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IMPORTANT, MUST BE READ... : Better Reboot Your Boeing 787 Every Three Weeks
Title : Better Reboot Your Boeing 787 Every Three Weeks

Better Reboot Your Boeing 787 Every Three Weeks

IMPORTANT, MUST BE READ... Crashing afterward a prolonged upwardly fourth dimension due to a counter rollover or other work is a classic error inward figurer software.  And, it simply flake the Boeing 787. Again.

The Problem:

The Boeing 787 aircraft has iii Flight Control Modules (FCMs) that are the dependent area of a novel FAA Airworthiness Directive.  Based on that judgement alone, y'all desire to brand certain whatever that involves gets fixed earlier y'all wing on a 787!

The FAA says at that topographic point is "a report" that all iii FCMs tin neglect at the same fourth dimension 22 days afterward they receive got been rebooted.  If y'all don't reboot the FCMs the FAA says this "could resultant inward flying command surfaces non moving inward reply to flying crew inputs for a brusque fourth dimension together with consequent temporary loss of controllability."  This is FAA-speak for the flat could crash. The're telling airlines to reboot the flat every 21 days to avoid this. Hope nobody forgets to produce that! (In fairness, I empathise it is probable that most planes larn rebooted to a greater extent than oft than this anyway. But this is non something y'all desire to larn out to chance.)

At this betoken nosotros tin only gauge at the cause, but the commons gauge is that it is a timer overflow problem. Let's hypothesize a 32-bit signed integer is counting the passing of fourth dimension inward milliseconds.  So a value of 32700 inward that counter is 32.700 seconds.

How long until it overflows 31 bits of counting into the 32nd bit, which is the sign bit?

0x7FFFFFFF = 2147483647 ==> 2147483.647 seconds
2147483.647 seconds * (1 min/60 sec) (1 hr/60 min)(1 day/24 hr) = 24.9 days

Hmm, a flake longer than the 22 days the FAA reports.  Some fourth dimension spent playing alongside diverse multipliers didn't look to give a probable candidate.  Possible factors if it is a timer rollover would include fixed betoken math (e.g., fourth dimension keeping inward 256ths of a second) or scaling from a 400 Hz aircraft air conditioning frequency. Or at that topographic point could survive about divided-down crystal oscillator frequency on the FCM that is involved.

Or, it could survive something completely different.  Maybe at that topographic point is retentiveness that records operating parameters periodically together with the arrangement crashes when that fills upwardly that retentiveness (for example, logs that larn downloaded every maintenance interval, alongside an expectation that the maintenance interval is to a greater extent than similar a few days than a few weeks).

For straightaway the campaign is a flake of a mystery to us.  I'll bet the FCM engineers receive got a pretty skillful thought at this point. No incertitude they'll number a ready equally fast equally they tin larn the FAA to review it.

But the big tidings is that for the minute time, the FAA is telling is telling the airlines they receive got to produce a maintenance reboot of their planes.  Last fourth dimension it was every 248 days. This fourth dimension it's every 21 days.

It's bad plenty that they receive got to reboot the infotainment systems ane time inward a while.  For flying controls, this is non skillful news. This is the form of work that should survive caught inward blueprint reviews.  Always mean value well-nigh what happens if whatever counter, timer, or information construction overflows.

Other Examples:

This is non the kickoff fourth dimension a work alongside long-running software has happened beyond the commons retentiveness leaks inward everyday applications.  Some examples are:

Timer rollover bugs:

• B787 needs to survive rebooted every 248 days due to a probable timer overflow põrnikas [Blog][NY Times] [FAA]
• Air Traffic command loses contact alongside 400 aircraft due to a 32-bit fourth dimension rollover inward 2004 [IEEE Spectrum]
• IBM: Interface adapters hang afterward 497 days of uptime [IBM]
• Windows 95: hang afterward 49.7 days without reboot, counting inward milliseconds [Microsoft]  (I met the engineers who constitute that one. And congratulated them on the pregnant feat of genuinely getting Windows 95 to run that long without crashing for another reason!)

There are too plenty of engagement roll-over bugs:

• NASA Deep Impact Comet Mission terminated unexpectedly when at 2**32 seconds afterward January 1, 2000 (a fourth dimension rollover bug). [Space.com] 

And floating-point roundoff issues (thanks to Dan for reminding me of this one):

• Patriot Missile mishap afterward operating for 100 hours without a maintenance reboot [GAO]

If y'all desire to dig further, at that topographic point is a "zoo" of related problems on Wikipedia:  "Time formatting together with storage bugs"

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