Chapter 2 - Theory and Aircraft Performance

1.  As discussed in Chapter 1, a very small amount of roughness, in thickness as low as 0.40 mm (1/64 in.), caused by ice, snow or frost, disrupts the air flow over the lift and control surfaces of an aircraft. The consequence of this roughness is severe lift loss, increased drag and impaired maneuverability, particularly during the take off and initial climb phases of flight. Ice can also interfere with the movement of control surfaces or add significantly to aircraft weight as well as block critical aircraft sensors. There is no such thing as an insignificant amount of ice.

2.  Ice can form even when the outside air temperature (OAT) is well above 
0°C (32°F). An aircraft equipped with wing fuel tanks may have fuel that is at a sufficiently low temperature such that it lowers the wing skin temperature to below the freezing point. This phenomenon is known as cold-soaking. Liquid water coming in contact with a wing, which is at a below freezing temperature, will freeze to the wing surfaces.

3.  Cold-soaking can be caused by fueling an aircraft with cold fuel. Where fuel tanks are located in the wings of aircraft, the temperature of the fuel greatly affects the temperature of the wing surface above and below these tanks. If there is rain or high humidity, ice can form on the cold-soaked wing and accumulate over time. This ice can be invisible to the eye and is often referred to as clear ice. Cold soaking can cause frost to form on the upper and lower wing under conditions of high relative humidity. This is one type of contamination that can occur in above freezing weather at airports where there is normally no need for deicing equipment, or where the equipment is deactivated for the summer. This contamination typically occurs where the fuel in the wing tanks becomes cold-soaked to below freezing temperatures because of low temperature fuel uplifted during the previous stop or cruise at altitude where low temperatures are encountered, or both, and a normal descent is made into a region of high humidity. In such instances, frost will form on the under and upper sides of the fuel tank region during the ground turn-around time, and tends to reform quickly even when removed.

4.  After a flight, the temperature of an aircraft and the fuel carried in the wing tanks may be considerably colder than the ambient temperature. An aircraft's cold-soaked wings conduct heat away from precipitation so that, depending on a number of factors, clear ice may form on some aircraft, particularly on wing areas above the fuel tanks. As well, cold soaking can cause ice to form due to humidity in the air when there is no precipitation, even when the temperature is above freezing. Such ice is difficult to see and in many instances cannot be detected other than by touch with the bare hand or by means of a special purpose ice detector such as a Ground Ice Detection System (GIDS). A layer of slush on the wing cannot be assumed to flow off the wing on takeoff and must be removed. This layer can also hide a dangerous sheet of ice beneath.

5.  Sheets of clear ice can dislodge from the wing or fuselage during takeoff or climb and can be ingested by aft fuselage mounted engines, thereby damaging or stopping them and can also cause impact damage to critical surfaces such as, the horizontal stabilizer.

6.  The formation of contamination on the wing is dependent on the type, depth and liquid content of precipitation, ambient air temperature and wing surface temperature. The following factors contribute to the formation intensity and the final thickness of the ice layer:

  • low temperature of the fuel uplifted by the aircraft during a ground stop and/or the long airborne time of the previous flight resulting in a situation that the remaining fuel in the wing tanks is subzero. Fuel temperature drops of up to 18°C have been recorded after a flight of two hours;
  • a large amount of cold fuel remaining in the wing tanks causing fuel to come in contact with the wing upper surface panels, especially in the wing root area;
  • weather conditions at the ground stop, wet snow, drizzle or rain with the ambient temperature around 0°C is very critical. Heavy freezing has been reported during drizzle or rain even in a temperature range between +8°C to +14°C.

7.  Skin temperature should be increased to preclude formation of ice or frost prior to take-off. This is often possible by refueling with warm fuel or using hot Freezing Point Depressant (FPD) fluids, or both.

8.  In any case, ice or frost formations on upper or lower wing surfaces must be removed prior to take-off. The exception is that take-off may be made with frost adhering to the underside of the wings provided it is conducted in accordance with the aircraft manufacturer's instructions.

9.  Aircraft certified for flight in known icing condition have been designed and have demonstrated system capability of providing some protection against the adverse effects of airframe icing in flight only. In addition, stall warning systems only give an effective warning under clean wing conditions.

10.  Frost, ice or snow formations on an aircraft may decrease the lift and alter the stall and handling characteristics. Aircraft may become airborne in ground effect but be unable to climb.