Who, what, why: What happens when airdrops land?
The UK and US have started dropping aid into Mount Sinjar in Iraq. But how effective are airdrops, asks Lucy Townsend.
Parcels including food, water purification cans and solar lanterns have been dropped to thousands of Yazidis fleeing the advance of Islamic State (IS) forces. Packed into crates and parachuted in, airdrops are described as the "least favoured option" for delivering aid.
"Wastage is huge," says Andrew MacLeod, who has coordinated air drops for organisations including the Red Cross and the Office for the Coordination of Humanitarian Affairs.
"Airdrops have the highest visibility on TV and the lowest impact on the ground. They are the least favoured option, used only in absolutely extreme circumstances where it is the only option."
Among the difficulties are high winds, which can blow packages off course, violence around the drop zone and damage to the crates as they hit the ground. Packages can also injure people on the ground. The RAF abandoned one planned drop into Mount Sinjar because too many people had gathered in the drop zone.
"After the Pakistan earthquake tents were dropped for people but they bounced down the hills, dropped into the rivers and got washed away," adds MacLeod.
- There are no reliable figures on wastage
- Airdrops are described as the "least effective" method of delivering aid
"In other cases there can be small scale rioting or violence around the site of the airdrop. You also have to ensure that you are not dropping into dangerous ground such as marshland where people may drown trying to collect it. Then there's also things like the high protein biscuits that are often dropped can cause constipation if too many are eaten.
"From a C130 aircraft the drop zone is probably multiple hundreds of metres long. The need for an airdrop has to be so urgent that it counters the high level of wastage."
Former RAF Hercules pilot John Gladston argues that military airdrops are usually much more effective. "Parachute airdrop is a much more reliable method than 'free-drop', where aid is simply pushed out the back of the aircraft. Modern parachute airdrop of secure, well-rigged loads is a safe, reliable and accurate way of delivering large quantities of supplies, particularly humanitarian aid."
Wheat, flour, rice and maize are the foods most commonly distributed by the World Food Programme (WFP). "They tend to be more robust things, but there is always wastage," says Gregory Barrow, from the WFP.
"It is the least effective way of delivering aid, but sometimes is the only option."
Subscribe to the BBC News Magazine's email newsletter to get articles sent to your inbox.