Need advice about negative FL?

[Warmer]

There is Table of cruising levels In Annex 2 Appendix 3.
Why there is FL -90 only?
How it could be -90 and why it is only value below 0? I tried to find answer somewhere on web...no chance.... :sad:
Thx in advance!

[WillDAQ]

If the pressure on the ground is significantly above 1013.25 (either due to weather or the ground being below sea level) then it's possible to get negative levels.

You'd just never normally see them as they're hidden by the transition altitude.

[HEATHROW DIRECTOR]

Minus flight levels are used by submarines :)

[le Pingouin]

Aside from it being a place filler to match FL 0 it means -1000ft based on 1013. If your surface pressure is 980hPa then FL -90 is ground level.

However it's not counting as you'd normally think it where -0.1 is 0.1 less than zero. You start at -1 & and increment up, so it follows the form of the positive FLs.

Just like:
FL 90 is 9000ft higher than FL 0
FL 80 is 8000ft higher than FL 0

Then:
FL -90 is 9000ft higher than FL -100
FL -80 is 8000ft higher than FL -100

Clear as mud?

[Warmer]

Well...According to the table:
FL 90 is 9 000ft higher than 0 :8, but it's not a point...
If I understand correctly FL -90 is 1000 ft below 0? Sorry, I am not a pilot, so is it an option of all altimeters to show -9000 ft Altitude [FL-90] below 0? And could it be more to the negative side...for example FL-80,-70... or what ever... Will it be processed by transponder and presented by Automated ATC system?

[le Pingouin]

Well...According to the table:
FL 90 is 9 000ft higher than 0 :8, but it's not a point...Ah shoot! Shouldn't do these things after a red or three. I'll fix the original.


If I understand correctly FL -90 is 1000 ft below 0? Sorry, I am not a pilot, so is it an option of all altimeters to show -9000 ft Altitude [FL-90] below 0? And could it be more to the negative side...for example FL-80,-70... or what ever... Will it be processed by transponder and presented by Automated ATC system?Yes, FL -90 is 1000ft below FL 0. I'm not a pilot either & in Oz we stop at FL110. FWIW negative altitudes are displayed on our system, although I suspect it counts backwards as you'd normally do, as in 100ft below A000 shows as -001.

[Warmer]

:ok: Thanks a lot...never met this kind of situation when Mode C indication was negative...But main idea is clear :D

[055166k]

In a low pressure situation it is not unusual to see minus mode C values...especially over the sea and low lying terrain. To correct an earlier contribution, submarines are not equipped with mode C transponders.....naval vessels are a regular feature around the UK coast and are frequently observed...particularly in The Channel. Sadly my ATC unit can find no dispensation in MATS 1 or MATS 2 or from SRG that would allow us to ignore warships.....therefore when providing deconfliction service we have to avoid.
n.b. codes are published in the assignment plan.

[Warmer]

Just to imagine...Is there any situation (Is it possible) to clear anybody to descend for example to FL -90...-80 ? This kind of level is presented in ANNEX, then You could use it...How it is..?:sad:

[HEATHROW DIRECTOR]

Sorr4y, in all my years in aviation I never heard of a minus FL so thanks to those who explained the situation.

055166k... Pair the codes of those naval vessels with interesting callsigns.. One afternoon we had Titanic, HMS Victory and Bismarck in the channel!!

[2 sheds]

In a low pressure situation it is not unusual to see minus mode C values...
You mean high pressure...WillDAQ had it correct.

2 s

[hvogt]

It seems the notation in question has been discontinued with amendment 42, which is good because I didn't understand it anyway.

What I don't get is how FL -90 can mean 1000 ft below FL 0. By my reckoning 1000 ft below FL 0 should be FL -10. Sorry Pingouin, I just can't follow you there.

[Spitoon]

Having looked at the actual document it just doesn't make any sense and I suspect it was nothing more than a typographical error. If it has been removed in an amendment without any other explanation it seems even more likely to be the case.

Nonetheless, many of the posts regarding the validity of negative FLs are quite correct. In certain circumstances an aircraft can fly at a negative FL and an encoding transponder will report this. But radar display systems will normally never display a negative FL because below TL the level data is converted into an altitude and operational procedures likewise use altitudes in such circumstances. FWIW, one radar display system I used enabled raw data to be viewed with a push of the button and negative FLs could often be seen.

[2 sheds]

What I don't get is how FL -90 can mean 1000 ft below FL 0.Because that is what the altimeter would (misleadingly) indicate, having "unwound" below zero, i.e. below the level of the 1013hPa datum.

I suspect that it has now been removed from Annex 2 because, in practical terms, even in the maximum recorded pressure and with a transition altitude as low as 1500ft, it would just not happen that even FL-95 would occur above the TA.

2 s

[hvogt]

Thanks, now I get the picture.

[le Pingouin]

It's most decidedly non-intuitive! It's counting Jim, but not as we know it :}

Ah bugger. It's correct that you need high pressure to get the -ve FLs & not low pressure. With high pressure the transition level goes up relative to the ground.