An altimeter vented to the cabin would be unable to pass the altimeter check. The same would be true if a blind encoder was used as it would not pass the correspondence test.
That is a very good point, and it speaks volumes about UK "maintenance practices / radio checks" on G-reg aircraft.
It doesn't wholly suprise me, given that the company which installed my TCAS system (a Garmin dealer, etc) was unable to carry out an ILS check (LOC and GS generated at the same time).
Here in the US, the altimeter check requirement only applies if the aircraft is being used for IFR. The transponder check is a use requirement, meaning that the transponder may not be used unless it has been checked within the last 24 calendar months.
Understood. I operate from an airport within a US Mode C veil, so unless the aircraft has never had an engine driven electrical system (which is the case for one of my aircraft) I need Mode C operating and a current Mode C transponder check. With the /X aircraft I typically tell ATC within a Mode C veil that I'm waivered negative transponder.
Here in the US, the altimeter check requirement only applies if the aircraft is being used for IFR. The transponder check is a use requirement, meaning that the transponder may not be used unless it has been checked within the last 24 calendar months. So for an aircraft only used for VFR and not flown above 10000 MSL or in Class A, B or C, the transponder is not required to be checked, but if installed, it may not be used unless it has been checked within the prescribed period. Since the sections requiring the altimeter and transponder checks are within 91.411 and 91.413 which would only apply to use in US airspace, I presume that in other country airspace there is a use requirement.
PS: An altimeter vented to the cabin would be unable to pass the altimeter check. The same would be true if a blind encoder was used as it would not pass the correspondence test.
I've seen a lot of VSI's installed vented to the cabin but relatively few altimeters or encoders.
My encoder hose layout was inspected by my own fair eyes... A transponder Mode C check must be done by a repair station in FAA land, one of few things where that is the case. In my situation an A&P buddy who used to work for one borrows their test hardware on a weekend and acts as their agent. We do the actual work at my place and I get the certificate in the mail. This way I get to see the data as it is produced. I get an additional check when ATC calls me as traffic to others. Seems to be right on.
My habit with ATC is to report altitude when making initial contact because my other aircraft has no transponder and its a necessity in that case. But with the pace of radio comm in my area, a more typical VFR call for landing is simply type, number, location, and ATIS. Mode C output is checked every two years for that reason, and validated. I do notice that when using Mode C to give traffic calls, towers explicitly say "indicating" when providing altitude info.
I don't recall having flown in an aircraft with two altimeters.
Yes, it is. There are many people who just don't take that serious enough.
many of those blind encoders use cabin pressure
That is simply a bad installation though. It's supposed to connect into the static tubing.
Well, many of those blind encoders use cabin pressure, witch is likeley not to be correct and well changes with the ventilation of the cockpit. Shouldn't be much an issue with a/c on the ground, though.
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.
danke schön Bosco
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.
