Perplexing Mode C problem with Madrid TMA controller

[richatom]

I had a perplexing issue with the Madrid TMA controller yesterday - I wonder if any ATC can explain the situation from ATC viewpoint.

I was flying at F110 and was handed to the Madrid TMA controller, about ten minutes before entering his TMA. After a couple of minutes, he called me and asked me to confirm my flight level. I confirmed F110, to which he replied that on his screen I was showing at F115. I replied that on three independent altimeters, all on 1013, I was at F110. He then replied that there must be a problem with my Mode C transponder. I told him that I was mode S, and there the conversation ended.

A couple of minutes later, I entered the Madrid TMA and as the transition level there is F140, I recalibrated to Q1029. I called the controller to tell him that I was now A110/Q1029, which he had asked me to do, and to which he thanked me and told me that I was now showing at the correct altitude on his screen.

Now it would seem to me that his SSR was unable to pick up the Mode S altimeter calibration, and that it assumed that I was calibrated at Q1029 instead of Q1013, hence I showed at the wrong flight level before entering his TMA.

Can any ATC shed any light on the likelihood of this, or offer any other explanation?

No other controller on the flight from Austria to Morocco had any problem at all so I am fairly sure the problem was not with my transponder.

[niknak]

Its just possible that the Madrid ATCO, or one of his colleagues, had entered the Madrid QNH into his radar instead of leaving at at the Standard Pressure Setting, and he hadn't reliased this when talking to you.
With such a high transition level, any readouts will be based on the QNH entered, if you don't put a QNH in the radar automatically reverts to the standard 1013.

[mr.777]

Difference between setting 1029 and 1013 would be about 480ft which sounds in line with what you described.

[LapSap]

Your altimeter encodes your altitude based on 1013, regardless of what you set on the subscale.
Most radar systems are fed the local QNH setting and automatically apply the correction if you are below the transition level. If, as you say, the Madrid transition level is F140, the Madrid controller's radar was correcting your "flight level" for what it assumes is an altitude (which is what it was going to be once you entered the TMA). As Mr. 777 says, the difference would be about 480 ft.
Clear as mud?

Edit: Thinking about it further- in fact on the controller's screen you were probably showing as A115 I would think - not F115 as he may have indicated.

[anotherthing]

Richatom

the fact you are Mode S equipped would not make a difference to the Madrid TMA controller - I don't think he has the equipment to decode it

[richatom]

Thank you to all - I realised that the likely anomaly was that the 28x(1029-1013)= 448ft - but what I am surprised to learn is that the SSR of a major TMA cannot differentiate between Modes C and S, and that the QNH has to be manually entered into the radar by the ATC. I thought that the whole idea of Mode S is that the altimeter calibration was also transmitted, thereby allowing the ATC to see the true altitude or flight level, whatever the altimeter calibration of the aircraft.

[Mister Geezer]

Most airfields will automatically show the level as an Altitude if you are below transition level.

Transition altitude at Madrid Barajas is 13,000. So I presume if you are below that then despite flying on 1013... your level will show as an Altitude on the radar.

[richatom]

Mister Geezer, that makes sense. But surely the Madrid controller must be well aware that aircraft outside of his TMA are on 1013, and therefore make allowances? He specifically asked me to make sure I was on 1013, so clearly he expected me to be on 1013.

[bekolblockage]

richatom
It would seem the controller perhaps doesn't fully understand the automatic correction his radar system is doing and the parameters it is working on or was confused by your reference to Flight Level. The fact that called 10 minutes outside the Madrid TMA obviously causes some ambiguity about what altimetry protocol you using.
As far as the Madrid radar is concerned, you were never at a "Flight Level". It was reading you at level that required QNH correction and displayed it accordingly I would suggest.

[Radarspod]

The transponder, regardless of whether Mode C or Mode S, will deliver the pressure altitude information always against 1013mB and the radar itself does no form of correction on that, passing the data onto RDP / display systems at the centre / tower as reported by the aircraft.The setting of the altimeter to a different QNH to 1013 will not affect the output of the transponder, as it is always referenced to 1013.

The RDP / display system can apply a QNH correction to display altitudes below transition rather than the reported flight level.

With Mode S enhanced surveillance, BDS 4,0 is downlinked which contains selected altitude / flight level information AND where available the barometric pressue setting in the cockpit. Unfortunately, baro setting does not feature in any of the european Mode S Enhanced surveillance mandates that I am aware of and as such generally is not provided or is unreliable. Otherwise it would be a useful piece of data. Transponder and avionics vendors would probably not be happy about providing it, as apparently it is an arse to implement.

[richatom]

Thanks Radarspod. Still a bit of a mystery though - when I was in the preceding airspace (transition level 50) the Madrid controller knew that I was on Q1013 as he specifically asked me to check that I was correctly calibrated. So if his radar was calibrated to Q1029 then surely it would have been immediately obvious to him why I was showing at F115?