FL flown vs true height

[UnderneathTheRadar]

Hi all,

Firstly, sorry if this question appears as SLF question but hopefully you'll see it isn't quite totally silly.

Have recently arrived in Hong Kong from LHR and during the later stages of the flight noticed that the pass display had at us at FL375. My shiny new hand held GPS (used with crews permission) had us at FL385 (or 385,500 feet give or take to be more precise).

Unfortunately it didn't occur to me to check the OAT and I have no way of knowing actual QNH but does this seem like a reasonable difference allowing for likely standard atmosphere deviations over northern China at this time of year?

One slightly related question, do a/c over Russia fly at Flight Levels measured in feet or metres? Is Mongolia the same?

Cheers in advance,

UTR.

[Air Soul]

Crikey, I didn't realise that Virgin were doing space flights yet

:ok:

AS

[UnderneathTheRadar]

Doh!

Please make self evident correction to first post!

UTR

[212man]

Well, a thousand feet (assuming you weren't in a Rutan machine) is only about 33 mb (HPa) so tht would equate to a QNH of about 980 HPa (mb), which is a perfectly normal differential from standard. Your GPs has no idea what the QNH is; it works purely on geometric height.

[idg]

QNH in northern China recently (and at this time of year generally) will be high. Roughly 1020+mb. Yes Russia and China both work on Metric F/Ls :ok:

[LGB]

You have to consider the pressure gradient all the way down. You cannot just take the QNH and do corrections at MSL. Where the temperature is lower than ISA, for instance, the isobars lie closer together, giving a low pressure at the same true altitude - the opposite for warmer than ISA. ISA deviation varies as you climb, making it more difficult to calculate.

So if there is a QNH of around 1020, and you measure a true altitude of 38500', my guess is that the averge ISA deviation is below ISA. (had to correct this once, hope I got it right this time!)

Can some met expert please see if the above is right (easy to fog it up ...)

By the way, an ICAO eastern level in meteres is 11400m = FL374. You should be able to find a list of this in any Jeppesen IFR binder (1).


Previous discussion as to why not make all aircraft fly true altitude instead of pressure is not just because of conservatism. For performance reasons this would be not be practical, optimum and max flight level etc.

[Old Smokey]

UnderneathTheRadar,

Whilst the QNH is not known for this discussion, idg is quite right in saying that it is typically in the region of 1020+ hPa in Northern China at this time of year.

At the fairly high levels that you describe, even small Temperature deviations from standard will usually have a much larger effect upon Indicated Vs True Altitude, than will QNH. A QNH of 1020 hPa would only account for 184 feet (56 M) of error.

For 37,500 feet Indicated Altitude to translate to 38,500 feet True Altitude would only require the Mean Temperature of the column of air beneath the aircraft to be 6.7°C above standard. This would not be unusual in winter conditions with a low Tropopause, leading to Temperatures significantly above standard above the Tropopause, even though they may be colder at much lower levels.

Regards,

Old Smokey

[jtr]

My 0.02 is that the shiney new GPS of yours is using the WGS84 model of earth shape, or similar, which assumes the earth is an ellipsoid, or something like that. As I understand it, the GPS altitude read out is quite likely to be wrong due to this. From first hand experience, I can tell you that on the A330/40 for example, if you look at the GPS altitude on the FMS it is often out by over 1000'.

[LGB]

jtr,

What do you mean by the GPS altitude being out by more than 1000', out compared to what - your pressure altitude? This is mainly due to the atmosphere not being the same as ISA, not geoid differences. You are comparing true altitude to pressure altitude, so of course they are not the same, whether Airbus GPS or a handheld in the cabin.