Special Condition for VTOL and Means of Compliance
IFA Comment: The use of large scale rechargeable lithium batteries is a considerable practical test for civil aviation. It’s good to see international regulators taking on this vital subject in recently published guidance material.
EASA updated on 21 June 2023 the “Third Publication of Means of Compliance with the Special Condition for VTOL”, Doc. No. MOC-3 SC-VTOL, Issue 2.
Statement of Issue
Substantial progress has been made in the development and integration of rechargeable lithium batteries in aviation. However, they still represent a significant fire hazard when used as power supply for systems in traditional aircraft due to their susceptibility to failures leading to self-sustaining increases in temperature and pressure (thermal runaway).
The recent use of lithium batteries as propulsion energy storage devices in electric and hybrid aircraft increases the importance of properly addressing this hazard, due to their novel function, higher capacity, higher specific energy, higher voltage, and the lack of significant service experience in this context.
Some of the most common root causes that could lead to a thermal runaway are (non-exhaustive list):
- Design and manufacturing issues
- Installation or maintenance issues
- Internal fault conditions ((cell manufacturing quality issues, dendrites…).
- External abuse conditions (external short-circuit, overcharge…).
- Physical damage during storage, transportation, service, or swapping.
- Heat sources (poor electrical connections, corrosion, short-circuits, arcs…).
Some of them can be mitigated through proper adoption of processes throughout design, manufacturing, installation, operation, and maintenance. Others cannot be completely avoided (i.e., cell internal short-circuit due to latent manufacturing defects) and their effect should be mitigated in-service.
Furthermore, the batteries and their protective layers/measures in propulsion battery systems represent a considerable part of the weight of the aircraft. Therefore, it is essential to define test requirements that ensure the adequate level of safety of the product for the intended operational conditions in a feasible way. This may include considerations on the time-to-land following the detection of a thermal runaway.
Link here to the EASA site to access the publication and all related content
