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Dual Altimeters

If I fly in IMC (not necessarily following IFR rules, but VFR rules), is this legal with one altimeter?

Notwithstanding that if you are in IMC you must comply with the IFR (even OCAS) so it's not legal no matter how many altimeters you have!....but I know that was not the point of your post....after reading all the posts and doing some more digging, I think Boscomantico is correct, there is no requirement for two altimeters in an Nreg...(non-RVSM)

YPJT, United Arab Emirates

It occurs to that when I went flying yesterday I had four pieces of equipment on board that could give my altitude in relation to terrain: one altimeter, a Garmin 196, an iPad plus external GPS, plus an iPhone in my pocket. I guess it would be five if you include Mode C plus radio to ATC. I've never flown in flight levels and probably never will, so it seems to me that the difference between GPS and QNH/altimeter setting would cause me no issues in the event of an altimeter failure.

I'd guess the tendency for EU aircraft to be fitted with two altimeters might be due to low and variable transition altitudes and all that stuff.

"Man with two watches doesn't know what time it is" :-)

"Man with two watches doesn't know what time it is" :-)

Absolutely! This has occured to me as well...which one do you trust? And you're right your GPS units will be accurate enough to keep you safe....although the Mode C gets it's FL from the encoding altimeter so that may not be any use...

Also I believe Europe will be changing the TA to 18,000ft per the US

YPJT, United Arab Emirates

I've understood that the Mode C encoder sends flight level data to ATC which is unaffected by altimeter setting. And that ATC corrects it to altitude. I'd be happy to be corrected!

I believe the only thing connected to my altimeter is a static hose.

That's correct....but I was assuming the altimeter was U/S....but yes it sends your Flight Level ie 29.92in or 1013hPa altitude is encoded with the transponder return

YPJT, United Arab Emirates

So I think the nice ATC person on the radio could tell me my indicated altitude in relation to terrain... as long as my radio didn't quit along with my altimeter and all three GPS's.

I can imagine this might be a very bad day :-)

Ha! Yes, you must have really upset the big man for that day to happen! :)

YPJT, United Arab Emirates

License privileges does not have anything to do directly with equipment requirements. AFAIK, no european country requires two altimeters for IFR as part of their "airspace" rules, only some do as part of their aircraft certification rules. So, again, no dual altimeters required for a (non RVSM) N-reg flying through Europe.

Mainz (EDFZ) & Egelsbach (EDFE), Germany

danke schön Bosco

YPJT, United Arab Emirates

I've understood that the Mode C encoder sends flight level data to ATC which is unaffected by altimeter setting. And that ATC corrects it to altitude. I'd be happy to be corrected!

I believe the only thing connected to my altimeter is a static hose.

Yes that is correct.

In the UK, ATC are not allowed to separate you from somebody else until they have got you to read them your altimeter reading, on the given QNH. This is even though the radar system can display your altitude, by correcting the Mode C (pressure altitude) for the known local QNH.

Incidentally, with the TCAS I now have, I see loads of examples where somebody's Mode C encoder is off by 200 or even 300ft. Just watching traffic sitting on the ground I see lots of +02 -02 (which is just in spec) and sometimes +03 or -03 (which isn't). It's curious why people fly around with that, because most Mode C/S transponders do display the pressure altitude (flight level) and if the QNH is anywhere near 1013, it should read the airfield altitude.

I would always want two altimeters. Sure, you cannot tell which one is right, but you can tell if one of them is stuck and that is probably a more likely failure mode.

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Shoreham EGKA, United Kingdom
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