Altimeter/Mode C are incorrect

[jraymond4321]

I purchased a RV-6a and it now has about 100 hours TT.

My altimeter is reporting 200-300 feet below actual altitude for the last 10 hours or so, it was accurate before.

I was going to adjust it, but it is a Falcon guage so I have to take the panel off and I haven't gotten around to it.

I was flying down to Miami Thursday, with flight following, and ATC said my mode C was reporting 7100, when I was actually at @7500 (verified with 2 GPS). On the way home, I checked when i left the tower, and same thing (mode C reported 2200, while I was at @2500).

Does this sound like any particular problem (ie. pitot leaking, or blockage)? I don't fully understand how the mode C works in conjuction with the pitot system, and if or how it is related to the altimeter.

I would like to have a better understanding before I bring it to the shop.

Additionally, would I need to go to an avionics shop for this, or my normal maintenance shop (transponder is avionics, but pitot would be maintenance)?

Thanks in advance

James Raymond
N436JE
Neptune Beach, FL

[osxuser]

So it's your altitude encoder which is reading low, not your altimeter? It's not too uncommon for them to be out of calibration, but just for kicks, make sure it's not connected to the PITOT line instead of the STATIC by accident.

[robertahegy]

Your Encoder is set for pressure altitude (29.92). Set your Altimeter to 29.92 and see if they agree. They won't agree if you have your alt. set at VFR settings (actual reported pressures). I would get a pitot static check and have a shop check the encoder. It may be the item needing adjustment. But first, verify your altimeter at a known field elevation and pressure. GPS's are not always precise at determining altitude.

Hope these checks help, if you haven't already tried them.

Roberta

[94kb]

Checking your altimeter against a controller is a good idea but it is not perfect. Your encoder responds to a pressure setting of 29.92 and only gives altitude reports with 100 ft increments. It changes at the 50 ft point.

The controller doesn't see you response directly. The ATC system adjusts the received mode C altitude for the local pressure in the area you are flying. If you are enroute, there could be some difference between your altimeter setting and the value entered into the ATC System. This shouldn't be much of a problem for a terminal area if you and the ATC System are set to the correct ATIS.

GPS altitudes are not as accurate as Lat Longs. Normally accuracy is about 1.5x the horizontal accuracy. This should be about 75 ft with SA turned off (as it is now) but the accuracy will vary according the current position of the constellation and your GPS receiver's selection of satellites to get the best Geometric Dilution of Precision (GDOP).


Ken

[Kevin Horton]

You say your altimeter and Mode C were working correctly until 10 hours ago. How do you know? By comparing them to the GPS?

Don't waste your time comparing your altimeter against the GPS. The GPS reports geometric altitude, plus or minus its error. The altimeter reports barometric altitude. Geometric altitude and barometric altitude are only the same thing if the temperature is the same as the standard atmosphere, all the way from the ground to the aircraft altitude. If the temperature is warmer than standard, the altimeter will be reading lower than the geometric altitude. The difference between barometric altitude and geometric altitude is approximately 3.5% for every 10 deg C that the temperature differs from standard (this is a gross simplification, that assumes that the altimeter setting comes from a place at sea level).

So, if 10 hours ago, the temperature was somewhere close to standard temperature, then I would expect your altimeter/transponder and GPS to be reporting roughly the same thing (plus or minus the normal errors associated with each device - perhaps 50 to 75 ft for the altimeter, 100 ft for the transponder, and 100 to 200 ft for the GPS). If the temperature has warmed up, then the GPS will be reporting geometric altitudes that are higher than the barometric altitudes reported by the altimeter and transponder. This is completely normal.

The altitude encoder and the altimeter should be seeing the same static pressure. So, they should be reporting the same altitude. If you have a static system problem, then there is no point asking the controller what the mode C says, as its output will be affected by whatever is going on with the static system.

Does the altimeter read the correct altitude on the ground at an airport that provides an altimeter setting? If not, then you have an altimeter problem.

Check for a static system leak.

[L.Adamson]

Originally Posted by 94kb

GPS altitudes are not as accurate as Lat Longs. Normally accuracy is about 1.5x the horizontal accuracy. This should be about 75 ft with SA turned off (as it is now) but the accuracy will vary according the current position of the constellation and your GPS receiver's selection of satellites to get the best Geometric Dilution of Precision (GDOP).
Ken

FWIW, I've been taking my Garmin 296 along, for a number of vehicle (land) trips around the Mt. West. Altitudes have ranged from 2000' to over 9000.

I wanted to check the Garmin's altitude readout using WAAS against posted altitude signs at mountain summits, state & federal parks, etc.

Now granted, the posted signs might be a few feet off, although some might read 8915', instead of rounded off at 8900'.

The "big" surprise, is that the Garmin, when using WAAS, was hitting these altitudes almost exactly, or perhaps three to five feet off, most of the time. A few times it would be around 15 feet off, and never worse than 35'. It also is consistant within 35' when walking around my yard on different days. But I think it's more accurate while on the move.

At this point, I'm starting to trust it, at least as much as an altimeter, and perhaps more. The "panel" page also keeps up while driving up and down the hills or making turns. If worse went to worse, I suppose I could now trust the panel page too!

L.Adamson

[Kevin Horton]

Sure, the GPS should provide a reasonably accurate geometric altitude. But, if you are flying, and ATC gives you an altitude, or you need to fly at a VFR cruising altitude, don't forget that those are barometric altitudes. The barometric altitude may differ by over a thousand feet from the geometric altitude if the temperature is significantly different than the standard atmosphere.

[4kilo]

Just to reinforce what Kevin is saying:

If you are flying GPS (geometric) altitudes, you may not be at the correct barometric altitude. This could cause a conflict with other traffic flying at the correct (baro) altitude. Since the difference between baro and geometric altitude becomes smaller and smaller the closer you get to the ground, comparing the two altitudes on the ground will not reveal the problem.

Pat

[L.Adamson]

Originally Posted by Kevin Horton

Sure, the GPS should provide a reasonably accurate geometric altitude. But, if you are flying, and ATC gives you an altitude, or you need to fly at a VFR cruising altitude, don't forget that those are barometric altitudes. The barometric altitude may differ by over a thousand feet from the geometric altitude if the temperature is significantly different than the standard atmosphere.

Not something I've gave a lot of thought too,

But for instance, I live in the Mt. West area with many airports ranging from 4000' to 7000'+ altitudes. Since it's common practice to set the altimeter to the "un-controlled" airport's runway altitude before takeoff, rather than relying on some report of barometric pressure; wouldn't it be very conceivable of having a drastic difference in VFR crusing altitudes, if mine started exact, and someone else was a 1000' off?

Or, am I just getting the wrong idea here?

L.Adamson

[pbesing]

1000 feet off? That would be an entire inch of mercury...I doubt others would be that far off. As far as temperature, what you need to be more concerned with is a change in temperature from one location to another. If the temperature is relatively constant accross a large area, that shouldn't be a problem..crossing a front or something, or entering a temperature inversion might have an effect, but temperature changes shouldn't make that much of a difference.