Use of transponder
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From: Earth
Use of transponder
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!
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!
Fleet Manager
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From: Ontario, Canada
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.
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.
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From: Ansião (PT)
"cavity" aka "magnetron", as in Cavity magnetron - Wikipedia, the free encyclopedia . 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?
Wonder if they're still in common use in transponders, I should expect today's electronics to create solid state transponders?
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From: germany
Jan Olieslagers: .....One of the last great British inventions, I should think.......
Thread Starter
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From: Earth
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...
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...
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From: Ansião (PT)
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.
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.
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From: Ansião (PT)
Mode A .... position data transmitted
Mode S .... Used in Europe.
Upto The Buffers
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From: Leeds/Bradford
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.
> 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.
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From: USA
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.
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.
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From: Swindon, Wiltshire
Don't fly with them any more - they are clueless... God knows what other bad habits or misinformation they will be imparting upon you.
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From: Amsterdam
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.
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.
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From: Midlands
“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
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
Avoid imitations
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From: Wandering the FIR and cyberspace often at highly unsociable times
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.
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.
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From: next door to the pub
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.
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Just as a pleasant request 
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.
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.
Last edited by mad_jock; 3rd August 2011 at 11:02. Reason: to add in mode A as well.
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From: Daventry UK
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!
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!
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From: UK
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...
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...
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.