By Rob Watson
Defence and security correspondent, BBC News
Yet another US helicopter has now been lost in Iraq.
Helicopters have been in insurgents' sights since the 2003 invasion
This time it was a CH-46 Sea Knight transport helicopter, which came down near Baghdad.
Al-Qaeda in Iraq claims to have brought it down, though the US military has indicated it may have been mechanical failure.
But whatever the reason, five helicopters have now been lost already this year with the US admitting at the weekend that the other four had been shot down.
Not surprisingly perhaps two key questions are now being asked.
First, are there any indications that the insurgents in Iraq have decided to step up attacks on US aircraft?
Second, have they developed new techniques or acquired new equipment to make any attacks more successful? Both questions are hard to answer definitively.
It is clearly the case that insurgents have wanted to shoot down US helicopters ever since the invasion in 2003.
Until now the US military has avoided losses by flying low and fast...but no method is entirely fail-safe
And as the US military does not provide details on the number of attacks on aircraft it is difficult to know whether or not there has been an upsurge.
Last weekend, a US military spokesman in Iraq Major General William Caldwell said it was premature to conclude that the threat posed to aircraft by insurgents had dramatically increased over the last few weeks.
But if it is hard to establish whether there is a new focus on targeting helicopters, have the insurgents got better at shooting them down?
In the past insurgents have tended to target helicopters using small arms fire, rocket propelled grenades and shoulder-fired missiles like the Soviet-era SA-7.
Certainly some insurgent groups have said they now have new ways to bring down aircraft, but it is not clear whether it is merely a boast or a reference to new anti-aircraft missiles.
Military analysts say they have seen no evidence of any new weapons, though they certainly do not rule out the possibility that it may well be just the insurgents good "luck" that accounts for this year's losses.
Relatively speaking the insurgents have had limited success in bringing helicopters down given the huge number of flights they have flown.
Flying low to the ground has been one way to evade fire
After 1.5 million hours of flying time, some 55 helicopters have been lost since May 2003, about half to enemy fire according to figures compiled by the Brookings Institution.
But the US military is not taking any chances. The US command in Iraq has already ordered changes in flight operations in the face of the recent losses.
Although they will not specify what those changes are, Major General William Caldwell said the US was "making adjustments in our tactics and techniques and procedures as to how we employ our helicopters".
There is no doubt helicopters are vulnerable if they can been seen and if enemies have the right weapons. Until now the US military has avoided losses by flying low and fast and by varying the routes and time of travel, but no method is entirely fail-safe.
What is also not in doubt is the importance of helicopters to US forces in Iraq. With travel by road long considered the most dangerous option helicopters have been the mainstay for getting around Iraq quickly and relatively safely.
It is hardly surprising the insurgents would want to make life as difficult for American forces in the air as it is for them on the ground.