British Airways is conducting its own tests of the impact of the ash cloud on aeroplane engines.

A BA Boeing 747 is flying over the Atlantic for up to four hours on Sunday.

BA's chief executive, Willie Walsh, who is a trained pilot, is on board, as is the airline's chief pilot.

The airline's engineers will work through the night to examine the impact of the flight on the 747's four engines. Results are unlikely till tomorrow.

Right now, BA and other airlines are planning on the assumption that they won't be allowed to fly till Thursday at the earliest.

As I pointed out on Saturday, the financial cost to the industry of the cessation of flights is immensely painful, at around £25m a day for BA and - according to the industry group IATA - at least £130m ($200m) a day for airlines collectively.

I would not be surprised if IATA on Monday were to call on European governments to provide financial support to airlines, which face a stiffer financial challenge than even after the collapse in passenger numbers after the 9/11 atrocity.

I would also expect BA and other airlines to urge the government to "stress test" the science that has led to the flying ban.

BA would want to highlight that in the US the authorities operate less constraining safety precautions after volcano eruptions.

"There are six active volcanoes in the world" said an airline executive. "We need to understand why the Icelandic eruption is seen by the authorities to be so much more dangerous than others".

Update 19:11: Airline executives and engineers want to know why they can't be given permission to fly at 20,000ft, below the ash cloud, till the cloud clears.

Also, they're worried that even if they are given permission to fly again in a few days, many people may decide to avoid air travel for an extended period on the fear that the plume and cloud will return.

"We need a permanent solution that reassures people in a proper way" said one airline boss. "Otherwise the damage to our businesses will be even more severe."

As I said earlier, there are deep concerns that some European airlines will be bankrupted by the disruption, unless they're given support by taxpayers.

In the UK, Easyjet is more robust than most.

Its losses are running at between £3m and £5m a day. But it has always operated with a massive liquidity buffer, so that it can withstand being grounded for up to six months.

As for food retailers, the big chains tell me that about one or two per cent of their produce is flown in.

"We're beginning to see a shortage of certain flowers, because Kenya supplies about half our stock at this time of year" said a supermarket boss. "And you'll begin to see less exotic fruit and out-of-season veg".

As for clothing retailers, the impact on them so far has been limited: some so-called "fast" fashion, based on what's in the latest shows, goes by air.

Right now, the biggest impact for business is the sheer number of executives who are stuck abroad, unable to come home.

"The real danger for them is that we'll discover we don't really need them," one business leader joked.

Update 21:38: The BA 747 has now landed at Cardiff after a two hour 46 minute flight, covering 550 miles to the west, over the Atlantic.

The plane took off from Heathrow, and flew through the no-fly zone.

It encountered no problems, no loss of engine performance, no damage to windows.

Engineers in Cardiff will now make a more detailed assessment of the Jumbo's engine over night.

Update 21:50: Earlier today a Met Office plane went through the cloud and encountered dangerous levels of ash.

Which shows that the issue isn't whether the cloud is real and dangerous - but whether its extent can be accurately mapped.

One possible solution is to put observation planes in the sky, to give a more detailed picture of the location of ash concentrations.

The government is therefore trying to obtain more observation planes, from the military in particular.