Use of transponder

[shumway76]

Hello!

A few questions regarding transponders:

1. After engine start, the transponder is set to STBY until entering the runway, when it is set to ON or ALT as required. What is the reason of setting the transponder to STBY? Why not from OFF to ON/ALT when entering the runway for takeoff?

2. What is the difference between ON & ALT as what I see on the C172 which I fly? Some instructors told me leave it at ON is sufficient, while others prefer ALT. Note that tower does not tell us whether to set transponder ON or ALT, but just gives the squawk code.

3. Upon landing & leaving the runway, transponder is set to STBY, then OFF after engine shutdown. But some instructors say it can be switched OFF immediately upon landing / leaving the runway.

Cheers!

[Pilot DAR]

Further to Silvaire's useful information;

If the aircraft you're flying has an "encoder", there should be a placard somewhere on the instrument panel, or face of the altimeter, saying so. If you find such a placard, you can assume that "ALT" will transmit your altitude corrected to 29.92", along with the transponder return. This has no direct benefit you you, but all kinds of benefit for others.

If you blunder into someone's controlled airspace, whoever is watching radar there can see you, and your altitude, and at least keep everyone else away from that #*#^ airplane.

In other airspace, those aircraft equipped with TCAS will see your altitude, and again, be able to separate themselves from you, even if you never know they are there.

Unless you are deliberately flying at an altitude you should not, and want to conceal this fact, operate in altitude mode.

If you cannot find the aforementioned placard, assume no encoder is equipped. Then "ON" and "ALT" have the same function.

If you have a total comm radio failure, select 7600 (well in North America, anyway). This will tell radar controllers that you've had a comm failure. Continue flying to where you have been cleared, if in controlled airspace, or leave controlled airspace my the least intrusive route.

If you have an emergency, for which "Mayday" is appropriate, select 7700. You'll get a lot on attention, much of which you'll never know about. If you're also squawking altitude, and you go down, radar will know that you have, which is what you want.

If in doubt, while flying in radar served controlled airspace, ask the controller what they would like you to do with the transponder.

My modest, non expert, understanding of the warm up for transponders, is that most have something inside known as "the cavity". It has properties similar to a vacuum tube, and warming up and cooling down are good things for it. A few minutes of each will do no harm.

[Jan Olieslagers]

"cavity" aka "magnetron", as in Cavity magnetron - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Magnetron) . One of the last great British inventions, I should think.

Wonder if they're still in common use in transponders, I should expect today's electronics to create solid state transponders?

[inbalance]

Jan Olieslagers: .....One of the last great British inventions, I should think.......


Wikipedia: The first simple, two-pole magnetron was developed in 1920 by Albert Hull at General Electric's Research Laboratories (Schenectady, New York), as an outgrowth of his work on the magnetic control of vacuum tubes in an attempt to work around the patents held by Lee De Forest (also US) on electrostatic control.

[Jan Olieslagers]

Yes, I saw it too. 't Was the Brits that first made it do something useful, though.

[shumway76]

Thanks for the feedback.

From my experience, STBY also is used whenever there is a requirement to change the squawk code in mid flight.

I wonder if most C172s have altitude encoding into the transponder ALT.

Also, if mode "C" is ALT, is mode "A" ON?
What about mode "S"
I believe modes "C", "A" & "S" are not available in most light aircraft, but it would be good to know what they are...

[Jan Olieslagers]

Again, Wikipedia can be your friend.

Mode C is an enhancement to mode A: it has all the info from mode A (which is 4 octal digits in 12 bits plus an 'ident') and on top it also carries altitude info from the encoder.

Mode S is quite recent, can carry lots more info (like gps coordinates, registration, flight number and much more). Amateurs capture the returns from mode S transponders to create websites like flightradar24.com

As a microlighter I am little acquainted with general aviation, but to my understanding the average 172-category plane in Western Europe carries a mode C transponder. Mode S being rather recent, its equipment is rather expensive.

[Jan Olieslagers]

Mode A .... position data transmitted Sorry to disagree: only mode S can transmit position information. Mode A and C will just answer the "blip" with one of their own, allowing the groundstation to calculate their position.

Mode S .... Used in Europe. And even only in certain countries, mainly in the North.

[Shunter]

Modern digital transponders don't require you to set standby when changing code. When the last digit of the new code is entered it is activated; there is no "crossover" as you would get changing individual digits on an old style transponder.

> Also, if mode "C" is ALT, is mode "A" ON?
Correct.

S stands for selective. You're probably best putting Mode S into google if you want a decent explanation. No difference in the way you physically use it and no particular benefit for pilots. You will however find that not carrying a Mode S transponder becomes more and more limiting in terms of where you can and can't fly.

[KKoran]

Silvaire1,

Mode S is not dead or even dying in the US. While the plan is to migrate to ADS-B for ATC surveillance, transponders will still be required. Mode S transponders with extended squitter will be one of the means for ADS datalink (the other being the Universal Access Transceiver) and will be mandatory for operations at or above 18,000 MSL. Mode S transponders are currently required on most aircraft operating under Parts 121 and 135 and are being installed more and more in Part 91 aircraft.

[stevelup]

Some instructors told me leave it at ON is sufficient, while others prefer ALT.

The instructors that fall into the first category above?

Don't fly with them any more - they are clueless... God knows what other bad habits or misinformation they will be imparting upon you.

[BackPacker]

Mode S is not just about the extended squitter (position information, heading, selected level etc.) but also about selective responses.

Mode A and C transponders will send a response everytime they are interrogated, either by a radar sweep or by a TCAS interrogation from a TCAS-equipped aircraft. As the responses are all on the same frequency, in busy airspace these responses may overlap, rendering both of them unreadable.

With mode S, an "all stations" interrogarion is only done every now and then. In between those, the equipment does selective interrogations, where only the station with a specific ID is asked to respond.

So in addition to making much more information available to ATC, they also help in avoiding frequency congestion.

Back to the original question. The transponder is also switched to STBY to verify you have the correct code set (1200, 7000 or an ATC-assigned code) during the pre-flight checks. The only reason I can think of to ever use OFF is when the aircraft doesn't have a Master Avionics switch, and you want all equipment to be off during engine start and shutdown (due to the potential for voltage spikes).

And another reason to use ON instead of ALT could be that ATC has found your altitude readout to be wrong, for instance because your encoder broke. They will ask you to turn off your altitude readout, but will want to keep your mode A 4-digit code visible. That's what you achieve in the ON setting. And for that same purpose, even mode S transponders have an ON and an ALT setting.

[Radix]

............

[Rod1]

“Apparently some transponders need to warm up briefly in standby (STBY) before they will operate in any other mode.”

The time depends on the age of the transponder. Because of the mode S uncertainty in the UK I fitted a second hand transponder and was given an old but serviceable encoder. The encoder requires 15 min to stabilise completely. I turn it from stby to alt as I line up as by then the errors will be quite small. The system is still working well after 5 years but may one day be upgraded to S.

Modern solid state units do not require warm up but banging an old style unit from off to Alt could spill some coffee as the errors will be 1000’s of feet.

Rod1

[ShyTorque]

Please use Mode C when flying in Class G airspace if your aircraft has it.

An increasing number of GA aircraft, especially helicopters, have TCAS or TAS, which can pick up the response from transponding aircraft and plot it on a small screen, with relative altitude in hundreds of feet. Other transponding aircraft can be detected up to twenty miles away, giving a much longer time for the pilot to visually detect and avoid it. The prime method of avoidance is by arranging an altitude difference because the equipment isn't so accurate in azimuth. This means an aircraft showing at the 12 o'clock position may actually be either left or right of the nose.

An aircraft with no Mode C / ALT still shows up but there is no indication of relative altitude. So the pilot has to scan a much larger quadrant of airspace because he has no real idea of where it is relative to himself (is this one up/down/same level?).

There are a number of pilots who like to talk about the "big sky" theory. After well over a decade of flying TCAS equipped GA helicopters (police and corporate) usually in Class G, I now know the UK sky is often far, far busier than those flying without the equipment probably realise.

We are forever taking avoiding action on aircraft when it's our right of way because pilots have obviously not seen us (either that or the pilots just don't comply with the rules of the air, which I'm less inclined to believe).

Yes, please use Mode C/Alt if you have it.

[Fly Through]

Just like to point out that a lot of transponders in use in GA aircraft need to be in 'standby' when altering the squawk, you'd be surprised how many times the alarms go off when people cycle through the emergency codes by accident.

[mad_jock]

Just as a pleasant request :ok:

Please select mode C or A for that matter on being cleared onto the runway.

With TCAS mode S has on ground mode which comunicates with other mode S transponders so that all the bells and whistles don't go off.

Mode C and A itself doesn't and if you turn it on while sitting at the hold point it triggers the TCAS on aircraft fitted on approach. If we are below 1100ft it will be a "traffic traffic" and above that possibly a Resolution advisory.

It gives you a bit of a jump and pulls your attention away from the approach.

Even if it does go off while you are lining up its doing its job warning us that there is a chance of collision. Also as well some say to turn it off in the circuit. Please don't it has allowed me to avoid situations occuring with light aircraft that could have got quite close if the TCAS hadn't triggered alerting us to previously unseen traffic.

Anyway this isn't a rant just a request with reasons for that request.

Also There are some airports with ground radar that needs mode A to work, off course in this situation please comply with local regulations.

[soaringhigh650]

Also another request:

Please select GND if you have it when you're taxying from the ramp to the runway threshold.

[david viewing]

Just a minor geeky point with apologies, but the 'cavity tube' used in older transponders like KT-76a is a travelling wave tube (a sort of microwave amplifier) rather than the magnetron as used in radars and given to the Americans during WWII.

As a kid I remember trying quite hard to obtain a magnetron from govt surplus shops for my home made radar project. Now every tip has a great pile of them in the form of microwave ovens!

[bookworm]

Also, if mode "C" is ALT, is mode "A" ON?
What about mode "S"
I believe modes "C", "A" & "S" are not available in most light aircraft, but it would be good to know what they are...

The truth is in fact a bit more complex than you're being told. Mode A, Mode C and Mode S are actually interrogation modes, sequences of bits/pulses sent by the ground radar to tell the transponder what data to put in its response.

In response to a Mode A interrogation, it returns the 4 octal digit code.

In response to a Mode C interrogation, it returns encoded altitude, provided that there is an altitude encoding input and the transponder is set to ALT. If there is no altitude input or it is set to ON, it does respond to Mode C interrogations, but with a blank "data unavailable" response. TCAS interrogations are Mode C.

There are different Mode S interrogations, some of which are all-call (all or many transponders reply), some of which are selective (only the transponder with the correct hex code replies).

A "Mode S transponder" is capable of replying to Mode A and Mode C interrogations, and even helpfully suppresses them if it is in ground mode. Older ("Mode A/C" or sometimes "ATCRBS") transponders reply to some Mode S interrogation modes.

[bookworm]

It is silly to have both, and transponder is inefficient in the sense of using a lot of electrical power unnecessarily. The move away from Mode S to UAT makes this a lot more likely.

Can you elaborate a bit, Silvaire1?

1090ES, the datalink by which a Mode S transponder is also capable of broadcasting its position and other information (ADS-B), operates in the same vicinity (1090 MHZ) as UAT (978 MHz). To send the same data over the same range at the same rate requires similar power. And what's more, ADS-B is broadcast, so you send the data whether it's needed or not. Why do you describe the transponder as "inefficient".

[ShyTorque]

Silvaire, How do you see other non-transponding aircraft being detected without the provision of radar?

Gliders, some light aircraft / microlight aircraft, aircraft with u/s transponders etc. also need to be considered if the ATC "known environment" is to be maintained.

[ShyTorque]

I fly one of those (non-electrical aircraft) in some exceptionally controlled airspace, minus transponder

You're fortunate to be able to do that. In UK we are now seeing the introduction of transponder mandatory zones (TMZ) around busy airfields (e.g. London Stansted)

[Jan Olieslagers]

Silvaire, what kind of "GPS data" would you want to download while airborne? If you mean a GPS receiver supplying raw GPS data to whatever more complex device to interpret and display it, the receiver will draw a max of 200 mA, amounting to +/- 2,5 Watts on a 12V system (which I think hefty already). A transponder will draw a lot more, though. Generally, transmitting takes much more electricity than receiving.

[BackPacker]

what is the size, weight and power consumption of the on-board hardware required to download GPS data? And how does that compare to the size, weight and power consumption of a transponder?

I'm currently on a weeks glider course. The gliders we fly all have a single lead-acid battery, weighing about one kilogram. This powers the mode-S transponder (which is not properly used here :ugh: and left in ACS mode - the equivalent of ALT - all day long, even when on the ground), the radio (which also doesn't see much use :ugh:), Flarm (the glider variant of TCAS, purely based on GPS position broadcasts) and the vario. Battery capacity is somewhere around 12 hours at least.

So we're not talking about excessive weights to carry transponders and the like, even in an unpowered aircraft.

[7AC]

I think Silvaire is onto something here, this sounds like something the good people at
Trig would be well able to come up with.

[LH2]

The instructors that fall into the first category above?

Don't fly with them any more - they are clueless... God knows what other bad habits or misinformation they will be imparting upon you.

Do you know the context in which this is alleged to happen? If not, you are making quite a jump to conclusions.

There are circumstances where mode A is mandated, even if mode C equipped. Usually it will be in the AIP, or by ATC request, or because you have a duff encoder which is not all that unusual.

[stevelup]

Do you know the context in which this is alleged to happen? If not, you are making quite a jump to conclusions.

There are circumstances where mode A is mandated, even if mode C equipped. Usually it will be in the AIP, or by ATC request, or because you have a duff encoder which is not all that unusual.

Clearly what you state above is true but it doesn't fit with...

What is the difference between ON & ALT as what I see on the C172 which I fly? Some instructors told me leave it at ON is sufficient, while others prefer ALT. Note that tower does not tell us whether to set transponder ON or ALT, but just gives the squawk code.

... which implies that the selection of ALT is down to the personal whim of the instructor!

Also, the fact that the original poster had not even had Mode A/C explained to them seems lax?

When I was training, one of the aircraft briefly had a faulty altitude encoder and we were told - for obvious reasons - not to select ALT in that aircraft. But the reason was explained and understood.

To just say 'ON is sufficient' without any kind of further explanation is deficient regardless of the other circumstances isn't it?

[FlyingStone]

There are circumstances where mode A is mandated, even if mode C equipped. Usually it will be in the AIP, or by ATC request, or because you have a duff encoder which is not all that unusual.

I've seen it written in AIP, but I think most people interpret it wrong. Usually, AIP states something like this "Transponder shall be set to mode A, code 7700." This is actually the requirement to operate the transponder in a way that it will reply to mode A interrogations. But there is no prohibition (at least I've yet to see it) to simultaneously reply to mode C and S interrogations. The same thing applies for example, if controller orders you to "squawk charlie" - you will set transponder to ALT, but on most S-mode transponders you will at the same time reply to mode S interrogations, in fact, on most of them it's not possible to select only mode A and C replies without disabling the mode S (e.g. deleting the ICAO24 code in the maintenance mode of the transponder). I agree on the part if you have a faulty encoder, usually the ATC request would be to switch to ON (mode A only or A-S with S-mode transponders) and usually this would be aircraft's last flight (except for ferry flight to an airport with maintenance facilities) in controlled airspace, at least in many parts of Europe, since controllers don't trust "odd VFR pilots" to maintain accurate altitude.

[ShyTorque]

To just say 'ON is sufficient' without any kind of further explanation is deficient regardless of the other circumstances isn't it?

Yes, it is deficient. Sadly some instructors, at least in UK, have little "operating" experience other than gaining the qualifications to instruct. They are barely off the bottom rung of the ladder.

[mad_jock]

The reasons why some instructors want it turn to mode A only is so that they can getaway with airspace infringments and also if they bust a not above clearance. There is also issues with not having any radar evidence if some one accuses you of a rule 5 bust. If you have carlie on they will find some radar that had you on. With it off its your word against whoever reported you that you were below 600ft agl.

There is no other reason for it.

It doesn't help either that some towers tell you to turn it either off or to mode A in the circuit thus removing any safe guards for commercial traffic.

[FlyingStone]

It doesn't help either that some towers tell you to turn it either off or to mode A in the circuit thus removing any safe guards for commercial traffic.

Airline pilots reporting TCAS TA/RA and submitting proximity reports where traffic was in normal traffic pattern on a CAVOK day with 100 km visibility don't help either.

[mad_jock]

You have to report RA's if you like it or not. Its a Manditory reporting item.

And if they feel that the aircraft is in danger they should also put a report in.

Whats a few reports compared to a collision in the ATZ which has happend quite a few times already this year.

[IO540]

Haven't read the whole thread but a transponder should be always ON.

If it is Mode C it should always be in ALT mode, unless otherwise requested by ATC.

To do otherwise is just nuts, because it makes a Traffic Service practically worthless so all you are doing is screwing others, and preventing TCAS systems seeing you, so you are not doing yourself any favours.

Yet loads of pilots continue to do it.

BTW transponders used a travelling wave tube, not a magnetron which is used for shorter wavelengths (radar). All modern transponders are solid state.