MOC VTOL.2325(a)(4) Fire Protection - Energy storage crash resistance

1.      Introduction and scope

VTOL.2325 (a)(4) requires that the energy storage system and its installation in the aircraft are designed to minimise the risk of post-crash fires in survivable emergency landings. The ultimate goal is to provide occupants with sufficient time to evacuate or be extracted from the aircraft following such events.

The similarity of VTOL capable aircraft and small rotorcraft justifies the consideration of the design and test criteria as being comparable and therefore applicable. These criteria, mainly contained in CS 27.952 Amdt. 6 and CS 27.561 Amdt. 6, have proven to be successful in a large number of accidents in preventing or delaying the onset of post-crash fires, thus maximising the occupant escape time after survivable emergency landings.

The main concern in small rotorcraft are crash-induced fuel leaks that quickly come in contact with ignition sources during or after impact. It is recognised that there are many possible energy sources in VTOL capable aircraft (fuel, electricity, gas) that require the need to consider other forms of fire initiation. However, they do not invalidate the defined emergency landing conditions for which the design needs to show its capability to minimise the risk of fire initiation.

The following accepted means of compliance with VTOL.2325(a)(4) therefore builds on the design and test criteria contained in CS 27.952 Amdt. 6 and CS 27.561 Amdt. 6, complementing or adapting them, whenever necessary to account for different energy sources.

In addition, this MOC also constitutes an accepted means of compliance with VTOL.2430(a)(6) regarding the energy retention capability of the energy storage and distribution system during a survivable emergency landing on land. Specific considerations for the demonstration of compliance with VTOL.2430(a)(6) of VTOL capable aircraft intended to be used for operations on water, emergency flotation or ditching as per VTOL.2310 or over water are provided in MOC VTOL.2430(a)(6).

2.      Energy Storage crash resistance

(a)      Unless other means that are acceptable to the Agency are employed to minimise the hazard to occupants caused by energy storage systems following an otherwise survivable impact (crash landing), the energy storage system should incorporate the design features of this MOC

(b)     These systems should be shown to be capable of sustaining the static and dynamic deceleration loads of this MOC, considered as ultimate loads acting alone, measured at the system component’s centre of gravity without structural damage to the energy storage system or their attachments that could cause any fire other than the contained battery fire allowed in point 3.(f)(2)(ii) of this MOC.

(c)      In addition, no harmful amounts of liquids or toxic fumes or gases should enter an occupied area or the evacuation path.

3.      Drop test requirements

Each energy storage system, or the most critical energy storage system, should be subject to a drop-test using the following methodology:

(a)      the drop height should be at least 15.2 m (50 ft);

(b)     the drop impact surface should be non-deforming;

(c)      the energy storage system should be charged or filled to its most critical condition expected during a crash;

(d)     the energy storage system should 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 energy storage system;

(e)     the energy storage system should drop freely in an orientation that is representative of a typical installation on the aircraft and impact in a horizontal position ±10°with regards to the horizontal axis of the VTOL; and

(f)      after the drop test there should be no risk of post-crash fire or other harmful release within a time frame compatible with the rescue of seriously injured occupants.

(1)     For liquid or gaseous fuels: no leakage of flammable fluids or gases.

(2)     For batteries:

(i)      structural damage should not lead to a fire, leakage of harmful fluids, fumes or gases; or

(ii)     any fire or leakage of harmful fluids, fumes or gases should be contained for at least 15 minutes in non-occupied areas and outside the evacuation path.

(3)     Any projectile release should not lead to serious injury to occupants or persons on ground.

4.      Energy storage system load factors

(a)      Except for energy storage systems located so that structural damage to the energy storage that could cause fire, leakage of harmful or flammable fluids or gases, or toxic fumes in occupied areas or the evacuation path is extremely remote, each energy storage system should be designed and installed to retain its contents under the following ultimate inertial load factors, acting alone.

(b)     For energy storage systems in the cabin:

(1)     Upward – 4 g.

(2)     Forward – 16 g. (18 g for CTOL)

(3)     Sideward – 8 g.

(4)     Downward – 20 g.

(5)     Rearward – 1.5 g.

(c)      For energy storage systems located above or adjacent the crew or passenger compartment that, if loosened, could injure an occupant in an emergency landing:

(1)     Upward – 1.5 g.

(2)     Forward – 12 g.

(3)     Sideward – 6 g.

(4)     Downward – 12 g.

(5)     Rearward – 1.5 g.

(d)     For energy storage systems in other areas:

(1)     Upward – 1.5 g.

(2)     Forward – 4 g.

(3)     Sideward – 2 g.

(4)     Downward – 4 g.

5.      Energy storage system isolation means

(a)      For liquid or gaseous fuel systems, self-sealing isolation means should be installed unless hazardous relative motion of energy storage system components to each other or to local aircraft structure is demonstrated to be extremely improbable or unless other means are provided.

(b)     The isolations means, such as a fuses, couplings or equivalent devices should be installed where structural deformation could lead to a hazard to the occupants due to high energy release or release of harmful amount of fluids or gases.

(c)      For liquid or gaseous fuel systems, the design and construction of the isolation means for fuel tank to fuel line connections, fuel tank to fuel tank interconnects, and other points in the fuel system should incorporate the following design features:

(1)     the load necessary to separate a breakaway coupling should be between 25 and 50% of the minimum ultimate failure load (ultimate strength) of the weakest component in the fuel-carrying line. The separation load should in no case be less than 1334 N (300 lb), regardless of the size of the fuel line;

(2)     a breakaway coupling should separate whenever its ultimate load (as defined in sub-paragraph 5(c)(1)) is applied in the failure modes most likely to occur;

(3)     all breakaway couplings should incorporate design provisions to visually ascertain that the coupling is locked together (leak-free) and is open during normal installation and service;

(4)     all breakaway couplings should incorporate design provisions to prevent uncoupling or unintended closing due to operational shocks, vibrations, or accelerations; and

(5)     no breakaway coupling design may allow the release of liquid or gaseous fuel once the coupling has performed its intended function.

(d)     For electrical energy storage systems:

(1)     During a crash landing in which structural damage could lead to the release of high energy, an isolation means should ensure that no energy can be released from the energy storage system which could lead to serious injury to occupants or persons on ground. Its activation should be:

(i)      automatic, unless this is demonstrated to be impractical, in which case other means acceptable to the Agency may be employed.

(ii)     indicated to the flight crew and rescue personnel.

(2)     A manual isolation means has to be safely accessible for the rescue personnel and be clearly marked.

(e)     All individual isolation means, such as fuses, emergency stop, breakaway couplings, coupling fuel feed systems, or equivalent means should be designed, tested, installed and maintained so that inadvertent activation in flight is minimised to the maximum extent practicable. It should be ensured that the isolation means are not degrading beyond an acceptable level in accordance with the reliability requirements for systems and the fatigue requirements for structural installations.

(f)      Alternatively, for gaseous or liquid fuels, equivalent means to the use of breakaway couplings should not create a survivable impact-induced load on the fuel line to which it is installed greater than 25 to 50% of the ultimate load (strength) of the weakest component of the line and should comply with the fatigue requirements of CS 27.571 Amdt. 6 without leaking.

6.      Frangible or deformable structural attachments

(a)      Frangible or locally deformable attachments of energy storage system components to local aircraft structure should be used unless hazardous relative motion of energy storage system components to local aircraft structure is demonstrated to be extremely improbable in an otherwise survivable impact.

(b)     The attachment of energy storage system components to local aircraft structure, whether frangible or locally deformable, should be designed such that separation or relative local deformation of the attachment of energy storage system components will occur without rupture or local tear-out of energy storage system components that will could cause leakage or high energy release.

(c)      The load required to separate a frangible energy storage system components attachment from its support structure, or to deform a locally deformable attachment relative to its support structure, should be between 25% and 50% of the minimum ultimate load (ultimate strength) of the weakest component in the attached system. In no case should the load be less than 1330 N (300 lbs).

(d)     A frangible or locally deformable energy storage system components attachment should separate or locally deform as intended whenever its ultimate load (as defined in sub-paragraph 6(c)) is applied in the modes most likely to occur.

(e)     All frangible or locally deformable energy storage system components attachments should comply with the fatigue requirements of CS 27.571 Amdt. 6.

7.      Separation of flammable fluids or gases and ignition sources

To provide maximum crash resistance, flammable fluids or gases should be located as far as practicable from all occupiable areas and from all potential ignition sources.

8.      Other basic mechanical design criteria

Battery system components, electrical wires, and electrical devices should be designed, constructed and installed, as far as practicable, to be crash resistant.

9.      Rigid or semi-rigid fuel tanks

Rigid or semi-rigid fuel tank or bladder walls should be impact and tear resistant.