Some time ago, at the Changchun Airshow, two X3-F eVTOL aircraft from Xpeng AeroHT collided mid-air, resulting in one aircraft crashing and causing a battery fire. This incident has drawn widespread attention within the industry. This article discusses the requirements and verification for eVTOL emergency landing and battery crashworthiness from an airworthiness perspective. A video of Archer’s battery drop test is appended at the end.
PART 01 Airworthiness Requirements for Crashworthiness and Emergency Landing of Traditional Helicopters
Relevant clauses in CCAR-27-R2 “Airworthiness Standards: Normal Category Rotorcraft” pertaining to crashworthiness and emergency landing requirements include: Emergency landing conditions – “§27.561 General,” “§27.562 Emergency landing dynamic conditions,” and “§27.952 Fuel system crash resistance.”
CCAR-27-R2 §27.561 requires that the rotorcraft must protect each occupant from serious injury during an emergency landing or ditching. It specifies that in a crash landing, every reasonable provision must be made to prevent serious injury to each occupant and defines the ultimate inertia load factors that different areas of the rotorcraft must withstand during an emergency landing. Provided occupants are correctly using seats, seat belts, and other safety facilities, these facilities must be capable of restraining the occupants and protecting them from injury by mass items located in the cabin interior, cabin exterior, and structure below the cabin floor.
Ultimate inertia load factors for mass items inside the cabin:
– (i) Upward 4g;
– (ii) Forward 16g;
– (iii) Lateral 8g;
– (iv) Downward 20g, after accounting for expected seat device displacement;
– (v) Rearward 1.5g.
Ultimate inertia load factors for mass items outside the cabin:
– (i) Upward 1.5g;
– (ii) Forward 12g;
– (iii) Lateral 6g;
– (iv) Downward 12g;
– (v) Rearward 1.5g.
Ultimate inertia load factors for the fuselage structure below the cabin floor:
– (ii) Forward 4g;
– (iii) Lateral 2g;
– (v) Downward 4g.
CCAR-27-R2 §27.562 specifies requirements for the dynamic testing of seats under emergency landing conditions for rotorcraft. Seats, restraint systems, and their attachments to the floor must be tested according to this clause to demonstrate their ability to reasonably protect occupants during an emergency crash. Compliance is demonstrated by conducting a dynamic seat impact test using a 77 kg (170 lb) Anthropomorphic Test Dummy (ATD) or its equivalent, simulating an occupant in a normal upright seated position.
CCAR-27-R2 §27.952 specifies design and verification requirements for the rotorcraft fuel system, including fuel tank drop test requirements, fuel tank load factors, fitting design and construction, and structural attachments. The aim is to minimize fuel leakage after a survivable crash and the occurrence of post-crash fires, thereby reducing casualties. Specific requirements for the fuel tank drop test include a minimum drop height of 15.2 meters (50 feet), an impact surface that must be non-deformable, the test tank being filled with 80% capacity of water, the tank being surrounded by structure simulating the contacting airframe structure, a drop attitude of ±10°, and the requirement that the tank must not leak after the test. It is worth noting that Light Sport Helicopters (LSH) are not subject to this requirement.
PART 02 Emergency Landing and Battery Crashworthiness Requirements for eVTOLs
I. Emergency Landing Conditions
The requirements for emergency landing of eVTOLs are similar to those for traditional helicopters. However, the published Special Conditions often adopt open-ended clauses based on general safety requirements, with specific design requirements and compliance methods provided in the relevant compliance documentation.
Taking the Special Condition for the EH-216S as an example:
PEU.C070 Emergency Conditions (corresponding to EASA SC-VTOL-02 VTOL.2270 Emergency conditions):
(a) The aircraft must protect each occupant from injuries preventing egress under the following conditions, even if the aircraft is damaged in an emergency landing:
(1) Proper use of safety equipment and features specified in the design;
(2) The occupant is subjected to the ultimate static inertia loads likely to occur during an emergency landing;
(3) Mass items inside the cabin or elsewhere that could cause injury to occupants, including components of the power and electrical systems, are subjected to the ultimate static inertia loads likely to occur during an emergency landing.
(b) The emergency landing conditions specified in (a)(1) and (a)(2) above must account for:
(1) The dynamic conditions likely to occur during an emergency landing;
(2) Loads experienced by occupants due to restraint or contact with interior objects must not exceed human injury criteria determined based on human tolerance.
(c) The aircraft must provide protection for all occupants under probable flight, ground, and emergency landing conditions.
(d) Each occupant protection system must perform its intended function without creating hazards that could cause secondary injury to occupants. When not in use, the occupant protection system must not hinder occupant egress or interfere with aircraft operation.
(e) Each baggage and cargo compartment must comply with the following:
(1) Designed based on its maximum load weight and the critical load distribution at the maximum load factors corresponding to the flight and ground load conditions determined per this Special Condition;
(2) Have means to prevent hazard to occupants from impact by contents or shifting of contents;
(3) Any controls, wires, lines, equipment, or accessories whose damage or failure could affect safe flight or landing must be protected;
(4) Any probable fire source must not affect continued safe flight or emergency landing.
(f) Any component of the power and electrical system whose damage or failure could affect safe operation or occupant safety must be protected.
The Means of Compliance (MOC) for EASA SC-VTOL indicates that CS-27 §§ 561 and 562 can be referenced as a compliance method.
Draft Special Condition for X3-F Aircraft X3.561 General:
(a) Although the aircraft may be damaged in a ground emergency landing condition, it must be designed in accordance with this clause to protect occupants under these conditions.
(b) The structure must be designed to give each occupant every reasonable chance of avoiding serious injury in an emergency landing under the following conditions:
(1) Proper use of seats, seat belts, and other safety facilities;
(2) The occupant experiences the following ultimate inertia load factors relative to the surrounding structure:
(i) Upward 4.0g;
(ii) Forward 9.0g;
(iii) Lateral 3.0g;
(iv) Downward 4.5g;
(v) Rearward 1.5g.
(c) The supporting structure must be designed to restrain, under any of the ultimate inertia loads specified in (b)(2) above, any items that could injure occupants if they broke loose in an emergency landing.
II. Battery Crashworthiness
The battery crashworthiness requirement for eVTOLs originates from EASA SC-VTOL-01 VTOL.2325 Fire Protection (a)(4). The Special Conditions for EHang EH216-S and Autoflight AE200-100 use similar wording. Using the EH216-S Special Condition as an example:
PEU.D025 Fire Protection
(a) The aircraft must be designed to minimize the risk of fire in the following situations:
(1) Fire caused by heat, radiant thermal energy, or system failures generated under probable operating conditions;
(2) Fire caused by flammable fluids, gases, or vapors;
(3) Situations where certain onboard systems (e.g., oxygen systems) due to their nature could propagate flames or become an ignition source;
(4) Survivable emergency landing.
The MOC SC-VTOL provides the compliance method for VTOL.2325 (a)(4). VTOL.2325 (a)(4) requires that the energy storage system and its installation in the aircraft be designed to minimize the risk of post-crash fire in a “survivable emergency landing,” with the ultimate goal of providing occupants sufficient time to evacuate or be rescued from the aircraft after such an event.
The compliance method requires that each energy storage system, or the most critical one, undergo a drop test. For eVTOLs, a battery drop test should be conducted, similar to the fuel tank drop test for CS-27 helicopters. The drop test conditions are as follows:
(a) The drop height must be at least 15.2 meters (50 feet);
(b) The impact surface must be non-deformable;
(c) The energy storage system must be energized or filled to the most hazardous state expected during the impact;
(d) The energy storage system must be enclosed within peripheral structure representative of the actual installation environment, unless it can be demonstrated that such structure lacks protrusions or other design features that could rupture the energy storage system;
(e) The energy storage system must be free-fallen oriented in a direction representative of the typical installation attitude of the aircraft, and impact with a horizontal attitude within ±10° relative to the VTOL horizontal axis;
(f) After the drop test, for a duration matching the time required for rescuing seriously injured occupants, there must be no risk of post-crash fire or other hazardous release:
(1) For liquid or gaseous fuels: There must be no leakage of flammable liquids or gases.
(2) For batteries:
(i) Structural damage must not lead to fire, or leakage of hazardous liquids, smoke, or gases.
(ii) For at least 15 minutes, any fire or leakage of hazardous liquids, smoke, or gases must be confined to uninhabited areas and outside the egress path.
(3) Any detached fragments must not cause serious injury to occupants or ground personnel.
PART 03 Battery Drop Test
One might wonder why the required drop test height is 15.2 meters (50 feet). The FAA analyzed 1,351 rotorcraft accidents between 1948 and 1974. A crash velocity corresponding to a 50-foot drop covers 99% of survivable crash envelopes, and passengers should be protected from the danger of post-crash fire, as illustrated in the figure below.
Finally, a video of Archer’s battery drop test is appended.
