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Altimeters convert a pressure measurement to an indication of altitude. When set to standard pressure setting (1013.25 hPa / 29.92"Hg), they assume the atmosphere is according to the ICAO Standard Atmosphere (ISA) and indicate the altitude accordingly. However, ISA has two altitude definitions; geometric altitude and geopotential altitude.

The ICAO Standard Atmosphere is a model of the atmosphere up to 80 kilometres high, based on the perfect gas law and the hydrostatic equation. In order to deal with the effects of reduction of gravity with height, it uses the concept of geopotential altitude H:

$$H= \frac{1}{g_0}\int_0^hgdh$$

with:

  • $h = \textrm{geometric altitude}$
  • $H = \textrm{geopotential altitude}$
  • $g_0 = 9.80665\textrm{ m/s, mean gravitational acceleration at mean sea level}$
  • $g = \textrm{local acceleration of gravity (height dependent)}$

Gravity is given by :

$$g = g_0 \left(\frac{r_e}{r_e + z}\right)^2$$ with:

  • $r_e = \textrm{the radius of the earth}$

And thus the relation between geopotential altitude and geometric altitude is given by:

$$H = h\frac{r_e}{r_e+h}$$ And $$h = H\frac{r_e}{r_e-H}$$

In the ISA model, the properties of the atmosphere (pressure, temperature, density) are all calculated with respect to geopotential height.

For a geopotential height of 12 500 meters (41 010.5 ft), which corresponds to a geometric height of 12524.6 meters (41091.3 ft), the ICAO atmosphere gives a pressure of 178.648 hPa.

My question is: what will a perfectly calibrated altimeter indicate when the pressure is 178.648 hPa? Does it indicate 41 010.5 ft (the geopotential altitude) or does it indicate 41091.3 ft (the geometric altitude)?

You might say that the difference is "only" 80 feet, but for RVSM operations that is quite significant.

The ICAO standard atmosphere (as described in ICAO Doc 7488) does not give the answer (at least I couldn't find it). And it includes tables for every 500 meters of both geopotential and geometric altitude, so that does not make it easier.

I'd like to know which altitude is indicated on an altimeter (ISA geopotential or ISA geometric) and where (in which official document) this is specified.

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  • $\begingroup$ Following the U.S. Standard Atmosphere 1976, 178.648 hPa (mbar) yields a pressure altitude of 41010.5 ft $\endgroup$ – selectstriker2 Feb 8 '18 at 14:20
  • $\begingroup$ @selectstriker2 Like the ICAO atmosphere, the US Standard Atmosphere 1976 yields both 41010.5 ft (geopotential) and 41091.3 ft (geometric). The calculations are the same, and the tabular data is provided for both geopotential and geometric data. $\endgroup$ – DeltaLima Feb 8 '18 at 21:46
  • $\begingroup$ I guess I should have stated that a calibrated TSO C106 altimeter should provide that altitude. @Gerry's answer is quite correct as to the driving document for that determination $\endgroup$ – selectstriker2 Feb 8 '18 at 22:24
  • $\begingroup$ Even if I didn't know the correct answer, I would strongly suspect it was geopotential altitude. With respect to it (H), pressure distribution is simpler and calculations are easier - particularly important when they have to be implemented on a mechanical device. One needs additional correction to convert to geometric altitude. $\endgroup$ – Zeus Feb 9 '18 at 0:01
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The TSO for Air Data Computers is TSO-C106. It specifies for the minimum performance standard "SAE Aerospace Standard 8002, Air Data Computer - Minimum Performance Standard."

From AS8002:

3.6 Altitude Reference:

Calibration of the static air sensing mechanism for pressure altitude outputs (including baro-corrected if supplied) shall be to the geopotential altitude tables of U.S. Standard Atmosphere, 1976.

While this is a US standard, the FAA asserts it is in compliance with ICAO Doc 7488.

For RVSM, additional requirements are levied against the installation primarily focused on reduction of static source error (SSE). That guidance is available in AC 91-85A.

It's important to remember that absolute accuracy at altitude is not as important as everyone adhering to the same reference.

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    $\begingroup$ In short: Above transision altitude it's the pressure that matters. Below it's whatever the local tower gives you as the pressure reading so that the elevations in the charts match the readings in the aircraft. Is that right? $\endgroup$ – yo' Feb 8 '18 at 14:47
  • $\begingroup$ Thank you for the quote from AS8002, exactly what I was looking for. Also the European equivalent (ETSO-C106) refers to SAE AS8002. $\endgroup$ – DeltaLima Feb 8 '18 at 15:54
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    $\begingroup$ @yo', it's right, but not what the question is about. The question is about how the “altitude” values in feet are assigned to the measured pressure values (it matters only a tiny bit, because at FL410 the difference is just 80 ft). $\endgroup$ – Jan Hudec Feb 8 '18 at 21:54
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In addition to the great answer provided by @Gerry, altimeters certified under TSO C10b (revised 1959) aren't required to be as precise. These are generally older analog altimeters. They must meet AS392C, which provides a table of values per NACA Report 1235, as well as tolerances.

The tolerance at 187.54 mbar (40,000 ft) is $\pm$ 230 ft!

C10 was revised to C, which requires the altimeter meet AS 8009C, Pressure Altimeter Systems, dated May 24, 2016.

4.4.1 Altitude Pressure Values

Altitude pressure values shall be in accordance with U.S. Standard Atmosphere, 1976 (Reference ICAO Standard Atmosphere Document 7488).

It also states that differential pressure calibration for SSEC should adhere to NASA Technical Note D-822, which does specify geopotential altitude:

Within an atmospheric layer throughout which the molecular scale temperature is a linear function of geopotential altitude, the hydrostatic equation and the perfect gas law yield the following expressions for pressure...

...

Pressure-altitude tables are given for altitudes up to 30 000 geopotential meters.

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  • $\begingroup$ Thanks for the extra info on TSO C10b. Would highly appreciate it if you or anybody else could find the relevant quote from AS8009C like in Gerry's answer for AS8002. $\endgroup$ – DeltaLima Feb 8 '18 at 22:29
  • $\begingroup$ It doesn't look like AS 8009C specifies geopotential or geometric $\endgroup$ – selectstriker2 Feb 8 '18 at 22:43
  • $\begingroup$ Does it refer to an atmosphere standard, or does it just provide a table? $\endgroup$ – DeltaLima Feb 8 '18 at 22:45
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    $\begingroup$ it differs from AS 392C (which provides the pressure next to the altitude) in that it only has Test Pressure Altitudes, which correspond to a given tolerance $\endgroup$ – selectstriker2 Feb 8 '18 at 22:47
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Modern air data systems are calibrated to geopotential height. That is, at some given pressure, the altimeter reads the geopotential altitude associated with that pressure in a standard atmosphere.

I doubt that any of us has ever flown in a standard atmosphere, therefore the altimeter reading we see is a pressure altitude associated with the non-standard atmosphere in which we find ourselves. You can't really know your actual geopotential height unless you also know the exact temperature profile from some datum to your position. And, if you have that temperature profile information, it can be quite tedious to determine your geopotential height.

As was mentioned elsewhere, none of this really matters as long as we all have the same Kollsman number set and obstruction clearance is not a factor.

Lower altitudes at low temperatures can require a prudent correction to published altitudes in order to maintain obstacle clearance.

(Derivations provided upon request)

Ed Lambert, PE

ATP A320, B747-4, DC9

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