Mode A code change on Approach (B747-400). Why?

[11K-AVML]

Hope someone here can enlighten me with an odd observation a colleague made today.

When landing at Heathrow, with the work load underway in the cockpit, would it be easy to misenter a new Mode A code? As far as I'm aware it's not standard practice to made a change at this late stage so I'm not really sure why this would happen, but I have seen it occur!

I'm wondering, purely for better understanding of how these things can happen, whether a Mode A code could be accidentally reset when a different task was intended or through RT confusion, or maybe there was a genuine reason? I can't really say what the codes were for sake of annominity of the a/c, but the a/c in question was a B747-400 if that helps with flight deck layout.

[Piper19]

My rough guess is that on frequency change (approach to tower) the tower asked for a new squawk. i've had that a few times on vfr flights during frequency handovers I had to recycle the squawk. But why is a 747 in mode A? I should say they never fly less than C? And Tcas equipped aircraft even have mode S? Or am I wrong here.

[TopBunk]

I have and continue to operate out of LHR as my main base for over 17 years (fog permitting) and have NEVER been asked to change transponder code on approach.

In the old days (prior to turning on the txpdr for taxi as we do now), there may have been the odd occasion when the wrong code was set and it had to be corrected on departure.

Maybe the confusion is about the A1000 code. This should be set when PARKED at LHR so that should the aircraft be towed atc can do some clever stuff/at leaast know it's a tow-er.?

[HEATHROW DIRECTOR]

I worked as an air traffic controller at Heathrow Tower for nearly 22 years and as a radar controller for 31 and can think of no possible reason for changing the code on final approach. Tower controllers have no requirement to do such a thing and the code-callsign pairing would be lost.

[Ziggy]

But why is a 747 in mode A? I should say they never fly less than C? And Tcas equipped aircraft even have mode S? Or am I wrong here.

Mode A is the basic mode, the four digit code. Every transponder transmits mode A. In some countries the phraseology is: "Airliner 123, squawk 3553 mode A" for any aircraft transmitting in any mode.
In addition to mode A most transponders can transmit mode C, which means the current altitude (actually the current flight level) of the aircraft will show up on the ATC radar screen.
Mode S is where the transponder is able to transmit selected data to the ground unit, like airspeed, selected altitude and so on.
I stand to be corrected but mode S is not a requirement for TCAS, just mode C.

I am not familiar with the ergonomics of the B747 flightdeck, but all I can think of is that the crew accidentally changed the transponder code during the approach, and ATC told them to change it back.

Regards,

Ziggy

[punkalouver]

I believe Mode S is a requirement for TCAS installment so the unit can communicate with another TCAS to coordinate RA's. A TCAS installation can see an aircraft with Mode C operating.

[11K-AVML]

Thank you all for your helpful and informative replies.
I did wonder if there was some kind of mistake with the resetting to a A1000 or similar at first, but this wasn't the case - it actually changed the the Mode A to that of another aircraft and so there were two a/c with the same call signs present. Very uncommon I know. I also know that both these aircraft were airbourne at the same time.
So I guess from your replies it looks even more likily to be a mistake. The exact cause of which still intrigues me.

I stand to be corrected but mode S is not a requirement for TCAS, just mode C.

I am not familiar with the ergonomics of the B747 flightdeck, but all I can think of is that the crew accidentally changed the transponder codebut during the approach, and ATC told them to change it back.

I believe Mode S is a requirement for TCAS installment so the unit can communicate with another TCAS to coordinate RA's. A TCAS installation can see an aircraft with Mode C operating.
Thanks punkalouver, but I have to side with Ziggys post; TCAS uses Mode C for communication and can also interpret the presence of Mode A codes in an a/c's vicinity. Mode S however, isn't currently a requirement for TCAS, quite simply because not enough a/c are fitted with Mode S for it to have a beneficial effect.

EDITTED TO ADD:
06/01/07
I take this back, particularly the last sentence. I have since learnt that the reason for my belief that few a/c transpond Mode S data has been shown to be inaccurate. I had been using software that showed me which aircraft were transmitting, but that program has been found to be faulty. Now it's been corrected I can see the reality is quite the reverse!:ugh:

[punkalouver]

"Aircraft equipped with TCAS II also require at least one Mode S transponder to provide the data communications needed to coordinate RAs with nearby aircraft."

Where did I get this information? From an NTSB recommendation. I am sure there are other sources on the web. Look at the very bottom line of page 1. It does say TCAS II. I believe you will find that most aircraft with TCAS installed have version II.

http://www.ntsb.gov/recs/letters/1999/A99_55_56.pdf

[Graybeard]

Transponders began life as military IFF, Identification, Friend or Foe, as most of you know. It was silent until interrogated by a ground based secondary radar, or IFF interrogator. The transponder receives on 1030 MHz and transmits on 1090, just the opposite of the IFF interrogator.

Mode A was the original, a 4 digit code. Mode C, altitude, was added to the reply later.

Mode S supplements the A and C replies, and was developed in the mid-1980s to provide extended info such as tail number to ground based interrogators, and to open the path for TCAS II. In fact, ground based interrogators fell years behind TCAS in deployment.

The TCAS II device is similar to a ground based interrogator: it transmits on 1030, and receives on 1090. It can receive replies from any IFF or transponder, but can only coordinate avoidance maneuvers with another TCAS II and Mode S equipped aircraft.

Coordination is dictated by the plane with the lowest tail number. The tail number, or aircraft Identification code is wired into the airplane. It's bizarre, but a Mode S transponder without its tail number will be blind to other aircraft, and its own TCAS will be inoperative. That's one more variable for the investigators of the Brasil midair.

Back to the original question: I can't imagine why ATC would be asking for a code change on final approach. Did they screw up and assign the same code to two different a/c?

GB

[bookworm]

I did wonder if there was some kind of mistake with the resetting to a A1000 or similar at first, but this wasn't the case - it actually changed the the Mode A to that of another aircraft and so there were two a/c with the same call signs present. Very uncommon I know. I also know that both these aircraft were airbourne at the same time.


A1000 is the Mode S conspicuity squawk (http://www.eurocontrol.int/msa/public/faq/faq.html#qa62). If the aircraft doesn't need a discrete Mode A code because it is being tracked using Mode S flight ID, it (along with any others) can be assigned A1000.

The only reason I can think of for reassigning in flight is to release the discrete Mode A code for re-use. But I have no idea if that is actually done, and final approach seems like a bad place to request it.

[bookworm]

Thanks punkalouver, but I have to side with Ziggys post; TCAS uses Mode C for communication and can also interpret the presence of Mode A codes in an a/c's vicinity.

TCAS does not interrogate in Mode A. It uses Mode C, even if the target transponder is incapable of encoding altitude (and the content of the response is therefore empty).

BTW, I think you misunderstood punkalouver's post. TCAS II can detect Mode A/C transponders (by interrogating using Mode C) but the aircraft using the TCAS II must have Mode S.

[punkalouver]

So what about the original TCAS? And does anybody still use it?

[punkalouver]

In case there are still any doubters about whether Mode S is necessary.
http://techaidproducts.com/PDFs/TCASUncovered.pdf

TCAS is what gave mode S its biggest boost.