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Since airliners are bound by their minimum equipment list, I was wondering, which parts/systems that are not on the minimum equipment list are often inoperative on normal commercial flights.

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    $\begingroup$ Do you mean dispatching with systems inoperative or systems that crap out during the flight? If it's inoperative on the ground, and it's not covered by an MEL, you're stuck. $\endgroup$ – John K Dec 14 '19 at 22:03
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    $\begingroup$ Yes I mean dispatching with systems inoperative. Systems that are not required for the flight by the minimum equipment list. $\endgroup$ – hph304j Dec 14 '19 at 22:11
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    $\begingroup$ this question is really broad. each aircraft types has its own equipments. you should restrict to one or two specific aircraft types (e.g. B737, A320, E190). Each airline may have its own restrictions (more restrictive than the manufacturer ones). Thus you may restrict to few airlines. $\endgroup$ – Manu H Dec 15 '19 at 12:31
  • $\begingroup$ The Blue Angels show team formerly used the Lockheed C-130T Hercules, nicknamed "Fat Albert", for Jet Assisted Take Off (JATO demonstrations). These ended in 2009 due to dwindling supplies of rockets. Hence, since then the JATO system is not operational. I do not know if this team is "commercial" enough. $\endgroup$ – h22 Dec 16 '19 at 18:40
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If a system goes south while sitting at the gate or the ramp, that brings on a cockpit indication, the crew will break out the MEL/CDL using whatever protocol or procedure they follow and if it's not covered, they are grounded until the system is made serviceable again and will take a significant dispatch delay, or worse, a cancellation.

The OEM will try to get as many items covered under the MEL as possible and will continuously try to expand it, because it has a big effect on overall dispatch reliability. An unreliable system system or subsystem that has a lot of failures will just be maintenance cost issue (and possibly minor delays) if it has MEL coverage, but if it doesn't, it becomes a cost plus dispatch delay/cancellation problem, much more expensive.

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    $\begingroup$ Are you sure it's not backwards: a non-MEL system is just mx, and a MEL system is mx+delay? $\endgroup$ – Therac Dec 14 '19 at 22:49
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    $\begingroup$ No a non-MEL item causes a delay or cancellation because it has to be repaired and made serviceable before you can fly. If it has an MEL, you can dispatch with it inoperative after doing a deactivation procedure or maybe just a simple crew procedure. In some cases you might take a minor delay if a tech has to go in and blank something off. $\endgroup$ – John K Dec 14 '19 at 23:34

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