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Maintaining assigned altitude - legal deviation limit before violation occurs?

Can anyone point me to the legal reference of the deviation limit before a violation occurs? I’m not talking about a violation actually being filed against the pilot, just the legal limit before one could be filed

LSZK, Switzerland

Is this for a formal “level bust” under IFR in CAS, or a general rule when flying under ATC direction, or something connected with infringing CAS?

AIUI there is a 200ft altimeter error to take into account to start with, but looking at some recent reports here this is clearly being disregarded in some infringement procedures.

Administrator
Shoreham EGKA, United Kingdom

ICAO Doc 4444 8.5.5.2.1
200ft RVSM 300ft non-RVSM

LPFR, Poland

Can anyone indicate a legal reference for the altitude deviation limit before a violation occurs? I’m not talking about a violation actually being filed against the pilot, just the legal limit before one could be filed. I believe it to be 300ft, but would like to find a reference.

The question arises from a couple comments in the EIR thread:

ArcticChiller wrote:

What irritated me was ATC when they called me to “immediately return to my FL, maintain strictly FLXX”, even though I was doing so within +-50ft. The controller said once again: “I see you at FLXX*1*, maintain strictly FLXX!”. The examiner and I looked puzzled as I couldn’t have kept altitude much more precisely (no autopilot, so I did have deviations of around 50 feet. The thing is, per my understanding a transponder is rounding up to the next one-hundred increment when at plus or minus 50ft of a flight level. That’s absolutely normal – or does anybody know what was going on there? I am positive that the transponder never showed an excess of 100 feet. To please the controller I pinned the altimeter hand to +-10ft and that made her calm down, apparently.

BackPacker wrote:

Wasn’t one of the improvements to mode-S, that the broadcasted FL is now +/- 25 feet, instead of +/- 100 feet for the older mode-C?

The Swiss ATC behaviour above is problematic for a number of reasons that boil down to the FL readout on the controller screen can only be resolved to at best 25ft. Or does someone know different?

1) mode-S can report to 25ft but it is subject to aircraft capability so not assured in all cases or legally required
2) mode-S or mode-C broadcasted altitude comes from the altitude encoder input. I know of at least 3 different possibilities for the resolution of this, but the bottom line is that 100ft is legal minimum resolution required:
- parallel grey code (old standard) is only transmitted by encoders in 100ft resolution
- serial RS232 encoder transmissions are normally to 10ft resolution. This is required for the transponder to report with 25ft resolution
- Shadin (for one) produces a converter that takes encoder grey-code and static port inputs and outputs altitude in serial RS232 format to 1ft resolution
4) altimeters are only graduated to 20ft…
5) how precise can one realistically set the Kollsmann window in turbulence? Anyone here claim he can set it to 1013.2 rather than 1013? Does it matter? Well, at 10’000 one hPa is about 38ft on a standard day.
6) legal altimeter accuracy required to be considered airworthy is 75ft (think about that next time you descend below 200ft DH without runway environment in sight)

However, the reason the ATC behaviour might lie elsewhere, which is perhaps subject for another thread. Recently a Skyguide controller was found guilty of “negligent disruption of public transport”. As one can imagine, this is making quite some waves in the aviation community, including Skyguide. Details are here for those wishing to google translate them.

LSZK, Switzerland

Sorry, but this was posted before I was finished writing it ….. :-(

@Peter, could you move the above into my second posted thread with the same subject and delete this thread?

[ done, as far as was possible without deleting other posts ]

Last Edited by chflyer at 10 May 07:32
LSZK, Switzerland

Peter wrote:

AIUI there is a 200ft altimeter error to take into account to start with,

60 ft for altimeters certified up to FL250 (IIRC), 75 ft for altimeters certified for higher altitudes. Add to that a QNH roundoff error of up to about 30 ft.

Last Edited by Airborne_Again at 10 May 08:17
ESKC (Uppsala/Sundbro), Sweden

chflyer wrote:

legal altimeter accuracy required to be considered airworthy is 75ft (think about that next time you descend below 200ft DH without runway environment in sight)

That is of course taken into account when determining the minimum obstacle clearance.

ESKC (Uppsala/Sundbro), Sweden

Peter wrote:

Is this for a formal “level bust” under IFR in CAS, or a general rule when flying under ATC direction, or something connected with infringing CAS?

Question is regarding the formal “level bust”.

LSZK, Switzerland

I think answers here will be country dependent.

As we have seen in e.g. this thread it appears generally true that ATC use the exact Mode C value to determine your flight level or use the local-QNH-corrected version of it if they want your altitude in order to determine whether you have committed a formal airspace infringement. No margin may be applied to this, even though a 200ft margin ought to be applied as per loco’s post above.

When it comes to level busts, I recall from one NATS presentation that they give you 200ft i.e. if they clear you to climb to FL200 and you climb to FL202 that is ok but FL203 will be a bust. But the threshold for the ATC authority reporting you to the CAA (which then decides whether to send you a letter, send you on some £xxx “education session”, or worse) may be something different.

And these things change all the time, according to the current policy on “pilot behaviour” and what to do about it.

Mode S transponders return the pressure altitude in 25ft steps but that doesn’t mean ATC systems see that, and quite separately it doesn’t mean ATC see it; in fact I am sure they see only 100ft resolution on the screen.

Administrator
Shoreham EGKA, United Kingdom

I found an answer in a Eurocontrol document. But it’s from 2004, I found it on this website of Eurocontrol.

(the foot note is: HEIDI – Harmonisation of European Incident Definitions for ATM.)

So, unless it changed since 2004, it probably is 300ft (and 200ft in RVSM airspace). Edit: So the controller’s reaction to my altitude deviation of +-50ft from assigned level was clearly a sign of the safety endangering attitude of Swiss ATC. I filed a report with that statement when it happened, but these reports probably don’t get read by the right people.

Last Edited by ArcticChiller at 10 May 09:35
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