Mode C below Transition Alt.

[engineoff]

Would be interested to know what the controller sees when an aircraft is displaying mode C yet flying below the trans alt. Is the level displayed on the screen always 1013mb based, or is it converted to QNH/QFE so the controller can quickly see if the a/c is at his designated alt/ht.
Tried getting a Mode C alt verification when flying on the QNH and took the controller a fair while to reply. Did he have to convert manually from 1013 to alt?

[ATCO Two]

Hi engineoff,

In the LTMA where the TA is 6000 ft, the actual QNH is input into the radar processor. The radar displays show altitude when the aircraft is below the TA and flight levels when above. Altitudes are displayed as two figures, e.g. 52 = 5200ft and flight levels as three figures, e.g. 070 = FL70. Hope this is useful.

[identnospeed]

engineoff,

If you were climbing at the time of the verification request then it would take the controller a little time, because our radar display is about 6 seconds behind what is actually happening. So if you are climbing at 3000 fpm you could be up to 300 ft higher than you are indicating on radar. Therefore the ATCO has to make an adjustment due to the time lag.

However, if you were level at the time, the most likely reason is that there was another a/c radar return above you (eg FL330) whose mode C was "garbling" with yours and the controller had to wait until your height readout was visible before giving a response.

Later

INS

[The Fat Controller]

From the Scottish perspective, we also input the QNH(Glasgow)on the Lowther Hill radar, for the TMA deps from PH/PF/PK, and have a little table for converting FL to altitude for those other radars that give us FL info, hope this answers your question.
In my previous life as an ATCO at EGPK, we could input our own QNH into the radar system.
Regards,

------------------
FATCO

[Baby Blue]

Interesting connundrum. From what I've seen ther is no standard across radar units. The problem is generated by the information you give the 'computer'(otherwise known as the TAB). Many units that operate outside Class A-E Airspace set the TAB to 1013. That gives them a "FL" readout regardless of whether the ac is above/below TA. This is useful when trying to avoid unknown traffic by 5000'. Alternatively, when controlling within a TMA, it is more useful to set the TAB to the QNH. In this circumstance all ac within that particular piece of sky should be under an Air Traffic Service and on the same altimeter setting. Therefore it's better to have a WYSIWYG.
Hopefully this has 'clarified' the situation . The most important thing to ensure is that the controller knows what is set in the TAB. Consequently, he/she can allow for the difference between QNH and SAS when necessary.

[ATCOMAN]

At fraggle we input the locat QNH and get altitude readouts below TL. The only problem we tend to get is with a TA of 3000ft, if you try to verify Mode C below that level, a lot of folks have already re-set the altimeter to 1013 so you get a Flight Level report when the radar is still giving you an altitude readout - hence pause while ATCO tries to do metal arithmatic and probably gives up! In my experience a mode C error is usually of very large amounts, not just a few hundred feet. Incidentally, can you do a Mode C check while the aircraft is still on the ground and indicating +/- 200ft of 0 ?

[engineoff]

Thanks for your replies to my initial post; very interesting to know what happens at the other end!
Perhaps should really start another post but would be interested to know the resolution of the Thames Radar/Heathrow Special facility. Went down the Thames on one of the heli-routes today and wondered if the controller is able to differentiate between traffic on one side of the river and different traffic on the other side, particularly at the Thames' narrowest points. Any information much appreciated.

[ATCO Two]

Hi again engineoff,

Resolution is quite good on the Heathrow 23 cm radar and we can differentiate between primary returns on the North and South banks of the Thames. The SSR labels will probably garble as you pass each other, but that is only a minor distraction. Why not come and see for yourself? E mail me if you're interested.