Number of altimeters

[freshgasflow]

I am interested in medical instrumentation and am keen to find out about aviation instrumentation as a comparitive study.
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I am curious about the number of altimeters in the big jets. I am getting conflicting information. I have been told by some that airliners have three altimeters so that if two don't agree, you have a third to compare to find out which of the two is correct. However, i have been told by others (quoting all sorts of rules) that this is not true, and that jet liners need to have only two (unless two are electronic, you need a standby one).

SO who is right ? And if only two are required, then how would one know which is the correct one?

[99jolegg]

As far as I know, and from a non-professional flyer, airliners have three altimeters. One will be found on the LHS PFD, one will be found on the RHS PFD and the third will be a traditional backup altimeter found on the center panel between the LHS and RHS.

The two altimeters on the PFDs are electronic nowadays. The backup will be simple retro style.

[Non-PC Plod]

Dont quote me on this, but I think UK CAA certification requires 3 altimeters for aircraft cleared to fly in instrument conditions, whilst FAA and most of the rest of the world only requires 2.
As for which one is correct: you check them both before takeoff, and hopefully they read pretty well the same. If they dont, the aircraft doesnt fly till the fault is rectified. If you have a failure in flight you will hopefully have some other cues. You have a vertical speed indicator, a radio altimeter, you may have 2 independent air data systems, which will tell you if they have failed or disagree with each other. You will know from the power and configuration of the aircraft whether it ought to be climbing or descending. You will probably have a GPS-derived height available for a rough cross-check.
Secondly, if you have a failure, it will probably be due to a blockage of the static air system (possibly by ice). So - is one of the pitot heaters not working? Otherwise if you deliberately climb or descend, and one of the altimeters moves, and the other doesnt, Bob's your Mum's very good friend. Alternatively, you can try the alternative static source, which in unpressurised aircraft will be cabin altitude. This again will give you a rough cross-check (may be a couple of hundred feet out due to the way the air presure varies inside and out).
There are many ways to skin tiddles!

[411A]

...whilst FAA and most of the rest of the world only requires 2.


Wrong.
With electric ADC's employed for each pilot, three are required, the one (standby) being directly connected to the static system, and therefore normally uncorrected for position/static system errors.
However, that 'third' altimeter (the standby) is normally connected to 28vdc (normally, essential/standby power), for a very important reason.
The internal vibrator, which acts to keep the internal mechanism properly...indicating.
And, yes, the forgoing is also a requirement, for quite a long time.
FAA and (nearly) all others.

[Dan Winterland]

It all depends on the certification process for each aviation authority. It is quite common for one authority to insist a manafacturer provides an different fit for operators under thier system.

But essentially what has been said here is correct. However, there can differences within fleets. My company has a fleet of modern Fly By Wire aircraft which were ordered at different times. This type has three ADCs (air Data Computers) which compute airspeed, altitude and rate of climb and feed them to the IRS (Inertial Reference System a very accurate navigation system which used laser ring gyros to compute attitude and position information) which then feed this ADIRS information to the displays in front of each pilot. Normally, the left seat occupant gets the info from ADIRS 1 and the the right seat from ADIRS 2. If an ADIRS fails, the info from 3 can be switched to that side. Also, there is a standby altimeter which in the older aircraft is a standard pressure instrument, the newer ones another minature LCD dispaly with a composite attitude/altitude/speed display - much like the main displays. But the really old aircraft also have a 5th altimeter in meters, as we do a lot of flying in China which uses the metric system and these aircraft don't have meters information on the main displays.

[frontlefthamster]

freshgasflow,

You need to bear in mind that safety assurance in aviation relates to the prevention of catastrophic events which kill many people, and thus provides considerable redundancy of systems. In medicine, problems such as malfunctioning of anaesthetic equipment will, at worst, kill one or two. Another relevant factor is that help, replcement equipment, expert assistance and so on are out of reach to the crew of an aircraft in flight, whereas in a medical facility they are usually readily available.