Experts: Why planes should stay grounded

Commercial European flights will be severely disrupted as long as some levels of ash are detectable in the air, the UK Civil Aviation Authority (CAA) told CNN on Monday.

Despite growing pressure from air travel groups such as the International Air Transport Association (IATA) and major airlines like British Airways, the CAA said that all current jet engine manufacturers specify zero levels of atmospheric ash for safe flying.

The CAA added that unless jet engine manufacturers changed their operating specifications, something it added was highly unlikely in the short-to-medium term, the restrictions will continue to apply.

Addressing calls from some quarters of the European travel industry to lift or ease flight restrictions, a CAA spokesman said: "We need evidence to prove that it is safe to fly... we have evidence that ash adversely affects aircraft and at the moment the manufacturers' guidelines are zero rating with respect to ash."

Would you be willing to fly through ash cloud?

The spokesman said that it would be very unlikely that NATS, the British air traffic control provider, would lift restrictions in the current circumstances.

Developments in volcanic ash affecting air travel

The CAA follows guidelines set by the International Civil Aviation Organization (ICAO), a U.N. agency that talks to aircraft manufacturers, airlines, governments and other organizations to draw up its safety regulations. All European states follow ICAO rules.

ICAO established the International Airways Volcano Watch (IAVW) system, which is collating data for the current volcanic ash spreading over large parts of northern Europe.

ICAO says on its Web site: "Since volcanic ash is composed of very abrasive silica materials, it can damage the airframe and flight surfaces, clog different systems, abrade cockpit windows and flame-out jet engines constituting a serious safety hazard."

NATS decides on whether to restrict flights in designated airspace based on information provided by the UK's Met Office.

The Met Office is currently conducting numerous tests every day to measure the presence of ash in the atmosphere. It does this by using "cloud-base" recorders at various locations to fire lasers into the air to detect particles and by research test flights.

Helen Chivers, a Met Office meteorologist, told CNN: "Ash is not restricted to a particular layer of the atmosphere; some is now being detected on the surface [ground] which means that aircraft would have to fly through it to reach their destinations."

UK sends in navy to help ash cloud crisis

The UK is sending Royal Navy vessels to bring home travelers stranded by the ash cloud disruption, British Prime Minister Gordon Brown said Monday.

The HMS Ark Royal and HMS Ocean were making their way back to British waters, while the Defense Ministry worked to pinpoint locations that most need help.

Transport across Europe has been crippled since the eruption beneath southern Iceland's Eyjafjallajokull glacier worsened last week, prompting local evacuations and shutting European airspace.

With planes languishing at airports, stranded travelers have crammed onto boats and trains and rented cars to reach their destinations.

"I believe this is one of the most serious transport disruptions we have faced," Brown said. "It's got financial consequences as well as human consequences and we will do everything in our power to make sure all the arrangements are in place to help people, where possible, to get back home."

Brown said he spoke to Spanish Prime Minister Jose Luis Rodriguez Zapatero about using airports in Spain -- which have been less affected by the air travel chaos -- as a hub through which to bring people back to Britain. Further information would be released later today, Brown said.

A third Royal Navy vessel, HMS Albion, en route to pick up troops in Spain, may also be able to assist, Brown said.

UK travel has been badly hit by the ash cloud, with airspace largely shut since last Thursday morning and not expected to open until 0000 Greenwich Mean Time on Tuesday at the earliest.

Some airlines have begun to push for the restrictions to be lifted.

A British Airways test Sunday found no damage to the aircraft at a variety of altitudes, the airline said in a statement Monday. It did not say what type of plane it flew.

"The analysis we have done so far, alongside that from other airlines' trial flights, provides fresh evidence that the current blanket restrictions on airspace are unnecessary," said Willie Walsh, British Airways chief executive.

European transport commissioner Siim Kallas said Monday the situation was "unsustainable."

"It is now clear that we cannot just wait until this ash cloud dissipates."

But, he added, "there can be no compromise on safety" and any decision to reopen airspace "must be based on scientific evidence."