Negative Flight Levels

[Beagle-eye]

I am doing some work on data from Air Traffic Control radar.

The technical specification for the data indicates that the Flight level can be negative. This has surprised me a little. :confused:

I can only assume that it may happen if an aircraft was landing at an airfield which was below MSL (Schipol ?).

Can anybody confirm that a negative flight level is/ is not possible and, if so, under what circumstances.

Many Thanks

B-E

[HEATHROW DIRECTOR]

It can also happen when a transponder is u/s. I've seen a fair number of light aircraft with altitudes registering as minus quantities, leading us to wonder if, in fact, we were seeing submarines!!

[Beagle-eye]

I've had a transponder go u/s on me and it has flipped between zero and something in the ionosphere. This was reported to me by ATC as there was no indication in the aircraft.

The FL data field (jn my radar data) has two check "bits". The highest bit indicated that the data is valid (i.e. within valid FL range) and the lowest bit indicates positive or negative FL. I'm confused by the need for the two bits. If a negative FL indicates a fault then why is there a validity bit :confused:

[Spitoon]

Every SSR I've ever had any dealings with worked internally on flight levels - because that's all the tx'der/altimeter in the aircraft report. Conversion to displayed altitude, height or whatever is done in the display processing and based on the datum selected by the user. Consequently there is plenty of time that an aircraft will be flying at a negative FL (as sensed by the encoding altimeter) although the pilot and controller will normally be happily working to some other, more useful pressure datum.

[PPRuNe Radar]

It's as Spitoon says, and can be very common depending on the radar equipment and its useage. In particular, area controllers operating primarily above the Transition Altitude and well clear of TMA operations have no real need for such a conversion from the reported Flight Level transmitted by the transponder to an altitude so it might not be displayed as anything except the former.

Imagine you are flying along at 900'AMSL quite comfortably on a QNH setting of 1043HPa (not unknown in the UK). Any radar without altitude conversion will be showing your Mode C as 000 ..... because with reference to the standard datum of 1013HPa, that's where you are. Now you drop down 100', and the radar will show you at -001, and so on. But amazingly you are still above terra firma. Similarly, if the pressure becomes even higher, then so will your indicated Flight Level on an unconverted radar screen.

[DFC]

High to low, watch out below and all that.

You have it the wrong way round.

QNH 983, and aircraft at 900ft QNH will be indicating FL18.

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Now if the QNH was 1043 your description would be correct.

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One good example is the QNE - the height indicated on the altimeter when the aircraft lands with the standard pressure setting 1013 (.2) set.

If the aerodrome is at sea level and the pressure is 1053 (max 1050 can be set on the pilot's altimeter)

The altimeter will show 1200ft below zero. However, remember how the altimeter works - it will not indicate -1200, it will indicate (-)8,800..........try it - zero the altimeter at say 3000ft and start descending.

The first mode C reading below zero is -900 then -800 and then -700. The ATC computer may cause something else to be displayed but that is what the encoder says if I remember correctly.

Regards,

DFC

[PPRuNe Radar]

You're right .... I knew I should have drawn it out like I always do normally :ok:

Now amended to prevent further confusion.

[Radarspod]

dammit, how did I miss that!!! And I've explained it a million times!:}

[Spitoon]

And the reason that I didn't give a numerical example is because I can never remember which way round it is and end up having to draw it out - and I was too lazy to do it to answer the question!

[Talkdownman]

Winding up altimeter subscale winds up Altitude.

[SINGAPURCANAC]

Apart from previously mentioned cases there should be one more reason.
Thales radar system that we use has some filters. One of these filters is level set up. Since unit is app we have to set up levels in such manner to cover our need. Top level is not problem .You may put whatever you want,usually FL300, but lower level is point. If you put FL 0 then reasonable assurance exists that you wouldn't be able to see all a/c especially immediately after departure/missed app point. So it is automatically set up at FL -30.
Also correction for QNH is mandatory to be done. The only problem atco is supposed to do it manually. Each time when QNH changes atco put new numbers into machine. Then favorite French machine calculate itself. No need for further atco intervention:)

[Odi]

983mb low? Up here in the Ice Station it's not unusual to have a QNH of less than 980mb. A few weeks ago we were giving a QNH in Sumburgh of 956mb with a Regional of less than 950mb.

Now that really does blow the mind when working out safe FLs!

[Bern Oulli]

I may have posted this once before but if you really want tp play around with an altimeter, sub-scales etc., go to http://www.flashsim.com/pubDown/f5alt/altimeter.html

Alternatively, go out for a brew.

[anotherthing]

It really is not difficult

"wind on pressure, wind on height" as stated above.

Its a very simple calculation, one that any ATCO should be able to do in their head, similarly the implication of certain differences in pressure should be understood (without having to work out actual figures each time) i.e. a pressure difference of 17 millibars compared to 16 etc.

[Spodman]

I understand it was left off the specification of our system, causing a system crash in testing...

[chevvron]

I suspect that Beagleye is referring to negative altitude rather than negative FL, which is entirely possible if the QNH hasn't been input into the radar processor as the display will only show altitude if converted from 1013 eg aircraft on QNH 1003 at sea level will show 300ft (approx) below sea level. (err have I got that right?)

[Beagle-eye]

Hi all

To pick up on what Chevron says .... the field allows for a negative flight level so I am trying to determine what would cause a negative flight level.

As background .... we take the FL and correct it for local barometric pressure (QNH) to get a reasonable approximation of height. We then use this to direct other sensors at the aircraft.

Thanks

B-E