*A Short Discussion of "Critical Loss of Thrust"*

With the change in the EASA CS23 from Amendment 4 [8] to Amendment 5 [9], the definition of the engine failure has changed. The former and well defined "failure of the critical engine" has been replaced by "critical loss of thrust" (CLT). On purpose, this term has not been specified any further. Therefore, the need to find a solution in order to assess distributed propulsion configurations and to define a "critical loss of thrust" scenario in accordance with EASA CS23.2115 [9] arises. An evaluation conducted by NASA with regard to the respective U.S. American specifications (which are almost identical to the European specifications), NASA experts have identified several certification gaps concerning both

the propulsion system and the whole aircraft certification in their analysis [10]. In order to overcome the propulsion system-related gaps, they recommend to conduct a Markov analysis to identify the most likely powertrain failure. Especially for hybrid configurations, Markov analysis can lead to the result that all powertrain components contribute to a system failure probability of less than 10 × <sup>10</sup>−<sup>9</sup> [11]. Therefore, the results of the Markov analysis suggest to neglect a deeper investigation of further CLT scenarios. A different approach to define the most critical scenario is found in Jézégou et al. [12]. First, top level aircraft functions (TLAFs) are identified. Next, the impact of a failure is correlated with these TLAFs in order to assess the criticality of the failure. For a design incorporating different energy generation paths, additionally the investigation should include multiple and possible cascading failures in order to identify the most critical scenario and the influence on the TLAFs. A TLAF interaction investigated in this paper is the suitability of the vertical tailplane (VTP) to counteract the resulting CLT torque, having been sized using conventional handbook methods within a preliminary aircraft design process. Based on the wiring possibilities depicted in Figure 1, a failure in the combustion engine also results in a failure of the connected electrical propulsor. Therefore, the impact of the CLT scenario on the required VTP size has to be taken into account within the aircraft design process.

(**a**) Configuration 1 (**b**) Configuration 2 (**c**) Configuration 3 (**d**) Configuration 4

**Figure 1.** PT2025 aircraft with on-wing (Configuration 1) and cross wiring option (Configuration 2), as well as the PT2025opt aircraft with on-wing (Configuration 3) and cross wiring option (Configuration 4), with the red and green lines symbolizing the independent wiring harnesses.

This is especially interesting for the provided GNOSIS configurations, as, due to their powertrain layout, the probability of a 50% thrust loss is equal to the probability for the loss of the combustion engine only, assumed to be around × <sup>10</sup><sup>−</sup>5. Therefore, in contrast to [6], the loss of two propulsors has to be investigated with regard to the torque they impose on the VTP. The most critical case for a CLT is considered to be the "go" case, where the engine failure occurs at a speed above V1 and the takeoff run has to be continued. Therefore, according to [6] the maximum thrust at *Vmc* = 1.2 · *Vs* is used within the scope of this paper.
