MIL-STD-810G – Part 23 – (ICING/FREEZING RAIN) Method 521.3

MIL-STD-810G covers Ice and Freezing Rain in Method 521.3.  521.3 is relatively short at 7 pages and covers the effects of ice and freezing rain on the operational capability of material.  Also included is evaluating the effectiveness of de-icing equipment and techniques. 

Icing and freezing rain occurs when the air is saturated with super-cooled water droplets which forms rime ice and/or glaze ice.  Rime ice is a white or milky accumulation and is lighter, softer and less transparent than glaze ice.  Glaze ice is a generally clear hard coating of ice.  Rime forms when super-cooled water drops freeze on exposed surfaces.  Glaze ice forms from a film of super-cooled water vapor freezing as a layer.

Method 521.3 can be applied to any material exposed to freezing rain conditions including large items such as vehicles, aircraft, and ships.  This method generally does not apply to electronic equipment such as rackmount computer systems.

Some adverse effects of ice and freezing rain include:

a. Binds moving parts together.
b. Adds weight to radar antennas, aerodynamic control surfaces, helicopter rotors, etc.
c. Increases footing hazards for personnel.
d. Interferes with clearances between moving parts.
e. Induces structural failures.
f. Reduces airflow efficiency as in cooling systems or filters.
g. Impedes visibility through windshields and optical devices.
h. Affects transmission of electromagnetic radiation.
i. Provides a source of potential damage to materiel from the employment of mechanical, manual, or chemical ice removal measures.
j. Reduces efficiency of aerodynamic lifting and control surfaces.
k. Reduces (aircraft) stall margins.

The principle goal of the test is to have super-cooled liquid water droplets impinge on the test item and freeze in place.  A chamber of suitable size capable of maintaining a temperature of -10 deg C (14 deg F) with the ability to spray cooled water on the test item is required.  The test can be performed outdoors if the ambient conditions are prone to icing.  Test conditions can be adjusted such as temperature, droplet size, water temperature, etc., to achieve satisfactory icing conditions.

MIL-STD-810G – Part 23 – (ICING/FREEZING RAIN) Method 521.3

MIL-STD-810G – Part 22 – (Temperature, Humidity, Vibration, and Altitude) Method 520.3

MIL-STD-810G covers the synergistic effects of Temperature, Humidity, Vibration and Altitude in Method 520.3. Method 520.3 is comprised of 22 pages.

810G covers each of these environmental factors individually in the following Methods:

Method 500.5 – Altitude

Method 501.5 – High Temperature

Method 502.5 – Low Temperature

Method 507.5 – Humidity

Method 514.6 – Vibration

It should be noted the above Methods are specific to the environmental factor being tested. For example, samples might be vibrated but will be done so at a benign temperature in a climate controlled facility. Conversely, a temperature test is done in a chamber mounted to the floor with no external vibration imparted on the sample.

Materials can exhibit much different characteristics at different temperatures such as thermal expansion or contraction causing binding, the material becoming much more brittle at a low temperature or much more ‘loose’ at high temperatures. This method is particularly useful in evaluating aircraft which may start a flight cycle hot from sitting on the ramp in the Middle East, climb to altitude with an external temperature of -65 deg F at 35,000 feet with vibration and shock from the engine and turbulence. Another example could be the SR71 which has to fuel immediately after take-off, then speed up to heat the air frame to seal the tanks due to thermal expansion.

Rugged computer systems, especially in flight environments, can be highly susceptible to the combined effects of altitude (less effective cooling), temperature (thermal contraction or chip failure), vibration (damage to components) and humidity (shorting).

Method 520.3 specifies tailoring the process to determine where these combined forcing functions of temperature, humidity, vibration, and altitude are foreseen in the life cycle of the materiel in the real world.

Some example failures are:

a. Shattering of glass vials and optical materiel. (Temperature/Vibration/Altitude)

b. Binding or loosening of moving parts. (Temperature/Vibration)

c. Separation of constituents. (Temperature/Humidity/Vibration/Altitude)

d. Performance degradation in electronic components due to parameter shifts. (Temperature/Humidity)

e. Electronic optical (fogging) or mechanical failures due to rapid water or frost formation.

(Temperature/Humidity)

f. Cracking of solid pellets or grains in explosives. (Temperature/Humidity/Vibration)

g. Differential contraction or expansion of dissimilar materials. (Temperature/Altitude)

h. Deformation or fracture of components. (Temperature/Vibration/Altitude)

i. Cracking of surface coatings. (Temperature/Humidity/ Vibration/Altitude)

j. Leakage of sealed compartments. (Temperature/Vibration//Altitude)

k. Failure due to inadequate heat dissipation. (Temperature/Vibration /Altitude)

The Method provides for three Procedures:

I. Engineering Development

Used to help find defects in a new design in the development state. The procedure is accelerated and failure-oriented to uncover design defects.

II. Flight or Operational Support

Used in preparation for, during and after flight or operational testing. Not accelerated and the test item can be moved from the test vehicle to the lab interchangeably to help determine failures or deficiencies.

III. Qualification

Used to demonstrate compliance with all contract requirements. Usually an accelerated test that emphasizes the most significant environmental stress conditions.

Follows is an example test profile combining all the environmental factors into one test cycle.

MIL-STD-810G – Part 22 – (Temperature, Humidity, Vibration, and Altitude) Method 520.3