Operational Airworthiness
The Clean Aircraft Concept
(This notice, supersedes Airworthiness Notice No. B017, Edition 1 dated 6 June 1994.)
Canadian Aviation Regulations (CARs) prohibit take-off when frost, ice or snow is adhering to any critical surface of the aircraft. This is referred to as "The Clean Aircraft Concept". The Clean Aircraft Concept is essential to the maintenance of flight safety.
Where frost, ice or snow may reasonably be expected to adhere to the aircraft, the Canadian Aviation Regulations require that an inspection or inspections be made before take-off or attempted take-off. The type and minimum number of inspections are indicated by the regulations, and depend on whether or not the operator has an approved Operator's Ground Icing Operations Program using the Ground Icing Operations Standard as specified in CAR 622.11 - Operating and Flight Rules Standards.
It is imperative that take-off not be attempted in any aircraft unless the pilot-in-command (PIC) has determined that all critical components of the aircraft are free of frost, ice or snow contamination. This requirement may be met if the PIC obtains verification from properly trained and qualified personnel that the aircraft is ready for flight.
Of the numerous techniques for complying with The Clean Aircraft Concept, the only method of ensuring flight safety in icing conditions is by inspecting critical aircraft surfaces and ensuring that they are clean before take-off regardless of the de-icing and anti-icing procedures used.
In all aviation operations, the pilot in command has the ultimate responsibility to determine if the aircraft is in a condition for safe flight.
Frozen Contaminants
Test data indicate that frost, ice or snow formations having a thickness and surface roughness similar to medium or coarse sandpaper, on the leading edge and upper surface of a wing, can reduce wing lift by as much as 30% and increase drag by 40%. Even small amounts of contamination such as this have caused and continue to cause aircraft accidents, which result in substantial damage and loss of life. A significant part of the loss of lift can be attributed to leading edge contamination. The changes in lift and drag significantly increase stall speed, reduce controllability and alter aircraft flight characteristics. Thicker or rougher frozen contaminants can have increasing effects on lift, drag, stall speed, stability and control.
More than 30 factors have been identified that can influence whether frost, ice or snow will accumulate, cause surface roughness on an aircraft and affect the anti-icing properties of freezing point depressant (FPD) fluids. These factors include ambient temperature; aircraft surface temperature; the de-icing and anti-icing fluid type, temperature and concentration; relative humidity; and wind speed and direction. Because many factors affect the accumulation of frozen contaminants on the aircraft surface, holdover times for FPD fluids should be considered as only guidelines unless the operator's program allows otherwise. Holdover time is the estimated time that an application of approved de-icing/anti-icing fluid is effective in preventing frost, ice or snow from adhering to treated surfaces. Holdover time is calculated as beginning at the start of the final application of an approved de-icing/anti-icing fluid, and as expiring when fluid is no longer effective. The fluid is no longer effective when its ability to absorb more precipitation has been exceeded. This produces a visible surface build-up of contamination.
The type of frost, ice or snow that can accumulate on an aircraft while on the ground is a key factor in determining the type of de-icing/anti-icing procedures that should be used.
Frost must be removed before take-off. Frost often collects around integral fuel cells, especially after a flight or following refuelling. Frost can be removed by placing the aircraft in a heated hangar or by other normal de-icing procedures. In some instances, 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.
Where conditions are such that ice or snow may reasonably be expected to adhere to the aircraft, the accumulation must be removed before take-off. Dry, powdery snow can be removed by blowing cold air or compressed nitrogen gas across the aircraft surface. In some circumstances, a shop broom could be employed to clean certain areas accessible from the ground. Heavy, wet snow or ice can be removed by placing the aircraft in a heated hangar, by using solutions of heated FPD fluids and water, by mechanical means such as brooms or squeegees, or a combination of all three methods. Should the aircraft be placed in a heated hangar, ensure it is completely dry when moved outside; otherwise, pooled water may re-freeze in critical areas or on critical surfaces.
The Cold-Soaking Phenomenon
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. 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. 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. A layer of slush on the wing can also hide a dangerous sheet of ice beneath.
The formation of ice 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 clear 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 12°C to 18°C have been recorded after a flight of two hours;
- - an abnormally 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.
Clear ice formations can also break loose at rotation or during flight causing engine damage on some aircraft types, primarily those with rear-mounted engines.
As well, 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 de-icing 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 re-form quickly even when removed.
In either case, ice or frost formations on upper or lower wing surfaces must be removed prior to take-off. 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. Skin temperature should be increased to preclude formation of ice prior to take-off. This is often possible by refuelling with warm fuel.
De-Icing and Anti-Icing Fluids
Frozen contaminants are most often removed in commercial operations by using FPD fluids. There are a number of FPD fluids available for use on commercial aircraft and, to a lesser extent, on general aviation aircraft. De-icing and anti-icing fluids should not be used unless approved by the aircraft manufacturer.
Although FPD fluids are highly soluble in water, they absorb or melt ice slowly. If frost, ice or snow is adhering to an aircraft surface, the accumulation can be melted by repeated application of proper quantities of FPD fluid. As the ice melts, the FPD mixes with the water, thereby diluting the FPD. As dilution occurs, the resulting mixture may begin to run off the aircraft. If all the ice is not melted, an additional application of FPD becomes necessary until the fluid penetrates to the aircraft surface. When all the ice has melted, the remaining liquid residue is a mixture of FPD and water at an unknown concentration. The resulting film could freeze (begin to crystallize) rapidly with only a slight temperature decrease. If the freezing point of the film is found to be insufficient, the de-icing procedure must be repeated until the freezing point of the remaining film is sufficient to ensure safe operation.
The de-icing process can be speeded up considerably by using the thermal energy of heated fluids and the physical energy of high-pressure spray equipment, as is the common practice.
Type I Fluids
These fluids in the concentrated form contain a minimum of 80% glycol and are considered "unthickened" because of their relatively low viscosity. These fluids are used for de-icing or anti-icing, but provide very limited anti-icing protection.
Type II Fluids
SAE Type II fluids will last longer in conditions of precipitation and afford greater margins of safety if they are used in accordance with aircraft manufacturers' recommendations.
Flight tests performed by manufacturers of transport category aircraft have shown that most SAE Type II fluid flows off lifting surfaces by rotation speeds (Vr), although some large aircraft do experience performance degradation and may require weight or other take-off compensation. Therefore, SAE Type II fluids should be used on aircraft with rotation speeds (Vr) above 100 knots. Degradation could be significant on aeroplanes with rotation speeds below 100 knots.
As with any de-icing or anti-icing fluid, SAE Type II fluids should not be applied unless the aircraft manufacturer has approved their use, regardless of rotation speed. Aircraft manufacturers' manuals may give further guidance on the acceptability of SAE Type II fluids for specific aircraft.
Some fluid residue may remain throughout the flight. The aircraft manufacturer should have determined that this residue will have little or no effect on aircraft performance or handling qualities in aerodynamically quiet areas. However, this residue should be cleaned periodically.
SAE Type II fluids contain no less than 50% glycol and have a minimum freeze point of -32°C. They are considered "thickened" because of added thickening agents that enable the fluid to be deposited in a thicker film and to remain on the aircraft surfaces until the time of take-off. These fluids are used for de-icing when heated, and anti-icing. Type II fluids provide greater protection (holdover time) against frost, ice or snow formation in conditions conducive to aircraft icing on the ground, than do Type I fluids.
These fluids are effective anti-icers because of their high viscosity and pseudo-plastic behaviour. They are designed to remain on the wings of an aircraft during ground operations or short-term storage, thereby providing some anti-icing protection, but to readily flow off the wings during take-off. When these fluids are subjected to shear stress, such as that experienced during a take-off run, their viscosity decreases drastically, allowing the fluids to flow off the wings and causing little adverse effect on the aircraft's aerodynamic performance.
The pseudo-plastic behaviour of SAE Type II fluids can be altered by improper de-icing/anti-icing equipment or handling. Therefore, some North American airlines have updated de-icing and anti-icing equipment, fluid storage facilities, de-icing and anti-icing procedures, quality control procedures and training programs to accommodate these distinct characteristics. Testing indicates that SAE Type II fluids, if applied with improper equipment, may lose 20% to 60% of their anti-icing performance.
SAE Type II fluids were introduced in North America ine 1985 with widespread use occurring in the 1990's. Similar fluids, but with slight differences in characteristics, have been developed, introduced, and used in Canada.
Type III Fluids
Type III is a thickened FPD fluid, which has properties that lie between Types I and II/IV fluids. Therefore, it provides a longer holdover time than a Type I fluid but less than Type I/IV fluid. Because of its flow off characteristics, it is suitable for aircraft that have a shorter ground roll to rotation. This should make it acceptable for some aircraft that have a Vr of less than 100 knots, typical of turboprop commuter aircraft.
Several years ago, a need was identified for a de-/anti-icing fluid that had longer holdover times than a Type I fluid, but a lower viscosity than a Type II or IV fluid, for use on aircraft with lower rotation speeds. The SAE had approved a specification in AMS1428A for Type III anti-icing fluids that can be used on those aircraft with rotation speeds of 100 knots or less. As of July 2004, one manufacturer produced a fluid, which met all the applicable requirements and qualified as a Type III fluid. Therefore, a new Type III generic fluid table was produced this year (2004) based on holdover times of this fluid.
Type IV Fluids
A significant advance is Type IV anti-icing fluid. These fluids meet the same fluid specifications as the Type II fluids and in addition, have a significantly longer holdover time. In recognition of the above, holdover timetables are available for Type IV.
The product is dyed green. It is believed that the green product will provide for application of a more consistent layer of fluid to the aircraft and will reduce the likelihood that fluid will be mistaken for ice. However, as these fluids do not flow as readily as conventional Type II fluid, caution should be exercised to ensure that enough fluid is used to give uniform coverage.
Research indicates that the effectiveness of Type IV fluid can be seriously diminished if proper procedures are not followed during application of Type I fluid.
All fluids users are advised to ensure that these fluids are applied evenly and thoroughly and that an adequate thickness has been applied in accordance with the manufacturer's recommendations. Particular attention should be paid to the leading edge area of the wing and horizontal stabilizer.
Further Information
Further information on aircraft critical surface contamination may be found in the training packages produced by Transport Canada Aviation "When In Doubt... Aircraft Critical Surface Contamination Training - Ground Crew, Small Aircraft and Large Aircraft" booklets and videocassettes. These videos and the accompanying booklets may be ordered from the Civil Aviation Communications Centre for a cost:
Telephone: 1-800-305-2059 or
(613) 993-7284 (in Ottawa)