A while ago I heard some stories about misbehaving avionics (software of course, what else?) caused by operation below sea level. Naturally, there are some airfields few feet below zero like many airfields in the Netherlands but there the altimeter will only show a negative altitude when on the ground, never in flight. If you really want to cruise below 0 for some time, Israel is the go to place. The Bar Yehuda Airfield in the Judean desert at -1266ft has a pattern altitude of -266ft. That leaves enough room for safe sub zero cruising.
But even in Germany you can (briefly) cruise below zero. Since Germany’s largest surface mine at Hambach is all the time in the news because of climate protests I had to look up how deep it is. It is even deep enough to fly below 0 MSL while still staying above 500 AGL:
And it is large enough to safely climb out again:
What surprises me is that is not a restricted area like the Polish and Czech mines.
Could be an interesting thread.
I wondered if gray code (the primary output of an encoding altimeter like a KEA130A) can represent negative numbers and on a quick google it appears that it cannot. Yet this would be such a huge problem (on the ground, as well as airborne) that an offset must have been built in. And sure enough an offset of 12 has been applied – here. So that is 12 × 25ft or 12 × 100ft?
The altimeter subscale is just a multiturn potentiometer which returns an analog voltage…
I doubt there are any issues with negative altitudes in glass cockpit avionics? you could mimic that by landing in Shoreham or LeTouquet and setting wrong QNH?
It’s possible to fly under sea levels in Jordan and/or Israël but most of those areas are restricted 
On further reading, the gray code in avionics supports down to -1200ft.
But lots of coders make mistakes, like that famous one where some airliner INS crashed at the 180 deg longitude point. And having done a fair bit of ARINC429 I can see how they did that.
But that’s a slightly different thing to flying below MSL. There are obviously not many places. But flying above MSL while getting the avionics to think you are below MSL is fairly easy.
not quite as deep 
Death Valley, -210’, and 40°C early morning…
I did a tour of Israel…back in 2012? It was not something easy to organise airport/airfield passes for to hire an aeroplane – its definitely “who you know”.. but not all of it is restricted, I think the Dead Sea area was do-able below sea level at that time? I know that I was briefed “Dont go north of Haifa, keep our side of the sea of galalee, and avoid the west Bank”.
Regards, SD..
I believe Schiphol EHAM is -11 ft MSL, Lelystad (EHLE) too!
EHLE is -12 so we beat Schiphol
Clipperstorch wrote:
The Bar Yehuda Airfield in the Judean desert at -1266ft
That will cause issues. ADS-B and 1090 Mhz Mode-S can handle down to -1000ft in 25ft increment mode and -1200ft in 100ft increment mode.
GPS seems to be able to handle negative values with no issues but some cheap chips might not do this properly.
FLARM can not handle negative values at all (and not above FL267 by the way)
GDL90 seems to be able to handle all negative values.
There are 2 ways this is done. Either it is positive values only but there is an offset. This is how the minus 1000 and minus 1200 limits happen. Or there is really a way to use every allowed number as a negative value (GPS, GDL90). Or there is nothing at all ;-)
Then regarding the software it is rarely tested so I assume many systems will have issues. (The same if you plan a flight past the north pole or the 180 meridian).
I think I can not assign a negative target value on our certified autopilot. Also setting a negative approach minimum in our G500 seems not possible. Not sure if vertical planning mode in the GTN750 supports negative values and the list goes on.
Further I assume naturally aspirated engines etc. could be overloaded as they would produce more power than rated.
I used to work for mining. Down there it was about 1,1bar and all kind of equipment gave up. My barometric watch did not display anything. Modern cars use down there would complain as their barometric sensors indicated “false” values etc.
Sebastian_G wrote:
Further I assume naturally aspirated engines etc. could be overloaded as they would produce more power than rated.
I don’t think that would be an issue as low temperatures have a much larger effect. E.g. 0°C at sea level and 1013 hPa is already lower than –1800 ft density altitude.
–20°C is not that uncommon in winter where I live and that gives a density altitude of –4500 ft. No one has second thoughts of applying full power…
