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The integrated standby instrument system (ISIS) is a standby instrument used in many large airliners that provides airspeed, altitude, and attitude information to the pilot in the case of electrical issues. The ISIS is an isolated system that uses its own pitot and electrical. If the ISIS can survive such dramatic failures, why don't we put that technology in the PFDs (Primary Flight Displays) so they to can maintain operation when electrical systems are down?

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    $\begingroup$ Its not that the Integrated Standby Instrument System (ISIS) is infallible, its that its failures are unrelated to the main system. ISIS can and will fail from time to time. But its failures should be unrelated to the main instruments, so that one of the systems is always working, and the chances of both failing at the same time is extremely remote. $\endgroup$
    – abelenky
    Jun 15, 2023 at 16:27
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    $\begingroup$ @abelenky, I'd upvote that as an answer. $\endgroup$ Jun 15, 2023 at 17:16
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    $\begingroup$ You may want to expand more acronyms, I came from HNQ having read PDF as Personal Flotation Device, $\endgroup$ Jun 16, 2023 at 20:11

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The value of the Integrated Standby Instrument System (ISIS) is not that it is infallible. Its value comes from its failures being unrelated to the primary system.

ISIS can and will fail from time to time. Its pitot system can get blocked, and its electrical system might have faults. But its failures should have no correlation to the main instruments. Ideally, both Primary and ISIS will work at all times, but if one system fails, the other one should work as a fully independent, unconnected system. The chances of both failing at the same time is extremely remote.

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  • $\begingroup$ Would it be a waste to give the PFD a backup battery for an extra layer of redundancy? $\endgroup$
    – Boeing787
    Jun 15, 2023 at 22:43
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    $\begingroup$ @Boeing787: Depending on the plane there is a very good chance that the PFD can already be powered by battery, by APU, or by the alternators on engine 1 or engine 2. Another battery would add a small bit of redundancy, at the cost of added complexity, but leave 500 other failure pathways unchanged. $\endgroup$
    – abelenky
    Jun 16, 2023 at 0:20
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And since the black boxes (CVR and FDR) can survive a crash, why not make the entire plane out of the same stuff?

Airplanes, as with nearly any complex technological system, is a balance between cost and benefit. Cost, in an aviation context, includes both direct $ costs and also the indirect cost of additional weight. Batteries sufficient to power large systems (primary flight displays and the avionic systems that provide the data form them) for a long time are heavy. Wiring, control systems, etc. all add to the weight. The benefit, in the case of a higher level of power backup for the PFD and related systems, is minimal, as the failure rate is relatively low (hopefully, by design) and ISIS needs to be there anyway for other types of failures. Plus with modern airliners there is a lot more that needs power than just the cockpit displays, which can be provided, in many airliners, by a Ram Air Turbine. If the RAT deploys then the key avionics will have power even if the engines can't be restarted, and key flight control surfaces will have power too.

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    $\begingroup$ The "data" is recoverable from the "black boxes". The boxes themselves are seldom in a working condition after a crash. They are build solidly enough not to get broken into thousands of piece, but to stay together enough they can be located. They also have a certain amount of fire resistance. $\endgroup$
    – Jim
    Jun 16, 2023 at 18:53
  • $\begingroup$ @Jim I am well aware of all of that. And the internal components can handle a lot more Gs than a human in a steel box ever could, so even if weight wasn't an issue, a big steel box around each passenger would not protect them effectively from a high-speed crash. $\endgroup$ Jun 16, 2023 at 19:11

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