Vintage Altimeter

[Potter1]

I’m currently restoring an Aeronca (Permit) and have got hold of an original altimeter in new condition. This instrument has no pressure scale and the adjustment knob rotates the dial, not the instrument hands.
One thought that crossed my mind, is it legal for me to install this instrument in the aircraft with no barometric scale, so no accurate way of setting the area or aerodrome pressure!...
Thanks,
P…

[OpenCirrus619]

I would suggest looking in the ANO (CAP 393 from the CAA web-site) as a first step - specifically Section 1: Schedule 4: Aircraft Equipment. This will indicate the minimum equipment required for various types of aircraft / flight.

The you want to check with your engineer / inspector - as he is the one who will sign it off. (No point in being able to say "but it says here" if he isn't going to sign the chit).

OC619

[BackPacker]

The environment of today is that people are discussing whether to implement Mode S, and when, for which types of flights/aircraft. There is a reason for this: traffic density, layering of airspace, accurate position/altitude reporting etc. In this environment, would you really want an altimeter in your aircraft which can only be set accurately if the aircraft is at a known height (ie. on the ground)?

Me personally, I would not dare take an aircraft with such an altimeter outside the local circuit although, technically, if you set it to the airfield elevation on take-off, write down the reported airfield QNH and adjust the altimeter with 30 feet up or down every time the QNH changes one millibar, it should theoretically display more or less correct throughout the flight. (Which way to adjust, and how to handle the transition layer, requires an amount of thinking I'm not quite capable of right now.)

If you want to go for the "original panel" look, why not install a small, modern altimeter somewhere more or less out of sight (or hidden behind a cover or something that's easy to remove - others do that too), and use that to calibrate your pride and joy in-flight? You could even get a mountaineering altimeter or a handheld Garmin eTrex GPS with internal barometer (I think the eTrex Vista has one) and use that for calibration.

All this is separate from the legal side though.

[DubTrub]

is it legal for me to install this instrument in the aircraft ... ?
Yes, if it is original, and certainly if it got its initial permit with that type of instrument installed.

Several Shuttleworth aeroplanes have no instruments at all and they have permits.

The only things not really allowed now for vintage aircraft are pyrotechics and radium-illuminated instruments.

[Potter1]

Many thanks for these, much appreciated.
Good idea about having a small second altimeter for calibration.
P...

[The Flying Pram]

The Type Approval sheet for my (permit) microlight says "Altimeter required (maybe wristwatch type)". As far as I'm aware the requirements for any permit A/C would be the same. The original altimeter on mine was a simple 0 - 8,000ft single dial type with no pressure scale. From memory it was about 2-2.5 inches in diameter. You could rotate it in its holder to zero it with airfield elevation. I believe it was intended for climbers etc. Having said that I certainly wouldn't want to fly with it these days!!