With the plethora of switches/knobs/buttons etc present in airplane cockpits, how can pilots be really sure that a button they just pushed (either to enable/disable some electronic system or move some non-visible part of the plane for example) has actually done its job correctly?

Is there some sort of visual or auditory feedback?

Or do they have to always assume that these have worked, until they realize otherwise by observing the airplane or other related systems behavior?

  • $\begingroup$ Pilots are trained on what every button and switch does. There will almost always be clues that the switch worked correctly either through an annunciation, a flight attitude change or sound. Sometimes the switch will move bavk to it's original position if it didnt activate the system correctly. $\endgroup$
    – wbeard52
    Dec 30 '19 at 14:32
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    $\begingroup$ Not an answer, per se, but if you have indicators that merely indicate that the switch was flipped, and not the state of the response, bad things can happen. $\endgroup$
    – Machavity
    Dec 30 '19 at 17:07
  • $\begingroup$ @Machavity see also the movie that came out one week prior, in which an indicator stuck and crew act on the misread... $\endgroup$ Dec 31 '19 at 14:48
  • $\begingroup$ @Harper-ReinstateMonica which movie? $\endgroup$ Jan 1 '20 at 1:39
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    $\begingroup$ @GuillermoGutiérrez The China Syndrome. Critical moment here. Beware the soundtrack "improvements" near the end. $\endgroup$ Jan 1 '20 at 4:39

Depends on the system in question, but typically the following indications will serve as a means to verify that a system is functioning correctly.

Activation lights - some cockpit switches will illuminate when commanded by crew. Granted this only can serve as an indication that the switch was engaged.

Indicator lights eg landing gear position lights, etc - remote switches on mechanical systems which are activated when said system are in a required positions or functioning. The switch then can illuminate lights on an instrument panel.

Mechanical indicators eg flap position indicators - system provides either a mechanical or otherwise signal back to an indicator which mechanically moves in unison with with the system.

Information Displays - integrated flight decks and multifunction displays take inputs from similar remote mechanical, thermal, pressure and electrical inputs from aircraft systems and display either textual or graphic indications on displays.

Kinsethetic reactions - eg lowering flaps will cause the airplane to decelerate and pitch up, landing gear actuation will cause sounds, vibrations, wind noises, etc.

Correct system function - eg autopilot mode engaged and the airplane is doing what the crew expected of the command.

  • 4
    $\begingroup$ Eyeballs... Look for the changes. I've flown a plane that had a mirror mounted so you could look for gear down. Didn't tell you locked and didn't show all of them, but at least it was something! $\endgroup$ Dec 30 '19 at 17:16
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    $\begingroup$ “Granted this only can serve as an indication that the switch was engaged.”—modern cockpit switches (always buttons) have two lights in them. The lower one is indicating non-default state (i.e. it says “on” on some buttons and “off” on others depending on what state it is expected to be in cruise; this is called “dark cockpit concept”) and the upper one says “fault” and lights if the system is not operated while commanded to operate. So if you press the button and the system does not respond, the “fault” light comes on and you know it does not work (there will also be a message on ECAM). $\endgroup$
    – Jan Hudec
    Dec 30 '19 at 21:44
  • $\begingroup$ Interesting..... $\endgroup$ Dec 30 '19 at 22:35

All flight guidance functions and their changes are shown on the Flight Mode Annunciator (FMA) and have corresponding switches on the Flight Control Unit (FCU).

The buttons on the FCU are usually lit when activated, and the mode is shown on the FMA.

Other systems, like pumps, generators etc. have pushbuttons that are lit when activated (or deactivated, depending on the system). Some have also a separate fault light on the button.

On Airbus aircraft, when doing a procedure according to ECAM, you get the first feedback from the ECAM screen when selecting a switch. The line on ECAM either disappears or changes color. The second feedback is on the button light. These two indications have different sources or sensors, so you can be relatively sure that the intended action is complete when getting the feefback, either one or both. In some systems both indications are required, others one is enough.
Non-critical systems like lights etc. do not necessarily have any feedback channel.

  • $\begingroup$ regarding the lit buttons (eg FCU), is this light actually a simple indication that the button was pushed (eg without necessarily meaning / guaranteeing that the related action actually took place) ? Or is there actually some kind of underlying validation mechanism/system that runs and confirms that the action was carried out successfully ? $\endgroup$ Dec 30 '19 at 13:39
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    $\begingroup$ At least in the Airbus family, there is some validation, the light doesn't go on unless the system does. Unfortunately I don't know more about those. Also if the desired function is broken, a separate fault indication will be displayed on the ECAM. $\endgroup$
    – Sami
    Dec 30 '19 at 14:00

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