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The FAA has issued an airworthiness directive for the 787:

SUMMARY: We are adopting a new airworthiness directive (AD) for all The Boeing Company Model 787 airplanes. This AD requires a repetitive maintenance task for electrical power deactivation on Model 787 airplanes. This AD was prompted by the determination that a Model 787 airplane that has been powered continuously for 248 days can lose all alternating current (AC) electrical power due to the generator control units (GCUs) simultaneously going into failsafe mode. This condition is caused by a software counter internal to the GCUs that will overflow after 248 days of continuous power. We are issuing this AD to prevent loss of all AC electrical power, which could result in loss of control of the airplane.

I'm curious: Are there any circumstances under which a 787 would remain powered continuously for 248 days?

[Edited to add a follow-on question:]

Boeing has said "If there is a definitive record of a powercycle within the last 120 days, no operator action is immediately required." Does that happen? 120 days?

What is the longest period a passenger aircraft might conceivably be continuously powered on?

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    $\begingroup$ 2147483648 centiseconds = 248.55 days (now we know they're using signed 32-bit integers........) $\endgroup$ – kevin May 1 '15 at 19:16
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    $\begingroup$ @kevin Or unsigned 32-bit integers with a timer interrupt period of 5 milliseconds. :) $\endgroup$ – reirab May 1 '15 at 19:52
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    $\begingroup$ "Ladies and gentlemen, may I have your attention, please? Our captain has just informed me that our aircraft is scheduled to undergo a normal, scheduled maintenance procedure known as 'rebooting'. During this procedure you may notice the engines shut down, the wind noise outside the aircraft increases in volume and frequency, and shouts of 'Oh my god! Oh my god! We're all gonna die!!' may emanate from the vicinity of the cockpit. Please rest assured that this is all totally normal and under control. And thank you for flying in our Boeing 787 today - now powered by Microsoft Windows..." :-) $\endgroup$ – Bob Jarvis - Reinstate Monica May 3 '15 at 1:56
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    $\begingroup$ On a more instructive note, I find it interesting that there are 21427200 seconds in 248 days. If the operating software of the plane measures time in 1/100ths of a second then it would overflow a 32 bit signed integer time field after about 248 days, 13 hours, and fourteen minutes. I don't know if this is the actual problem but A) it's certainly coincidental that the timespan-to-overflow under this assumption is so close, and B) that the Patriot missile batteries used in the first Gulf War suffered from a similar overflow problem. Boeing's mileage may vary... $\endgroup$ – Bob Jarvis - Reinstate Monica May 3 '15 at 2:02
  • $\begingroup$ @BobJarvis: Wouldn't the wind noise decrease in volume and frequency with the engines shut down (due to the plane slowing down from aerodynamic drag)? $\endgroup$ – Sean Feb 16 at 22:30
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Aircrews seldom have to power off an aircraft completely (also known as a cold and dark cockpit). Airliners usually stay powered on at the gate. This is known as a "short turn-around": engines are stopped, the APU is stopped, but electrical power and air conditioning is still supplied by ground equipment.

This has the advantage of minimizing turn-around time (after all, airliners are meant to stay in the air to make money). The cabin also has to be cleaned and prepared for the next flight at this time, so something must power the lights.

It is theoretically possible to stay on this way for 248 days. However, airliners have scheduled maintenance, which completely powers off the aircraft. They are also powered off if the expected time at the gate is long. So, in reality, it's very unlikely to have an airliner powered continuously for 248 days.

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    $\begingroup$ I'm guessing you meant to say "Aircrews seldom have to power off an aircraft completely..."? What are typical scheduled maintenance intervals? $\endgroup$ – John Wiseman May 1 '15 at 19:28
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    $\begingroup$ @JohnWiseman the engineers can power off or on aircrafts if necessary. Scheduled maintenance intervals are usually named "A", "B", "C" or "D" checks. "A" is the simplest, performed every 250 flight hours; while "D" is the most comprehensive. For details see en.wikipedia.org/wiki/Aircraft_maintenance_checks $\endgroup$ – kevin May 1 '15 at 19:33
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There have been issues with the 787 giving "nuisance" messages after starting up. One solution would be to start the process earlier, to leave time to deal with them. Another is to just never shut it down. However, the plane does need to be shut down sometimes for regular maintenance. A Boeing spokesperson said:

No airplane in the fleet experienced that condition.

The issue was discovered during lab testing, and while it would be unusual for a plane to be on for 248 days straight, it's not impossible, so Murphy says that it will occur at the worst possible time. So in the interest of safety, the FAA is making operators aware so that they can be sure to shut down their planes regularly and avoid the issue.

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    $\begingroup$ I suppose a response of "have you tried switching your aeroplane off and back on again" wouldn't be considered an appropriate way to deal with these "nuisance" messages. $\endgroup$ – Dawood says reinstate Monica May 2 '15 at 0:38
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    $\begingroup$ I've been sitting on a Boeing 757 when they changed the fan tray (presumably in the electronics rack) and the error messages persisted, so "switch it off and back on" is exactly what they did. Takes a surprising time for everything to spin down, but it worked, and off we went. Presumably 757s don't have ctrl, alt, and delete keys! $\endgroup$ – Brian Drummond May 2 '15 at 11:02
  • $\begingroup$ Why did this answer make me think of Microsoft products? </snark> $\endgroup$ – Brian Topping May 2 '15 at 14:50
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    $\begingroup$ @BrianTopping Microsoft is headquartered close to where Boeing was founded... $\endgroup$ – FreeMan May 2 '15 at 15:28
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This is a bug Boeing found in the 787. This bug, which was found during laboratory testing, is:

... plane’s power control units could shut down power generators if they were powered without interruption for 248 days, or about eight months

This bug wasn't found in any airplanes in line operations. Boeing states that:

... power was shut down in all airplanes in service in the course of the regular maintenance schedule ...

Airliners stay on for very long times, but their proper maintenance requires them to be power cycled regularly. About this issue, Boeing says:

If there is a definitive record of a powercycle within the last 120 days, no operator action is immediately required.

So, the answer to your question is there aren't any.

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    $\begingroup$ "So, the answer to your question is there aren't any." If the A check doesn't require power-off, and the B check is done approximately every 180 days (according to @kevin's Wikipedia link above) it would be theoretically possible for Third-World Airlines Inc to "accidentally" file the paperwork for the B check without actually doing it, and trigger the problem. That is very unlikely to be a current problem with a new aircraft type, but 40 years from now some of them will still be flying having been sold on as "pre-owned". $\endgroup$ – alephzero May 1 '15 at 22:36
  • $\begingroup$ @alephzero: This might be a feature and not a bug for the "accidental file" problem. If it weren't for innocents being involved, it certainly would highlight the problem airlines. $\endgroup$ – Brian Topping May 2 '15 at 14:53
  • $\begingroup$ @alephzero: I wonder if a reasonable fix might be to add some code which, if the plain has gone more than 180 days since the last reboot, would start showing a message indicating that the plain must rebooted within the next 60/59/58/etc. days (leaving 8 days' cushion at the end). I would think two months' notice that the system must be rebooted would be sufficient to ensure that someone would notice. $\endgroup$ – supercat May 2 '15 at 16:19
  • $\begingroup$ At an air museum, an employee once told me that software used in commercial airliners undergoes around 10 years of testing before it is deployed. That's probably why they just issued a policy. Chances are, they'll go to 64-bit (or implement such a warning) in, well, 10 years? :) $\endgroup$ – wizonesolutions May 2 '15 at 17:35
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    $\begingroup$ @supercat - the more reasonable fix would be to fix the wrapping counter to stop overflowing every 248 days. If they are going to push out a fix, it might as well be a real fix and not a band-aid. $\endgroup$ – Johnny May 3 '15 at 2:07

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