JB747 - over the last few years I've been spending a bunch of time flying out of HKG into Chinese cities like Chengdu, Nanjing, Beijing, Shanghai, etc. Recently I've noticed there appears to be a transition point after leaving Hong Kong where assigned altitudes move from an assigned flight level in feet (i.e. FL350 or 10,650m) to an assigned flight level which is metric (i.e. 10,700m or FL351). What is the reason behind this and are metric flight levels utilised in other airspace? Are there any specific require for the aircraft and/or tech crew to fly in this airspace? Does this create any confusion when transitioning from non-metric airspace to metric airspace?
There are a number of altitude standards in use around the world.
The most common until recently was generally described as ICAO standard levels. That divided aircraft up by track, using 360º to 179º for easterly, and 180º to 359º for westerly tracks. The westerly set were called ICAO even (even though you'll see they aren't) and the easterly tracks ICAO odd.
ICAO even levels are F200, F220, F240, F260, F280, F310, F350, F390, F430.
ICAO odd levels are F210, F230, F250, F270, F290, F330, F370, F410. There are lower levels too, but not relevant to the discussion.
As you can see, that kept aircraft in opposite directions 1,000 feet apart up to F290, when it switched to 2,000 feet separation. This was all well and good, but as aircraft systems became more accurate, and the skies more crowded, it became necessary to better utilise that space between levels. So, RVSM, reduced vertical separation, was introduced. That basically had the effect of separating all levels by 1,000 feet. And, making things a little more sensible, RVSM even, is still the westerly tracks, but now the level are all actually evens.
I think the reason the old English 'feet' are still used is because 1,000 feet happens to be both a nice separation, and also an easy number to work with.
But, in China and the USSR (and perhaps other places that I don't go) metric altitudes were in use. They had their own version of EVENS/ODDs, with 600m between aircraft on opposite tracks. When RVSM was introduced, that came down to 300m.
On flights from Oz to London, until about a year ago, at various times, we'd fly in ICAO, RVSM and metric RVSM airspace. Countries that were previously part of the USSR, switched to RVSM about a year ago, and most airspace is now using RVSM. I don't know whether Russia switched...haven't been there lately.
China still uses the metric altitudes, so any time you transit from one system to another, there has to be an adjustment area where ATC instruct aircraft to climb or descend to their new level. Leaving HK, HK ATC will hand you over to China at the correct level, whilst in the other direction Chinese ATC make the adjustment. Is there potential for it to go wrong...you bet there is.
So how do we fly these altitudes? Well, we have the ability to select a metric altitude display, but it's only a small addition to the normal displays. The MCP and everything else remains in feet. We have to convert cleared metric altitudes to feet (using a conversion chart), and we then set and fly the altitudes in feet. When there, the metric display should be close to the desired number, but it won't be exactly correct.