Question about Mode C

PanJet

PanJet

Well-Known Member
So basically I've always understood that a Mode C transponder has kind of a built in altimeter that always reads pressure altitude, which it then in turn squawks. Then, ATC knows your true altitude because they also know the altimeter setting they gave you, so their computers automatically recalculate the pressure altitude the transponder is squawking for differences in pressure.

Today, however, my new instructor who is helping me get my CFII, tells me that I'm wrong, and that the transponder is somehow linked directly to the altimeter. I ask him why then does it squawk pressure altitude, and he says it has something to do with the way it's connected to the altimeter, and that the actual altitude that the transponder is squawking is true altitude even though it displays pressure altitude (for those of us with digital transponders).

I still believe I'm right (and even if I'm not my explanation made more sense than my instructor's because he couldn't figure out where the conversion came in), but I'd like some clarification and or correction to be sure. Also, if I'm correct that a transponder has a built in altimeter of sorts, where does it get its static pressure from?
 
You're right, the transponder gets its altitude information from a blind encoder which only reads pressure altitude. It is not connected to the altimeter on the panel.
I pulled out a cessna 172 POH and according to that it is connected to the static system, so the altimeter and blind encoder are both fed from the same source
 
The transponder displays pressure altitude and It's not directly connected to the altimeter but It's connected to the same static system so in case the static port gets blocked the alternate source will actually feed the transponder as well.
 
My understanding:

The transponder has its own pressure sensor. It is always set to 29.92. The transponder sends its data out to ATC who's computers then correct the data for that day's atmospheric conditions.

I would love to learn more if anyone knows more specifics.
 
Here's what you do. Stick your head under the panel and find the static lines running between the VSI and Altimeter and elsewhere. If it's a blind encoder setup, you will find a 1"x2"x3" or so little aluminum box with a static line running into it. This should be hard mounted somewhere up under the panel but is usually in a fairly easy to get at place. That is the "blind encoder" that senses altitude and "digitizes" the information and sends it to the transponder.

Most light aircraft have this sort of setup. Larger and newer aircraft sometimes will have expensive altimeters that don't require a seperate blind encoder. Perhaps this is what your CFII is thinking of.

Anyhow, if you find the little box under the panel I'm talking about, point it out to him and ask him what it does and why there is a static line running to it?

After doing a google search, it's took me about 20 seconds to find a link with some pics.

http://www.aircraftspruce.com/menus/av/encoder.html
 
On some light planes you will see a sticker on the altimeter, telling you that it is an encoding altimeter. Why you would care I don't know, but I have flown several Pipers with the sticker.
 
The blind encoder uses pressure altitude rec'd from a static port on the outside of the aircraft or somewhere on the static line. As the pressure altitude is relayed to ATC after its rec'd a set of Mode C interrogation signals it replies back to ATC your pressure altitude rounded to the nearest hundred feet.
 
I was flying tonight and was worried about the altitude difference between my altimeter and my transponder and couldn't wait to get on land and search JC for this post. Anyway, I was flying tonight and the reported altimeter setting was 30.15 and my altimeter was reading 3,000, but my transponder was reading 2,600. Is this because of the explanations in the previous post...that being that the transponder is reading altitude based on pressure altitude? So, basically, when flying, I should go by my altimeter, not the transponder altitude reading?
 
I'd definately rely on your altimeter. If you were assigned 10,000' you'd never want to fly according to your xpdr. Make sure your altimeter is correct though and there is no error within your pitot static system or local altimeter setting. The pressure differential is computed by ATC not your xpdr. A pressure altitude reading is sent to ATC where it is then adjusted by their computers which have the current altimeter setting.
 
John_Mitchell said:
I was flying tonight and was worried about the altitude difference between my altimeter and my transponder and couldn't wait to get on land and search JC for this post. Anyway, I was flying tonight and the reported altimeter setting was 30.15 and my altimeter was reading 3,000, but my transponder was reading 2,600. Is this because of the explanations in the previous post...that being that the transponder is reading altitude based on pressure altitude? So, basically, when flying, I should go by my altimeter, not the transponder altitude reading?
Click to expand...
What Merit said. Just the difference between 29.92 and 30.15 is about -230', so I would not worry what your transponder is saying. They almost never agree with the altimeter.
 
PanJet said:
So basically I've always understood that a Mode C transponder has kind of a built in altimeter that always reads pressure altitude, which it then in turn squawks. Then, ATC knows your true altitude because they also know the altimeter setting they gave you, so their computers automatically recalculate the pressure altitude the transponder is squawking for differences in pressure.

Today, however, my new instructor who is helping me get my CFII, tells me that I'm wrong, and that the transponder is somehow linked directly to the altimeter. I ask him why then does it squawk pressure altitude, and he says it has something to do with the way it's connected to the altimeter, and that the actual altitude that the transponder is squawking is true altitude even though it displays pressure altitude (for those of us with digital transponders).

I still believe I'm right (and even if I'm not my explanation made more sense than my instructor's because he couldn't figure out where the conversion came in), but I'd like some clarification and or correction to be sure. Also, if I'm correct that a transponder has a built in altimeter of sorts, where does it get its static pressure from?
Click to expand...
Actually you are both wrong (assuming a typical GA small airplane setup). The transponder does not have an altimeter built in, nor is it connected to the altimeter. Most GA airplanes have a device called a blind encoder. This is a seperate part from both the transponder and the altimeter. The encoder is connected to the static line and has a pressure altimeter. There is a digitizer inside the encoder that converts the pressure altitude to electronic form and sends that information via data link cable electronically to the transponder. From that point on you are correct, that the transponder sents the pressure altitude along with the squawk and ATC's computers will convert it to true altitude.

Now, some more sophistocated setups have what's called an encoding altimeter where the transponder does get the altitude information directly from the altimeter. That might be what your instructor is referring to, but most trainiers do not have this setup.
 
ATC's computers do not recalculate to find "true" altitude since they're primarily interested in seperation. Since everyone's transponder uses 2992 they can determine different planes relative altitude to one another, even if it is off from its true altitude by a little. At least, this is my understanding.
 
SpiraMirabilis said:
ATC's computers do not recalculate to find "true" altitude since they're primarily interested in seperation. Since everyone's transponder uses 2992 they can determine different planes relative altitude to one another, even if it is off from its true altitude by a little. At least, this is my understanding.
Click to expand...

visit a tracon or center sometime. They recieve the pressure atitlude and convert it using the altimiter setting. The datablocks on the scopes display this altitude, not the pressure altitude.
 
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