ADS-B Out and altitude reporting

[Fearless Tower]

Pardon my ignorance if this has been addressed before. I just got the Twin Beech officially ADS-B out (had to install the wire from the 530W to the 330ES).

So….now I'm traceable even when VFR flying and not talking to anybody

When I looked up my flight home from the avionics shop on FlightAware, the altitude reported was about 200' lower than I was flying (800 vs 1000' AGL). Now, that reported altitude would coincide with pressure alt. Is that what it is supposed to be reporting?

I ask, because in the past (just tracking myself on IFR flights), my ATC recorded altitude is almost always the altitude I see on the altimeter.

 

[bnt83]

Pardon my ignorance if this has been addressed before. I just got the Twin Beech officially ADS-B out (had to install the wire from the 530W to the 330ES).

So….now I'm traceable even when VFR flying and not talking to anybody

When I looked up my flight home from the avionics shop on FlightAware, the altitude reported was about 200' lower than I was flying (800 vs 1000' AGL). Now, that reported altitude would coincide with pressure alt. Is that what it is supposed to be reporting?

I ask, because in the past (just tracking myself on IFR flights), my ATC recorded altitude is almost always the altitude I see on the altimeter.

All blind encoders output pressure altitude referenced at 29.92.

 

[eman1200]

yep, same for me. PA, I believe.

 

[BGF_Yankee]

I've been wondering this and whether or not I had an encoder problem as well. I'm almost always showing a 100 to 200 foot difference off of what I see on my altimeter cruising along at. I think once I saw 400-500. I guess I could always just ask ATC, but for whatever reason I never think about it when I get in the airplane...only after the flight when I'm reviewing the tracking data.

 

[Baron62]

Your encoder sends standard Pressure Altitude (29.92) out. That is what is on flightaware and ATC recieves. The controller's system corrects for current atmospheric pressure setting and the pilot does the same via the Kollsman window.

 

[bnt83]

The possibly of difference between the pilot's mechanical altimeter and the blind encoder is why ATC asks "say altitude", they are checking what you read off against what they see on the scope. If wildly different something is messed up.

 

[Morgan3820]

The possibly of difference between the pilot's mechanical altimeter and the blind encoder is why ATC asks "say altitude", they are checking what you read off against what they see on the scope. If wildly different something is messed up.

I am having similar concerns. At what difference should I be concerned?

 

[bnt83]

I am having similar concerns. At what difference should I be concerned?

Set your mechanical altimeter to 29.92 and read the difference...

 

[bnt83]

 

[Morgan3820]

Set your mechanical altimeter to 29.92 and read the difference...

I did that and the difference was 200'. Should I be concerned as I recall ATC gets concerned at 300' off assigned altitude.

 

[denverpilot]

I did that and the difference was 200'. Should I be concerned as I recall ATC gets concerned at 300' off assigned altitude.

One or the other is off. Time for a pitot-static check.

 

[Morgan3820]

One or the other is off. Time for a pitot-static check.

It had one just a month ago. It is going back in. I am going to upgrade the encoder while I am at it.

 

[John Collins]

I did that and the difference was 200'. Should I be concerned as I recall ATC gets concerned at 300' off assigned altitude.

When you set your altimeter to 29.92, it should agree with your encoder output within 125 feet.

(c) Automatic Pressure Altitude Reporting Equipment and ATC Transponder System Integration Test. The test must be conducted by an appropriately rated person under the conditions specified in paragraph (a). Measure the automatic pressure altitude at the output of the installed ATC transponder when interrogated on Mode C at a sufficient number of test points to ensure that the altitude reporting equipment, altimeters, and ATC transponders perform their intended functions as installed in the aircraft. The difference between the automatic reporting output and the altitude displayed at the altimeter shall not exceed 125 feet.