Mode C question

[XL319]

This may seem a silly question....but is there any way you can check to see if your transponder is Mode C. Thanks (flying to France at weekend, but unsure as to charlie)

[IO540]

If it has a switch position, or a button, marked "ALT" then it is Mode C.

However, the marking could be different.

ATC (a radar unit) can confirm whether it is working OK.

[tmmorris]

Not really, unless you have a friend with TCAS or ask a controller when airborne...

Or start pulling the panel apart, but that doesn't sound like a good idea.
I suggest you take off, select mode A, then as soon as you can get a radar service switch to mode C and ask the controller to verify if mode C is displaying (and matches your altitude). If not, go back to mode A: I've known ac with no mode C encoder transmit spurious altitudes when mode C is selected, which is worse than just mode A, obviously.

Tim

(PS IO540 beat me to it: but just because it says it has mode C, it doesn't mean the encoder works...)

[dublinpilot]

Well....my thoughts.

1. Does it have an ALT selection switch/button? If not, then there is no mode C, but having an ALT doesn't mean that the mode c is actually hooked up.

2. If it has the ALT selection switch/button, then there is often a plackard noting that Mode C info is correct up to 25000ft or something like that. Seeing that would imply that you do have mode C.

3. I don't know for sure, but I would suspect that if you have the ALT switch/button but the Mode C wasn't wired up, that there would be some sort of plackard to report it.

4. Ask the aircraft owner/operator (I'm assuming you don't own it.)

5. All else failing, fly it aircraft and contact a radar unit and ask if they are getting a mode C response from you.


...after that I'm out of suggestions!

dp

[XL319]

I know it's not Mode A, but it does have a ALT setting which i always use anyway. I was under the impression it was Mode C but a controller the other day asked what height i was at....which got me thinking!!!:ugh:

I dont own it, the owner states it is Charlie. May be best to ask once i have FIS

[IO540]

A radar controller can see your Mode C altitude return but he still has to ask you what your altimeter is reading; it's in his book of rules.

To be fair, many transponders are knackered and return a completely wrong altitude.

[Capt Pit Bull]

Incidentally, Mode C is not the same as altitude reporting.

This can cause endless confusion when reading specifications or trying to understand control panels etc.

An aircraft without an altitude encoder will still reply to Mode C interrogations, it just doesn't include any data inside the pulse frames.

In otherwords, XL319, the question I think you are trying to ask is whether or not the aircraft has altitude encoding, rather than does it have an "A+C" setting on its control head.

Been a while since I've flow a light aircraft, but if I recall correctly they are usually placarded to say there is an altitude encoder. If there is no placard, you might be wise to assume there is no encoder.

The owner / operator should be able to tell you. If you're the owner, check with an avionic engineer.

pb

[XL319]

Yeah probably was aiming for altitude encoding. This then brings up the question.....if no encoder but mode C...is it ok for France?

Thanks for that. Will double check with a controller also.

[Fuji Abound]

If it is reporting an incorrect altitude it is probably the encoder that is at fault. This may or may not be part of the transponder unit itself.

The more modern mode S offerings from Garmin and others display the altitude being reported on the transponder which is a useful check.

[Tall_guy_in_a_152]

My aircraft is in the "Mode C without altitude encoder" category. This does not meet the French requirement, but I have never had a problem there. The only time I had aggro from ATC in France was when I was VFR in class G and Seine Information would not accept that I did not have mode C available! The lady controller called me up to apologise ten minutes later and agreed that she was wrong to demand it in the circumstances!

The French rules state something along the lines that it can be over-ridden by a competent authority. I guess this includes ATC and so, whilst there is no guarantee, they will try to accomodate you in controlled airspace regardless.

Before anyone asks, we are waiting for clarification of the Mode C/S situation in UK and France before we decide to spend a few hundred pounds fitting the altitude encoder.

[IO540]

What is the point in Mode C without an altitude encoder?

[NutLoose]

Have a look under the panel, if you have a pipe tapped off the static side of the system....(follow the pipe work from the altimeter) and look for a box, about 4 inches by 6 inch with the pipe going into it and a wide connector plug on the other side, that is the altitude encoder..... failing that is it not in the log books or on the C of A renewal paperwork

[Vedeneyev]

I've always wondered why most transponders don't have an onboard altitude encoder, particularly the more modern ones now the electronics and transducers are so minimal? In fact, is there even such a thing as a transponder that includes an encoder?

[gasax]

Why are so many transponders capable of Mode C but not enabled? The £600 odd that most 'avionics' engineers' will charge has a bit to do with it.

The encoder is abot £130, the wiring loom is a bit fidddly but nothing your average DYIer could n't manage. The static connection very easy. But try and do it on a c of A aircraft and it all gets expensive.

Certainly you would have thought combined transponders and encoders would be childs play, but with the natural conservatism of the US manufacturers it has yet to happen (why bother when you can keep selling the same old rubbish.

However at least one to my knowledge does have it - the new Filser Mode S boxes have an encoder built-in and apparently very straighforward installation - i.e. live, earth, aerial, static. Progress at last!

[bookworm]

What is the point in Mode C without an altitude encoder?

See Capt Pit Bull's post above. The transponder still needs to reply to Mode C interrogations even if it has no altitude information available.

[Navex77]

When you switch to ALT mode, that is already in MODE C. Your squawk code will appear on controller's screen when you are about 300' of height.

[UV]

XL 319
Simple...
It should be labelled "NO Altitude" ......
UV