This question is specific to Bombardier C-Series aircraft which has three modes: Flight control computer normal mode, FCC Direct mode and REU direct mode: (I am not sure if other aircraft have an intermediate mode before switching to alternate law)

Control law gains are computed for both FCC normal and FCC direct mode even if the computer is in normal mode. Direct mode commands are outputted when the switch happens from normal to direct.

Fly By Wire Flight control computer switches from normal law to alternative law under certain conditions (when minimum required sensor set is lost for instance). So I assume the control laws are also switched from normal to direct. Are the direct mode control gains computed and monitored when the flight control system is under normal law?

  • $\begingroup$ When the system is in normal law, it is in normal law, not direct law, so what do you think it should be doing with direct mode control gain? $\endgroup$
    – Jan Hudec
    Commented Mar 30, 2016 at 12:01
  • $\begingroup$ There is nothing to compute on direct mode control gains. In direct mode, the output equals the input. And, somewhat notably, with centred stick the both normal and direct law outputs are zero, because normal law keeps the aircraft trimmed for its current speed, so unless the stick is off centre (which is rare), switch to direct law won't actually move the surfaces anyway. $\endgroup$
    – Jan Hudec
    Commented Mar 30, 2016 at 12:03
  • $\begingroup$ Bombardier aircraft has one more intermediate mode called Flight Control Computer Direct Mode (before switching to REU* direct mode, where pilot inputs are directly translated into surface movements). This is a mode with reduced functionality but still FBW system computes the gains. Hence we have two control laws: Normal and Direct mode control laws. Is it not the case usually? *REU - Remote electronics unit $\endgroup$ Commented Mar 30, 2016 at 12:34
  • $\begingroup$ You may want to edit your question to specify that you're specifically interested in the Bombardier FBW systems, then, to help reduce confusion. $\endgroup$
    – FreeMan
    Commented Mar 30, 2016 at 12:35
  • $\begingroup$ @user2927392, you tagged the question “airbus”, so I am talking about the Airbus modes. Bombardier control modes, and their implementation, are different from Airbus ones (and I know absolutely nothing about them; I only read up on the Airbus ones). $\endgroup$
    – Jan Hudec
    Commented Mar 30, 2016 at 12:39


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