Altitude Reporting & When To Worry

[Marauder]

When I flew IFR yesterday one of the controllers told me that I was showing 200' above my assigned altitude. He also said it wasn't a problem but thought I should know.  As I was being handed over from controller to controller I asked what my altitude was showing and they all said I was 200' higher than what I was showing in the plane (even when I flew different altitudes). I think the snitch goes off somewhere over 300', right?

 

They were all Philly controllers, so I am left to wonder if it was something to do with Philly or do I have a legitimate problem. Any thoughts?

[Cruiser]

when was your last static system/transponder check?

 

Your encoder reports pressure altitude. The ATC computers adjust their readouts by the "altimeter setting" you dial in the Kollsman window. The equipment is allowed 125 foot tolerance, I think ATC looks when you get 250 off. They usually ask me to confirm when other traffic is involved. 

 

Your altimeter could be off, your encoder could be off or Philly's computer could be off.(not likely) if you were the only one they were having problems with.

[BorealOne]

Technically, any deviation from assigned altitude is a deviation from ATC clearances.

 

But for ATC, it's all about Mode C tolerances on their scopes. If Mode C is more than 300' out of what the pilot says, it's not valid Mode C for ATC purposes, so they are more likely to make an issue of it.

 

If you are 200', I'd suggest getting it calibrated, as it doesn't fix itself.

 

http://www.jetlaw.com/insights/wp-content/uploads/altitude-deviations-mar-08.pdf

[John Pleisse]

Philly is pretty strict, especially if you are VFR. They usually transition traffic 5,000 and 6,000 only. I was with a Buddy in an Arrow picking up ice (no much, but enough to ask for a deviation)...they really made us stay at 5,000 way too long. I can see why they would be tight with altimeter.

[Marauder]

when was your last static system/transponder check?

Your encoder reports pressure altitude. The ATC computers adjust their readouts by the "altimeter setting" you dial in the Kollsman window. The equipment is allowed 125 foot tolerance, I think ATC looks when you get 250 off. They usually ask me to confirm when other traffic is involved.

Your altimeter could be off, your encoder could be off or Philly's computer could be off.(not likely) if you were the only one they were having problems with.

The last IFR check on the transponder and pitot/static was December 2013. You do bring up an interesting point though. Which Kollsman? The one on the Aspen or the analog one? I usually set both but I would think one would be considered primary.

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[Marauder]

Philly is pretty strict, especially if you are VFR. They usually transition traffic 5,000 and 6,000 only. I was with a Buddy in an Arrow picking up ice (no much, but enough to ask for a deviation)...they really made us stay at 5,000 way too long. I can see why they would be tight with altimeter.

I was on an IFR flight plan and at 5000'. When this weather breaks, I will try heading out to Harrisburg and see what they report. The last time I can remember a controller commenting on my altitude was just before I replaced my aging King unit.

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[N601RX]

Does your transponder or other device display the output of the blind encoder?  If so does it match the altimeter when set to 29.92?

[Danb]

Chris, It maybe a good idea try try another center, I flew from Ilg to Det and Philly was off a couple hundred feet. Harrisburg, Washington,Cleveland and Detroit had me at my assigned altitude? It's not always our stuff.

[Cruiser]

Look on your altimeter recertification record, when I had the Aspen, they certified both it and the altimeter. Actually, I wrote the numbers while the avionics tech watched the readouts.

[jlunseth]

Don't know what kind of xpondr you have, I have an old King Mode C.  I had that problem show up once in the nearly 5 years I have had the aircraft.  It was reporting me consistently 600 feet lower than actual.  I checked everything, reset the xponder, same result.  Had it checked by my avionics shop, they are pretty good, they found nothing.  It never happened again.  Don't ask me why it happened. 

[Marauder]

Does your transponder or other device display the output of the blind encoder? If so does it match the altimeter when set to 29.92?

Nope, my transponder truly is blind

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[yvesg]

I was having similar issues on my trip to California a month ago. The mode C encoder I have is the Ameriking, which is the cheapest one available. I was 2-3 hundread feet higher than what was reported and the worst was always when it was really cold and at the beginning of the flight. These things have the pressure sensor in a small oven which is supposed to keep it at the right temperature.

Also, the box is located near a vent with a draft almost permanently. I think it was just too cold, because I only get such issues when it is minus 20 C or less. These boxes are not rated below minus 20 C anyway.

I will replace it with another brand and change the installation to stop that draft. This shall take care of the problem I hope.

Yves

[Marauder]

Chris, It maybe a good idea try try another center, I flew from Ilg to Det and Philly was off a couple hundred feet. Harrisburg, Washington,Cleveland and Detroit had me at my assigned altitude? It's not always our stuff.

Thanks Dan. I never had anyone else ever mention it in the years I have flown here. I will be quizzing the controllers a bit on my next flight. BTW -- the next time you get an urge to fly somewhere, let's meet up for a meal.

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[Marauder]

Look on your altimeter recertification record, when I had the Aspen, they certified both it and the altimeter. Actually, I wrote the numbers while the avionics tech watched the readouts.

Will do. I saw the entry but didn't read it too closely.

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[Marauder]

Don't know what kind of xpondr you have, I have an old King Mode C. I had that problem show up once in the nearly 5 years I have had the aircraft. It was reporting me consistently 600 feet lower than actual. I checked everything, reset the xponder, same result. Had it checked by my avionics shop, they are pretty good, they found nothing. It never happened again. Don't ask me why it happened.

I have a Narco AT-150'that I installed new back in 1993 or 94. The blind encoder was re-used.

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[carqwik]

Check your flight log graph on Flightaware....it shows the altitude captured by ATC computers.  If your log shows you say 300' higher (or lower) than what you thought you were flying at (assuming correct baro utilized) then you've got an issue somewhere.  The way I understand it, the encoder sends out an altitude reading based on 2992 pressure altitude which is corrected by the ATC software to show your altitude for any given radar sector altimeter setting...at least for those flying below the flight levels. 

[RocketAviator]

If I recall correctly Mode C transponders only report pressure altitude calibrated to 29.92". The ATC decoding system which receives the Mode C reply automatically applies the appropriate correction for local barometric pressure.   You can set your Baro to anything you want while in level flight and the transponder reports the same.  I have but dont often use a Zeon XRX and it indicates the Mode C output.

[Jerry 5TJ]

....I was with a Buddy in an Arrow picking up ice (no much, but enough to ask for a deviation)...they really made us stay at 5,000 way too long.....

If you tell ATC you are getting ice in a non-FIKI plane and they don't respond - my experience is they listen up when you say ice - then say the E word and get out of that situation.

[wishboneash]

My altimeter is off by about 40 ft when on the ground (field level is 400ft). At 8,000 to 10,000 ft it is off by 150-200ft. I usually fly at 100 ft below ATC assigned altitude which puts me reasonably close to the correct assigned altitude.

[BigTex]

When you check in with ATC, they'll quite often tell you what altitude they have you at... I guess you can adjust accordingly.

[flyboy0681]

For those of you that have a Monroy ATD-300, you can look at the display to see what pressure altitude the transponder is transmitting to ATC.