Mode C Transponder question

[Walboy]

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[AustinPilot]

Transponders always use pressure altitude. You should not try to fly an altitude based on what the transponder is reporting.

ATC will correct the pressure altitude with the area's local altimeter setting. Did you adjust your altimeter enroute to match the local value? Usually they will give it to you when you come on frequency.

At 8000 and 10000 feet, my altimeter reads 30 and 40 lower. So, if I fly at the assigned altitude with indicated altitude exactly where is should be, I'm 30 or 40 feet high.

I don't really understand what this means. If you are flying 8000 indicated then what are you using to determine that you are 30 or 40 feet high?

 

[Stewartb]

Even when my pitot-static system is newly re-certified my transponder can deviate +/- 100' from my correctly set altimeter. ATC doesn't comment even when they call me to other traffic at 100' higher than the airspace ceiling, which I'm not violating. For that reason I always give called traffic a pretty big vertical window rather than zeroing in on the altitude ATC reports them. Maybe an ATC guy can comment about acceptable deviations in the reporting? One time my encoder reported me at 3100' when I was approaching at 1200'. In that case ATC did call me to verify. I volunteered to turn off Mode C. They did not direct me to do so once I'd called my altitude. I thought that was odd.

 

[MAKG1]

I once flew a 172 with a transponder that reported altitude 200 feet high. Under a Class B 1500 foot floor, at 1400. Approach asked me repeatedly to "say altitude," which I did. After several times, I asked what they thought my altitude was, and that's how I found out the transponder was AFU. ATC neither asked me to turn off altitude reporting nor busted me for anything. But they did report my altitude to other traffic as it was reported by the transponder. Given that I was in busy airspace close under a Class B floor, other traffic was closer than that report would have implied.

 

[Velocity173]

Not sure if Yuma is still using the old ATCRBS (TPX-42A) but that system requires the controller to type in the actual altimeter setting to get a correct Mode C. If they've upgraded to he newer systems, then it gets an automatic feed from the local barometer.

If you were less than 300 ft of your reported altitude then you have a valid Mode C. In this case the controller saw fit to inform you anyway. Just maybe a hint that your static or your encoder might be going bad.

Generally PT isn't allowed in the radar room because of a distraction to others but it all depends who signed on as radar sup. It does happen on occasion though.

 

[flyingron]

There is a table in my aircraft log book from the altimeter cert that records the error. This is pretty standard, much like a compass deviation card.

Your altimeter won't pass certification if it's not "close enough." There's no point in applying the correction.

 

[luvflyin]

Even when my pitot-static system is newly re-certified my transponder can deviate +/- 100' from my correctly set altimeter. ATC doesn't comment even when they call me to other traffic at 100' higher than the airspace ceiling, which I'm not violating. For that reason I always give called traffic a pretty big vertical window rather than zeroing in on the altitude ATC reports them. Maybe an ATC guy can comment about acceptable deviations in the reporting? One time my encoder reported me at 3100' when I was approaching at 1200'. In that case ATC did call me to verify. I volunteered to turn off Mode C. They did not direct me to do so once I'd called my altitude. I thought that was odd.

Don't know about that 3100 vs 1200. He may have just transposed numbers in his head for a second. ATC may use a Mode C altitude that differs by less than 300' from reported altitude and shouldn't have you stop altitude squawk until it does. A lot of things can affect altitude. Older systems required manual altimeter setting input from the controller. Those altimeter settings typically came in hourly. Rapidly rising or falling pressures combined with the timeliness of readings can add up. Another factor is pressure gradients. If there is a steep gradient and your flying quite a distance from the source of the altimeter setting, that's going to add up to depending on which way the gradient is "leaning." Another factor that can throw things off is Bernoulli effect. If your flying over rough terrain it can add up to if the wind is blowing at a pretty good clip. Think of the mountains as the top of a wing and the mountain passes as the Venturi of a carburetor. Increased velocity = lower pressure. "High to low, watch out below." It is the primary reason for the 2000' terrain clearance rule in Designated Mountainous Areas. While that shouldn't affect Mode C read outs much because both your Altimeter and the encoding altimeter are going up and down together, differences between the accuracies could show some difference.

 

[John Collins]

An IFR altimeter test will check many factors. Scale error can be up to 20 feet at sea level and rises to 60 feet at 8000 MSL and 110 feet at 18000 MSL. The calibration of the Kollsman adjustment is to be within 25 feet at selected pressures. The correlation with an encoder has to be within 125 feet. Other checks of friction, leakage, and hysteresis are also conducted. So in a worse case situation at 8000 feet, your altimeter could be off by 60 feet and another 25 feet because of the altimeter setting, for an allowed 85 feet and this can be off the encoder by 125 feet or 210 feet. If your altitude is compared to that of another aircraft, under the worst case conditions, you could be at the same altitude and but have pressure altitudes off by 400 feet. Fortunately, the worst case conditions don't occur that often and most of the time, the pressure altitude will be within +/-100 feet.

 

[Stewartb]

Don't know about that 3100 vs 1200. He may have just transposed numbers in his head for a second. ATC may use a Mode C altitude that differs by less than 300' from reported altitude and shouldn't have you stop altitude squawk until it does. A lot of things can affect altitude. Older systems required manual altimeter setting input from the controller. Those altimeter settings typically came in hourly. Rapidly rising or falling pressures combined with the timeliness of readings can add up. Another factor is pressure gradients. If there is a steep gradient and your flying quite a distance from the source of the altimeter setting, that's going to add up to depending on which way the gradient is "leaning." Another factor that can throw things off is Bernoulli effect. If your flying over rough terrain it can add up to if the wind is blowing at a pretty good clip. Think of the mountains as the top of a wing and the mountain passes as the Venturi of a carburetor. Increased velocity = lower pressure. "High to low, watch out below." It is the primary reason for the 2000' terrain clearance rule in Designated Mountainous Areas. While that shouldn't affect Mode C read outs much because both your Altimeter and the encoding altimeter are going up and down together, differences between the accuracies could show some difference.

In the 3100' example my equipment was broken. ATC radar had me dancing with heavy 747s coning into ANC so they asked. I turned Mode C off and had it repaired before I used it again. Lake Hood airspace does not require a transponder so turning it off was not a problem.

 

[luvflyin]

In the 3100' example my equipment was broken. ATC radar had me dancing with heavy 747s coning into ANC so they asked. I turned Mode C off and had it repaired before I used it again. Lake Hood airspace does not require a transponder so turning it off was not a problem.

You defiantly needed to stop altitude squawk. It sounds you were on top of it right away so the controller didn't need to tell you to do it

 

[Shawn]

I had a similar problem with a GTX330. Reporting altitude was off as much as 300' at times compared to altimeter and even GPS altitude...and the error varied at different altitudes. Had same problems as MAKG going through Bravo airspace and getting called out on assigned altitude busts.

For me it ended up being a small leak in the pitot system. Easy fix.

Since the transponder is working off of pressure altitude you should be able to set your altimeter to 29.92 while in flight and your altimeter and transponder altitude should be the same to see if you have a calibration error between the two.