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What's the mechanism of the altimeters in big jets?

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Main instruments

In large aircraft, main altimeters are not separate mechanical altimeters. Instead the pitots, static probes and angle of attack probes, which are redundant, have their outputs digitalized by analog to digital converters (ADM: air data modules):

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Pitot/static pressure converter, source

Digital data are send using a bus and processed by air data units, often coupled with the inertial units, sometimes also with GNSS (ADIRU/GNADIRU for Airbus).

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A330 air probes and ADIRU, from A330 FCOM

ADIRU are computers, located in the avionics bay:

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A330 ADIRU, source

They process probes inputs and compute altitude, airspeed, vertical speed, angle of attack, wind drift, air temperature, etc.

ADIRU information can be displayed on various screens. On large aircraft the content of the screens is managed to cope with failures, information can be rerouted to working displays. In normal mode, data from the left set of probes is displayed on the left seat screens, and data from the right set on the right seat screens.

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Source: A330 FCOM

Altitude is usually displayed on PFD (primary displays). Here seen from the right seat:

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A330 F/O screen, source: Flickr, photo by swiss_a320

Standby instruments

Analog standby instruments are still fed with air pipes from pitot and static standby probes, in particular the standby altimeter is a barometric instrument, using an aneroid capsule.

Radioaltimeters

A word about radioaltimeters (RA), as they were mentioned in a comment. Radioaltimeters are not altimeters, this is a misnomer, they measure height above ground level (AGL). In aviation, the word altitude is used exclusively for a measure of height relative to mean sea level.

That said, there is not much to say, RA are exactly the same than GA or business jets aircraft. They are used for landing, to evaluate the height above the threshold, in particular the decision height which is the point where the approach must be aborted if all conditions for a landing are not met. As their name implies, this is a radar measure (roundtrip time of a wobble radio signal).

RA are one of the data sources for "one thousand", "twenty", "minimums", etc, callouts (as @Bianfable mentions barometric altitudes can also be used). On large aircraft RA are coupled with the autopilot mode to monitor the approach and trigger various alarms.


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    $\begingroup$ Why no radar altimeters? $\endgroup$ Nov 29 '21 at 15:32
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    $\begingroup$ "angle of attack probes, which are redundant": Wouldn't one wish they had been. seattletimes.com/business/boeing-aerospace/… $\endgroup$ Nov 29 '21 at 17:26
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    $\begingroup$ Great answer, but you may want to clarify that the use of the term "altitude" in aviation is used "...exclusively for a measure of height relative to mean sea level." There is absolute altitude, pressure altitude, and density altitude. These altitudes are not a measure of height relative to mean sea level. Thanks $\endgroup$
    – 757toga
    Nov 29 '21 at 22:12
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    $\begingroup$ I would remove the image for a piezoelectric sensor, since that type of sensor cannot typically measure steady state pressure. $\endgroup$
    – Sanchises
    Nov 30 '21 at 9:30
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    $\begingroup$ @Bianfable: Thanks. note in this section I just emphasize RA are not used for navigation like barometric altimeters. The similarity between RA and barometric minimums is another whole topic. However I improved the last sentence to not exclude pressure-altitude from callout sources. $\endgroup$
    – mins
    Nov 30 '21 at 10:54
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The Captain's and First Officer's electric altimeters have their pressure data (from static ports) processed by separate Air Data Computers (ADC or ADIRU, etc. nomenclature depending on the Manufacturer).

Generally, there is a Standby Altimeter which is mechanical (pneumatic) with pressure received from alternate static ports.

Another type of altimeter is called a radio altimeter (sometimes called a radar altimeter), which uses radio signals (from antennas located on the bottom of the aircraft) to measure the absolute altitude/height of the aircraft directly above the terrain/surface below. It integrates with several aircraft systems. Of note, it is used by pilots to define the minimum altitude/height allowed to continue the descent before visually sighting specific elements of the landing environment for certain types of instrument approaches. Lastly, the display in the cockpit typically will display altitudes below 2500 ft (sometimes 5000 ft) above the surface directly below the aircraft.

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    $\begingroup$ There are radar altimeters as well. $\endgroup$ Nov 29 '21 at 17:10
  • $\begingroup$ @PeterMortensen I agree and will add some info. Thanks $\endgroup$
    – 757toga
    Nov 29 '21 at 17:18

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