Altimetry Q for Mr ATCO

[HectorPascal]

Chaps, please clear up a slight disagreement.
If I'm cleared to FL 70 and I leave QNH (say 1000) set and climb to 7000' what do you get for a mode 'C' readout?
A. FL 70
B. 7000'
C. FL 6.61 (actual FL)
Any responses gratefully received.

[fly4fud]

I'm no ATCO (in fact more of an ATCO nightmare... :) ), but I know that all altitude infos are sent in standard setting, i.e. 1013 format. Smaller A/Cs (paperbag types) have so called encoders or encoding altimeters, bigger ones ADC. All those gizmos will transmit you altitude in 1013 format :cool:

[cossack]

Altimetry is a real headscratcher sometimes!

When QNH is 1000mb the transition level (first available flight level above transition altitude) is FL 65 (assuming transition altitude is 6000 feet), ie you've lost half a flight level.

If you've climbed to 7000 feet QNH 1000, you're actually at FL74 (as near as dammit).
On a radar display, you will show as FL 074.

If the QNH was 1026 and you climbed to 7000 feet you would show at FL 066.

Most radars will show you at an altitude when you are below the transition altitude and a FL when above it regardless of what you have set on your altimeter.

I hope this is of some help.

[ 13 July 2001: Message edited by: cossack ]

[U R NumberOne]

Just to follow on from what cossack has said. In order for the radar to do this the ATCO must input the QNH whenever it changes - saves us a whole lot of 30 times table calculations!

[cossack]

UR No 1
Our TELS do that for us! Usually very quickly too! They must be waiting like a coiled spring! :D

[gul dukat]

Hi Hector! heres a plan !! why not hop in and VISIT an atc unit ! We would be pleased to show you round! even demonstrate the old changing of the qnh !

[HEATHROW DIRECTOR]

Ahh.. but apparently you should not believe everything you see on a radar. Some years ago I had the doubtful privilege of showing some ant-noise people round Heathrow Approach. They were blabbing on about aircraft regularly flying down Windsor High Street at 3-500 feet and I attempted to re-assure them that the actual altitude was more likely to be 1200-1500ft, as clearly indicated on our SSR labels. "Poppycock", cried the leader "You people can just change those settings at will to confuse us". Unfortunately nothing any of us could tell him would convince him otherwise.

[Red Spitfire Driver]

Just to answer the question.

If you are flying beneath the LTMA on the QNH - on the radar your mode C will show (say) 25 - for 2,500ft.
When you climb and get up to the Flight Levels (say) FL65 the radar will show 065 which tells us radar chappies that it is a Flight Level. :cool:

[MightyGem]

The mode c readout on our transponder is based on 1013, regardless of what the bar alt is reading. So, as it's the transponder that sends the signal and not the bar alt, surely a FL must appear on the screen.

[cossack]

MG
As UR No 1 said earlier, the radar processor has the QNH input into it and so calculates your altitude before displaying it on our screens.
If you're flying outside the area covered by the processor you will be displayed as a FL, but within it you will be altitude.