Altimeter calibration

Phil Burgess · Post by **Phil Burgess** » Thu Jan 17, 2008 10:02 pm

The altimeter in my Pitts has recently started under reading by about 120feet at sea level. (if I set zero feet it's about 4mbar less than QFE) I think it is a falcon guage as supplied by A/C spruce, though I will check when I remove it. I have a brand new altimeter to be fitted at the weekend.

Is there anything that can be done with the old one to get it back into cal? Or are we not allowed to open instruments ourselves as builders... It could be that if theres some scope for adjustment inside, I could rotate the mbar scale until it corresponds with the correct altitude etc. If I can fix it, it might find it's way into my Monoplane project.

Andrew Leak · Post by **Andrew Leak** » Thu Jan 17, 2008 11:42 pm

Phil, simple answer. NO. Don't take it upon yourself to 'calibrate' the instrument unless you have a clear idea of what you are doing and also a Druk pressure tester. There is probably a good reason why it has 'lost' 120 ft and as this is one of the most important flight instruments (as a former instrument repairer/seller) I suggest you send it to Cair Aviation or Chris Qualye at [email protected] (no business interests may I add!!). Its fairly inexpensive to have it professionally calibrated after all it's your life. I have seen many permit instruments under/over read through the years and as stiction is also a problem - particularly for aerobatic aircraft where altitude change is fairly rapid - I can only hope you get it fixed properly or consign it to the bin.

Andrew

Mark A · Post by **Mark A** » Fri Jan 18, 2008 10:11 am

Some altimeters have a keeper plat under the setting knob, whereby removing the plate allows you to pull out the setting knob and adjust the altimeter without setting the pressure sub-scale (or vice-versa, I can't remember).

Obviously doing that won't address a faulty capsule in the altimeter, but may get you sorted if its just a small slip of the needle.

Andrew Leak · Post by **Andrew Leak** » Fri Jan 18, 2008 11:01 am

Mark, you are correct with the keeper plate on military pressure altimeters, which is used to adjust the millibar scale for the correct pressure setting generally used on the bench. Civilian produced altimeters like Aerosonics etc use a keeper 'screw' by the side of the adjustment knob (which locates a small bar to retain the position of the knob and is inaccessible unless removed from panel)

However, I must say again that it is NOT good practice to fiddle with altimeters unless you are competent at calibration. If this self repair becomes common practice - with potentially disastrous consequences, we will more than likely all have to start coughing up quadruple prices for TSO'd instruments (as do the EASA/CAA licenced aircraft) as the powers that be will just stop us from using non approved equipment. This I cannot afford and I am sure other members will feel the same, so just let the professionals do their job and fly safe. After all, would you start to 're-calibrate' your GPS if it started playing up? I certainly would resist it.

Andrew

Phil Burgess · Post by **Phil Burgess** » Fri Jan 18, 2008 12:41 pm

I'm already resigned to the fact that this altimeter is not up to the standard required for use in my Pitts, that is why I am replacing it with a brand new item. The requirement for the altimeter in my Taylor Monoplane is much less stringent (I think) as even if it's lucky, it will only have 1/3rd the climb and descent performance of the Pitts. I fly in class G airspace almost exclusively and therefore I wonder just how accurate an altimeter needs to be, in order to be "fit for purpose". In the Pitts I want to be beyond any doubt that when I set the apropriate setting on the mbar scale when flying away to do a display away from base that when it reads 100' I'm at 100' not -20'...

I'm an avioncs engineer by trade and could get my hands on a druck relatively easily if necessary. Though I have no practical experience of opening an altimeter for cal as yet - that is why I asked the question.

Incidentally how much would a professional organisation charge for such a cal or repair given that these items are not all that expensive to replace with new?

Andrew Leak · Post by **Andrew Leak** » Fri Jan 18, 2008 11:39 pm

Last time I sent a unit in it was around £35 but it was fairly sound and only needed a 'tweak'. As you can pick up a non approved unit for around £120 it might make better sense to buy a new one and dump the old one on Ebay with a disclaimer. Many appear like this and sometimes get bids around £50! The maths is easy and you've got rid of the hassle of P&P and the waiting time to get your unit back.

Andrew

Phil Burgess · Post by **Phil Burgess** » Tue Jan 22, 2008 9:55 pm

Ok - new Altimeter fitted to the Pitts, just need some good WX to test it...

Might have fixed the old one; upon removing the screw adjacent to the subscale knob, it is possible to pull the knob out slightly and turn the subscale without moving the pointers. I will check the calibration of this instrument and if it is reasonable I will use it in my Taylor Monoplane project with no further adjustment. If not I will send it away to be adjusted by a suitable engineer.