taken from BBC :
Flights across Britain were temporarily grounded following a computer breakdown at an air traffic control centre.
The problem happened at 0605 BST on Wednesday at West Drayton, near London's Heathrow Airport.
It meant that flight data normally produced electronically had to be prepared by hand.
It is a very creaky system that has been patched together over a number of years
Ian Simile, Air Traffic Controllers' Union
The system was fully operational again by just after 0640 BST, but air traffic control ran at 70% of its usual capacity until about 1000 BST.
Travellers at airports across the country suffered delays to their journeys, but by 0945 BST the majority of flights were leaving on time again.
Initially, passengers at Heathrow, Gatwick and Stansted airports had faced waits of up to two hours, while those at Manchester Airport had faced delays of up to three hours.
All flights from Birmingham International Airport were suspended after the breakdown happened, but services had almost returned to normal by 0915 BST.
It is the second time in just two weeks the system has failed.
Safety "was not compromised"
The first problem occurred in the run-up to the busy Easter bank holiday, and left thousands of travellers stranded in airports around the world.
Richard Everitt, chief executive of National Air Traffic Services (Nats) told the BBC that particular incident had not been caused by a computer glitch.
It had been caused by the incorrect inputting of data, he said.
Neither problem was connected with computers at the new Swanwick air traffic control centre in Hampshire, which was dogged by software problems before it opened six years late in January.
Ian Simile, a spokesman for the Air Traffic Controllers' Union, described the computer system at West Drayton as "creaky" and said an investigation should be carried out as to why it had broken down again.
He told BBC Radio 4's Today programme: "It is a very creaky system that has been patched together over a number of years.
"It does seem to be susceptible to crashing."
He stressed the breakdown did not compromise safety.
"Planes are still seen on the radar, they know where every plane is and they are being kept a safe distance apart."
These incidents are a further setback for the newly-privatised National Air Traffic Services (Nats) which has had to receive a £30m bail-out from the government after running into financial difficulties following the events of 11 September.
A group of seven airlines, including British Airways and Virgin Atlantic, has taken a 46% stake in Nats.
On Tuesday, one of these partner airlines - low-cost carrier easyJet, said it was considering writing off its investment.