Why set transponder to standby while taxiing?

[tom775257]

Hello,
I have been taught to set the transponder to standby as I taxi the aircraft, then as I pass the hold turn the control to test then release to ‘Alt.’ The question is why bother setting it to standby for taxiing. All the instructors I have asked seem to recommend this action. Anyone know why? Unless the TXPR is using thermionic valves (obviously not), I can’t think why?
Cheers,
Tom.

[Keef]

I suspect some transponders do still use thermionic valves in the TX output stages.

Ours is reasonably modern, but it won't transmit for a couple of minutes after powering up. So it goes to standby when we've started the engine, and to "Alt" when we line up for takeoff.

[Sir George Cayley]

Have you thought about what might be landing as you approach the hold

Think TCAS -
Think TA -
Think expensive Go Around!

Any help?


Sir George Cayley

The air is a navigable ocean that laps at everyones door

[rodan]

Also, think radar controller cursing the secondary data from taxiing a/c cluttering the middle of his display if there is an SSR radar head on the airfield.

[bingoboy]

Yes Rodan is right --- lets all try to clog screens as much as possible until logic prevails re mode S !!!!!!!!!!!

[rustle]

bingoboy

"...lets all try to clog screens as much as possible until logic prevails ..."

You're not a fireman are you? :D

How many responses have you sent the CAA about Mode S?

(I assume you were having a laugh)

[Final 3 Greens]

Tom

I agree with the other postsers, but additionally if you set the transponder to standby as you taxi, you also double check by default that the previous pilot hasn't left it on 'alt' after shutdown - see previous posts on the consequences of that.

In the perfect world, one should check all equipmenet is off before start up, but the transponder is very easy to forget.

[tom775257]

Hello all,
Sorry if I didn’t make myself clear…I never plan on taxiing around with transponder on. I was asking ‘why bother to change it from off to standby for taxi'. I guess Final 3 Greens point is very valid regarding it offers a method of double checking the transponder is not squawking by setting to standby…
Keef: That is interesting. Thermionic valves seems a very backward technology to be using though, very prone to damage from vibrations. But it would explain the delay. Bizarre.
Thanks for the replies,
Tom.
:)

[Dan Winterland]

I think the standby mode is really there to stop the transponder from transmitting while you change codes preventing inadvertant emergency code selections. I still teach my students to switch to standby while changing codes. I know that most ATC SSR heads now have a few seconds delay before displaying - and in the case of emergency codes - actioning indications, but IMHO it's still good practice should you not change sqwawk fast enough or get distracted part way through.

As standby is there, I see no reason not to use it on the ground prior to departure - at least the kit will be warmed up.

Some larger airports are now ditching their ASMI (airfield surface movement indicator) radars in favour of a new system which uses mode S. LHR is one of these, transponders are now set ot the start of taxy and go off when on the gate.

You won't get a TCAS Resolution Advisory on approach from an aircraft holding as RAs are inhibited below 800' on most TCAS systems. On the aircaft I fly, you get no audio either. You just get the yellow traffic squares with alt readout. A Go Around won't happen in this situation.

As an aside, the word 'Squawk' comes from the wartime codes for Identification Friend or Foe (IFF). During WW2, most classified kit was given a codename. In the case of IFF (which is still essentially the same as SSR, just a slightly different frequency and only 76 codes) it was 'Parrott'. Your 'Parrott' replied to an identification request by 'Squawking'.

[Fuji Abound]

I have always understood many older transponders have a thermionic valve and the valve does not take kindly to being suddenly switched on, whereas in stand by mode the valve has a chance to warm up. Myth or fact?

[wet wet wet]

Another point, not mentioned so far, is that transponders go into a self test mode when first switched on, i.e. by switching to 'standby'. The self-testing takes quite a few seconds and when this is going on the interrogation light is lit continuously. So, by switching to standby for the taxy, the transponder should have completed its self check and be ready to transpond normally when you depart.

[bertiethebadger]

I believe that the compass is calabrated with all electrical instruments switched on. So if the transponder is off, the compass may not read true.

Given the importance of the compass & that this is when you initially tune the DI, it would be important for the transponder to be on.

I was always told to turn the transponder from standby to alt when leaving the airfield / circuit

[Min Sink]

bertiethebadger

New advice on when to switch the transponder from SBY to ALT or ON

From http://www.pilotweb.co.uk/news_story.cfm?id=3043

Pilots are being asked to change the way transponders are used while in the circuit for a UK airfield.
The CAA says that, with immediate effect, you should now select and transmit code 7000 with altitude reporting (Mode C), if fitted, while airborne at all times other than the following, when:

- Air Traffic has allocated a different code;

- a special-purpose code (such as the distress code of 7700) is more appropriate;

- the aircraft is operating within a visual circuit and local procedures require the transponder to be set to Standby.

The change is a result of the increasing use of airborne collision avoidance systems (ACAS) in commercial aircraft.

MS

[Vedeneyev]

Transponders do indeed have a thermonic valve, just like an old cathode ray tube. There's like a box of electrons in there that bounce around but don't start bouncing around until the source of electrons is hot, and just like old TV's, transponders take time to warm up before they respond to SSR Interrogations. Turn the transponder to standby when taxi-ing, then it's fully warmed up when you switch it to 'On/Alt' during your pre-takeoff immediate actions. Turning it 'On/Alt' straight from 'Off' won't damage it, it will just take 30secs to 1min to respond to SSR Interrogations (not good if you're on a radar assisted departure). Turning it on whilst on the ground will inevitably clutter radar screens. If you're not using your transponder whilst airborne, you'd better leave on 'STBY' rather than 'Off' in case you need to suddenly switch it on and dial in 7700.

[Keef]

Tom - I have seen ads for "all solid state" transponders, but they are pretty new. For the sort of pulse power needed at 1GHz, thermionic devices are probably preferable, and are actually pretty robust. I'm still using a transmitting valve here that has "1943" and the W^D symbol on it.

Dan - are there any SSR units on airfields, apart from the one at EGLL? I thought it was all piped from remote SSR units these days.

Anyway, SOP in our group is "lights - camera - action" when taking off. If I'm in the air, Mode C is on. Nobody ever complained, and some military folks in Scotland seemed very glad of the Mode C...

[A and C]

Dispite the talk of thermosomething valves the real reason for "walming up" the transponder it the mode C encoder works at a constant temp and to do this it is fitted with a small heater and this takes time to get up to the correct working temp.

Next time your aircraft is in the sheds for the radio annual check if you watch the transponder check you will see that the mode A code works almost instantly but the mode C will drift around for about 5 min before reaching a realistic reading.

[tom775257]

Here’s a question then…
In what format is the SSR data sent (mode C), I assume it can’t be digital? How is the data encoded in analogue? A bit like ‘teletext’???
What power output is being transmitted?
Cheers,
Tom.
:confused:

[Keef]

Tom

Mode C is sent in pulses - so the analogue/digital argument is irrelevant. If you had to pick one, you'd say "digital".

For a simple explanation look here:
http://www.airsport-corp.com/modec.htm

The power output is 150 to 350 watts - but it's pulsed, so the "mean" power is a lot less. depending on how often it's interrogated and the code it happens to be sending. But it's the instantaneous power that defines the output stage rating, and 350 watt UHF power transistors are expensive, hence the use of thermionic devices in most transponders. Those need a finite time to warm up. That's the main reason for switching the device to standby (rather than OFF) once the engine is started.

For info on all-solid-state (ie non-thermionic PA stage) transponders, see here: http://www.ultralight.nu/garmin.htm


A&C
I wasn't aware that encoders are heated - any more than altimeters are. Where does that information come from? Is that for ones mounted in unheated/unpressurised avionics bays, rather than those mounted behind the panel and connected to the static line?

[A and C]

Most blind encoders used in GA aircraft are heated , but some of the older ones such as those built by Transcal and most encoding altimeters use a notched wheel and optical reader to encode the altitude data these type do NOT need a walm up.

The more modern encoders have a baro unit that is individualy calibrated and corrected by an eprom and these units must work at the calibration temp and so have a heater unit built in and must have time to walm up.
Most of them can be fitted in or out side the pressurised cabin.
The types that I have found (in todays research ) use this system are Ameriking , Narco , King and the Shadin ADA200 air data computor (if you are going mode S).