Fuel System Drop Test — Alternative Fluid

CS 27/29.952(a) requires to drop test each tank, or the most critical tank, as following:

Unless other means acceptable to the Agency are employed to minimise the hazard of fuel fires to occupants following an otherwise survivable impact (crash landing), the fuel systems must incorporate the design features of this paragraph. These systems must be shown to be capable of sustaining the static and dynamic deceleration loads of this paragraph, considered as ultimate loads acting alone, measured at the system component’s centre of gravity without structural damage to system components, fuel tanks, or their attachments that would leak fuel to an ignition source.

1. Drop test requirements. Each tank, or the most critical tank, must be drop-tested as follows:
   1. The drop height must be at least 15.2 m (50 ft).
   2. The drop impact surface must be non-deforming.
   3. The tank must be filled with water to 80% of the normal, full capacity.
   4. The tank must be enclosed in a surrounding structure representative of the installation unless it can be established that the surrounding structure is free of projections or other design features likely to contribute to rupture of the tank.
   5. The tank must drop freely and impact in a horizontal position ±10°.
   6. After the drop test there must be no leakage.

For some specific fuel system designs, filling the fuel system as required by CS 27/29.952 (a)(3) to 80% with water for each bladder tank would require a blockage of connector pipes between the bladder tanks.

It is proposed that instead of filling each tank of the system to 80% with water to alternatively allow filling the tank system with an alternative drop test fluid of equivalent mass and relevant other characteristics.

The alternative drop test fluid shall be similar to the approved fuel type in terms of density and viscosity, thus representative for the actual operation of the helicopter and for the fuel system crash test. 

The alternative drop test fluid shall be as safe as the normally required water in terms of handling characteristics and environmental considerations. The test article setup shall avoid local reinforcements like e.g. plugs in any connecting pipes that could stiffen the fuel system.

Considering all the above, the following Equivalent Safety Finding is proposed.