The design assurance level, also called development assurance level, is assigned during the system safety assessment. Each failure condition of a function is classified by its effect: catastrophic (level A), hazardous (B), major (C), minor (D) or no safety effect (E). The more severe the effect, the more rigorous the process used to develop the item, because errors made during development cannot be found by random testing alone.
The level is a process requirement, not a measure of reliability. In DO-178C the number of objectives grows from 26 at level D to 69 at level B and 71 at level A, and the share that must be satisfied by someone independent of the author rises from 2 to 30. DO-254 uses the same five letters for complex electronic hardware. The system-level guidance that assigns the levels is SAE ARP4754A, with ARP4761 for the safety assessment.
In drones and eVTOL aircraft, the level a component receives depends on the architecture and on the aircraft category: a flight-control computer whose loss is catastrophic is a candidate for level A or B, while a payload processor may need only C or D. A datasheet states it as “DAL A”, “DAL B” and so on, usually separately for software and for hardware.
A frequent mistake is to treat a higher letter as a better product. A level is only meaningful against the failure condition it was assigned for; a part developed to DAL A for one function does not prove suitability for another aircraft, and architecture (redundancy, monitoring) can lower the level a single item needs. Always check which standard the level refers to.
