By Patrick Jackson
BBC News Online
Terror scares involving airliners like that of the Lufthansa plane forced back by Israel have regularly made the headlines since 9/11 - though the irony is that those who attack airliners for real invariably give no warning.
Few terror targets are more vulnerable than a plane in the air
When two airliners were destroyed over Russia in August it took many days before the authorities were even prepared to admit they had been bombed.
Yet once an alert has gone up from the ground no airline pilot can afford to take the risk of continuing a flight, knowing that ultimate responsibility for the lives of those on board rests on his or her shoulders.
Equally, the combat pilot sent up to "escort" that suspect Boeing or Airbus cannot rule out the possibility of the order coming in to shoot it out of the air.
Often fighters are scrambled because a simple communications error by an airline pilot gives the impression of a rogue plane being in the air.
However, the oldest form of scare is still that caused by anonymous bomb threats.
It is, in the words of Flight International magazine's David Learmount, usually the work of some "sad" prankster rather than any organised attempt to sow panic.
Traditionally, bombers who do issue warnings have been very careful to make them instantly credible. The classic example is that of the codewords used by IRA callers in the UK which the security forces would have recognised.
However, genuine bomb warnings have been largely confined to land targets such as railway stations.
EUROPE'S STRING OF BOMB THREATS
5 October: Lufthansa airliner bound for Tel Aviv diverted to Cyprus after Israel denies entry to airspace
4 October: bomb threats divert a New York-bound Singapore Airlines plane to
Manchester Airport and a London-bound Greek Olympic Airlines plane to Corfu
30 September: a British Airways flight from Berlin to
London is diverted to Amsterdam
28 September: New York-bound Olympic Airlines plane diverted to Ireland's Shannon Airport
26 September: New York-bound Olympic Airlines plane diverted to UK's Stansted Airport
When a warning comes in affecting a plane in the air, a difficult decision must be made rapidly as to whether it is credible.
"If there is a bomb on-board, what are the passengers' relatives going to think of you for not acting?" David Learmount told BBC News Online.
"This is what makes the airlines so incredibly vulnerable."
He suggested the stream of airliner scares in Europe which began at the end of September may have been caused by a nuisance call, taken up by "copycats".
Since 9/11, nations have been pooling information on nuisance callers in order to build up a profile for sussing out future hoaxers, Flight International's safety editor said.
Air traffic controllers would decide on the basis of such a profile whether to inform a pilot of a threat received.
Other flight security advances include the G8's Secure and Facilitated International Travel Initiative launched in June which provides, among other things, for the creation of a "24-hour aviation point of contact network to address imminent threats to airlines".
One of the most dramatic sights in the skies since 9/11 is that of fighter planes tailing diverted airliners.
Combat pilots are now scrambled routinely for airliner alerts
In the Lufthansa case, the pilot diverted to Larnaca Airport after Israel denied access to its airspace and scrambled two F16 fighters.
Cyprus strongly protested, accusing the Israeli air force of violating its sovereign airspace.
David Learmount casts doubts on reports of fighters "forcing" planes to land - they are sent out primarily, he says, to destroy airliners which pose a real threat - whether they are genuinely carrying bombs or are being used by suicide hijackers as weapons in themselves.
There are no cases on record of fighters downing planes on grounds of anti-terrorism.
However, the Soviet Union did deliberately shoot down two South Korean civilian airliners which violated its airspace - forcing one to land in 1978 and destroying the other in 1983, with the loss of 269 lives.
Diverted flights cost airlines dear, given the fees airports charge for unscheduled landings and the cost of accommodation for stranded passengers, extra aviation fuel and lost flight hours.
However, as David Learmount explained, air forces are increasingly reluctant to foot the bill for alerts caused by simple pilot error:
"At least once or twice a week, somewhere in northern Europe, combat planes intercept a civil airliner. Why? Usually because the pilots have made a very simple mistake...
"They make a one-digit error with the radio frequency and all of a sudden nobody can talk to this plane."
Fortunately, such mistakes are usually resolved by contacting the airliner through the international emergency frequency - but only after valuable air force resources being wasted.
According to Mr Learmount, some European air forces are now considering charging airlines for the cost of intercepts which arise from pilot error.