A07_Restraint_-_Section_5.3.3_of_MIL-STD-1791C.PDF

MIL-STD-1791C

w/Change 1

5.3.3 Restraint requirements.

Cargo items, in their shipping configuration(s), shall be capable of being restrained under the conditions shown in Table IV and Table V. After an encounter with such conditions the cargo item shall maintain its unitary integrity, not pose a hazard to the aircraft or aircrew, nor prevent egress or rescue from a downed aircraft. Cargo items exclusively loaded on HCU-6/E pallets and restrained by MIL-N-27444 nets do not have to meet the requirements of 5.3.3.1, 5.3.3.2, or 5.3.3.3.

NOTE: It is recommend that supplemental air transport restraint provisions be added to sides of the load that have mission requirements to be airdropped, be transported in multiple units, or be transported in commercial transport aircraft. The supplemental provisions are to provide total restraint when the primary front and rear provisions that are called for in MIL-STD-209 cannot be fully utilized. For airdrop, having tiedown provisions on the side allows easier access to the airdrop platform tiedown rings. For air transport, the packing multiple units into the aircraft reduces access to the forward and rear cargo floor tiedown provisions, whereas the floor provisions on the sides are available. For airlift in commercial aircraft, the load needs more tiedown provisions because the rated strength of the aircraft floor tiedowns are lower than the ones on the military aircraft. The supplemental provisions are most useful to loads weighing over 10,000 pounds.

5.3.3.1 Restraint conditions.

The forces are considered to be applied independently. All directions are relative to the aircraft.

If the item can be loaded facing either direction then the forward restraint requirement applies to restraining the item against movement toward the front of the aircraft, regardless of which way the cargo item is loaded/facing. Similarly, the aft restraint requirement means restraining an item against movement toward the aft end of the aircraft, regardless of which way the item is loaded/facing.

TABLE IV. Restraint load factors.

Direction G-Load Up1/ & 4/ 2.0G

3.7G (nuclear cargo) Down1/ & 2/ 4.5G Forward3/ 3.0G

2.0G (with forward cargo barrier on KC-135, KC-10) 1.5G (with forward barrier on CRAF airplanes) 9G (see conditions specified in A.4.1.4 Restraint levels when cargo is behind passengers)

Aft3/ 1.5G

Lateral3/ 1.5G

1/ These are limit loads. Ultimate require at least a 1.5 factor of safety times limit load (e.g. 2G up × 1.5 = 3G Up Ultimate). Item function should be maintained up to limit load.

2/ Vehicles and other equipment shall be capable of withstanding the downward load factor without damage to their wheels, suspension systems, or support (see A.4.1.8).

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3/ These are ultimate loads. The item need not remain functional but shall maintain unitary integrity; tiedown provisions may yield but not break.

4/ The C-130J and J-30 require 3G Up restraint, depending on weight and location (see Figure B-101 for details).

The force shall be held a minimum of 6 seconds (see MIL-STD-209).

Alternatively, cargo items may be tested to withstanding the following changes in velocity (∆ V) within 0.1 second. Onset/decay rates are described in the guidance in Appendix A, section A.4.1.8. The final velocity must be held long enough for an adequate cargo response to the input.

TABLE V. Restraint velocity changes.

Direction ∆ V Up1/ 10.0 ft./sec

(15.2 ft./sec for nuclear cargo)

Down1/ & 2/ 11.5 ft./sec

Forward3/ 10.0 ft./sec

7.5 ft./sec (with forward cargo barrier on KC-135, KC-10)

5.0 ft./sec (with forward barrier on CRAF aircraft)

30.0 ft./sec (if cargo is to be carried aft of personnel on any aircraft)

Aft3/ 5.0 ft./sec

Lateral3/ 5.0 ft./sec

1/ These are limit loads and require at least a 1.5 factor of safety be applied without reaching ultimate strength. Item function should be maintained.

2/ Vehicles and other equipment shall be capable of withstanding the downward load factor without damage to their wheels, suspension systems, or support (see A.4.1.8).

3/ These are ultimate loads. The item need not remain functional but must maintain unitary integrity; tiedown provisions may yield but not break.

Internally carried equipment is not required to meet the load factors in Tables IV and V. However, the primary cargo item is then additionally required to contain all loose objects produced by the loads to prevent them from becoming a hazard in the cargo compartment.

5.3.3.2 Tiedown provisions.

Equipment shall be provided with no less than four tiedown provisions or locations to adequately restrain the equipment subjected to the accelerations specified in restraint criteria. Provisions may consist of tiedown rings, structural cutouts, axles, frame members, or special equipment.

Note that axles are allowed to provide up to half of the total required restraint in any direction up to the axle rating (e.g., total restraint from all axles can only provide a maximum 1.5G forward restraint if the axles have excess capacity). If the item has a suspension, then the axles provide no vertical restraint. This prevents the item from separating from its axle. Tiedown provisions shall accommodate both ends of MIL-DTL-25959 and MIL-PRF-27260 tiedown devices and shall be marked in accordance with marking requirements (see 5.3.4). The tiedown provisions shall be capable of accepting the maximum number of tiedown devices as required by the tiedown pattern. These tiedown provisions shall be suitable for use in conjunction with the tiedown provisions on the aircraft floor. Aircraft floor tiedowns have a capacity of 10,000 pounds or 25,000 pounds, depending on the specific aircraft (see Appendix B for specific aircraft tiedown pattern).

NOTE: Supplemental restraints are restraint provisions in addition to the front and rear primary tiedown provisions. The idea is to provide a substitute for the primary tiedowns, when space is limited in the aircraft, when aircraft tiedowns are not available to the front and rear primary tiedowns, or where other cargo could prevent using the primary tiedowns. Supplemental tiedowns are added to the sides of the item. If the item has an airdrop requirement, supplemental tiedown provisions allow easier access to the airdrop platform tiedown rings. It is recommended that equipment weighing over 10,000 lbs. be equipped with supplemental tiedowns.

Ultimate strength for each tiedown provision shall be at least 1.5 times to limit load.

It is recommended that tiedown rings provide the same strength at all angles (see Figure 2 for illustration). Any specific directional strength limits for tiedowns must be identified to prevent damage because it is nominally assumed that a tiedown has the same strength in all directions (see Figure 3 for example tiedown provisions).

The provisions shall be located symmetrically about the equipment longitudinal centerline.

Provisions shall be located to provide restraint in both the fore and aft direction of the equipment.

The area of action for each provision is illustrated on Figure 2. Longitudinal spacing between provisions shall be no less than 20 inches for 10,000 pound provisions, and 40 inches for 20,000 pound or stronger provisions. If a provision’s location is suitable for use in only one direction, an additional provision must be provided for use in the opposite direction. Restraint devices should not contact any other part of the equipment. When a restraint device must contact a part of the equipment, testing must demonstrate that the affected part(s) has sufficient strength to withstand the force exerted to prevent permanent deformation of any part of the equipment, and that contact will not adversely affect the material of the tiedown device.

Provisions shall be located on the chassis of wheeled vehicles or the hull of tracked vehicles.

Provisions shall be no higher than 6 feet on the equipment and no less than 2 feet above ground level. Provision placement should take into consideration vertical center of gravity of the equipment to prevent tipping.

Welded-on tiedown provisions not installed in accordance with applicable American Welding Society specifications, shall be derated by a factor of two due to difficulty in verifying quality of the weld for the life of the item. This may be alleviated by performing Non-Destructive Evaluation to show that the weld is of adequate quality to maintain the rated capacity.

FIGURE 2. Tiedown rings.

Other Tiedown Provisions Preferred Tiedown Provisions:

Example Configurations

FIGURE 3. Example tiedown provisions.

5.3.3.3 Cargo without tiedown provisions.

Items without tie-down provisions shall be capable of being restrained against and withstanding the forces imposed by aircraft flight and maneuvering operations as shown and stated in 5.3.3.1 for restraining loads. The requestor shall identify structural hard points where straps/chains may be wrapped around the item and pulled tight. The hard points shall be marked as tiedown points and marked with rated capacities (see 5.3.4.1).

5.3.3.4 Structure.

The item shall remain in one piece (unitary integrity) and be capable of being unloaded in the same manner in which it was loaded (maintain loadability), unless otherwise detailed in the loading plan (e.g., a palletized item shall not crush or penetrate the pallet and render it immobile, thus requiring removal via forklift. A wheeled or tracked vehicle shall not crush its own suspension and render itself immobile. A self-propelled vehicle shall remain capable of driving off, if it was driven on, etc.).

Military rolling stock gross weight shall not exceed the vehicle’s tactical (cross-country) rating. If the tactical rating is exceeded, test or complete analysis shall be conducted to verify that the item can withstand the load factors and velocity changes.

Commercial rolling stock weight shall not exceed 80 percent of the manufacturer’s gross vehicle weight rating (GVWR). If 80 percent of the GVWR is exceeded, test or complete analysis is required to verify that the item can withstand the load factors and velocity changes.

5.3.3.5 Verification methods.

Compliance with the restraint requirements shall be assured when it can be demonstrated by engineering analysis or actual test that the item in its shipping configuration and restrained in a manner representative of how it would be restrained for flight can be subjected to the stated static and dynamic loads without loss of structural or unitary integrity of the item and without incurring damage to the aircraft.

Loss of structural or unitary integrity during analysis or test shall be cause for rejection of the load for airlift. Loss of loadability during verification of the vertical up or vertical down restraint criteria requirement shall be sufficient to cause rejection of the load for airlift.

For palletized items in which the item is restrained to the pallet, the item’s center of gravity location shall be verified by analysis or test.

If a vehicle does not have a cross-country weight rating and its weight or axle/suspension loading exceeds 80 percent of its highway gross vehicle rating, test or complete analysis is required to verify that the item can withstand the downward load factor. The vehicle suspension can also be demonstrated using the U.S. Army cross-country test at Aberdeen Proving Ground, MD (see A.4.1.8).

The requirements of 5.3.3.2 shall be met when it can be shown by engineering analysis or actual test that the proposed tiedown provisions are adequate in strength, location, size, and number to accept the required MIL-DTL-25959 and MIL-PRF-27260 tiedown devices and provide the restraint required in 5.3.3.1. Cargo without tiedown provisions must be similarly verified using proposed tiedown locations instead of provisions.

5.3.4 Markings.

Equipment shall be marked in accordance with the provisions of MIL-STD-129 and MIL-STD-209, as appropriate, to provide the information necessary to facilitate loading and restraining the item in the aircraft. Unless otherwise specified, the marking shall be stenciled in an appropriate location and/or provided on the vehicle data plate in accordance with MIL-STD-130. Markings shall include at least those defined below: