Power down your 787

[Pigeon]

Boeing 787s must be turned off and on every 51 days to prevent 'misleading data' being shown to pilots

The power cycling is needed to prevent stale data from populating the aircraft's systems, a problem that has occurred on different 787 systems in the past.

According to the directive itself, if the aircraft is powered on for more than 51 days this can lead to "display of misleading data" to the pilots, with that data including airspeed, attitude, altitude and engine operating indications. On top of all that, the stall warning horn and overspeed horn also stop working.

Link

[Pigeon]

32 bit register for the clock

CrazyRocketEngineer

These are all pretty understandable consequences of the overflowing counter though. Not necessarily very "interesting" aside from the 32bit counter.

They use a 32bit millisecond counter which overflows. That in turn is used as time base for calculating airspeed and engine RPMs ("engine operating conditions"). Attitude and Altitude are a bit more tricky, but I assume they use some sort of sensor fusion that involves some time-dependent measurements. Stall warning and and overspeed horn are also pretty understandable, since these directly depend on airspeed, attitude and altitude.

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UKSFM99

If it's a millisecond clock with a 32bit counter, it would reset every 49 days, not 51....wtf Boeing.

232 = 4294967296 milliseconds with 32bits 4294967296 / 1000 = 4294967.296 seconds 4294967.296/60 = 71582.78826 minutes 71582.78826/60 = 1193.046471 hours 1193.046471/24 = 49.7 days!

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FCCorippus

the clock is probably 1.024 not 1. Someone pointed this out in another thread about this but I'm way too lazy to find it.

Link

[Pigeon]

Johnmad

This is most likely not a bug. As someone who has worked with flight critical software. that code is often run on ancient and proven hardware. So it's possible that the hardware doesn't support 64bit registers and even if it did the extremely strict coding rules could forbid mixing 32/64bit registers and also force every value to be of type signed.even though a time value should not be negative. All these rules make for extremely safe software but with some limitations. I can assure you that someone at Boeing has decided on the max supported uptime and that should be reflected in the start/shutdown procedure and maintenance.

[Pigeon]

2^32 mils is 49.71 days

2^64 mils is 584,554,531 years