Originally Posted by
DozyWannabe
It is if you read what I said.
It may not be the only reason it is there, but that is the only reason it *has* to be there in a computer-controlled backdriven design.
I read what was written, and I still say it was disingenuous. As it relates to backdriven controls, the PFC off switch is there because
Boeing required it - not the FAA nor EASA.
Originally Posted by DozyWannabe
The presence of force-feedback on the B777's PFCs is in fact the reason that design requires a bypass mode (the oft-discussed "Big Red Button") for safety reasons while the Airbus design does not. The bypass mode exists purely to counter the scenario in which the computer controlling the force-feedback may fail.
The FAR 25/EASA type certificate limitations of what you refer to as "force-feedback" on the 777 are satisfied through the use of slip clutches in the case of AFDS back drive actuators going 'rogue' due to computer/electronic malfunction. (no relation to the FBW PFCs).
The elevator feel units are mostly mechanical devices that get their input from the ACE and not the PFC directly. During a fallback to DIRECT MODE (your "big red button") the feel units are still in operation, but the airspeed-commanded variable resistance is replaced by a flap position based command.
EDIT: Additional info: Even with a "full" command from the PFC via the ACE, the effect on the controls from the feel unit is limited by mechanical design. The mechanism, by design is unable to place the full feel load (upwards of 100lbs) at small deflection positions. All of the feel units and backdrives on the 777 can be overridden by control inputs, albeit, at slightly higher than normal command pressures. Should one set of backdrives or a feel unit jams, there is the breakout mechanism allowing one guy to fly the aircraft unimpeded.
The system goes out of its way to discourage dual input with visual and aural warnings, so it follows that the only way it should be happening is with the full consent and intention of both flight crew.
I like this explanation much better. Thanks!
I appreciate your expert knowledge of critical computing systems and otherwise find your posts informative.
