What’s the setting range altimeters can handle? Is there a standard requiring a certain range for aircraft to be certified? Are there many occasions where a QNH is so high or low that airplanes have problems handling that?
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4$\begingroup$ Do you mean the range for the reference pressure allowed in the Kollsman window? $\endgroup$– minsJan 3, 2019 at 17:41
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1$\begingroup$ Or do you simply mean the certified altitude range of an altimeter? $\endgroup$– selectstriker2Jan 3, 2019 at 18:21
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2$\begingroup$ @mins yes, exactly, I meant the reference pressure. sorry for the confusion $\endgroup$– FlorianJan 7, 2019 at 14:29
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1$\begingroup$ I edited your question accordingly. Probably the answer is in "Pressure Altimeter Systems", SAE AS8009C, if someone with the document can confirm. I wouldn't be surprised the range is 28.1 in to 30.99 in as mentioned in SAE AS392C, related to sensitive altimeters. $\endgroup$– minsJan 7, 2019 at 20:02
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1$\begingroup$ @Gerry, TSO C10b was updated to rev C (2016), which now references AS8009C instead of AS392C. $\endgroup$– selectstriker2Jan 8, 2019 at 14:52
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3 Answers
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There are actually multiple different answers depending on the specific type of altimeter system and what standard applies.
The stand-alone altimeters in older aircraft and small GA aircraft (panel mounted "steam gauge") are certificated to SAE AS392C, Altimeter, Pressure Actuated Sensitive Type. The specified scale range is "at least 28.1 - 30.99 inches of Hg (946 - 1049 millibars)".
This standard was updated in 2016 to require compliance with SAE AS8009C, Pressure Altitude Systems. Altimeters built to this standard have a specified range of 27.50 - 31.50 inches of Hg (931.3 to 1066.7 mb).
Aircraft with "glass cockpits" use Air Data Computers certificated to SAE AS8002A, AIR DATA COMPUTER - MINIMUM PERFORMANCE STANDARD. Barometric corrected altitude correction is covered by Table 2, which defines a range of 22.00 - 30.98 inches of Hg or 745 - 1049 millibars.
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$\begingroup$ Do Air Data Computers (TSO C106 / SAE AS8002) actually use the whole range defined in table 2? The ones I have experience with only do 28.00 to 31.00 $\endgroup$ Jan 8, 2019 at 14:30
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2$\begingroup$ @selectstriker2 Thanks for the note on AS8009C. I've updated the answer. The ADCs I've seen will handle a baro input over the whole range. But looking around, I find the BARO setting control panels are OEM defined parts and don't use the full range (where I was able to find the range) so your experience is accurate for those aircraft. $\endgroup$– GerryJan 8, 2019 at 19:28
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1$\begingroup$ +1: Interesting answer, what about the second question (QNH not in the setting range). I read Denver is way below 27.5 (24/25) in Hg. $\endgroup$– minsJan 8, 2019 at 19:45
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$\begingroup$ @mins, I doubt any place is often at QNH below 27.5 as that basically means a hurricane or a very severe storm. And QFE —which Denver does have 24–25—is not used in the USA. Which leaves the question about places where they do regularly use QFE. $\endgroup$ Jan 8, 2019 at 20:04
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$\begingroup$ @mins QNH setting adjusts to MSL, so Denver is still in the range for the baro setting. To set it to display QFE in Denver you would need an ADC that allows settings in the 24-25 inch range. I have seen NOTAMs that prohibited that enforced the FAR 91.144 restrictions 2NinerRomeo mentioned. $\endgroup$– GerryJan 8, 2019 at 20:06
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The pressure ranges for altimeters in the United States are set forth in a technical standing order. For example TSO-C10b in turn references SAE standard AS392C, which indicates the following performance parameters:
Type I : range to 35,000 feet
Type II: range to 50,000 feet
Both adjustable from 28.1 to 30.99 inHg.
These parameters have regulatory consequences, notably FAR 91.144, which restricts flight operations when the barometric pressure exceeds the upper limit set in the TSO. I have found no regulation for the bottom end, I suppose because the weather would be discouragingly foul.
answered Jan 8, 2019 at 0:07
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It is very dependent on the type of altimeter. A lot of standard GA altimeters are rated from -15,000ft to 50,000ft based on a mechanical range rather than a pressure limitation.
For example, on an older model of an aircraft from 1979 a barometer reading of 25.69 was taken from the aircrafts altimeter. At uncorrected station pressure, this would be 71,000ft.
Thus altimeter limits are altitude based, not pressure based*
*In most cases.
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$\begingroup$ The question is about the setting range (the value the reference pressure can take), not the altimeter range. $\endgroup$– minsJan 8, 2019 at 8:44