Peter: GPS altitude is good for obstacle clearance (although GNUD might be an issue), as a GPS altitude is more accurate than a barometric one.
The barometric altitude is inaccurate, the farther you are from the QNH provider, the less accurate is your altitude indication.
For vertical separation with other aircrafts, it is not an issue at all: you’re not interested in you actual altitude, you just want your altimeter to be exactly as inaccurate as the one of the aircraft about to collide with you, and that’s the very principle of barometric altimeters. But your GPS altimeter is too accurate for the purpose of vertical separation. Although you might derive a formula to recalculate a barometric altitude from a GPS one, the required data (actual temperature profile) is not available, so it’s not possible to make an accurate enough calculation.
Cobalt wrote:
So a completely seperate pitot-static system would make the second altimeter hugely more useful than just the altimeter.
I believe this is what aircraft starting as small as the Cessna Caravan have. A pitot/static probe on the co-pilots side. Gives full reduncancy for the primary pressure instruments (ASI, ALT, VSI).
Unless they both ice up 
When I’m flying a jet with three altimeters, the two primary altimeters get set to ‘STD’ when we’re cleared to a flight level. The ‘third’ is set to ‘STD’ once we’re above MSA. The whole point of having three altimeters is so that if one fails, you should always have two reading the same enabling you to identify the dud. Being able to do this is especially important in RVSM airspace.
In something like a King Air with just two altimeters, then I’ll set the primary to ‘STD’ when cleard to a flight level and then set the secondary to ‘STD’ when above departure MSA.
My own practice is to leave QNH on Alt2, but I’m a Brit…
You got me thinking about redundancy though. We actually have three altimeters — two in the panel, and one in the encoder that reads out on the transponder. When flying FLs, I occasionally crosscheck Alt1 against the encoder output. ATC, however, checks it on a very regular basis, and shouts at me if it doesn’t correspond with my cleared level, which is on Alt1. So on balance, I’m not too worried about redundancy.
Cobalt wrote:
Number of accidents…
Every single accident is one too much if it can easily be avoided. And you are certainly right about that second pitot-static system. A backup instrument is really only useful if it has it’s own source of data. Installing a second static port would certainly not add much to the overall cost of any aeroplane.
Peter wrote:
An alternate pitot would be a much higher priority to me i.e. having a second ASI.
I would always take the second altimeter instead. Proper training in flying pitch/power more then replaces the second ASI.
I guess the days of the 5000$ encoding altimeter will soon be over.
Sure – because you paid €50k for the box with the big piece of glass on it 
And then you pay more €€€ for a dealer to look at it whenever it needs looking at – because nobody else can. Whereas a KEA130A can be overhauled in the USA for $3k and is dead easy to change. (N-regs are required to have the static system test after any disturbance of the static piping).
the number of accidents because of pitot/static failure are probably a magnitude higher than the other two, though.
If you include some really dumb airliner crashes…
I am not aware of GA crashes. Have there been any?
So a completely seperate pitot-static system would make the second altimeter hugely more useful than just the altimeter.
That’s a good point and I wonder why. Maybe because in the unpressurised context the cabin vent (“alternate static”) is thought to be good enough. It’s only maybe 100-200ft off. It would be dead easy to install another static vent right next to the existing one(s).
An alternate pitot would be a much higher priority to me i.e. having a second ASI.
This thread reminds me of this one
Number of accidents because of broken altimeter = ?
Number of accidents because of defect in pitot-static system = ?
Number of accidents because of lack of altitude awareness when flying at a low flight level = ?
All are rare; the number of accidents because of pitot/static failure are probably a magnitude higher than the other two, though. So a completely seperate pitot-static system would make the second altimeter hugely more useful than just the altimeter.
C.
Peter wrote:
The cheapest encoding one is perhaps the KEA130A which is about $5000 new (can’t find a price right now).
Wow. For less than that price you can get a proper TSOed air data computer that can do much more for you than just encode an altitude (e.g. http://www.aircraftspruce.com/catalog/avpages/sandia735.php?recfer=5964). I guess the days of the 5000$ encoding altimeter will soon be over.
That’s clever but possible only if they are all encoding types.
“Simple” planes have non-encoding ones because they are cheap – below €1000 normally.
The cheapest encoding one is perhaps the KEA130A which is about $5000 new (can’t find a price right now). These are normally used as #1 and they drive the autopilot, and maybe the transponder, TCAS, etc. Then #2 is a cheapo one.
On a long high altitude flight the QNH is soon meaningless so both of mine with be set to FL. And then the Q becomes: what do you do about terrain clearance in an emergency? A: use the GPS altitude
Michael_J wrote:
Imagine your primary altimeter goes stuck or fails. What would you like the second one to show you?
The problem is that when the two altimeters show different values (deliberately or not) you will never notice that the one you use for flying is defective or set wrongly. This is why real airplanes have a comparison monitor that will display a big red/orange/amber alert across the primary flight display if the two (or more) altimeters show different numbers.
