Why mode A when squawking 7700?

[AlexEvans]

According to my CD-Rom on Air Law, when squawking 7700 (mayday), 7600 (radio failure) or 7500 (unlawful interference), it is to be done in mode Alpha.

The narration says so.

The set in the example picture has an 'Alt' setting which I guess is mode Charlie but is selected to 'On' which I assume is mode Alpha, confirming the narration.

Why not squawk 7700 in mode Charlie during an emergency?

[BackPacker]

It might just have been awkward writing.

SSR Mode A = four digit squawk code
SSR Mode C = pressure altitude

You cannot use mode C without using mode A, but you can use mode A exclusively. But when they refer to mode A 7700, they may just want to indicate that your mode A readout should be set to 7700. This might not automatically imply that mode C should be turned off.

At least, I cannot think of any reason NOT to use mode C in an emergency. If nothing else, they can follow your flight down, determine your descend angle from the radar tapes and extrapolate your flight path to the point of impact if you're lost in the ground clutter or other surface effect that might degrade radar performance.

Perhaps if you ask this in the ATC forum you'll get a more enlightening answer.

[dublinpilot]

BackPacker has it right.

This confused me too when I was training!

When you squwak mode c, you are actually squwaking mode A (code) AND mode C (altidude).

If you have mode C, then continue to use it in an emergency ;)

dp

[bookworm]

You cannot use mode C without using mode A, but you can use mode A exclusively.

In fact with most modern transponders you can't, at least you can't suppress replies to Mode C interrogations. If you select On rather than Alt, or if the transponder has no altitude encoder hooked up to it, the transponder will respond to Mode C interrogations with framing pulses but no information pulses. TCAS relies on this (it doesn't use Mode A) to display targets with no altitude info.

As BackPacker says, when they refer to "mode A 7700" they just want you to set that squawk, they're not asking you to switch away from Alt.

[BackPacker]

Bookworm, if you have an old transponder which only supports mode A, will TCAS react to it?

[bookworm]

Bookworm, if you have an old transponder which only supports mode A, will TCAS react to it?
Taking your question literally, I don't know. I don't think so, because I think TCAS will only "react" to responses to its own interrogations, but I'm not 100% sure. TCAD (which just listens but doesn't interrogate) has special algorithms for distinguishing a Mode A from a Mode C response.

Bear in mind that the
current TSO for transponders (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgtso.nsf/de2f3bfa11f1ee2a85256bd400466f2d/acbba541ff4c071d86256dc10067f209!OpenDocument) dates from 1973, and the requirement to support Mode C may be considerably older than that. So by "old" we may be talking museum-piece here.

[niknak]

From an ATC perspective it's useful if you have mode C selected but it's not that important.
If you are squawking any of the above codes, mode A or C, every ATC unit equipped with Secondary Radar will move every other aircraft out of your way to allow you to get on with the matter in hand.
Equally, if you are near an aerodrome we know has no secondary radar, or any radar at all, we will tell them and do the same.

Either way, it's your emergency - you aviate, we'll do the best we can to do the rest...

[MikeJ]

The correct responses to this post only show that conventions should be changed. You cannot squawk mode C without also squawking mode A, as has been made clear above. Also, unless you have got Mode S, you cannot squawk anything that is not mode A.

Not that long ago, when signing out from a radar unit, I was told "squawk 7000 mode alpha". My reaction was to take it to mean - NOT mode C, so I also switched from ALT mode. Otherwise, why would the "mode alpha" instruction be given?

I suggest that whatever the technical definitions, ATC requests should be simple: Mode C means switch ALT on, Mode A means switch it off. Then we will all know what to do!

[ShyTorque]

"Bookworm, if you have an old transponder which only supports mode A, will TCAS react to it?"

Yes, it will. It will assume that the other aircraft is at the same level and play safe by giving an alert against it. This is a problem because the pilot of the TCAS equipped aircraft gets the alert but does not know whether to look up, down, or level! It is not a good idea to rely on azimuth avoidance because the equipment is unreliable around the 12 o'clock position. A mistakenly applied 'avoidance' turn could put the two aircraft on a collision course, rather than into safety.

Which is why, if a transponder has Mode C/Alt facility, it is better for that mode to be selected, rather than just plain vanilla mode A.

[bookworm]

I think you miss the point of my posts above, ShyTorque.

Which is why, if a transponder has Mode C/Alt facility, it is better for that mode to be selected, rather than just plain vanilla mode A

All transponders flying today (anything that meets the TSO-C74b/c which has been around since 1973) respond to Mode C interrogation even when they are switched to On rather than Alt, and even if they have no encoder attached. Here are the relevant bits from the TSO:

2.4 Interrogation. The equipment must accept and reply to interrogations on at least Modes 3/A and C

2.14 Pressure-Altitude Transmission. ... The transponder must be provided with a means to remove the information pulses from the Mode C reply when requested by Air Traffic Control. The transponder must continue transmitting the framing pulses on Mode C when the information pulses have been removed or are not provided.

(This is from TSO-C74c. I believe TSO-C74b, which shows as historical on the FAA site, also requires Mode C framing pulses but with no information pulses as it is for transponders that cannot take an encoder.)

There's no doubt that these show on TCAS, and that for all the reasons that you correctly describe it's better to have them on Alt if they have an encoder attached.

The question I was answering was about pre-1975 transponders that are incapable of responding to Mode C interrogation at all.

[norilsk]

But if you have a very old transponder like mine it has only mode A & B and mode b has been obsolete for years. Yes 7700, 7600 7500 still produce the required effect!

[ShyTorque]

Bookworm,

I think you missed the point that I wasn't actually replying to your post!

Backpacker asked that particular question; you said you didn't know the answer, which related to TCAS, not ATC radar displays.

[bookworm]

ShyTorque

I'm sorry if I misunderstood. I assumed that when BackPacker used the word "old" transponder, it was in contrast to my "modern" (by which I meant TSO-74b or c). The transponders that your reply refers to are "modern" in that sense of the word, even if they are 30 years old!

An "old" transponder that replies to only Mode A will not be detected by TCAS alone, as TCAS interrogates in Mode C only. The issue I didn't know the answer to is whether TCAS would "react" to a Mode A reply triggered by a different interrogator.

[ShyTorque]

Bookworm, Thanks.

So what would you expect the TCAS to indicate with regard to aircraft without a Mode C? :confused:

[mm_flynn]

Am I correct that

These are two different protocols, but use the same SSR frequencies ( a bit like AM vs FM on a given frequency)
All 'Modern' (in the Bookeworm defined sense) transponders will sense and respond to either a Mode A or Mode C interrogator. In addition, the response will be appropriate for the interrogator (that is even if you have Alt selected the transponder's response to a Mode A interrogation will be a Mode A response)
The Alt switch simply fills in or blanks the Mode C framing pulses when responding to Mode C
A transponder which can only reply to Mode A interrogators (i.e Bookworm's Old) wouldn't respond to TCAS (as it is a Mode C/S interrogator) but would respond to most ground stations (because they are Mode A/C interrogators)
we don't seem to know if TCAS would 'see' the Mode A response to a Mode A interrogator.
We all use a historic shorthand which is technically wrong and use 'Mode A' to refer to a modern transponder which is only outputing the 4 digit squawk and framing pulses and a 'Mode C' to refer to one outputing altitude data as well as the 4 digit code. Even though in both cases the transponder is responding to Mode C interrogations

Do most of the ground radar units in the UK still interrogate in Mode A ?

[eharding]

In SEP world, IMHO, if you have to use to word 'Mayday' on your own behalf, then don't dick about with the gadgets, just look out and pick a space on the ground to drop yourself into. You can apologise for your failure to adhere to some bullsh!t regulation afterwards.

[bookworm]

mm_flynn
Certainly a good summary of my understanding, with the proviso on 4 that I don't know if "old" transponders that reply only to Mode A ever existed -- I don't have the TSOs.
On 6 to be absolutely clear, the "shorthand Mode C transponder" replies to a Mode A interrogation with the 12-bit squawk code as information pulses between framing pulses, and to a a Mode C interrogation with the encoded altitude between framing pulses. The "shorthand Mode A transponder" also replies to a Mode A interrogation with the 12-bit squawk code as information pulses between framing pulses, and but a Mode C interrogation with nothing between framing pulses (though I would like to look at TSO-C47b to be 100% sure of that [edited to add that ICAO Annex 10 Vol IV 3.1.1.7.12 seems to confirm it]).
Do most of the ground radar units in the UK still interrogate in Mode A ?
Yes, all of them do, even the ones that also use Mode S, for part of their cycle at least. More here (http://www.eurocontrol.int/msa/gallery/content/public/documents/Principles%20of%20Mode%20S%20Operation%20and%20Interrogator% 20Codes%202.pdf).

[NoTurningBack]

Hi,
In answer to your question about TCAS 'seeing' Mode A replies.
All interrogators operate at 1030 MHz and all replies are at 1090 Mhz.
So TCAS will receive the Mode A reply but will not be able to determine
range from it as the reply is not syncronised to a TCAS interrogation.
Most replies of this type are rejected by the receiving processor in a
section known as the de-fruiter (FRUIT= False Replies Unsynchronised In Time).
The TCAD systems measure the amplitude of all replies regardless of their mode and use that for the determination of range (all transponders having a standard output power). Signal amplitude decays with range in a logarithmic manner. TCAD makes a lot assumptions regarding a clear path to the receiving antenna.
I hope that is of some help.
NTB

[ShyTorque]

So - what is indicated by a 'contact' on a TCAS screen not showing a relative altitude?

[bookworm]

So - what is indicated by a 'contact' on a TCAS screen not showing a relative altitude?

That the aircraft has no encoding altimeter or that the transponder is switched to On rather than Alt.

[ShyTorque]

Bookworm,

Precisely, that was the reason for my initial reply to Backpacker and my plea for pilots with Mode C fitted to select that, rather than using Mode A only. :D