Explain my transponder?

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yachtjim

Pre-takeoff checklist
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Jim
Since I bought my plane I have had controllers complaining to me that my altitude shows about 400' too high when I am at my typical cruising altitudes of 7500 to 8500'. When i first was told about this I took it to an avionics shop that said they looked at it and it was fine. I kept flying and the controllers kept complaining. At this year's annual I had the IFR cert done and they of course test the transponder for this. The same avionics shop said they found it to be 200' off and adjusted it. Said they " ran it all the way up to 15,000'". So i figured i was good to go.

Last night while flying to Boston I get the same complaint from a NY Controller. This guy just requested that I turn off my mode C. So I switched my Narco transponder from Alt to On. He said to let each of the remaining controllers know mode C was off.

So this brings up the question, when do I need to use Mode C. I fly in the Mid atlantic area inside of the DC mode c veil so I assume it needs to be on then. What about outside of there?

What do the controllers see on their radar if mode c is off? Just my position but not my altitude?

Any suggestions as to how I can better explain my issue to the avionics shop? I've now tried twice to get it fixed, and they keep saying its fine.
 
I'd start by looking at the test card that is completed during the 411/413 checks, just to see if any anomalies are mentioned.

yachtjim said:
Since I bought my plane I have had controllers complaining to me that my altitude shows about 400' too high when I am at my typical cruising altitudes of 7500 to 8500'. When i first was told about this I took it to an avionics shop that said they looked at it and it was fine. I kept flying and the controllers kept complaining. At this year's annual I had the IFR cert done and they of course test the transponder for this. The same avionics shop said they found it to be 200' off and adjusted it. Said they " ran it all the way up to 15,000'". So i figured i was good to go.

Last night while flying to Boston I get the same complaint from a NY Controller. This guy just requested that I turn off my mode C. So I switched my Narco transponder from Alt to On. He said to let each of the remaining controllers know mode C was off.

So this brings up the question, when do I need to use Mode C. I fly in the Mid atlantic area inside of the DC mode c veil so I assume it needs to be on then. What about outside of there?

What do the controllers see on their radar if mode c is off? Just my position but not my altitude?

Any suggestions as to how I can better explain my issue to the avionics shop? I've now tried twice to get it fixed, and they keep saying its fine.
Click to expand...
 
Get a better avionics shop. My guess i you have a static leak to the encoder that they missed. Unfortunately, this won't show up as a correspondence error on the ground but they're supposed to perform separate tests to find this.

When you stop altitude squawk, the transponder only replies to mode A replies, which means they get your squawk code as well as whether the ident button has been pressed recently. Either a mode A or C reply will yield the (horizontal) position information.

Unless you have some indication (other than controller complaints) that your mode C is inoperative, I would operate it unless the controller (as did in the case you mention) tells you to stop altitude squawk.
 
yachtjim said:
Since I bought my plane I have had controllers complaining to me that my altitude shows about 400' too high when I am at my typical cruising altitudes of 7500 to 8500'. When i first was told about this I took it to an avionics shop that said they looked at it and it was fine. I kept flying and the controllers kept complaining. At this year's annual I had the IFR cert done and they of course test the transponder for this. The same avionics shop said they found it to be 200' off and adjusted it. Said they " ran it all the way up to 15,000'". So i figured i was good to go.

Last night while flying to Boston I get the same complaint from a NY Controller. This guy just requested that I turn off my mode C. So I switched my Narco transponder from Alt to On. He said to let each of the remaining controllers know mode C was off.

So this brings up the question, when do I need to use Mode C. I fly in the Mid atlantic area inside of the DC mode c veil so I assume it needs to be on then. What about outside of there?

What do the controllers see on their radar if mode c is off? Just my position but not my altitude?

Any suggestions as to how I can better explain my issue to the avionics shop? I've now tried twice to get it fixed, and they keep saying its fine.
Click to expand...

Have you had the altimeter checked?

With Mode C off controllers will see your target and a data clock without Mode C reported altitude. They may enter your verbally reported altitute into the data block.

Because you have an encoder you're required to have it on when in controlled airspace, unless directed otherwise by ATC.
 
Ron Levy said:
OTOH, it might be your altimeter, not the encoder/transponder.
Click to expand...

Ah, you beat me to it!

If its your altimetery system then that means its indicating off and your really the 400' too high. :yikes:

A crude check of this can be done on the ground. Set the altimeter to the field baro and then read the altitude. Is it very close to field elevation or is it 400 feet off?
 
Sumtin ain't right..........

The IFR cert check is a transponder and static/ pitot test.... Any hiccups in the entire system should appear during that test... :dunno:
 
My altimeter shows almost the exact altitude as the gps. A leak to the encoder could be the issue. When I was digging around behind the dash during the last annual I replaced every hose clamp that I could find on the pitot static system. Most were missing! I will examine all of the hoses to see if one is leaking or still missing a clamp.
 
When you get through tinkering with the static system, you might want to get another full 91.411/413 systems check.
 
N801BH said:
Sumtin ain't right..........

The IFR cert check is a transponder and static/ pitot test.... Any hiccups in the entire system should appear during that test... :dunno:
Click to expand...

Nothing in the pitot system is tested.
 
Well I'm here to save the day with some very technical advice....
Did you check to see if your controllers were high? :rofl:


All kidding aside, does this problem appear at all altitudes or just above 8,500msl?
 
flav8r said:
Well I'm here to save the day with some very technical advice....
Did you check to see if your controllers were high? :rofl:
Click to expand...

I think what he really needs is some compliments as to the accuracy of his instruments.
 
ATC skin reflection radar gives ATC slant range distance. The addition of Mode C gave them your Pressure Altitude (referenced to 29.92 in.)

You fly to MSL presumably setting the barometric correction on your altimeter to whatever nearby ground stations are reporting.
 
flav8r said:
Well I'm here to save the day with some very technical advice....
Did you check to see if your controllers were high? :rofl:


All kidding aside, does this problem appear at all altitudes or just above 8,500msl?
Click to expand...

I generally always cruise at 7500 plus so this is the only time I am straight and level with flight following. Other times I'm either climbing or descending. So I am not sure if it's inaccurate at lower altitudes.
 
I'd probably consider getting rid of the Narco AND the Avionics shop.
 
N801BH said:
Probably so.... When they bless my Transponder they do a complete pitot /staic system test for another 20 bucks so I always say... Do it.
Click to expand...
Are you really sure they are checking the pitot system, not just the transponder, altimeter, and static system per 91.411 and 91.413?
 
I had serious ATC/altitude problems earlier this year. ATC was seeing my alt bouncing between 6500 & 8500 within seconds while I was level at 6500. Turned out to be a bad encoder.
 
Ron Levy said:
Are you really sure they are checking the pitot system, not just the transponder, altimeter, and static system per 91.411 and 91.413?
Click to expand...


Yup..... real sure.... He dials in a certain speed on the test instrument that is plugged into the pitot tube and the ASI moves to that figure +/- 3mph. I am sitting in the left seat when he does it. Simple but effective, and it is nice to know every two years my pitot system is ok..:yes:

It is probably not required though.
 
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JOhnH said:
I'd probably consider getting rid of the Narco AND the Avionics shop.
Click to expand...

Replacing the transponder won't do any good if you stick with the same encoder / altimeter.

Many encoders are temperature sensitive and the include a heater inside to keep them at a constant temperature - if the heater is not working you could get different results in a room temperature shop compared to whatever temperature it is seeing at altitude (could be hotter or colder depending on where it is located).
 
N801BH said:
Probably so.... When they bless my Transponder they do a complete pitot /staic system test for another 20 bucks so I always say... Do it.
Click to expand...

Exactly, when time comes to do any of it I do all of it.
 
Capt. Geoffrey Thorpe said:
Replacing the transponder won't do any good if you stick with the same encoder / altimeter.

Many encoders are temperature sensitive and the include a heater inside to keep them at a constant temperature - if the heater is not working you could get different results in a room temperature shop compared to whatever temperature it is seeing at altitude (could be hotter or colder depending on where it is located).
Click to expand...

The transponder in my Travelair had tubes that did that quite well.:eek:
 
I had a similar issue, then the unit finally went completely on the fritz. It was a bad encoder. Cost a couple hundred to replace and no issues since.
 
How does the entire system work? What are all the components in play to ultimately spit out an altitude that atc sees?
 
There are two parts here indicating altitude. The encoder and the altimeter. You need to figure out which one is reading off.
Check EACH before replacing anything.

And keep in mind that GPS alt is not related to baro alt, and a 400' error is more than within the margins of GPS/Baro difference. Your GPS alt won't tell you if the altimeter or encoder is wrong.
 
In a typical light plane, it starts with the static pressure system which senses outside air pressure via the static port(s). Static pressure is distributed via a system of plastic tubes around the airplane. One branch leads to the encoding altimeter, which has two components -- a pressure sensor and a digitizer. The sensed pressure is converted to pressure altitude based on the standard 29.92 atmospheric pressure, and that is converted to electronic digital form. The digital pressure altitude is sent by wire to the transponder, which adds the altitude data to the 4-digit code in the return pulse sent down to the Secondary Surveillance Radar (SSR) on the ground.

The code and pressure altitude received by the SSR then go to the computer generating the image on the controller's scope. The computer uses the code to link your blip to your data (call sign, aircraft type, etc). It corrects your transmitte pressure altitude to MSL altitude based on the local altimeter setting for that sector (which is also displayed for the controller to read to you on initial contact), and that MSL altitude is added to the data block displayed on the scope.
 
yachtjim said:
How does the entire system work? What are all the components in play to ultimately spit out an altitude that atc sees?
Click to expand...

The altimeter and the encoder are connected to the static system and measure the static pressure which is assumed to correlate to an altitude. The altimeter displays the altitude on the face (based on the setting in the Kolsman window), and the encoder sends a binary altitude signal (based a 29.92 altimeter setting) to the transponder through a parallel port (more or less).

Note: In some cases, the encoder is built into an altimeter.

The encoder typically has two adjustment screws - one for low altitude and one for high (I assume an offset and slope correction).

The transponder has knobs or buttons for you to set a squawk code and also can read the binary (modified grey code) altitude from the encoder. Each digit in the squawk code is between 0 and 7 - three binary bits per digit. I forget how many bits are in the altitude code.

When the transponder gets a request, it sends back the squawk code and the coded signal from the encoder.

If what you see on your altimeter does not match what ATC is seeing (with allowances for altimeter settings) then most likely it is the encoder or the altimeter.

When your shop did the IFR cert, they should have hooked up a vacuum pump to the static system. Then, as they reduce the pressure in the static system they can compare the static pressure (show as an altitude on a "master" gage) with the altimeter reading and the signal from the transponder (They have a radar transmitter / receiver pointed at your airplane also). If there is an error, they should be able to determine which is wrong - the altimeter or the encoder. If the encoder is off, they can tweak it to bring it back into spec. (usually). Apparently, this was done once to your system.

You will find that encoders are surprisingly affordable given that they are aircraft avionic parts example: http://www.bennettavionics.com/ssd120.html. Likely yours is getting flaky. The internal pressure sensors are sensitive to things like temperature and something may be going wrong with the temperature control or some other part of the analog circuitry.
 
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Ok, so it it sounds like the encoder is definitely the flaky part. The piece of the puzzle I was missing was that atc would convert the msl altitude based on standard pressure by using the actual pressure of my location.

One of you guys mentioned a high altitude and low altitude adjustment for the encoder. Perhaps this is a good place to start.

I am not sure what all was tested when they did the ifr cert. I assumed they would test everything, not sure why they wouldn't, but this has all been a learning experience. So I'll make sure they test the encoder.
 
yachtjim said:
Ok, so it it sounds like the encoder is definitely the flaky part. The piece of the puzzle I was missing was that atc would convert the msl altitude based on standard pressure by using the actual pressure of my location.

One of you guys mentioned a high altitude and low altitude adjustment for the encoder. Perhaps this is a good place to start.

I am not sure what all was tested when they did the ifr cert. I assumed they would test everything, not sure why they wouldn't, but this has all been a learning experience. So I'll make sure they test the encoder.
Click to expand...
The encoder is tested as part of thethe 91.411 altimeter/static system and altitude reporting (so-called "IFR") cert.
 
somorris said:
I had a similar issue, then the unit finally went completely on the fritz. It was a bad encoder. Cost a couple hundred to replace and no issues since.
Click to expand...

And encoders are substantially cheaper than transponders, too. You may want to get a 2nd opinion from another avionics shop.
 
yachtjim said:
..... I took it to an avionics shop that said they looked at it and it was fine. I kept flying and the controllers kept complaining. At this year's annual I had the IFR cert done and they of course test the transponder for this. The same avionics shop said they found it to be 200' off and adjusted it. Said they " ran it all the way up to 15,000'". So i figured i was good to go.


Any suggestions as to how I can better explain my issue to the avionics shop? I've now tried twice to get it fixed, and they keep saying its fine.
Click to expand...

The only common denominator is that avionics shop,, the plane has been tested two times and still is reading wrong? I agree with the others... find a different shop.
 
If it's a Garmin GTX 330, FIRE the puppy. My GTX 330 failed. The replacement GTX 330 ES just failed again. I'm going to a competitor. THE QA at Garmin really really sucks. The electrical bus is perfect. Yes, I have a master and we never flip the alternators while the master is on.

The 430W and all the Bendix-King stuff keeps on trucking.

The transponders, however, sucketh majorly.
 
bbchien said:
If it's a Garmin GTX 330, FIRE the puppy. My GTX 330 failed. The replacement GTX 330 ES just failed again. I'm going to a competitor. THE QA at Garmin really really sucks. The electrical bus is perfect. Yes, I have a master and we never flip the alternators while the master is on.

The 430W and all the Bendix-King stuff keeps on trucking.

The transponders, however, sucketh majorly.
Click to expand...

They want you to buy the 750 with plug on TXP.
 
Yep it's temp sensitive, I've had ATC ask me what I'm doing in the Alpha, when my altimeter was reading 17.5.
 
NineThreeKilo said:
Yep it's temp sensitive, I've had ATC ask me what I'm doing in the Alpha, when my altimeter was reading 17.5.
Click to expand...

Highly doubt that would be a temp issue.

Yes temperature affects actual altitude, but the temperature swings at FL180 are not so much and besides, a super non-standard temperature would affect both your indicated altitude AND your encoded altitude AND other aircraft in the area.

If ATC claims your in the Alpha it's because you're in the Alpha. If you got away with blaming it on temperature then great for you.

An incorrect baro setting or a localized non-standard pressure in the area is much more likely the issue than temperature.
 
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Ron Levy said:
The encoder is tested as part of thethe 91.411 altimeter/static system and altitude reporting (so-called "IFR") cert.
Click to expand...

When the Encoder is working properly, does that mean the whole system is?

How do we know the transponder is responding to interrogation?
 
bbchien said:
If it's a Garmin GTX 330, FIRE the puppy. My GTX 330 failed. The replacement GTX 330 ES just failed again. I'm going to a competitor. THE QA at Garmin really really sucks. The electrical bus is perfect. Yes, I have a master and we never flip the alternators while the master is on.

The 430W and all the Bendix-King stuff keeps on trucking.

The transponders, however, sucketh majorly.
Click to expand...
I've got six years and nearly 1000 hours on my GTX330, so YMMV.
 
Tom-D said:
When the Encoder is working properly, does that mean the whole system is?
Click to expand...
No, but if it isn't, the whole system won't pass the tests.
How do we know the transponder is responding to interrogation?
Click to expand...
Because the only observed problem is a 400 foot differential between what the panel altimeter says and what the controller sees. If the transponder wasn't responding to interrogation, the controller wouldn't see anything at all beyond a primary return.

Point is, there are several items which might cause the observed condition, and without further research, the specific problem cannot be determined. Might be the encoder, might be the altimeter, might be the transponder, might be the wiring, might be the plumbing -- just can't tell from the reported discrepancy.
 
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