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I've been told anecdotes like this several times on the interviews. Interviewers used this story as an argument in favor of defensive software development.

Here is an explanation from one of the numerous occurrences on the web:

This story was told by people from Motorola and is supposedly included in every microcontroller training course Motorola gives.

Test flights of F-16's were being conducted in Israel. The F-16's were doing low height rounds. On approach to the Dead Sea, the whole navigation system suddenly reset itself. The daring pilot landed the bird. HQ called up Motorola and ordered a team on the spot ASAP. The ground tests went perfectly, but every time the bird went airborn, it rebooted.

The pilots were getting restless. Flying on the border of hostile territory without navcom, with the Arabs pointing their earth-to-air missiles at anything that moves, wasn't that pleasant. Neither was debugging the whole navcom in-flight. Then someone figured it out.

The height of the Dead Sea relative to world sea level is -400 meters. As soon as the F-16 reached sea level, the navcom did a divide by zero, crashed, and rebooted.

When I tried to search for information on this topic I couldn't find out any references to mentioned Motorola training courses. I also failed to find any references to real-life events (dates, places, crash reports).

My question is: Were there any documented cases of aircraft navigation system crashes due to zero/negative height level?

Additional question that emerges from the quote above: Are there any Motorola microcontroller training courses that mention "Dead Sea case" as an example?

Related discussion on snopes forums

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default locale
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    Why would you need to divide anything by current altitude? I cannot think of any measure that would require that. – vartec Aug 12 '13 at 11:52
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    BTW. Another variant of the story is that the F-16s autopilot turned plane upside-down as soon as it crossed the equator. – vartec Aug 12 '13 at 12:05
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    @vartec, maybe a bug in the `SubmarineMode()` method :) – Benjol Aug 12 '13 at 12:11
  • @vartec descent speed/distance to ground = 1/time until you hit ground, once this reaches below some value sound warning. we avoid the test for 0 for descent speed and use only 1 comparison for an *amazing* micro-optimization – ratchet freak Aug 12 '13 at 12:15
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    if aircraft electronics could not deal with below sealevel altitudes, there'd be hundreds of aircraft crashes in the Netherlands every single day. Schiphol, Amsterdam International, is situated at -12 feet. Rotterdam International I think is at -7 but I don't remember exactly. Lelystad is at -10 or so (but has no scheduled flights, only executive and private aircraft), etc. etc. – jwenting Aug 12 '13 at 12:23
  • @ratchetfreak: but that would use radio-altimeter, which would give positive value. Besides, I'd imagine that EGPWS is completely separate from NAV/COM. – vartec Aug 12 '13 at 12:32
  • @jwenting on approach pilots request the current atmospheric pressure on the runway so they can fine tune the altimeter (and will use the radio-altimeter on final approach) – ratchet freak Aug 12 '13 at 12:40
  • @ratchetfreak I know that quite well, thank you. Yet the assumption of the question was that the simple fact that an area is below sealevel is enough to cause an aircraft to crash because of electronics failure, which is clearly bogus. – jwenting Aug 12 '13 at 13:00
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    There are so manny things wrong with this that its hard to know where to start. The main indication of urban legend is the rather vague terms in which the story is told. "The whole navigation system reset" is the sort of thing you say in a made-up story, but if you really know what you are talking about then you are much more precise. Aircraft nav system record several different kinds of altitude, and almost all of them can go negative at some point - frequently enough for it to be tested. – DJClayworth Aug 12 '13 at 13:59
  • Another reason this is going to be hard to validate is the secretive nature of Military technology. Just finding out whether Motorola manufacture any parts of the F16's avionics is hard enough. I like the subject of this question, but I fear its not going to get a decent answer. – Jamiec Aug 12 '13 at 14:02
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    For a 'divide by zero' to occur, the altitude has to hit EXACTLY zero - not going to happen every time (nav systems check sensors periodically, so there is no guarantee there will be an exact zero readingon any sample as the aircraft passes through zero ASL). Nav system don't get debugged in the field, they get debugged on simulators (because nav techs don't know how to fly military planes). Also I've been to the dead sea and I don't believe there is an airfield, let alone a military one, in the area. – DJClayworth Aug 12 '13 at 14:02
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    @DJClayworth Bar Yehuda, -378m. 1,200m runway, it could operate a F-16. – Loren Pechtel May 17 '23 at 05:14
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    There was a story about a flight simulator reacting very violently to crossing the equator. Violently enough to not test it on a real airplane (simulator and real airplane are supposed to act 100% identical). Supposedly caused by “up” being at least slightly to the north in the northern hemisphere, but being slightly “south” after you cross the equator. – gnasher729 May 28 '23 at 08:27
  • Slightly different from the report linked elsewhere since the problem was found in a flight simulator which apparently tried to turn the airplane upside down. Which would be very, very unpleasant. – gnasher729 May 28 '23 at 08:44

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I found two similar cases.

The USS Yorktown (CG-48) suffered a divide-by-zero error that brought down its entire network and caused its propulsion to fail. (ref, ref)

A group of 8 F-22 fighters had their systems fail when they crossed the international date line. (ref, ref)

I could find no evidence that a 0 foot altitude has caused a system failure at the dead sea or anywhere else.

  • yeah, I actually found the link to USS Yorktown article. Thanks for the reference on F-22 squadron – default locale Aug 12 '13 at 17:24
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    Adding a minor data-point: I found a Snopes forum poster claiming the story was propagated in a non-fiction book by Tom Clancy. I searched the obvious candidate: "Fighter Wing: A Guided Tour of an Airforce Combat Wing", but it didn't appear in there. – Oddthinking Aug 13 '13 at 01:14
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Not like the story, but negative altitudes have been problematic

This article reports that the aircraft navigation computer on a Lockheed-Martin C-130J Hercules started misbehaving when landing at the Dead Sea, which is at -1 240 ft MSL.

We had operated the aircraft as planned but now that we were on the ground the aircraft systems presented a myriad of navigation errors.

Where’s the Nav???

The aircraft navigation computer was unresponsive to our inputs and would not allow us to see the pre-programmed route we loaded for our return to base. The computer acted like we had run the aircraft into the ground. We immediately got the checklist out and began troubleshooting. The aircraft was unable to locate any GPS satellites, accept any updates to its navigation solution, and there were no NAVAIDs to tune. We realized we were going to have to make it back to base in the old school way. We pulled out the chart and made a plan to takeoff and fly following the road to the West back to base. We completed running our takeoff data performance numbers, configured the aircraft and commenced a maximum effort takeoff roll.

The aircraft climbed out and as we reached -300 MSL everything came back. The nav computer came back online and our GPS position confirmed the base was 20 miles to West.

According to the article, the crew reported this error and the manufacturer amended the flight manual with a caution, pending future updates to the navigation computer's software.

We got our own ‘Warning’ in the manual

Most warnings in flight manuals are due to someone doing something wrong. We contributed to one for doing something right! After the aircraft manufacturer reviewed the reports coming out of the Dead Sea airland operations they realized the navigation computer was not fit for operating below 400 feet MSL. The company immediately issued a change to the manual with a WARNING that the aircraft not be operated below an altitude of -400 MSL. I’m sure when they release the next version it will include updated navigation for flying to the Dead Sea and earth’s other extreme low elevation locations

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  • they can indeed be problematic, but more often than not it's because pilots disregard the negative altitude, thinking it's a glitch. Seen this happen at Amsterdam, with some pilots going around when they're suddenly over a runway that's way below them, while some others suddenly hit the brakes hard and drop very rapidly, with predictable results for the shock absorbers, tyres, and brakes. – jwenting May 16 '23 at 13:20