Mode S v TCAS & ALT v FLT Level

[Ahh-40612]

Following on from a recent "incident" I am looking for an expert to help with a couple of Mode S queries.
Traffic wrongly cleared to FL50 (should have been 5000ft) against someone else's traffic at 4000ft - low pressure = loss of separation. About 600ft on this occasion.
Mode S indicates 50 and 40 - so no way of knowing what pressure setting this relates to.
STCA went red - How did it know?

What would the respective TCAS's make of the situation?

Many thanks to anyone able to help.

[zkdli]

Ahh- 40612,
The incident you are referring to happened in TC airspace. The way you have explained it or had it explained to you is confusing.
First if mode "S" indicated 40 and 50 then there was 1000ft between them. However, if you are referring to the Selected Flight Level parameters that are downlinked from the transponder, this is different information and relates to the FL set by the pilots on the autopilot. This does not take into account the pressure setting on the altimeter of the aircraft.
What happens in the situation you describe is that the higher one is flying the Standard Pressure Setting and the lower aircraft is flying on the QNH.
The STCA works off the information transmitted from the alitmeter and notices the reduction in separation and activates, In the situation you describe this is what occurred.
Hold breath and wait for the discussion:O

[Ahh-40612]

Sorry
I was referring to the SFL indication we get on the radar.
There is no way of knowing what pressure setting this is based upon.

And on this occasion LL radar believed the aircraft was descending to 5000FT against their traffic at 4000FT.

[PPRuNe Radar]

Not a TC expert, but the following might be relevant.

1. Mode 'S' (and plain vanilla Mode 'C') will always transmit an actual level using the transponder with reference to the Standard Pressure Setting. It does not care what the pilot has set as the pressure datum on the aircraft altimeter.

2. The Selected Flight Level is what the pilot has set in his FMS or Mode Control Panel, or whatever piece of kit gives him an interface to his autopilot and management systems. It has no datum attached to it, except that the pilot (or system) will be using whatever setting is on the altimeter to monitor or achieve level compliance.

3. STCA processing is carried out on the 'raw' Mode 'C' or 'S' actual level information. In otherwords, using the reported Flight Levels from the aircrafts' transponders. An entirely separate piece of software carries out the altitude/Flight Level conversion for display to the ATCO. Thus, regardless of the displayed level datum being shown to the controller, STCA will always be working to a common value when deciding whether to give an alert or not.

[BEXIL160]

I've been waiting to post this for a while.....

http://www.c141heaven.com/64/pic_64_0612.html

Doubt very much wether it ever had Mode S:) :) :)

(slightly off topic.... sorry couldn't resist!)

Rgds BEX

[Minesapint]

Doubt if that had 64 code IFF!

Mode S altitude updates are every 25' as opposed (I think) to 100'. That should make STCA a little better later on. Its interesting to look at the Mode S monitoring data, things seem to be improving and the transponder faults are being sorted.