Obviously a useful anecdote for anyone in the UK wondering why skies are clear and planes are on the ground...

I've heard of that anecdote but I still am not sure why it applies to the whole of Europe, let alone England.

Is there even a standard for how much volcanic dust there can be before airports are closed? How do they know we're currently over the limit? If they answer is that they don't really know, how will they know when it's over?

France, Belgium and Netherlands (at least) have been conducting test flights during the weekend: http://www.reuters.com/article/idUSTRE63G1U920100417

Finnish Air Force had some fighter jets in air on Thursday 15th, when the ash first hit Finland, and they've published some photos from inside the engines after some contact with ash:

http://www.flightglobal.com/articles/2010/04/16/340727/pictu...

Cool that they released the photos. Any insight into what we're seeing there, and how it poses a risk?

As far as I understand, the volcanic ash melts in the combustion chamber (where temperature is like 1000 C), and then later can solidify on surfaces, and may block cooling ducts and such.

As far as I know there are no standards, but people are currently working very hard to figure some out.

Worth it just for this bit:

Despite the lack of time, Moody made an announcement to the passengers that has been described as "a masterpiece of understatement":[3][4] “Ladies and gentlemen, this is your captain speaking. We have a small problem. All four engines have stopped. We are doing our damnedest to get them under control. I trust you are not in too much distress.

I've always found the use of understatement to be an interesting difference between British and American English.

My favo(u)?rite phrase from the article was "a bit like negotiating one's way up a badger's arse". I'll be looking for an excuse to use that in my next business meeting.

In the Jakarta incident, the pilot descended 6000 feet because the copilot's oxygen mask had a problem.

Was this a wise thing to do? What does the physics say?

Yes. The physics says there is more air lower down.

I don't really understand your question. Are you wondering if maybe staying high is better because you can fly longer?

That's true, although I assume as they went down they gained in speed, which also lets them fly longer.

all things being equal though if you have to descend faster than 'best glide speed' the speed gain isn't worth the altitude loss. but not passing out due to oxygen deprivation is...

What about for the purposes of restarting the engines?

Lower is better, max height for a restart depends on the engine but is 20,000-30,000 ft

I assume there is a front-loaded initial startup cost of oxygen to initiate combustion that is difficult to do at higher altitudes primarily because the turbines are not spinning (and therefore aren't pulling oxygen into the engines).

Air speed might make it easier to get the turbines up to speed again, but I would make an uneducated guess that the oxygen concentration of the surrounding atmosphere is more important; I think it's likely the engines will contribute much more to turbine rotation than air speed.

Moreover, the engines won't come up to full power instantly; they will need time to throttle up as well... in this case, glide time is more important than air speed.

Ergo, the altitude is more important for the purposes of restarting the engines. (... Except for the "tangential" need that the operator needs to be conscious to restart the engines, and thus needs sufficient oxygen concentrations in the cabin)