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ProfChrisReed
30th Aug 2009, 18:37
Apologies for posting what is probably a very simple query, but I can't find an answer via Google or a PPRuNe search.

I suspect the altimeter in my glider is overreading, by cross-checking with logs of flights (against both GPS altitude and barometric altitude, as both are logged). The overreading seems to increase with altitude, and at 4,000 ft appears to be >10%.

This started happening quite suddenly. Could it be something as simple as a kinked static tube, or is it likely that the (fairly old) instrument is failing? If the latter, is repair worth investigating, or am I looking at a replacement.

ProfChrisReed
30th Aug 2009, 21:37
Port is connected to a pair of static sources.

Of course (doh!) I could disconnect the tube next time I fly, which would leave the port open to cockpit ambient - if that largely removed the errors I'd know where the problem is.

Engineers are thin on the ground where I fly, so any other suggestions would be welcome.

john_tullamarine
30th Aug 2009, 23:13
If the problem is recent, then the position of the static port is unlikely to be involved.

(a) have you done any mods etc altering the exterior of the glider in the area in front of the static port ? and we are talking quite a way in front, not just a few inches.

(b) is there any recent damage to the skins near and forward of the static port ?

(c) have you checked the static line from the port to the instruments ?

(d) was there any change in the performance of the ASI/VSI at about the same time as the altimeter ?

(e) quick check is to get the local tug to fly form and compare all the pressure instruments during a descent

(f) otherwise, afraid you will either have to get the gauge cal checked or install a known cal unit for comparison test

(g) .. and, on what basis are you presuming that the error is not with the baro unit ? I think one would not be terribly interested in GPS altitude other than as a very gross check.


10% at 4000 feet is ridiculous and you ought not to tolerate any suspicion of such an error quantum.

ProfChrisReed
31st Aug 2009, 11:00
Thanks for the replies.

The problem only manifested itself in the last couple of days. In gliding, most of the time the altimeter is largely irrelevant. It's main uses are (a) avoiding airspace, but where I was flying there wasn't any, and (b) deciding if you have enough height to get home. Thus on an average flight I might only look at the instrument a dozen times.

It was only in the last 20 mins or so of flying yesterday that my suspicions crystallised and I compared the altimeter to the GPS height display. I was logging barometric height and GPS height from two separate sources, and a review of those logs indicate they always agreed to within 80 ft or so. No changes to the aircraft, and all other instruments read OK.

It must be either a static problem, which I can check by flying with the port open to the cockpit, or a defect has developed in the instrument.

Lightning Mate
31st Aug 2009, 11:34
Food for thought.

The GPS height computed is the height above the WGS 84 ellipsoid, and doesn't change with atmospheric pressure changes.

MaxHelixAngle
31st Aug 2009, 12:05
As Lightning Mate has alluded to you're error may not be an error at all. Altimeter's are merely barometers scaled in feet and measure Pressure Altitude whereas GPS receiver's use satellite triangulation to approximate true altitude, based on either the WGS84 ellipsoid or the ellipsoid corrected for local gravitational changes (changes to sea level).

A slight simplification; the pressure altimeter assumes a constant pressure lapse rate of approximately 30ft per hectopascal, this is only correct at one temperature, ISA. At temperatures above ISA the air is less dense and the actual pressure lapse rate will be smaller and the converse is true for cold air. Should the temperature differ from ISA then pressure altitude will differ from true altitude. The magnitude of the error will depend on the ISA deviation as well as the height above the aerodrome from which the QNH has been determined (suspect this is why you're error is greater at 4000ft). In warm air the altimeter will under-read and in cold air will over read.

True altitude can be calculated by the following formula and then compared to GPS true altitude for a rough confidence check:
True altitude = Pressure altitude + (Height above airfield * ISA DEV) / (273+OAT)

Cheers,
MHA

displaced gangster
1st Sep 2009, 00:46
One of the better web sites.

www.luizmonteiro.com (http://www.luizmonteiro.com)

:ok:

muduckace
2nd Sep 2009, 21:34
Of course (doh!) I could disconnect the tube next time I fly, which would leave the port open to cockpit ambient - if that largely removed the errors I'd know where the problem is.


Please do not do that, the instrument is calibrated to the instalation. As stated above check for damage around the static port. Have the system ground tested. Another thing that may show the difference between line damage or a leak (assuming you'r cockpit goes to ambient). Try to maintain GPS altitude for as long and steady as possible, if the altimeter does not change it is likely the indicator. If it slowly matches GPS a leak or blockage is possible.

ProfChrisReed
5th Sep 2009, 20:20
This is a glider, so I'm certain it's not calibrated to the aircraft.

Until recently, when we fell under EASA, things were pretty simple: buy new altimer, bolt to panel, attach tube to static, fly.

Now I have a lot of hoops to jump through, so want to check as much as I can before putting it in the hands of a professional.

Disconnecting the static won't harm the instrument and doesn't even require me to remove the panel (reach behind, tug gently), so I will try it tomorrow and report back. Any errors I get (and the flight will be largely between 50 and 65kt, so I doubt they will be large, and anyway I can open the DV panel to equalise cockpit and outside air pressure) must be no worse than the current suspected errors!

Thanks again for all who have offered suggestions.

I think the instrument is about 40 years old, so death from natural causes seems most likely, but will see tomorrow ...

[Edited to add, will of course keep away from any airspace where I might actually need to rely on the instrument]

Loose rivets
5th Sep 2009, 20:26
Some mud-dobbing critters modified my Rallye Club for me. However, we're talking about an over-read here aren't we?

Oh, and edit to say, I used to commute to work night and day it this, and let down off the Clacton VOR into a meadow home bound. So, the altimeter needed to be spot on.



http://img.photobucket.com/albums/v703/walnaze/Statictube.jpg

ProfChrisReed
6th Sep 2009, 18:27
Yuk! Nasty critters.

Flew today with the static disconnected, and the problem persists - confirmed by cross-check with the tug altimeter.

Looks like 40 years is the natural life span for an altimeter.

Keith.Williams.
6th Sep 2009, 19:42
You could make a comparison with the altimeter in the tug while both aircraft are parked side by side on the ground.

Then change both subscales to another number and compare the indications again.

Do this over a fairly wide range of subscale settings and see how the two instruments compare.

This will take out all transient effects and it will cost you nothing.

Then if the readings are very different from yours quietly swap your altimeter with that from the tug !! :eek::=:hmm: