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I have a discussion going on with a flying club member.

He states that altimeters may have an error margin of up to +/- 200ft. Example: ATC instructs you to fly 1500 feet, you would still stick to their instructions when flying 1300/1700ft as they could not find out what the actual altitude is indicated on the altimeter, but only get Transponder FL reading + correct from pressure deviation. We both agree, though, that sticking to the altitude indicated on the altimeter is the best thing to do.

It's more of a hypothetical discussion: Assume the altimeter displays 200ft more than the actual altitude (while correct QNH/Altimeter setting is dialed in), it could mean I'm intersecting the vertical border of a restricted area, believing I'm 200ft above it.

We tried to look up in EASA documents, but weren't able to find any guidance. The farthest we come are manufacturer tolerances upon delivering the devices and that systems need to be calibrated. Can somebody shed some light on this and reference the legally relevant documents?

Assumptions: VFR flight in Europe/under EASA regulations, daytime, SEP.

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  • $\begingroup$ Related: aviation.stackexchange.com/questions/71702/… $\endgroup$
    – Jamiec ♦
    Commented May 18, 2022 at 12:43
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    $\begingroup$ He may be conflating checkride or ATC standards for how closely a pilot is expected to maintain altitude, with tolerances for how accurate the instrument is required to be. $\endgroup$
    – Ralph J ♦
    Commented May 18, 2022 at 13:35

2 Answers 2

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The relevant information is contained within ICAO Document 8168 Volume 1 Part 3

A serviceable altimeter indicates the elevation of the point selected, plus the height of the altimeter above this point, within a tolerance of:
a) ±20 m or 60 ft for altimeters with a test range of 0 to 9 000 m (0 to 30 000 ft); and
b) ±25 m or 80 ft for altimeters with a test range of 0 to 15 000 m (0 to 50 000 ft).

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  • $\begingroup$ I believe it is also worth pointing out that those values are only applicable for aerodrome elevations up to 3500 feet as stated in Notes 2 and 3 just below the cited paragraph in ICAO Doc. 8168. For higher airport elevations (up to 15,000 feet), up to +- 225 feet from the correct reading is acceptable (Table III-1-3-2). $\endgroup$
    – Darjan
    Commented Sep 5, 2023 at 13:26
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Depending how the altimeter was certified, it shouldn't be that far off unless you are up at 35,000 ft. Altimeters that are certified under ETSO-C10B/C must meet AS8009B/C. If the altimeter is also certified under ETSO-C106A, then some of the tolerances at the higher altitudes are tighter as well.

Altitude Equivalent pressure (inches of mercury) Tolerance ±(feet)
−1,000 31.018 20
0 29.921 20
500 29.385 20
1,000 28.856 20
1,500 28.335 25
2,000 27.821 30
3,000 26.817 30
4,000 25.842 35
6,000 23.978 40
8,000 22.225 60
10,000 20.577 80
12,000 19.029 90
14,000 17.577 100
16,000 16.216 110
18,000 14.942 120
20,000 13.750 130
22,000 12.636 140
25,000 11.104 155
30,000 8.885 180
35,000 7.041 205
40,000 5.538 230
45,000 4.355 255
50,000 3.425 280

https://www.ecfr.gov/current/title-14/chapter-I/subchapter-C/part-43#Appendix-E-to-Part-43

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    $\begingroup$ These tolerances are quite a bit narrower than those for the popular answer. As of this comment, I'm the only upvoter for this answer. Any idea why no one else appreciates this answer? $\endgroup$
    – Amazon Dies In Darkness
    Commented Jul 16, 2023 at 10:50
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    $\begingroup$ I suspect because the other answer is what a pilot is going to check during pre-flight (no special equipment needed) so that may seem acceptable to the larger number of pilots here. My answer is in regards to the actual altimeter system from someone who has helped design various altimeters, and I suspect there are fewer of us here. $\endgroup$
    – selectstriker2
    Commented Jul 17, 2023 at 13:51
  • $\begingroup$ Thanks for the info. I'm glad the people designing the altimeters have such detailed certifications. $\endgroup$
    – Amazon Dies In Darkness
    Commented Jul 17, 2023 at 15:40

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