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Blackout Bug: Boeing 737 cockpit screens go blank if pilots land on specific runways

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Blackout Bug: Boeing 737 cockpit screens go blank if pilots land on specific runways

Old 8th Jan 2020, 11:56
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Blackout Bug: Boeing 737 cockpit screens go blank if pilots land on specific runways

https://www.theregister.co.uk/2020/0...een_blank_bug/

Seven runways, of which five are in the US, and two in South America - in Colombia and Guyana respectively trigger the bug. Instrument approach procedures guide pilots to safe landings in all weather conditions regardless of visibility."All six display units (DUs) blanked with a selected instrument approach to a runway with a 270-degree true heading, and all six DUs stayed blank until a different runway was selected," noted the FAA's airworthiness directive, summarising three incidents that occurred on scheduled 737 flights to Barrow, Alaska, in 2019.
https://www.federalregister.gov/docu...pany-airplanes

This AD requires revising the airplane flight manual (AFM) to prohibit selection of certain runways for airplanes equipped with certain software. This AD was prompted by reports of display electronic unit (DEU) software errors on airplanes with a selected instrument approach to a specific runway. The FAA is issuing this AD to address the unsafe condition on these products.
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Old 8th Jan 2020, 12:38
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OMG!!!
This is symptomatic treatment at its best. What about fixing that f***ing display software?
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Old 8th Jan 2020, 13:05
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Originally Posted by BDAttitude View Post
OMG!!!
This is symptomatic treatment at its best. What about fixing that f***ing display software?
Seems they already have
"The FAA has confirmed that the faulty version of DEU software has already been removed from all airplanes conducting scheduled airline service into the affected airports. This AD is intended to address unscheduled diversions and Boeing Business Jet (BBJ) flights into the affected airports"

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Old 8th Jan 2020, 13:13
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This is the problem when you keep building code on top of legacy code. Certain combinations can trigger unintended consequences.
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Old 8th Jan 2020, 13:36
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Originally Posted by malanda View Post
Seems they already have
"The FAA has confirmed that the faulty version of DEU software has already been removed from all airplanes conducting scheduled airline service into the affected airports. This AD is intended to address unscheduled diversions and Boeing Business Jet (BBJ) flights into the affected airports"
So there's a fix and they do not mandate it?!

Some combination of data have triggered this bug and 7 approches have been identified.
So who guarantees that no 7 + x approches exist right now or are inserted later with DB updates?

Still find it strange.
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Old 8th Jan 2020, 16:12
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747s could lose all 6 DUs until the EIUs got changed. Happened twice in-flight, IIRC.
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Old 8th Jan 2020, 17:01
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Originally Posted by BDAttitude View Post
So who guarantees that no 7 + x approches exist right now or are inserted later with DB updates?
Runways tend not to move overnight.

It would be a fairly trivial task to crunch one of the FMS data providers' runway databases and determine which ones are aligned at exactly 270 (plus or minus whatever the tolerance is).
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Old 8th Jan 2020, 17:15
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Originally Posted by DaveReidUK View Post
Runways tend not to move overnight.

It would be a fairly trivial task to crunch one of the FMS data providers' runway databases and determine which ones are aligned at exactly 270 (plus or minus whatever the tolerance is).
That's just about every second round here - the others being 07 to 09 - pedominantly westerly winds . Who needs FMS approach page anyway .
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Old 8th Jan 2020, 18:03
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You have to be carefull you do not fix the problem but cause a worse one.
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Old 8th Jan 2020, 18:49
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Originally Posted by BDAttitude View Post
That's just about every second round here - the others being 07 to 09 - pedominantly westerly winds . Who needs FMS approach page anyway .
From the AD:
"Not all runways with a 270-degree true heading are susceptible; only seven runways worldwide, as identified in this AD, have latitude and longitude values that cause the blanking behavior"

I don't see any obvious pattern in the lat/long values. A very curious bug.
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Old 8th Jan 2020, 19:38
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Originally Posted by malanda View Post
From the AD:
"Not all runways with a 270-degree true heading are susceptible; only seven runways worldwide, as identified in this AD, have latitude and longitude values that cause the blanking behavior"

I don't see any obvious pattern in the lat/long values. A very curious bug.
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Western longitude and rather large variation is common.
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Old 8th Jan 2020, 20:51
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Originally Posted by BDAttitude View Post
So there's a fix and they do not mandate it?!

Some combination of data have triggered this bug and 7 approches have been identified.
So who guarantees that no 7 + x approches exist right now or are inserted later with DB updates?

Still find it strange.
No direct knowledge of this case, but sometimes a software update may require an associated hardware update. So while installing the new s/w is easy and cheap, the associated h/w may not be. If an operator never plans to operate into one of the seven affect airports, they may not want to be bothered (or pay for) the associated changes.
Shortly after EIS of the 747-400, Boeing came out with an EICAS s/w update to correct some issues with the original s/w. However it wasn't just a s/w update, it required a hardware change as well. Well, there was a certain operator who couldn't be bothered to update the hardware and so wouldn't incorporate the update. This operator also happened to be the launch customer and so had most of the affected aircraft. So finally the FAA issued an AD mandating the new s/w. Due to the unfortunate wording of the AD, it meant that every single time Boeing updated the 747-400 EICAS software, Boeing had to obtain an 'Alternate Method of Compliance' (AMOC) to the AD (which isn't trivial).
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Old 8th Jan 2020, 21:20
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I’ve been operating strictly to company and Boeing SOPs for the last 30 years on different Boeing products.(to cover mine and everyone else’s back including Boeing) I’m old enough and have so much experience that I’m beginning to not trust certain things that I always took for granted, and would maybe now go for my own very experienced seat of the pants flying instinct instead of Boeing QRH etc to save the day .. Not the way it should be I know !. Maybe Boeing can’t be trusted anymore, and that makes it up to me to be safe these days. Any thoughts. ?
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Old 9th Jan 2020, 03:12
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I'm a software developer, and this makes me very angry. The fact that a "magic value" can trigger a bug like this indicates that there is something rotten in the fundamental approach the developers of this particular software took.

There had to be multiple pairs of eyes looking it over, too, and either no one was confident enough to speak up and say "Hey, this isn't the right way to do this", or they were and were then overruled by someone with more authority than sense.

To be clear, I see this kind of thing all the time. I just don't work with safety-critical software, and I had hoped the standards were materially different for that.

I'd love to see the technical explanation of the bug and/or overview of the code, but I suspect it would just make me angrier.

I also quote this post from The Register article's comment page, with which I agree completely:
The bug itself is very troubling, but what is much troubling is what the bug implies about the quality of the software at large. First it implies a lack of bounds checking on the display unit. Second, a lack of testing on the display data inputs. Third, no checking of what the FMS is pushing out. Fourth, no one bothered to write a rational error-handling on the display unit in case values were out-of-bounds (especially since this causes the entire display to go dark instead of just a single value).

But what really worries me is that a software glitch can trigger a failure of all display units simultaneously, rather than just ones showing specific pages. SO would a bad value that is only displayed on one of those tertiary EIS display pages cause the displays to go out as well? What about garbage from the WX Radar?
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Old 9th Jan 2020, 03:18
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Originally Posted by DaveReidUK View Post
Runways tend not to move overnight.

It would be a fairly trivial task to crunch one of the FMS data providers' runway databases and determine which ones are aligned at exactly 270 (plus or minus whatever the tolerance is).
True, but the magnetic heading can effectively change overnight. It's a weird side effect of the magnetic poles not staying put.

Edit: Came across the following website because a certain drone suddenly was unable to fly when the magnetic database changed on Jan1. https://www.ngdc.noaa.gov/geomag/WMM/ I don't know the particular correlation to the avionics side of this but looking at the loops and whorls of the magnetic field leaves me dizzy. They track not only the current offsets but the rate at which the offsets change; apparently 9 dimensions all together.
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Old 9th Jan 2020, 05:27
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Originally Posted by MechEngr View Post
True, but the magnetic heading can effectively change overnight. It's a weird side effect of the magnetic poles not staying put.

Edit: Came across the following website because a certain drone suddenly was unable to fly when the magnetic database changed on Jan1. https://www.ngdc.noaa.gov/geomag/WMM/ I don't know the particular correlation to the avionics side of this but looking at the loops and whorls of the magnetic field leaves me dizzy. They track not only the current offsets but the rate at which the offsets change; apparently 9 dimensions all together.
I think the article related the bug to the true direction, not the magnetic direction of a runway.
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Old 9th Jan 2020, 06:56
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So a combination of particular magnetic headings, latitude, longitude and mag variation might cause the Nav display to output nonsense, but all six displays blanked??

Why the PFD?. Why the Engine/system display?

Please tell me this is untrue or that at least the PFDs are given a higher level of coding security.
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Old 9th Jan 2020, 07:18
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Originally Posted by ReturningVector View Post
I think the article related the bug to the true direction, not the magnetic direction of a runway.
Ahh, true heading - OK.

What is unclear is why there is also a connection to latitude and longitude.

I looked at the airports - based on data from https://www.airnav.com these are the deviations in degrees from 270:

82V -0.009377087
KBJJ 0.001984966
KCIU -0.024015687
KCNM 0.017476942
PABR -0.00926391

AirNav provides Lat/Long for each end of the runway which I converted with ATAN to a degrees variation. It did not list the last two.

SKLM, is listed as runway true heading: 272.4 on SKLM - Jorge Isaac Airport and
SYCJ is listed at 271.9

So it's a mystery. While I can imagine truncating the true heading to one place would be a problem, I can't see where they would truncate the true heading by more than 2 degrees.
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Old 9th Jan 2020, 07:53
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Originally Posted by Uplinker View Post
So a combination of particular magnetic headings, latitude, longitude and mag variation might cause the Nav display to output nonsense, but all six displays blanked??

Why the PFD?. Why the Engine/system display?

Please tell me this is untrue or that at least the PFDs are given a higher level of coding security.
I expect it failed in some section that would ordinarily be labeled "THIS CANNOT FAIL," not because it is impervious to failure but because there is no sensible response if it does. AT&T managed to crash almost all of its communications network because of a that kind of bug and, unlike airport information, they were in control of every detail of their hardware operation. See All Circuits are Busy Now: The 1990 AT&T Long Distance Network Collapse

The 737 is rumored to be running an 80286 processor which is a well understood design. However I don't see the architecture for the software. It should have an independent watchdog timer processor to automatically reboot if there is a fundamental software failure, but maybe only the thread/task/subsystem to update the displays failed and the watchdog reset and all other controls is still running. I've seen software fail because it was awaiting feedback that never happened. It may be that one display got a message it could not handle and never responded. That would quickly stop the task to update the displays. If the displays are programmed right they should have their own watchdog timers to stop displaying when too long a time has passed without an update rather than freezing and giving the impression they are still functioning.

There was a question of why not every runway was tried on the software. While hindsight is great on this sort of problem, it also suggests trying every combination of: altitude, airspeed, pitch, AoA, lat, long, N1, fuel load, weight, true airspeed, relative airspeed, and on and on.

The reason I wondered about the magnetic heading earlier is because this last year cannot be the first time a 737NG flew into Barrow, Alaska. Can it?

The last two airports on the list are nearly 2 degrees from a true heading of 270 degrees. It's not likely for that to result in a divide by zero problem.

It's a shame, but I doubt the actual source of the problem will be revealed.
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Old 9th Jan 2020, 08:26
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Originally Posted by MechEngr View Post
Ahh, true heading - OK.

What is unclear is why there is also a connection to latitude and longitude.

I looked at the airports - based on data from https://www.airnav.com these are the deviations in degrees from 270:

82V -0.009377087
KBJJ 0.001984966
KCIU -0.024015687
KCNM 0.017476942
PABR -0.00926391

AirNav provides Lat/Long for each end of the runway which I converted with ATAN to a degrees variation. It did not list the last two.

SKLM, is listed as runway true heading: 272.4 on SKLM - Jorge Isaac Airport and
SYCJ is listed at 271.9

So it's a mystery. While I can imagine truncating the true heading to one place would be a problem, I can't see where they would truncate the true heading by more than 2 degrees.
It probably depends which FMS database you use - I've just run some numbers from the one I use (not AirNav) and the true headings for the 7 runways in question range from 269.968 to 270.012.

As for the FAA's assertion that "only seven runways worldwide, as identified in this AD, have latitude and longitude values that cause the blanking behavior", I suspect that only runways of over a certain length (5000 feet?) at civil airports have been considered. There are around 150 runways in total with true headings in the above range, but once short runways and military fields are excluded (and, for some reason, airports with parallel runways) there are only about a dozen left worldwide.

I don't think that mag variation has anything to do with the criteria.
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