Radar Processing Facility

[Capt Chambo]

There is currently a thread running on the Questions forum about a bmi A320/321 and the use of the transponder without Mode C.

Would an ATC'er like to explain what is meant by"....... to squak mode A only to check some radar processing facilities,"?

As I have also posted on the Questions forum the transponder fit on the bmi A320/321's have no facility to isolate the Mode C.

Your opinions would be welcome.

[The Fat Controller]

Hi Capt Chambo it's FATCO again.
As I mentioned in my other post, it was the night that RVSM was introduced, and the "attention getter" for non-rvsm aircraft in Scottish airspace had been changed on our radar displays.
Basically if an aircraft is flying at FL195 or above and the flight plan has the aircraft as non-rvsm a little equals sign (=) appears to warn us.
The sign also appears if an aircraft drops it's mode C at any level, and all I was doing was asking someone to stop mode C to see if this happened instantly, and it does, there was nothing sinister in my request, but it has brought to light the other issue of the selection of mode C on the aircraft in question.
Hope this fully answers your question.
Regards,

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FATCO

[Capt Chambo]

FATCO thanks for your reply. I didn't think for a moment that there was anything sinister in your original question.

As I have posted in the other forum, non of the bmi A320/321's have the capability to isolate the Mode C. Although it would appear that other A320/321 operators may have a different transponder fit.

If nothing else your original question has got us all into the books!

[aluminium persuader]

Capt C -
So what do you do if ATC find that your mode C is mis-reading and instruct you to squawk A only? Do you think that company engineering believe that having two transponders will render the chances of dodgy mode C on both almost nil, or is it something in the regs? (Not hostile btw, just curious). Every a/c I've flown has had switchable mode C, usually along the lines of off/on/alt/tst

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Once more unto the breach, Dear Friends...

[Capt Chambo]

Aluminium Persuader.

You ask a good question and I hold my hands up and say I don't know the definitive answer. I don't believe that we can squawk A only, as I have said before we have no Mode C switch. Once the transponder is on, it is in mode C.

However the chances of us getting into a situation where the height readout is suspect without being able to correct it must be fairly small statistically.

The aircraft has two transponders which are normally fed their Altitude readout from their respective ADR. In the event that the one transponder fails, we switch to the second one. However either transponder can get altitude data from the "spare" ADR. I don't know how you work it out statistically but you would need either both transponders to somehow scramble the altitude information, or you would need to lose all three ADR's, before we are in a situation where we cannot find a way of giving you a proper height readout.

If you were to lose all three ADR's then you are down to standby instruments only.

However reading the manual about emergencies, under the section about the loss of all three ADR's, the book says that amongst the list of inoperative items is "ATC ALTI MODE". Now maybe I am reading more into this than is really there, but the inference there is that the Altitude mode is lost but not necessarily the Alpha mode!

Good questions! I wish I knew the answer!

[BEXIL160]

...all...

Actually there is a (little used oft forgotten) procedure that ATC can use if your mode C is wrong and you can't switch it off without turning the mode A off as well. We can ask you to set 0000, which should tell everybody looking at their "tube" that your mode C is a bit wonky and should be disregarded.

This way at least we (in ATC) keep some sort of SSR target, and bearing in mind the loss of Primary Radar cover out to the west of these fair isles, isn't such a bad idea.

Rgds
BEX

[Spodman]

Have come across this a few times, our manual says this:

"2.1.10.8 If, following confirmation of the correct pressure setting, the discrepancy continues to exist, the following action shall be taken according to circumstances:

a. request the pilot to stop Mode C transmission, provided this does not interrupt the operation on the transponder on Mode A and notify the next control positions or ATC unit concerned with the aircraft of the action taken; or
b. inform the pilot of the discrepancy and request that Mode C operation continue in order to prevent loss of position and identity information on the aircraft and notify the next control positions or ATC unit concerned with the aircraft of the action taken."

There are other occasions we can't communicate what we want from the pilot, and then just go with "b".


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I am not an animal, I am an ATC.

[ferris]

G'day Spodman,
how's things down there in wine country?
I'm enjoying life in the sandpit.
Ferris