2.2.2. Safety/Acceptable Level of Safety

Acceptable Level of Safety (ALoS) is primarily related to aircraft accident prevention, and its definition depends on the region, regulatory framework, safety records, etc. During the MMEL development process, crew performance and workload must be considered along with the e ffects of system/componen<sup>t</sup> failures on landing and take-o ff and their impact on the aircraft and its occupants [4]. In the context of MEL, ALoS can be maintained by adjusting aircraft operational limitations, transferring a system or component's function to another functioning system or a system that provides the information needed or performs similar functions as long as the crew workload changes are acceptable, and the training provided to the crew remains su fficient and relevant [4].

Another way of maintaining an ALoS during MEL application is by developing maintenance actions based on the MMEL (e.g., deactivating and securing the system/componen<sup>t</sup> concerned). For instance, for the Bombardier CL600 concerned, to maintain an ALoS on a lavatory fire extinguishing system when it is inoperative a placarding procedure is carried out: Associated lavatory door is locked CLOSED and placarded, "INOPERATIVE–DO NOT ENTER" [21]. Another example regarding the Airbus A320 is the Emergency Locator Transmitter (ELT) which broadcasts signals in the event of an accident; to maintain the ALoS, the inoperative ELT must be deactivated and repaired within 90 days [22].
