EASA publishes means of compliance for thermal runaway

EASA has published its Means of Compliance with the Special Condition for VTOL focused on thermal runaway.

The publication, its third means of compliance (MOC) ‘Doc. No. MOC-3 SC-VTOL, Issue 2’, focuses specifically on mitigating risks associated with thermal runaway. The MOC was previously put out to the industry for review and EASA published it alongside those responses. There are comments from Boeing, EP Systems, Ampaire, Diamond Aircraft among others. EASA plan to publish all MOCs in one document at a later stage. 

In its introduction EASA recognises “big progress” has been made in the development and integration of rechargeable lithium batteries. However, they still represent a significant fire hazard when used as power supply for systems in traditional aircraft due to thermal runaway. Now the recent use of lithium batteries as propulsion energy storage devices in electric aircraft increases the importance of properly addressing this hazard, it said. This is due to their novel function, higher capacity, higher specific energy, higher voltage and the lack of significant propulsion service experience.

“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.” the MOC stated.

“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.”

The considerations referring to time-to-land is a key point for OEMs. Weight saving is one of the fundamental aspects of aircraft design, therefore the inferred flexibility of EASA to test and approve an aircraft’s battery propulsion system safety level based on the conditions it will operate in is good news for OEMs.

Industry commenters were mixed in their reaction to the MOC proposed initially. The General Aviation Manufacturers Association (GAMA) said the MOC is mostly focused on thermal runaway whilst yet there are many factors to consider in a compliance demonstration for a battery system. “Additionally, the MOC refers to the multiple layers of protection several times but does not expand on them. This can cause issues in design development where an applicant will not fully understand these multiple layers. In order to achieve the proper safety level the multiple layer approach is needed,” GAMA stated.

Yves Morier, former principal advisor to the Certification Director and Flight standards and director, New Technologies at EASA said the MOC is a good document that proposes two means of compliance: RTCA DO-311A section 2.4.5.5. ‘Thermal Runaway Containment Test’ and Battery Thermal Runaway Containment for Continued Safe Flight and Landing (CSFL) Time Tests.

“The second one was developed because of identified limitations of the RTCA test that are explained in the MOC. Although it is presented as an alternative, I would encourage to use the second one in particular for the EVTOL operated above congested areas (enhanced category of the SC-VTOL),” said Morier. 

He said it is important to realise that the MOC focuses on thermal runaways and that additional MOCs will be developed to cover further issues as described in the comment response document by EASA: “This Means of Compliance is not addressing neither superseding other tests needed for the certification of propulsion battery systems (i.e. external short circuit, available system capacity and energy, protections testing, battery system crashworthiness tests…)”.

“Therefore, other MOCs will need to be developed to address these aspects. EASA is working to develop the other MOCs either internally or in collaboration with EUROCAE,” Morier continued.

“A very thorough comment response document (CRD) has been published as the draft MOC generated a lot of interest and generated a healthy debate. As usual the CRD gives good insights on the intentions of EASA and is of great help to whomever wish to study the MOC in depth.”

Download the MOC and the comment document via the EASA website.