ALTERNATE LAW, DIRECT LAW AND MECHANICAL BACKUP
Control-law reconfiguration depends on the particular combination of failed references, computers, surfaces and supplies. A single universal list of normal, alternate and direct labels hides important differences. In the supplied alternate-law description, pitch can retain a load-factor-related objective and automatic trim while lateral control follows a more direct surface relationship. Some protections are lost or replaced by stability functions. The displayed law and remaining status must be read with the initiating failures, not inferred solely from whether the autopilot is available. [73, PDF pp. 27–32]
Low-speed stability is not normal angle-of-attack protection. The source describes a tendency to oppose continued deceleration, with conventional stall warning, rather than an equivalent protected boundary. It also identifies an alternate-law form without those reduced stability functions. High-speed stability and warnings similarly must not be equated with retained normal-law protection. A pilot can otherwise expect a limiting response that the active law no longer supplies. Use the actual PFD indications and applicable abnormal procedure to establish the remaining envelope-management task. [73, PDF pp. 29–32]
Direct law establishes a more direct relationship between stick input and surface command, subject to the reference’s configuration/centre-of-gravity scheduling. Automatic pitch trim is not available in that described pitch-direct state, and the source identifies the USE MAN PITCH TRIM message. Warnings can remain even though flight-envelope protections do not. Landing-gear/configuration transitions can cause further reconfiguration in specified failure cases; the exact transition is effectivity-dependent. Do not assume that every alternate-law approach remains unchanged as the aircraft is configured. [73, PDF pp. 29–33] [87, PDF pp. 62–64]
Mechanical backup is a further distinct provision using the retained mechanical control paths identified in the source, including stabiliser and rudder functions with their power dependencies. It is not a normal direct-law mode and does not restore unavailable hydraulic energy. Abnormal-attitude reconfiguration also has its own logic; recovery from the attitude need not restore the original normal-law state. These distinctions matter when planning a simulator demonstration. First establish that the particular failure and mode are implemented, then use the correct procedure and a defined initial condition rather than creating a composite scenario whose resulting law cannot be explained. [73, PDF pp. 5, 33] [86, Failures Guide; scenario selection scope]