What the "S" is going on at LHR?

[Bof]

Guy I know said he was going into LHR the other day, and picked up a message on the ATIS for pilots to leave their Transponders on Mode S after landing with appropriate Mode 3A set.

Apparently, this confused all of them on the flight deck and he asked me as an 'old and bold' if I knew why, especially as he said they couldn't find a Mode S switch on any of the panels!!. Clearly I am too 'old and bold' as I didn't know either. Not wishing to appear too ignorant, I airily replied that I wasn't quite sure, but would find out.

So guys, can you illucidate? Why the Mode S?

[Duke of Burgundy]

Bof - It`s so that their new Surface Movement Radar system is able to maintain the identity of each aircraft as it moves around the airport.

From what I understand the Mode S "squitter" transmits the aircraft`s flight plan call-sign once per second and it is this which the system uses.

I`m sure one of the tower experts will give the Full Monty about this in time but I hope this is enough to be going on with.

[ATCO Two]

Heathrow has installed an A-SMGCS - Advanced Surface Movement Guidance and Control System - the first in the world. Without going into too much technical detail, A-SMGCS is a surface movement radar system that displays aircraft callsigns and allocated stand numbers on the ground radar screen. When a pilot switches on the mode S transponder with the mode A code selected, the mode A code is transmitted via mode S, and a label is generated which is derived from the code/callsign database. If the transponder is switched off the label on the display will be lost. If A0000, A2000 or standby is selected, the
A-SMGCS will attempt to display the aircraft registration derived from the system database.
Another feature of the equipment is the RIMCAS - Runway Incursion Monitoring and Collision Avoidance System. This provides audible and visual warnings in the event of runway incursions.

The message is therefore, leave the transponder on after landing until parked on the stand, and also select the transponder on with the allocated code during pushback. Hopefully the service pilots receive from Air and GMc in the dark and in LVPs will improve markedly.

[250 kts]

I assume all the operators were informed this was being introduced and of the procedures and this was just one crew which got through the net!!!

If not LHR management need a good kicking for not promulgating the information correctly. Mind you from what I hear the NATS management at LHR need a good kicking anyway-eh, Gonzo??

[Bof]

Many thanks for the info guys - the system works!

[Departures Beckham]

ATCOtwo: Does the "Mode S and Mode 3a" parts have any significance or is the transponder just left on in the same way it is through flight?

[ATCO Two]

Hi DB,

As far as I am aware the transponder is just left on with the mode A code selected. The mode S capability is probably transparent to the pilot. Perhaps an engineer can confirm.

[Gonzo]

250kts,

As with any change in procedures, some crews got it, some didn't (most did, and they've been periodically asking us for the last few months if they should squawk on the ground, so we know some sort of advance notice was out there!

And what have I done now to provoke your curiosity?

Or do you know something I don't?????????????

Gonzo.

[GTH]

There is more information on this subject at
http://www.pprune.org/forums/showthr...threadid=51765

As you can see we have been trying to get the word out for quite a while. For most airlines all that was required was to keep the transponder in the normal position until on stand. The squat switch prevents problems with other radars.

[Willie Eckerslike]

Mode S is a function which allows Transponders to transmit data as well as the standard code (mode A) & altitude (mode C). It is used mainly by TCAS to allow two TCAS systems to co-ordinate avoidance manouvres between them.

Most Transponders have the ability to deselect either RAs or TAs & RAs. This is normally used when operating engine out as the 2 TCAS systems may co-ordinate an RA which demands a high rate of climb or descent which may be undesirable on one engine. With your mode S switched off the other aircraft will, on getting no response from your TCAS, initiate an independant RA.

So to operate your transponder in mode S leave it in the normal inflight (TCAS on) position.

[Departures Beckham]

I know that TA stands for Traffic Alert, but what does RA stand for? Also, does a TCAS get data relating to the position of other aircraft from equipment aboard the other aircraft (e.g. altimemter) or does the TCAS calculate the aircrafts position itself (in a similar way to primary radar).

Thanks in advance.

[Scott Voigt]

Departures;

Actually TA stands for Traffic Advisory. RA stands for Resolution Advisory. When you get a TA, the box tells you that there is traffic out there and to start looking. When you get an RA the box tells you to do something. It can be to climb, to descend or to monitor vertical rate, which basicly means don't touch anything or the citation gets it <G>....


regards

Scott

[ATCO Two]

It has been noticed that since the instruction to select mode A (with allocated code) at pushback, or leave mode A selected after landing has been removed from the Heathrow ATIS, compliance with this requirement has lapsed considerably. Please guys make it part of your SOPs when operating into Heathrow. Many thanks.