2.1 Selecting the Icing/Freezing Rain Method
After examining requirements documents and applying the tailoring process in Part One of this Standard to determine where icing/freezing rain is anticipated in the life cycle of materiel, use the following to confirm the need for this Method and to place it in sequence with other methods. This Method is designed to determine if materiel can operate after ice accumulation from rain, drizzle, fog, splash or other sources. When ice removal is required before operation, use integral deicing equipment or expedients normally available to the user in the field. Evaluate deicing equipment and expedients to assess their effectiveness and the potential for damage that may degrade performance.
2.1.1 Effects of Icing/Freezing Rain
Ice formation can impede materiel operation and survival and affect the safety of operating personnel. Consider the following typical problems to help determine if this Method is appropriate for the materiel being tested. This list is not intended to be all-inclusive.
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a. Binds moving parts together.
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b. Adds weight to radar antennas, aerodynamic control surfaces, helicopter rotors, etc.
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c. Increases footing hazards for personnel.
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d. Interferes with clearances between moving parts.
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e. Induces structural failures.
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f. Reduces airflow efficiency as in cooling systems or filters.
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g. Impedes visibility through windshields and optical devices.
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h. Affects transmission of electromagnetic radiation.
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i. Provides a source of potential damage to materiel from the employment of mechanical, manual, or chemical ice removal measures.
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j. Reduces efficiency of aerodynamic lifting and control surfaces.
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k. Reduces (aircraft) stall margins.
2.1.2 Sequence Among Other Methods
- a. General. Use the anticipated life cycle sequence of events as a general sequence guide (see Part One, paragraph 5.5).
- b. Unique to this Method.
- (1) One approach is to conserve test item life by applying what are perceived to be the least damaging environments first. For this approach, generally apply the icing/freezing rain following rain tests, but prior to salt fog tests, because residual salts could impair the formation of ice. Also, apply this test prior to dynamic tests that could loosen components.
- (2) Another approach is to apply environments to maximize the likelihood of disclosing synergetic effects. For this approach, subject the test item to the dynamic tests prior to conducting the icing test.
2.2 Selecting Procedure Variations
This Method has one procedure. However, the test procedure may be varied. Before conducting this test, complete the tailoring process by selecting specific procedure variations (special test conditions/techniques for this procedure) based on requirements documents, Life Cycle Environmental Profile (LCEP), and information provided within this Method. Consider the following in light of the operational purpose and life cycle of the materiel.
2.2.1 Ice Formation
2.2.1.1 Principal Causes
A buildup of ice occurs in four principal ways:
- a. From rain, drizzle, or fog falling on materiel whose temperature is at or below freezing.
- b. From sublimation.
- c. From freezing rain or freezing drizzle falling on materiel at or near freezing.
- d. From sea spray and splash that coats materiel when the materiel temperature is below freezing.
2.2.1.2 Types of Ice
Two types of ice are commonly encountered: rime ice (opaque/granular) and glaze ice (clear/smooth). Published extremes for ice accretion may be used for calculating design and structural evaluations but are not considered practical for establishing test conditions due to the large thicknesses involved unless the test is intended to provide practical confirmation of design calculations.
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a. Rime Ice: A white or milky and opaque granular deposit of ice formed by a rapid freezing of supercooled water drops as they impinge upon an exposed object. Rime ice is lighter, softer, and less transparent than glaze. Rime is composed essentially of discrete ice granules and has densities ranging from 0.2 g/cm3 (soft rime) to almost 0.5 g/cm3 (hard rime). Factors that favor rime formation are small drop size, slow accretion, a high degree of supercooling, and rapid dissipation of latent heat of fusion. The opposite effects favor glaze formation.
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(1) Hard Rime: Opaque, granular masses of rime deposited chiefly on vertical surfaces by dense, supercooled fog. Hard rime is more compact and amorphous than soft rime, and builds out into the wind as glazed cones or feathers. The icing of ships and shoreline structures by supercooled spray from the sea usually has the characteristics of hard rime.
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(2) Soft Rime: A white, opaque coating of fine rime deposited chiefly on vertical surfaces, especially on points and edges of objects, generally in supercooled fog. On the windward side, soft rime may grow to very thick layers, long feathery cones, or needles pointing into the wind and having a structure similar to that of frost.
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b. Glaze Ice: A coating of ice, generally clear and smooth but usually containing some air pockets, formed on exposed objects by the freezing of a film of supercooled water vapor. Glaze is denser, harder, and more transparent than rime. Its density may range from 0.6 to 0.9 g/cm3. Factors that favor glaze formation are large drop size, rapid accretion, slight supercooling, and slow dissipation of heat of fusion. The opposite effects favor rime formation. Glaze occurs when rain or drizzle freezes on objects, and is clear and nearly as dense as pure ice. Since glaze ice is more difficult to remove, it is structurally a more significant factor.
2.2.2 Configuration and Orientation
Consider the following factors:
- a. Whether or not the test item receives icing on all sides and the top.
- b. Whether or not the test item is in its deployment configuration. If required, perform tests in other configurations such as for shipping or outside storage.
2.2.3 Test Temperature
Test temperatures that may be used to produce the required environmental conditions are recommended in the test procedure. The recommended temperatures of the chamber and water may have to be adjusted for different size facilities to prevent premature freezing of the water droplets before they come in contact with the test item. However, do not use an initial test item temperature below 0 °C (32 °F) to allow water to penetrate (cracks, seams, etc.) prior to freezing.
2.2.4 Water Delivery Rate
The objective is to produce a clear, uniform coating of glaze ice. Any delivery rate that produces a uniform coating of glaze ice is acceptable. A water delivery rate of 25 mm/h (1 in/h) is suggested in the test procedure and is based on data from previous testing.
2.2.5 Water Delivery Method
Any of the following water delivery systems can be used as long as the water is delivered as a uniform spray:
- a. Nozzle arrays directing spray to the top, sides, front, and rear of the test item.
- b. Nozzle arrays that direct spray straight down onto the test item. Side-spray coverage is achieved by using wind or an additional hand-held nozzle. Minimize any wind in order to maintain uniform ice accretion.
- c. A single nozzle directing the spray over the appropriate surfaces of the test item.
2.2.6 Droplet Size
The droplet size range may have to be adjusted for different size facilities. A fine spray in the range of 1.0 mm to 1.5 mm diameter nominal droplet size has produced satisfactory icing in some facilities.
2.2.7 Ice Thickness
Unless specifically measured data for the anticipated situation are available, the following ice thicknesses are recommended (paragraph 6.1, reference b):
- a. 6 mm (0.24 in) - represents general conditions, light loading.
- b. 13 mm (0.5 in) - represents general conditions, medium loading.
- c. 37 mm (1.5 in) - represents heavy ground loading and marine mast loading.
- d. 75 mm (3 in) - represents extremely heavy ground loading and marine deck loading.
2.3 Operational Considerations
- a. Some materiel covered with ice may be expected to operate immediately without first undergoing de-icing procedures; other materiel would not be expected to operate until some form of de-icing has taken place (e.g., aircraft ailerons (flaps) prior to flight).
- b. Ice removal, if required, may include built-in ice-removal systems, prescribed means that could be expected to be employed in the field, or a combination of these.
c. The correct operation of anti-ice systems such as pre-heated surfaces.