Faulty transponder on club aircraft

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yak-aviation

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The scenario is this. Rented a PA-28 from my club and set off with a mate for a 200m x-country. Upon arrival, was told that someone was on the phone waiting to talk to me. I was somewhat surprised and impressed, although soon told that commercial traffic had been diverted because I was flying at 20,000ft. Hhhmmm, I explained I was squawking charlie in a PA-28 and that they should know that I'm a PA-28 and obviously incapable of reaching 20,000ft. Response was akin to beep-beep-silence-beeeeepppp.

To the crux of the question then, how does one check for faulty trx altitude reporting ?!
 
Do you have the equivalent of 91.411 & .413 checks over there? In the meantime, Skip's suggestion is valid and you probably only need to do it once to know if you have a problem.
 
Yes, to be fair, Skip did kindly answer my question. And it wouldn't be onerous to call ATC except I rarely fly the same a/c so that would be a pain. I'll look into the checks you mention, I haven't heard of them but will check. Thanks both, Alex
 
When I hear ATC tell another pilot there's traffic, I know he's referring to me, and his altitude report is more than a couple hundred feet different than it should be? I know I have an encoder that needs adjusting so I call ATC and offer to turn mode C off and I get it fixed.
 
If you're on flight following and the error is 200 feet or more, you'll very likely find out you have a problem.

"Cessna 123XY maintain VFR cruise altitude."

Or they may just ask you your altitude.

Or you will hear yourself as a traffic call to someone else, with the wrong altitude.

I found a 200 foot error once when I flew at 1400 feet under a 1500 foot class B floor. I had to respond to "say altitude" several times during a 17 mile flight.
 
Good point, we're always talking to somebody and it would have been quite easy for a controller to confirm/check with me en route.
 
yak-aviation said:
The scenario is this. Rented a PA-28 from my club and set off with a mate for a 200m x-country. Upon arrival, was told that someone was on the phone waiting to talk to me. I was somewhat surprised and impressed, although soon told that commercial traffic had been diverted because I was flying at 20,000ft. Hhhmmm, I explained I was squawking charlie in a PA-28 and that they should know that I'm a PA-28 and obviously incapable of reaching 20,000ft. Response was akin to beep-beep-silence-beeeeepppp.

To the crux of the question then, how does one check for faulty trx altitude reporting ?!
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Why not request flight following? The first thing they do is verify your altitude readout.
 
I'm surprised my old NARCO transponder is still working. Started a slush fund to replace it. On occasion I do ask the tower if they are picking up transponder when departing class D airspace.
 
yak-aviation said:
The scenario is this. Rented a PA-28 from my club and set off with a mate for a 200m x-country. Upon arrival, was told that someone was on the phone waiting to talk to me. I was somewhat surprised and impressed, although soon told that commercial traffic had been diverted because I was flying at 20,000ft. Hhhmmm, I explained I was squawking charlie in a PA-28 and that they should know that I'm a PA-28 and obviously incapable of reaching 20,000ft. Response was akin to beep-beep-silence-beeeeepppp.

To the crux of the question then, how does one check for faulty trx altitude reporting ?!
Click to expand...

Thats why all newer transponders display pressure altitude. Its kinda like all newer VHF com radios will only transmit for 30 seconds before it goes into stuck-mic-mode.
 
What transponder?

Many of the ones with digital screens, like most garmin, you can display your pressure altitude readout, it should give you a heads up if it's sending FUBAR info out to ATC, I don't often check it, but when scrolling from the flight time to the stop watch feature on my GTX I'll glance over it.

GTX345_Pressure_Alt.png
 
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Do you keep a calulator around to convert it to a useful value? Pressure altitude is what our equipment reports to ATC and isn't the same as what your altimeter reports unless you have a 29.92 day. When ATC calls traffic they adjust it to what your altimeter is reading.
 
Most of the club aircraft I've flown have Garmin GTX327's that report pressure altitude. So if I was flying at 20,000ft, are you saying it should have been visible on the unit?
 
Stewartb said:
Do you keep a calulator around to convert it to a useful value? Pressure altitude is what our equipment reports to ATC and isn't the same as what your altimeter reports unless you have a 29.92 day. When ATC calls traffic they adjust it to what your altimeter is reading.
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Unless it's a end of then world level event, there will be a good difference between, say crusing around at 5500 and a pressure altitude of FL200.
 
Stewartb said:
Do you keep a calulator around to convert it to a useful value? Pressure altitude is what our equipment reports to ATC and isn't the same as what your altimeter reports unless you have a 29.92 day. When ATC calls traffic they adjust it to what your altimeter is reading.
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Jot down the altimeter setting then turn it to 29.92.
 
Stewartb said:
New guys can read and learn. The majority of our planes have separate altitude encoders. The transponder is only the messenger.

https://www.avweb.com/news/avionics/183231-1.html
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You mean a fault within the transponder itself cannot screw up the reported pressure altitude? What converts the data given by the encoder to RF?

I reaslise the most likely reason is harness issues (especially if gray code data)
 
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yak-aviation said:
Hhhmmm, I explained I was squawking charlie in a PA-28 and that they should know that I'm a PA-28 and obviously incapable of reaching 20,000ft. Response was akin to beep-beep-silence-beeeeepppp.
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If this was a Mode C transponder, and you weren't on flight following, then they would not have known your aircraft type. A Mode C transponder only transmits your beacon code (I'm assuming 1200 in this case), your pressure altitude, and an Ident bit. If you had a Mode S transponder then it should transmit aircraft type.
 
I just saw in another post that it was a Garmin GTX327 which is a Mode C transponder. They would not have known your aircraft type without you telling them.
 
Altimeter mis-reporting of 300-400' is far more common and not that easy to figure out, until you fly in a itty bitty corridor with a 500' range from floor to ceiling right between fighter jets and heavy transports. In my experience, at least. My inlet crossings are always at or near the top of our airspace at 1200'. Fighters and other military traffic crosses perpendicular at 1400'. ATC gets nervous when you're squawking 1500-1600'.

I don't know how they do it in Europe but here in the states if you're squawking FL200 and they don't know exactly who you are and where you're going they'd be on the radio asking, wouldn't they?
 
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Stewartb said:
Do you keep a calulator around to convert it to a useful value? Pressure altitude is what our equipment reports to ATC and isn't the same as what your altimeter reports unless you have a 29.92 day. When ATC calls traffic they adjust it to what your altimeter is reading.
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It's 1 inch per thousand feet. That's pretty simple math. No calculator required.
 
In my experience ATC is happy to verify altitude because an accurate transponder makes their job easier.

Part of the process of flying a different aircraft for me is to check the maintenance logs to make certain the transponder has been tested and singed off in the preceding 24 months and the aircraft is airworthy and the condition inspection is up to date.

I have been told that simply putting an INOP sticker on an out of certification transponder and not using it is not sufficient compliance.

This situation may have been a recent failure.

In my opinion it should be repaired or removed for the aircraft to be airworthy.
 
Vance Breese said:
In my experience ATC is happy to verify altitude because an accurate transponder makes their job easier.

Part of the process of flying a different aircraft for me is to check the maintenance logs to make certain the transponder has been tested and singed off in the preceding 24 months and the aircraft is airworthy and the condition inspection is up to date.

I have been told that simply putting an INOP sticker on an out of certification transponder and not using it is not sufficient compliance.

This situation may have been a recent failure.

In my opinion it should be repaired or removed for the aircraft to be airworthy.
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You've been told wrong and your opinion is just that, your opinion. If I have a coffee maker installed in my aircraft and it is dirty, I can placard it "do not use" and never turn it on. Doesn't make my aircraft unairworthy. The regulation requires a transponder that hasn't been tested and certified in 24 months not to be used. Doesn't make the aircraft unairworthy.
 
Witmo said:
You've been told wrong and your opinion is just that, your opinion. If I have a coffee maker installed in my aircraft and it is dirty, I can placard it "do not use" and never turn it on. Doesn't make my aircraft unairworthy. The regulation requires a transponder that hasn't been tested and certified in 24 months not to be used. Doesn't make the aircraft unairworthy.
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It looks like he flies in the UK. I wonder if transponders are required equipment over there.
 
We have TMZs - Transponder Mandatory Zones. The UK requires the carriage of radio navigation equipment in notified airspace. Pressure-altitude reporting transponders must be capable of operating in Modes A and C, and have the capability and functionality prescribed for Mode S. The requirements for transponders within a TMZ are as follows -

a) All flights operating in airspace designated by the competent authority as a TMZ shall carry and operate Secondary Surveillance Radar (SSR) transponders capable of operating on Modes A and C or on Mode S, unless in compliance with alternative provisions prescribed for that particular airspace by the Air Navigation Service Provider (ANSP)

b) A pilot wishing to operate in a TMZ without serviceable transponder equipment may be granted access subject to specific arrangements agreed with the TMZ Controlling Authority.

Mostly it's a requirement with an exception to manage without, but it's not the norm.
 
Witmo said:
You've been told wrong and your opinion is just that, your opinion. If I have a coffee maker installed in my aircraft and it is dirty, I can placard it "do not use" and never turn it on. Doesn't make my aircraft unairworthy. The regulation requires a transponder that hasn't been tested and certified in 24 months not to be used. Doesn't make the aircraft unairworthy.
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Actually it does unless the transponder is disabled or removed, AND placarded inop. In this case, it must also be logged by an A&P ( transponders aren't allowed as preventive maintenance). See 14 CFR 91.209(d).
 
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