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What is ARP-4754A, DO-178C/DO-254 and other standards for ?

This article is intended for people who wonder where these aviation standards or "norms" come from and what their purpose is. This article deliberately takes shortcuts and simplifications to get to the point of the message. In particular, it is possible, in theory, to propose alternative methods to the standards mentioned, but in reality, this only happens very rarely in a few specific cases.


The law

Depending on the country, the laws are identified via various media, for example the CFR: Code of Federal Regulations in the USA. It is a little more complicated for the states of the European Union, but concerning the laws relating to aeronautics, the content of these laws is more or less the same in most countries, particularly in the USA and in Europe.

The laws concerning aeronautics have as origin, for almost all of them, the safety of people.


Safety first

Under the law, and among other laws, the development of an aircraft, system or equipment on board an aircraft, requires safety analyses. The standards (guides) recognized and applied by the aircraft manufacturers and their suppliers for the realization of these analyses are ARP-4754A and ARP-4761. These analyses start at the aircraft level and then continue at the systems and equipment level.

From these analyses, a criticality level is determined for each system and equipment. The highest criticality level is "Catastrophic", the lowest is "no effect on safety". For example, it can be understood that the total loss of flight controls (in flight phase) is catastrophic.

Using a method proposed in ARP4754A, starting from criticality and architecture, a Design Assurance Level (DAL) is determined for each system and equipment.


Standards

Laws are written in non-technical forms, such as "critical systems must be safe". Standards attempt to translate these high-level rules into objectives and activities that can be understood, and for which evidence can be provided. The standards are written by working groups involving aircraft manufacturers (Airbus, Boeing, ...), system manufacturers (Thales, Safran, Honeywell, ...) and authorities (EASA, FAA, ...).


DAL

The DAL (Design Assurance Level) is arbitrarily represented by a letter: A for the highest, E for the lowest. The DAL being linked to the criticality, it is fundamental to understand the following concept:

- The higher the DAL, the more numerous and thorough the verification activities will be

- The higher the DAL, the more independence is needed in these verifications

These objectives aim to guarantee a certain level of confidence (Design Assurance) in the design of the system, equipment, electronics or software under consideration. The higher the DAL, the higher the level of confidence should be.


ARP-4754A, DO-178C/DO-254

These standards, but others as well (DO-200, DO-326, DO-330, DO-356...), identify objectives based on the DAL as well as the need for independence to meet these objectives. Each standard contains, towards the end of the document, tables of objectives. Knowing the DAL, it is sufficient to look in the columns "DAL" what applies to the project. Let's take a concrete example with DO-178C:


The "only" difficulty you may encounter is whether what you have put in place to satisfy an objective is acceptable or possibly partly unnecessary.


Conclusion, responsibility

In conclusion, the applicable aviation standards are dictated by laws that originate in the safety of people. It is important for the stakeholders to understand that they have a responsibility for their correct application. It is important to keep in mind that the majority of systems on board aircraft are critical systems.







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