ATC Altitude Reporting

[Brian Scranton]

On a few flights now, ATC has told me I am 300-400 feet (low) off my assigned altitude. My altimeter is set on the ground based on my airport ATIS and it matches the field elevation. My Garmin 330 transponder shows FL pressure altitude correctly, for example I can set 29.92 and my altimeter will match what my 330 is showing. And I always thought that ATC sees what my 330 is coding...but adjusts based on local altimeter under FL180. 

For example, yesterday, I was level at 15,000 on my altimeter, set to local (30.16), and was told I was at 14,600...GPS altitude was 16,000...and when I looked at my transponder alt, it showed FL146...but that's pressure alt...and anything under FL180 should NOT be pressure alt, right? 

Any ideas? 

 

 

[Robert C.]

Brian,

Maybe I misunderstand the scenario, but if you are at 15,000 under ATC control (IFR flight plan I assume) then you would have received an altimeter setting from Center or Approach. So you shouldn't be on the ATIS alt setting. Or was that simply saying that you checked on the ground and that ground elevation and altimeter indication are the same when using ATIS setting?

If the latter than it does sound like a transponder issue because you are correct that it shouldn't show pressure altitude below the flight levels.

[Andy95W]

Most altitude encoders have more than one calibration adjustment.  Does ATC tell you about altitude discrepancies at lower altitudes, like 3,000-4,000 feet, or is it just the higher altitudes above 10 or 12?  If so, then it will probably just be a simple adjustment by your avionics shop.  If it is always off by 400', and that always corresponds to pressure altitude, then you may have something else wonky with your altitude encoder.

Please keep us posted.

[Brian Scranton]

3 minutes ago, Robert C. said:

Brian,

Maybe I misunderstand the scenario, but if you are at 15,000 under ATC control (IFR flight plan I assume) then you would have received an altimeter setting from Center or Approach. So you shouldn't be on the ATIS alt setting. Or was that simply saying that you checked on the ground and that ground elevation and altimeter indication are the same when using ATIS setting?

If the latter than it does sound like a transponder issue because you are correct that it shouldn't show pressure altitude below the flight levels.

I'll set it to ATC's altimeter setting for sure--but I check on the ground and the ATIS/altimeter is correct (accurate within 25 feet to the field elevation) and then I'll set to 29.92 and the pressure alt as shown on the transponder is correct. 

 

2 minutes ago, N1395W said:

Most altitude encoders have more than one calibration adjustment.  Does ATC tell you about altitude discrepancies at lower altitudes, like 3,000-4,000 feet, or is it just the higher altitudes above 10 or 12?  If so, then it will probably just be a simple adjustment by your avionics shop.  If it is always off by 400', and that always corresponds to pressure altitude, then you may have something else wonky with your altitude encoder.

Please keep us posted.

I don't hear them complain until I am above 12,000 feet...

Thanks for your help guys. FYI, I had the pitot/static and transponder check done in February...

[Bob - S50]

When was your last IFR certification?  May be time to have the altitude encoder AND your altimeter checked.

[Brian Scranton]

42 minutes ago, Bob - S50 said:

When was your last IFR certification?  May be time to have the altitude encoder AND your altimeter checked.

February. Since everything syncs up on the ground, ie the altimeter reads correctly and the pressure alt does as well on the transponder I will go out this week sometime and go fly a few different altitudes to see if that stays constant and note where the change happens. I suspect a faulty encoder. But I suppose it could be a leak in the static line as well...

Looking back at FlightAware, it doesn't seem to happen on flights below 10,000...

[Bob_Belville]

I think I'd talk to the shop that did your recent IFR certification about what ATC is reporting. They might offer to verify that you're in spec all the way to FL180 even if they might not tell you that  it is possible they did not check it all the way up the first time.

[Andy95W]

Most systems I've seen have two calibration points, a low and a high altitude.  If both of those are correct, the rationale is that the points in between will also be accurate.

It sounds like your high altitude calibration may have drifted (or your avionics shop bumped it by mistake).  A static leak in an unpressurized airplane would normally show a slightly higher altitude if the leak were in the warmer-temperature cabin of the airplane.  Since yours shows 400' low, I will be surprised if a leak were causing the discrepancy (but I wouldn't put money on it either).

[Brian Scranton]

The shop in up in Warshington state--too far to fly just for this. I have a more local options but want to do some investigation first to help limit their troubleshooting. 

Andy, the calibration points are on the encoder, right? Not on the 330 itself...I am not sure what encoder I have--i'll have to look under the panel and see if I can locate it.  

[Piloto]

Brian it is likely that your encoder is conveying altitude via a 10 wires interface. If one of these wires is intermittent it will show an altitude discrepancy at some altitudes only. Try cleaning the transponder contacts and encoder contacts with contact cleaner. With time the contact pins build up oxidation that causes intermittent problems.

José

[Andy95W]

Brian- yes, the adjustments are on the encoder itself.  

[yvesg]

Could tnis be your alternate static valve opened?

Yves

[Brian Scranton]

6 minutes ago, yvesg said:

Could tnis be your alternate static valve opened?

Yves

I checked that! But good thought. Merci bien!

[takair]

Jose may be right.  Have your avionics shop check the A1 pulse.  This pulse happens to cut off in at 14800 ft.  If you are perfectly level at 15000, if it dropped out, your altitude happens to read 14500, but that assumes perfect 15000.  I suspect you may have an intermittent A1.  Looked at one of your flights and it jumped around a lot. This supports a pulse problem vs a calibration problem.

[takair]

This could be as simple as a seating problem.  First check to make sure transponder is not loose.  If not loose, make sure one of the other radios is not preventing seating.  Also, The back of the 300 series racks can bow a little and may prevent the connector from seating evenly.  By the way, which encoder do you have?

[Piloto]

Just now, takair said:

This could be as simple as a seating problem.  First check to make sure transponder is not loose.  If not loose, make sure one of the other radios is not preventing seating.  Also, The back of the 300 series racks can bow a little and may prevent the connector from seating evenly.  By the way, which encoder do you have?

I have a similar problem in my rack. To get them all the way back I insert them in parallel until both seat together simultaneously. The problem is related to the unit bezel dimensions.

José 

[Brian Scranton]

10 hours ago, takair said:

This could be as simple as a seating problem.  First check to make sure transponder is not loose.  If not loose, make sure one of the other radios is not preventing seating.  Also, The back of the 300 series racks can bow a little and may prevent the connector from seating evenly.  By the way, which encoder do you have?

That's a great question--I have zero idea what encoder I have. I have to dig into my logs, which are at the airport--or duck under the dash and locate the unit. 

[takair]

8 hours ago, Brian Scranton said:

That's a great question--I have zero idea what encoder I have. I have to dig into my logs, which are at the airport--or duck under the dash and locate the unit. 

Not critical...would just look at seating and that one wire...to start.

[cbarry]

If you have recently upgraded for ADSB out (if you have the GTX330 ES), then the  altitude variance could be as simple as the need for a GPS and/or transponder software update.  I say this because I've had a similar experience.

[Brian Scranton]

Because my gps shows higher than my altimeter my buddy suggested a static leak. I'm going to fly tomorrow and I'll report back!

[takair]

1 hour ago, Brian Scranton said:

Because my gps shows higher than my altimeter my buddy suggested a static leak. I'm going to fly tomorrow and I'll report back!

Don't forget that most IFR GPS's are augmented by the encoder to help with altitude precision....  I would expect a static leak to be a gradual and consistent offset, yours seems to have a step change.  

[Piloto]

1 hour ago, Brian Scranton said:

Because my gps shows higher than my altimeter my buddy suggested a static leak. I'm going to fly tomorrow and I'll report back!

Above 3000ft AGL it is normal to have a GPS vs baro altitude discrepancy. The baro altimeter relies on an ideal atmosphere to indicate the correct altitude. To overcome this you can find the baro corrected altitude using your E6B calculator. Enter the OAT and altitude as indicated on the E6B calculator. The corrected altitude should match the GPS altitude. One way to find if you have a static leak is by opening and opening and closing the alternate air valve while in flight. The altimeter and airspeed readings should change.

José

[Zwaustin]

My GTX330 was reading 200-300 off above 10k feet. It became a problem on some flights as ATC kept telling it was 300 off sometimes asking me to turn it to standby so it wouldn't alert them (my assumption). At my annual in June some minor adjustments were made to the unit and i have not had the issue since (60 days). Yours is also most likely is a simple setting issue. 

[Brian Scranton]

Twas the AR500 (ar stands for archaic) encoder. The high limit needed adjustment and the encoder didn't get out of whack until it warmed up--at first it only showed an 80 ft differential and after an hour it was up to ~150ft at 20,000ft. Plus the altimeter was 40ft off the other way, so...problem solved I think. I'll know more once I go high. Tested at 8000ft and ATC had me at 7960 (close enough!)

No static leak. No transponder issues.