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Primary Function

Primary Function is a function that is installed to comply with the applicable regulations for the required function and that provides the most pertinent controls or information instantly and directly to the pilot. For example, the PFD is a single physical unit that always provides the primary display of all the following: altitude, airspeed, aircraft heading (direction) and attitude located directly in front of the pilot in a fixed layout in accordance with 14 CFR 23.1321. For controls such as brake or engine controls, the primary may be the backup systems. For example, a mechanical backup brake system could be considered to be the primary with regard to meeting the requirements and the electronic brake system would be the secondary.

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