Altimeter Accuracy

[Targetbuster]

Question: When you set your altimeter to Barometric pressure according to ATIS or other accurate info service, what is the general deviation you see on the altimeter vs. your known altitude? I'm curious because I noticed >100 ft variations while doing ground preps the other day on the aircraft I fly. The handbook of aeronautical knowledge says variations of >75 ft are not acceptable.
I also notice a considerable deviation from GPS altitude.
So, I guess my question is: should a properly barometrically adjusted altimeter be showing relatively the same thing as the GPS? My thought would be yes (within 50 ft or so).

Thanks,

Target

[drseti]

should a properly barometrically adjusted altimeter be showing relatively the same thing as the GPS?

GPS altitude accuracy can vary widely, due to a number of different factors (all beyond your control). The spec on a pressure altimeter is indeed +- 75 feet, when current barometric pressure is set in the Kolzman window. And the appropriate way to check it is against published field elevation. If you're outside that 75 foot limit, you can have an instrument shop recalibrate it for you. (This should be done anyway as part of your 2-year altimeter/transponder/encoder/static system check required by FAR 91.411 and performed in accordance with FAR part 43, Appendix E).

If this is a rental aircraft, ask the FBO for a look at the airframe logbook. The above check should be logged within the previous 24 calendar months. If it isn't, you might make a gentle suggestion that it be done, while mentioning that the altimeter seems to be out of spec. Although most renter pilots consider maintenance the owner/operator's responsibility, FAR 91.3(a) and FAR 91.7(b) can be interpreted as requiring the Pilot in Command to verify that all required maintenance and inspections have been performed.

[Jim Stewart]

A little bird told me how to adjust a Falcon altimeter so that the field elevation and indicated altitude match if you know the barometric pressure..

http://www.falcongauge.com/PDFs/instruc ... adjust.pdf

[zdc]

Having the pitot/static system checked every 24 months is only mandatory if you fly IFR.

Next time you fly, make sure you have the proper local altimeter setting then contact ATC and ask for a Mode C readout. That should give you a good idea about the accuracy of your altimeter.

[zdc]

A little bird told me how to adjust a Falcon altimeter so that the field elevation and indicated altitude match if you know the barometric pressure..

http://www.falcongauge.com/PDFs/instruc ... adjust.pdf

You need to be careful what airport elevation you are using when making the adjustment. Field elevations depicted on charts or the AFD are elevations at the Airport Reference Point [ARP] which have different locations at different airports.

[Targetbuster]

Thanks for the replies. I did call an avionics shop and asked what the tolerances were and the avionics technician told me that at sea level, the variance in a properly set altimeter should be no more than 20 ft +- known field elevation.
It is a rental aircraft and I've been dropping gentle hints that I suspect the altimeter is out of spec but this discussion will help make my point. Interestingly, none of the instructors I've talked to seem to think there is a problem.
But the last time I sat in the plane I called the local ATIS got the baro setting and dialed it in. The altimeter read 120' below known field altitude. And that is 22' so it should be within the 20' parameter at that MSL.
I hate not having a reliable altimeter. Seems like the most important instrument, even for VFR flight, to me.

[zdc]

Thanks for the replies. I did call an avionics shop and asked what the tolerances were and the avionics technician told me that at sea level, the variance in a properly set altimeter should be no more than 20 ft +- known field elevation.
It is a rental aircraft and I've been dropping gentle hints that I suspect the altimeter is out of spec but this discussion will help make my point. Interestingly, none of the instructors I've talked to seem to think there is a problem.
But the last time I sat in the plane I called the local ATIS got the baro setting and dialed it in. The altimeter read 120' below known field altitude. And that is 22' so it should be within the 20' parameter at that MSL.
I hate not having a reliable altimeter. Seems like the most important instrument, even for VFR flight, to me.

Are you flying a Sport Plane? If you are , some of these acft are equiped with non TSO'd gauges which are not of the highest quality.

[Targetbuster]

Are you flying a Sport Plane? If you are , some of these acft are equiped with non TSO'd gauges which are not of the highest quality.

Yep, a sportstar. Sure hope you're not right about the quality of the altimeter.

[drseti]

Having the pitot/static system checked every 24 months is only mandatory if you fly IFR.

It's also required if you operate a transponder with altitude reporting (which is itself required in some airspace). VFR certification is to a less stringent standard than IFR, but it is still required. It's there in FAR part 43 somewhere (can't look it up right now since my FAR/AMT is at work, and I just got home).

[drseti]

Yep, a sportstar.

I had my SportStar's pitot-static, transponder, and altimeter check (to VFR standards) done about a month ago. Local instrument shop charged $200; calibrated the altimeter and encoder up to 14,000 ft (service ceiling of my SportStar is 13k and change). I have the Dynon D100-D120 combo, with the digital altimeter, and also the standard analog gauges (altimeter and airspeed). Instrument shop certified the analog altimeter. It seems to be reasonable quality (though maybe not TSO'd) and the Dynon seems to track it really closely.

[Targetbuster]

[quote="drseti] It seems to be reasonable quality (though maybe not TSO'd) and the Dynon seems to track it really closely.[/quote]

Good to know. Thanks again for all the replies.
The piece on calibrating one's own altimeter is interesting and if the plane were mine, I'd do it. For now, I'm just going to write this issue up next time I fly and let the maintenance dept deal with it. I think I have more than enough co-inciding opinions to justify a write up on this.

[Jim Stewart]

Yep, a sportstar.

I had my SportStar's pitot-static, transponder, and altimeter check (to VFR standards) done about a month ago. Local instrument shop charged $200; calibrated the altimeter and encoder up to 14,000 ft (service ceiling of my SportStar is 13k and change). I have the Dynon D100-D120 combo, with the digital altimeter, and also the standard analog gauges (altimeter and airspeed). Instrument shop certified the analog altimeter. It seems to be reasonable quality (though maybe not TSO'd) and the Dynon seems to track it really closely.

You paid more (and got more) than the FAR's require. The FAR-mandated VFR-only tests are all electronic and pretty much automated. They don't check the altitude accuracy other than the reported airport altitude during the test. My guy charges $75 and did it in about 15 minutes. He joked that the paperwork takes longer than the test.

[drseti]

You paid more (and got more) than the FAR's require.

That could well be.

They don't check the altitude accuracy other than the reported airport altitude during the test.

Gee, I wonder how that could possibly satisfy ATC's Mode C requirements? Or don't they care if your indicated altitude is really correct??

[Jim Stewart]

I'm no expert on the matter, but I believe the reasoning was more along the lines of making sure your transponder doesn't jam or hide the signal from other aircraft. The early transponders were a mess of analog adjustments and were prone to drift and fail in ways that would paint a broad arc on on the radar scope, covering other aircraft. Making sure your transponder isn't doing that would probably be the most important reason to do the check.

There is the assumption that every time you obtain an ATC clearance, you will be comparing your altimeter to the reported altitude and that would be a self-check.

[zdc]

Having the pitot/static system checked every 24 months is only mandatory if you fly IFR.

It's also required if you operate a transponder with altitude reporting (which is itself required in some airspace). VFR certification is to a less stringent standard than IFR, but it is still required. It's there in FAR part 43 somewhere (can't look it up right now since my FAR/AMT is at work, and I just got home).

I'm confused by this response. If you use a xpndr it has to be checked every 24 months. If you fly VFR only, the pitot/static system does not require a check. Part 43 describes how the checks are accomplished, not what checks are required.

Since you rent an airplane to the public, it is probably wise to have the pitot/static check done anyway.

As an aircraft owner who flys only VFR, I see no reason to expend extra dollars for a check that is not required, unless I start to notice a significant discrepency.