Transponders

[devarshpatel19]

Hello,

Can any one tell me what is the difference between ALT mode, AUTO mode and ON mode of Transponder?

Thanks!

[hetfield]

Guess it's pretty basic.

ALT = altitude reporting (MODE C)
AUTO = air/grd switches to ON/OFF
ON = ON

[Wodrick]

Never seen precisely those markings however ALT is Mode C i.e. altitude reporting only.
ON is forced into the normal modes, and AUTO will be off until some criteria is met to indicate aircraft not parked, so engines running, park brake off, or even weight on wheels will depend on type.

[Checkboard]

ON is mode A : returns the code set on the box only.
ALT is mode A and mode C: returns the code on the box, and the aircraft altitude.

[V1... Ooops]

AUTO is typically used on newer transponders that comply with the Mode S EHS (Enhanced Surveillance) specification. On the aircraft that I fly, the transponder will automatically switch between flight mode (emitting a squitter every second, unsolicited) and ground mode (responding to specifically addressed inquiries only) when it is in the AUTO position.

I don't think the TSO for transponders specifies exactly what AUTO should do, or even if the transponder must/should have an AUTO mode, so it would be best if you carefully checked the documentation for the specific aircraft you are flying.

[Sciolistes]

and ground mode (responding to specifically addressed inquiries only) when it is in the AUTO position.

I don't think the TSO for transponders specifies exactly what AUTO should do, or even if the transponder must/should have an AUTO mode, so it would be best if you carefully checked the documentation for the specific aircraft you are flying.
I have read so much contradictory information regarding the use of transponders on the ground, the issue seems to be a bit of a dog's breakfast.

On the one hand one hand Mode S should be used (TA/RA option on many) then only ground can see your returns but you get TAs landing traffic even though they don't see you. Although I can watch some (very few) aircraft land and taxi off the runway on the traffic display!?

On the other hand some airports specifically require Mode A on the ground, but then other documentation suggests that can trigger TAs for landing traffic.

Our FCOMs for different aircraft with different radio fits are also unclear about the state of the transponder on the ground in various modes.

On that basis I reckon that the transponder should be on SBY on the ground until line up, unless local procedures dictate otherwise, regardless of options like AUTO, etc.

[Piltdown Man]

And running on with Checkboard's reply, AUTO is (probably) Mode A & C automatically selected when airborne. However, as is always the case, the definitive answer is in the appropriate manual.

Here's my aircraft's transponder settings:

STANDBY: The XPDR system is energized but does not transmit the altitude data. TCAS Disabled.
ALT-OFF: The XPDR system transmits the reply in Modes A and S, and does not transmit altitude data. TCAS Disabled.
ALT-ON: The XPDR system transmits the reply in Modes A, C and S transmitting altitude data. TCAS Disabled.
TA: The XPDR system transmits the reply in Modes A, C and S transmitting altitude data. TCAS is in the TA (Traffic Advisory) mode only.
TA/RA: The XPDR system transmits the reply in Modes A, C and S transmitting altitude data. TCAS is in the TA/RA (Resolution Advisory) mode.

In normal operation, we only select ALT OFF and TA/RA, the STANDBY function being automatically selected on shutdown.

PM

[josh121.5]

hope u guys dont mind answer the following :

1. what is mode S ?

2. If you forgot to turn on ur transponder during the checks in an airline and take-off and if the tower advises you to chek the transponder , do u have to fill in an incident report in your company ??

3. in GA aircrafts we often could hear the Atc tellin us to Recycle the transponder quite often, due to the top condition of our transponder:ugh:so just wondering does it ever happen in an airline ? as in do they ask you to recycle the transponder ??

[FlyingTinCans]

1/ Mode S is an upgrade from mode C transponders, and allows aircraft to be TCAS equipped, it also gives the ATCO more information such as aircraft callsign, speed, and even the altitude the pilots have programmed into the autopilot (to cross check clearances).

2/ Only if the safety of the flight had been affected.

3/ Of course it can happen, rarely though, and like anything with transport category aircraft, we have a backup transponder onboard

[de facto]

ATC cant see the altitude you set in your FMS,only the one set on your MCP.

[Sciolistes]

Mode S is an upgrade from mode C transponders, and allows aircraft to be TCAS equipped, it also gives the ATCO more information such as aircraft callsign, speed, and even the altitude the pilots have programmed into the autopilot (to cross check clearances).
I was not aware of that! Is this standard for all Mode S aircraft, or for those specifically equipped for FANS?

[FlyingTinCans]

The basics behind mode S is the only thing that is standard, there have been enhancements and add-ons since it was implimented, TCAS I, TCAS II, enhanced mode s, datalink, ADS-B... It will depend on the transponders age and the regulations the a/c flys under.

[V1... Ooops]

...I have read so much contradictory information regarding the use of transponders on the ground...

On the one hand one hand Mode S should be used (TA/RA option on many) then only ground can see your returns but you get TAs landing traffic even though they don't see you. Although I can watch some (very few) aircraft land and taxi off the runway on the traffic display!?

I don't think that the companies that put together the fully integrated avionics systems have yet fine-tuned how TCAS should behave on the ground when the transponder is in ground mode.

In other words, if the aircraft has a fully integrated avionics system and the aircraft passes its air/ground status to the transponder, the transponder should be left in the 'ON' position (in practice, Mode S) at all times, and never turned off. The transponder will then do what I said it does, which is respond to specifically directed interrogations only when it is on the ground.

But... It seems that the TCAS, which is an entirely independent system that only uses the transponder for the purpose of sending and receiving information, is not being supplied with the air/ground status of the aircraft, and as a result, it is not going into a "watch things only" mode when the aircraft is on the ground. Whether this is by design (meaning, the TSO for the TCAS specifies that it must be fully functional while on the ground), or by error of omission (the avionics companies have not yet sorted out the small details), I don't know. But, so far as transponders only are concerned, if it is a fully integrated system and the transponder is supplied with air/ground status, then it should not be turned off or placed in standby when the aircraft is on the ground.

Michael

[Rotorgoat8]

Normally ATIS for a particular airport will state transponders ON for ground Ops. This means they have the capability to monitor ground movement of taxiing aircraft. RA's are normally inhibited on final approach so the ground Ops in effect would not affect arriving aircraft.

[STBYRUD]

...though even a yellow TRAFFIC on the ND can be a bit unnerving, especially during low vis ;)