, etc.

6. Recirculated snow--whipped up into the air by engine, propeller, or rotor wash.

6. High relative humidity with temperatures below the dew or frost point can cause frost. This is common during overnight storage after descending from higher altitudes, especially on lower wing surfaces in the vicinity of cold-soaked fuel cells.

7. Frost--a crystallized deposit formed from water vapor on surfaces at or below 32 degrees F.

8. Clear ice--usually around integral fuel tanks, difficult to see, and usually detectable only by touch or ice detector.