2.1 Selecting the Low Pressure (Altitude) Method
After examining the requirements documents, and applying the tailoring process in Part One of this Standard to determine where low pressure is foreseen in the life cycle of the materiel, use the following to aid in selecting this Method and placing it in sequence with other methods. Based upon the LCEP, there may be a requirement to conduct this Method in combination with other Methods within this standard (i.e., high temperature, low temperature, or vibration).
2.1.1 Effects of Low Pressure Environments
In addition to thermal effects (see Methods 501.7 and 502.7), 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 and some of the examples may overlap the categories.
2.1.1.1 Physical/Chemical
- a. Leakage of gases or fluids from gasket-sealed enclosures.
- b. Deformation, rupture or explosion of sealed containers.
- c. Change in physical and chemical properties of low-density materials.
MIL-STD-810H
METHOD 500.6
- d. Overheating of materiel due to reduced heat transfer.
- e. Evaporation of lubricants.
- f. Erratic starting and operation of engines.
- g. Failure of hermetic seals.
2.1.1.2 Electrical
Erratic operation or malfunction of materiel resulting from arcing or corona.
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. Normally, this Method is performed early in a test sequence because of both its limited damage potential, and its generally early occurrence in the life cycle. However, other testing may contribute significantly to the effects of low pressure on the test item (see paragraph 2.1.1), and may have to be conducted before this Method. For example:
- (1) Low temperature and high temperature testing may affect seals.
- (2) Dynamic tests may affect the structural integrity of the test item.
- (3) Aging of non-metallic components may reduce their strength.
2.2 Selecting Procedures
This Method includes four low pressure tests: Procedure I (Storage); Procedure II (Operation); Procedure III (Rapid Decompression), and Procedure IV (Explosive Decompression). Based on the test data requirements, determine which of the test procedures or combination of procedures is applicable.
NOTE: For Procedure II, Method 520.5 may be used in addition to this Method when considering the potential synergistic and/or flight safety effects. However, Method 520 is NOT a substitute for Method 500.
2.2.1 Procedure selection considerations
Differences among the low pressure test procedures are explained below. Select the procedure that represents the most severe exposure anticipated. When selecting a procedure, consider:
- a. The materiel configuration.
- b. The logistical and operational requirements (purpose) of the materiel.
- c. The operational purpose of the materiel.
- d. The test data required to determine if the operational purpose of the materiel has been met.
- e. Procedure sequence.
- f. Whether the cargo compartment is pressurized.
2.2.2 Difference among procedures
- a. Procedure I - Storage/Air Transport. Procedure I is appropriate if the materiel is to be transported or stored at high ground elevations or transported by air in its shipping/storage configuration. Evaluate the materiel with respect to known effects of low pressure (paragraph 2.1.1) and the LCEP (Part One, paragraph 4.2.2.3.1) to determine if this procedure is appropriate.
- b. Procedure II - Operation/Air Carriage. Use Procedure II to determine the performance of the materiel under low pressure conditions. It may be preceded by Procedure I. If there are no low pressure storage, rapid, or explosive decompression requirements, this procedure can stand alone.
MIL-STD-810H
METHOD 500.6
- c. Procedure III - Rapid Decompression. Use Procedure III to determine if a rapid decrease in pressure of the surrounding environment will cause a materiel reaction that would endanger nearby personnel or the platform (ground vehicle or aircraft) in which it is being transported. This procedure may be preceded by Procedure I and/or Procedure II.
- d. Procedure IV - Explosive Decompression. (See paragraph 1.3.) Procedure IV is similar to Procedure III except that it involves an "instantaneous" decrease in the pressure of the surrounding environment. NOTE: This procedure is more appropriate for items such as sealed cockpit equipment whose failure could endanger cockpit personnel. Since one purpose of this test is to ensure failure of the materiel does not endanger personnel, and a catastrophic failure severe enough to cause an explosive decompression of the cargo compartment would, most likely, bring down the aircraft, carefully consider the appropriateness of application of this procedure for large cargo items. This procedure may be preceded by Procedure I and/or Procedure II.
NOTE: After either decompression test, a potential safety problem could exist that is not obvious. Exercise caution during the post-test operational check.
2.3 Determine Test Levels and Conditions
Having selected this Method and relevant procedures (based on the materiel's requirements documents and the tailoring process), it is necessary to complete the tailoring process by selecting specific parameter levels and special test conditions/techniques for these procedures based on requirements documents and Life Cycle Environmental Profile (LCEP), (see Part One, Figure 1-1), and information provided with this procedure. From these sources of information, determine the functions to be performed by the materiel in low pressure environments or following storage in low pressure environments. Determine the test parameters such as test pressure and temperature, rate of change of pressure (and temperature if appropriate), duration of exposure, and test item configuration.
2.3.1 Test Pressure (Altitude) and Temperature
Base determination of the specific test pressures (altitude) and temperatures on the anticipated deployment or flight profile of the test item. See Method 520.5, Table 520.5-2, for pressure versus altitude conversion equations.
- a. Ground areas. If measured data are not available, temperatures may be obtained for appropriate ground elevations and geographical locations from STANAG 4370, AECTP 230 (paragraph 6.1, reference b). The highest elevation currently contemplated for ground military operations (materiel operating and non-operating) is 4,572 m (15,000 ft), with an equivalent air pressure of 57.2 kPa (8.3 psia) (see paragraph 6.1, reference c).
- b. Transport aircraft cargo compartment pressure conditions. The test pressure used for each of the four procedures in this Method will vary greatly for each test item. Compartments normally pressurized may not be in certain situations. There are many different types of cargo transport aircraft on which materiel could be transported, and many different types of pressurization systems. Most pressurization systems provide outside atmospheric pressure in the cargo compartment (no pressure differential between the inside and outside of the aircraft) up to a particular altitude, and then maintain a specific pressure above that altitude. The pressure inside the cargo department is known as “cabin altitude.” Subject the test item to the most likely anticipated conditions. Unless the materiel has been designed for transport on a particular aircraft with unique cabin altitude requirements, use the following guidance:
- (1) For Procedures I and II, unless otherwise identified, use 4,572 m (15,000 ft) for the cabin altitude (corresponding pressure in a standard atmosphere: 57.2 kPa or 8.3 psia).
- (2) For Procedures III and IV, use 2,438m (8,000 ft) for the initial cabin altitude (75.2 kPa or 10.9 psia), and 12,192 m (40,000 ft) for the final cabin altitude after decompression (18.8 kPa or 2.73 psia).
NOTE: Cargo aircraft may transport cargo in either pressurized or un-pressurized conditions for various reasons including fuel economy
MIL-STD-810H
METHOD 500.6
- c. Transport aircraft cargo compartment temperature conditions. The range of temperatures associated with the various low pressure situations varies widely, primarily depending on the capabilities of the environmental control system within the cargo compartment of the various aircraft. Obtain the test temperatures from measured data or from appropriate national sources.
- d. Transport aircraft cargo compartment humidity conditions. The humidity exposure associated with the various low pressure situations will also vary widely. If humidity has been identified as an environment of concern in the LCEP, humidity levels should come from measured data or from appropriate national sources.
2.3.2 Altitude Change Rate
If a specific rate of altitude change (climb/descent rate) is not known or specified in the requirements document, the following guidance is offered: In general, and with the exception of the explosive decompression test, do not use a rate of altitude change that exceeds 10 m/s (32.8 ft/sec.) unless justified by the anticipated deployment platform. In a full military power takeoff, military transport aircraft normally have an average altitude change rate of 7.6 m/s (25 ft/sec.). Use the value of 10 m/s (32.8 ft/sec.) for ground deployment tests (for standardization purposes) unless otherwise specified.
2.3.3 Decompression Rate
There are several conditions for which the rapid rate of decompression may vary. These include:
- a. Sufficient damage to the aircraft cockpit or other critical small compartments causing virtually instantaneous decompression (explosive decompression -- to be accomplished in 0.1 second or less). This procedure is not intended to be used for materiel transported in the cargo bay.
- b. Relatively minor damage caused by foreign objects through which decompression could occur at a slower rate than above (rapid decompression -- not more than 15 seconds).
2.3.4 Test Duration
For Procedure I, use a test duration representative of the anticipated service environment but, if this is extensive, use a test duration of at least one hour that has historically been considered adequate for most materiel. Once the test pressure has been reached and any required functions performed, Procedures II, III, and IV do not require extended periods at the test pressure. In some cases, there may be a need to tailor Procedure II to account for test item stabilization (see Part One, paragraph 5.4.1).
2.3.5 Test Item Configuration
Determine the test item configuration based on the realistic configuration(s) of the materiel as anticipated for transportation, storage, or operation. As a minimum, consider the following configurations:
- a. In a shipping/storage container or transit case.
- b. In its normal operating configuration (realistic or with restraints, such as with openings that are normally covered).
2.3.6 Humidity
Although various levels of humidity commonly exist in the natural environment, there is no requirement to include it in this Method because of the complexities involved in controlling combinations of temperature, air pressure, and relative humidity. However, this Method may be tailored to accommodate temperature and humidity if so identified in the LCEP as a non-operational environment of concern. Method 520.5 does include this combination for an operational environment and which requires the development of a tailored test profile. MIL-HDBK-310 (paragraph 6.1, reference a) includes data on humidity at altitude.