The collision of two Royal Navy helicopters in the Gulf has once again highlighted the dangers of flying such aircraft in wartime.
Seven soldiers were killed in Saturday's crash
Simon Turner, a former RAF Harrier pilot with experience of helicopter operations, explains the difficulties faced by pilots.
People will be asking how such a tragic collision can happen, given the wealth of technology employed by coalition forces. Was it bad luck or bad practice?
Accident investigators will be attempting to answer these questions, but will not get all the answers for some time.
In deploying its air power, the Navy's priority will be the defence of its fleet.
All possible measures will be taken to keep the position of the fleet a secret and not compromise it by careless operations. Some of these measures may have had some bearing on the collision.
An enemy will attempt to find opposing ships in many ways, one of which is to use radars to track aircraft heading to and from the ship. The aircraft will counter this by staying low, under enemy radar horizons, until safely distant from the ship.
Commercial aircraft avoid each other by vertical as well as lateral separation.
They may simultaneously pass over the same piece of ground but be separated by 1000 feet. The risk of being forced to operate under enemy radar is that all your aircraft are at similar heights.
The crews would have known the risks and done their best to combat them - but when circumstances, and ultimately luck, conspire against you, accidents like this can happen
A ship's defences need to have freedom to fire on incoming threats. So they channel incoming friendly aircraft down narrow corridors to approach and leave the ship. But aircraft have limited scope for lateral avoidance when in the corridor.
Another danger is that the lights of a ship are easily seen in an otherwise very dark sea. Aircraft and ships will therefore operate with very low intensity lights.
In most cases, the pilots will be using night vision goggles. These enhance safety by significantly improving vision during night operations.
However, the image they provide is not equivalent to daylight viewing with the naked eye. Also, they limit the pilots field of view to about 40 degrees - the equivalent of looking through a few toilet rolls while driving a car. You have to keep your head moving to scan an area of interest. All peripheral vision is lost.
Additionally, a ship's position and its own systems can be compromised by electronic emissions. So approaching aircraft will most likely switch off their emitters, such as radars, as they enter the ships approach corridor. Crews are responsible for using their eyes to see and avoid conflicting traffic.
A ship's sharp-eyed radar operator may assist if two friendly aircraft are seen on his scope approaching a collision. However, considering all factors, this is not a viable measure in preventing such accidents.
Saturday's collision happened at 0150 GMT. Given the location of the accident, the sun would have been just below the horizon.
In this position the sun would have been visible as a slightly ruddy sky to the naked eye, but would dazzle like a bright sun if viewed through night vision goggles.
ROYAL NAVY SEA KING
Early warning role since 1982
Detects low-flying attack aircraft
300 in UK service
Although there would have been a good moon some of its effect may have been blocked by cloud, smoke, sea fog or mist.
With a visibility of one nautical mile, a sensible figure for helicopter operations, two helicopters on a collision course at Sea King speeds would be closing at about 240 knots. This would give 15 seconds to react.
When two aircraft are on a collision course their relative position to each other stays the same. There is no apparent movement in their position in the windscreen of the other. The only change is that they grow in size. If this point happens to be behind a fixed part of the cockpit, such as a metal strut, then they may not see each other until it is too late.
Given that we want aircraft to operate in such difficult circumstances, what procedures are used to help prevent collisions?
"See and avoid" is the main collision prevention measure taken. In most cases this is effective but military operations are often risky by design and defence of the fleet is always the highest priority.
Consequently, the helicopter crews involved in this incident would have known the risks and done their best to combat them. But when circumstances, and ultimately luck, conspire against you, accidents like this can happen.