What does the Transponder Mode C transmits?

[JABBARA]

I am flying for an airline and have several questions concerning the operations of some systems. Here is the first one:
(Sorry the question is a little be long because English is not my native language and I tried to be rather explanatory. I hope I have succeded)

In the company, most of the pilots think that the Mode C transmits the altitude whatsoever the altimeter is indicating as set by barometric scale. In another word, they say, if the pilot forgets to reset the altimeter to standard (1013 Mb) above the transition altitude (e.g. the altimeter barometric set-ting is forgotten at QNH 998 Mb) and he levels off at FL 150 indicated, in this case, the barometric altitude will be 15000, but the actual FL will be about FL 145 (because 15 Mb error will be about 500 feet ~ 30 x 15 Mb), and despite the fact that the vertical separation with another airplane below (i.e at FL 140) will be less than 1000 feet, the radar controller will be monitoring as if everything is normal without becoming aware of the problem.

The point I do not agree with them is : The Controller will be aware of the problem because the transponder always (regardless the altimeter setting on subscale), transmits the altitude with reference to standard setting. In another word, it always transmits the FL. So, despite the pilot has forgotten to set the altimeter to 1013 rather than 998 above transition altitude, the controller will read FL as 145 while the pilot is flying at FL 150 indicated. But all these may also bring another question to minds: What about if the things occur in the reverse way? I have the answer for this too. If a pilot forgets to set QNH below transition level, the controller will be again aware of the problem because the radar computer will automatically apply the QNH correction upon the received transponder information send by the airplane.
I remember, I have read some similar things regarding the radar operation from an USAF document at my initial flight training days, but now I am unable to reach the same document.

Now please someone would tell me, how correct I am. Or what is the right?

Thanks in advance.

[PPRuNe Radar]

You are correct. Mode C is always transmitted based on the Standard Pressure Setting. :ok:

[JABBARA]

Pprune Radar Thank you for your reply.

In fact I was expecting more replys from the other members as well. I really wonder whether my English is not so good to explain what I want to ask?

[EternalKnight]

I know that in the aircraft I fly, as soon as the master bus is turned on, the altimeter asks for the current barometer setting. I have never had an ATC get mad at me for having it improperly set, but our school stresses that each time the master bus is turned on, the altimeter baro setting should be set to the current baro setting.

[Milt]

Ok then - where does Mode C get its reading from ?

Is there a separate altimeter always set on 1013 or is there a crafty pick off from one of the cockpit altimeters.

Raises the question re the Orange Boxes also and who calibrates them/it.

[andyb79]

encoding altimiters send the info to the transponder as a flight level regardless of the setting on the subscale. the controllers then work out the appropriate altitude on the ground, whether this is done by the computer or the controller i dont know( any ATCO's wish to comment?)

imagine the chaos if it transmitted the altitude indicated, with people operating on QNE,QNH and QFE in the same area!

about the orange box, thats a good question. i would imagine that it gets its info (in at least duplex) from the altimiters as a FL, with any discrepances being worked out by some rather clever people in a lab after the event! remember that the box itself is just a data recorder so therefore doesnt need calibrating,just checked to ensure its operating on all channels/inputs.

[Smurfjet]

Computers do the job. Our system is divided into grids with each one the appropriate altimeter setting. When the a/c is in that grid, the computer takes the mode C altitude and adjusts it with the grid's setting and we have the "real" altitude. So if your altimeter has the wrong QNH, you will not be at the appropriate altitude.

:cool:

[Spitoon]

Having sat in front of a radar for many years, I can confirm that mode C data are based on ISA. On any modern radar display system the transition altitude and QNH are input (the former as part of the system configuration, the latter either manually or automatically) and the display makes the necessary conversion below TA. On the system that I'm used to a FL is shown as a number and an altitude is prefixed by an 'A'.

If the pilot has the wrong altimeter setting selected or has a duff altimeter it is soon fairly obvious to the controller - because the pilot's level reports don't tie up with the radar display. Before mode C data can be used for separation it must be verified by the controller, i.e. correlation between the radar display and pilot report must be confirmed.

[JABBARA]

Thank you Spitoon.

That was the thing which I was expecting to hear. :ok: Meanwhile I am happy because my English is understandable.:)