The "MIL-DTL-27422 Military specification: Tank, fuel, impact resistant, ballistic tolerant, aircraft" standard developed by the United States Department of Defense includes crash resistant, ballistic tolerant fuel cells for use in fixed wing and rotary wing aircraft (including tilt rotor). Requirements and verification tests are explained. The exact design criteria, such as fuel cell dimensions, total weight, and interface requirements, depend on the intended aircraft specified in the contract or purchase order.
Briefly, this specification covers crashworthy fuel tanks for use in aircraft.
It has been deemed appropriate for crash-resistant fuel tanks to be of the following types and classes:
This standard establishes provisions regarding pre-production inspection.
The requirements of this standard apply only to fuel tanks mounted inside aircraft. A tank consists of an interconnected cell or group of cells and its attached components.
Specifications and standards for required goods and services not specified herein are selected in accordance with the “MIL-STD-143B Military standard: Standards and specifications, order of priority for selection” standard.
The materials and processes used by the tank manufacturer must be suitable for the purpose and in accordance with the applicable official specifications. Materials conforming to the contractor's specifications may be used, provided the specifications contain provisions for adequate testing. Use of the Contractor's specifications does not constitute a waiver of legal control. The use of magnesium is prohibited. Cadmium coated parts are not used where the parts are normally exposed to fuel.
Tanks must be suitable for use with fuels that comply with MIL-G-5572 and MIL-T-5624 standards. It is designed together with the housing structure to prevent loads from concentrating on joints, seams, connection parts and concentrated bending. The assembly, consisting of the tank and aircraft structure in which it is mounted, must have the necessary strength to adequately absorb the stresses caused by:
Class A tanks, all external dimensions of the cell (including connection points) shall not be less than the corresponding dimensions of the cell space. The amount of oversize between ports and between any port and the nearest edge must not exceed 6,35 mm. Mounting alignment must be done manually, without the use of force tools. There should be no signs of stress wrinkles after installation.
The dimensions of class B tanks must comply with the requirements specified in the procurement activity.
For type I tanks the inner layer layer and barrier, if used, shall sufficiently restrict the diffusion of the fuel to prevent activation of the sealant and for both type I and type II tanks the diffusion rate shall be no more than 24 gram per square meter per 0,70 hours.
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