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 172510 2nd Aug 2012 08:45

ICAO Altimeter

I've passed my ATPL(A) and I'm now training (part time, as aviation is just a hobby) for CPL(A).

- What accuracy is required for an altimeter? Where is it officially documented?
- I suppose that the setting knob of an altimeter is linear, ie that the altitude indication variation rate is 27.31ft/hPa whatever the altitude reading is. Where is this officially documented?
- What is the official definition of QNH, and where is it officially documented? I assume that the only good definition of QNH would be, the altimeter setting that would make an ideal ICAO altimeter read the altitude of the ARP (or RWY Threshold).
I've tried to search the forum with "27.31" but it does not work. May be there's a trick I don't know about to do a search for numbers.

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 DaveReidUK 2nd Aug 2012 10:07

The ICAO standard is ISA (the International Standard Atmosphere).

International Standard Atmosphere - Wikipedia, the free encyclopedia

 172510 2nd Aug 2012 11:30

I thank you for the link, but it's of no help at all.
I have read ICAO doc 7488, and I think I now know enough about ICAO atmosphere.
Obviously my English is not good enough and my question was not understood.

My questions refers to aviation altimeters.

When set to 1013.25 setting, a perfect aviation altimeter should read if the static pressure is P<264.36hPa
Altitude(in feet)=(1-(P/1013,25)^(1/5.2561))*283.16/.0065/.3048
I call below that function Altitude=ICAO(P)
That's the ICAO definition of ICAO atmosphere for altitude <11km, ie pressure<264.36hPa (see Doc 7488)
The derivative of that function when altitude=0 is 27.31ft/hPa
My question is
What a perfect aviation altimeter should read if the altimeter setting is s and the static pressure P?

My guess is it should read
Altitude(in feet)=ICAO(P)+27.31*(s-1031.25)

Am I right, and where is it officially documented?

My other question is: what is the accuracy required to legally fly IFR? And where is it documented.

May last question is: What is the QNH?
My guess is QNH=round down ((Altitude of the station - ICAO(P))/27.31+1031.25)
Am I right? And where is it officially documented?

 Capn Bloggs 2nd Aug 2012 12:49

My other question is: what is the accuracy required to legally fly IFR? And where is it documented.
The required accuracy of Australian IFR altimeters is in the AIP:

When two altimeters are required for the category of operation, one of the altimeters must read the nominated elevation to within 60FT. When the remaining altimeter has an error between 60FT and 75FT, flight under the IFR to the first point of landing, where the accuracy of the altimeter can be re-checked, is approved. In the event that the altimeter shows an error in excess of 60FT on the second check, the altimeter must be considered unserviceable for flight under the IFR.

 hvogt 2nd Aug 2012 21:06

The accuracy of altimeters appears to be more a question of certification standards, and I very much doubt there will be any ICAO standard or recommended practice (SARP) pertinent to this question. From the operational point of view an altimeter is fine when it indicates the elevation within the tolerances given in PANS-OPS, Volume I, Part III, Section 1, Chapter 3, Section 3.2. At least this what ICAO requires; of course Bloggs has already mentioned national requirements which may be different.

With regard to your question concerning the definition of 'QNH' your assumtion is very close to the definition in PANS-OPS, Volume I, Part 1, Section 1, Chapter 1: 'Altimeter sub-scale setting to obtain elevation when on the ground'.

 Oktas8 4th Aug 2012 07:31

If I understand correctly:

My guess is it should read Altitude(in feet)=ICAO(P)+27.31*(s-1031.25)
It should read altitude in the standard atmosphere. If it doesn't, it isn't a "sensitive altimeter"! Hence I think that the answer to your question is actually the definition of the standard atmosphere - which I know you have already studied.

QNH is the pressure at the measuring station, reduced to sea level according to ISA. That definition is found in any meteorological textbook or dictionary, and is internationally accepted just as the values for ISA are internationally accepted.

(As an aside, QFH is the pressure at the measuring station, reduced to sea level according to the actual temperature of the air. Therefore QNH does not account for density fluctuations, but QFH does. Aviation is not interested in QFH.)

Since the rate of pressure change varies with height, the value of 27ft/hPa is valid only at ISA sea level. Altimeter movement is not constant at different pressures; 30ft/hPa is a better average value throughout the lowest 5000 foot layer. 100ft/hPa is appropriate to higher altitudes.

Each ICAO State will publish maintenance standards which altimeters must meet when undergoing bench testing every two years or so. If neither your regulator nor your Ops Manual states a pre-flight accuracy (checked before take-off), you might consult your national maintenance regulations. 50' or 60' maximum error on the ground is reasonable around the world for IFR I believe.

Hope that helps,
O8

 ft 7th Aug 2012 07:38

There is of course a standard laying down the specifications which an altimeter has to be designed and manufactured to in order to be approved. Unfortunately, it is one of those non-free standards which will set you back a significant amount. I'm on a dumbphone ATM so I won't research the reference, but I think it is in the EASA certification standard documents (and thus probably in the relevant FARs as well). Could have been an SAE one, can't quite recall.

The operational checks recommended are something entirely different.

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