TXPDR use at LHR?

[WOK]

Anyone from 121.9 or .7 listening?????

A recent change in procs has us selecting the txpdr 'On' before pushback (rather than lining up) to provide a secondary return to Ground movement radar.

It is implied that, while taxying, we should squawk 0000+modeC.
Is this right? Or could we select the squawk issued with IFR clnc?

And, for anyone working outside LHR: Would this procedure cause you grief, since SOPs now have us squawking on the ground at every field we fly to.

Thanks, if you can help.

PS I always thought A0000 meant "I'm not sure if my mode C is accurate"?

[ATCO Two]

Hi Wok,

This instruction applies to mode S transponders only, (permanently allocated internal code to each aircraft).

The instruction states :-
Heathrow Airport is equipped with Mode S Surface Movement Radar.

Pilots must ensure that:-

1. The aircraft transponder is set to transmit mode S signals from the commencement of pushback.
2. The aircraft transponder is set to transmit mode S signals after landing continuously until the aircraft is fully parked on stand.

[WOK]

Thanks, I understand the Mode S use.

In order to activate Mode S we also have to activate Modes A&C.

My real question is: Does it matter what Mode A we squawk while taxying?

It's actually a bit awkward to squawk 0000 until line up, because when you do line up you have:

(i) Stowed the Comms log on which the clnc is recorded, and

(ii) Forgotten the allocated squawk.......

Can we squawk the allocated A code while we taxy??????

Thanks

[M.Mouse]

How do you do it on a 747-400 or 777? Both these aircraft switch the transponder on or off when the aircraft takes off or lands.

I also have heard our new procedure is ammoying some other airfields. Should it only be at LHR?

[ATCO Two]

Squawk the currently assigned code while taxying and only do so at Heathrow (at the moment). Not sure about the B744 and B772 situation - perhaps an engineer could provide some input?

[WOK]

Thanks! I'll suggest to fleet mgmt they contact Heathrow twr with a view to changing the notice.

FWIW I believed that 777, 744s were being (or have been) modded to remove the Nose gear compression inhibit from the txpdr. I believe that BA's 767s have already been modded, but I can't remember why the mod was done. (I don't think it was connected with Mode S gnd use).

As for other a/fields, I can only say that JFK haven't yet complained about the Mode A/C clutter, but I guess that 99.9% of their tfc doesn't squawk til line-up.

Thanks again, folks.

[GTH]

WOK
I am the project manager for the system being put in at Heathrow that is causing this change.
We may need to check the wording of the instruction to make sure it is clear.

Just some background information in case anyone is interested. The new system uses multilateration
to calculate the position of aircraft from their Mode S transmissions. It consists of 15 receivers
around the airport feeding data back to the control tower. The positional information will be displayed as
part of the ASMGCS (SMR to you and me) displays in the VCR.

One of the features of Mode S transponders is that they transmit a squitter message once per second,
containing the Mode S address (which is fixed and unique to each aircraft). This happens even when the
aircraft is on the ground, as long as the txpdr is not switched off or to standby, and it is this signal that we
use to calculate positions. Note that for all aircraft with weight-on-wheels switches, the Mode A/C txpdr is
disabled when on the ground (assuming it has been set up correctly). Therefore, leaving your transponder
on should not cause clutter on radar displays.

As for your Mode A code, you should squawk your allocated code when taxiing. We only want you to use 0000
(or 2000) when you reach the stand. By the way, 0000 is a general use code for local allocation. As I say, we
need to check our wording , because if you squawk 0000 whilst taxiing we can't display your callsign.

Finally, this type of system is currectly being installed at at least 5 European airports (Frankfurt, Zurich, Schiphol,
Brussels, Paris CDG), and eventually up to 60 US airports. Therefore the procedures for Heathrow will become
widespread.

Graeme Henderson

[RaRadar]

I'm involved with installing the new Mode S radars, terminal and en-route not multi-lateration, and we can now see aircraft moving around the airport on Mode S. Unfortunately some aircraft do not disable their transponders when the get to the stand and sit there squawking hour after hour.

I've not seen the situation at LHR but certainly the case at LGW, FRA and DUS.

As there's no response to Mode A/C, RF interference is minimal but 20/30 labels virtually on top of each other tends to clutter the center of the screen! I suspect a gentle reminder to turn off may be forthcoming soon.

[Mick Stability]

Thanks very much for the authoritative info chaps. Unfortunately we only get a series of intructions from our technical staff with no rhyme or reasons. I suspect they know as little as we do.

Could we perhaps persuade you to give us chapter and verse, and also the full list of airfields participating? I'm one of those people who are delighted to jump on the bandwagon, but would like to know the itinerary first.

[Sniff]

Out of curiosity, will it be required for a/c under tow to squalk mode S in the future?? Maybe we will see towing A/C nav lights at night again (as opposed to nothing with a towbarless tractor)

[GTH]

Mick
I can't give you a definitive list of airports taking part because there is no European programme
for this (unlike in the US). The airports listed above (plus Vienna who I forgot), have all at least
gone to tender. Frankfurt, Zurich and Heathrow have all got systems installed that are being
optimised/tested now. We have been working with these airports to devise common procedures. Eurocontrol may be taking a lead on this soon.

Sniff
There are no plans to change procedures for towing aircraft at the moment. We are looking at a number
of options, in particular placing low power transponders on vehicles, including tugs. There is
a regulatory issue before we can do this on a large scale.

Graeme Henderson

[HEATHROW DIRECTOR]

I believe our surveillance radars are gated to cut out SSR returns on the ground at Heathrow, and probably other large airfields. We still see SOS appearing when the guys in the radio workshops muck sbout though!

[WOK]

GTH, RaRadar:

Thanks for the definitive answer, chaps.

Another change to SOPs imminent by the looks of it!

[NW1]

Hi WOK,

(Would send this by email instead of boring everyone else with it but I couldn't make it work ......)

I had also heard some say that the new xpdr. handling could be interpreted as needing to keep 0000 until the before TO Cx., which is a pain as you say, so we mentioned it on the sim. the other day. The official word is that 0000 is only needed before you get the clearance and the code is entered on receipt of the clnc. as before. 0000 is there as an obviously "null" entry to make it clear that you haven't entered the correct code yet. I try to remember to write the code on the t/o proforma anyway because I've had the xpdr reset itself during taxi and I don't like scrabbling for the comms. log at lineup either!

Hope that helps.