But while they may look the same because of their characteristic tandem rotor blades, the Sea Knight is much older and smaller.

The CH-46 has been in service since the 1960s and can be most simply described as a flying bus. It ferries troops and kit to and from the front line.

Each of its six rotor blades is more than 25ft long and it can lift up to 5,000lbs (2,270 kgs).

Typically, it will carry up to 25 combat-ready troops and can remain airborne for two hours on its internal tanks.

What has made the helicopter so useful to the US Navy is its high manoeuvrability - meaning that it can quickly drop off or pick up troops and equipment in combat scenarios which may prove more difficult for other helicopters.

The first 600 aircraft entered service during the Vietnam War and it was generally regarded as reliable. But for a number of years, the US military has sought to retire the CH-46 as it has become increasingly dogged by age-related mechanical failures.

Its planned replacement, the V-22 Osprey - a helicopter-plane hybrid - has had problems of its own so a changeover date is still not set.

In 2001 A Marine Corps CH-46 crashed, killing three Marines and injuring two. The following year, the US Navy command grounded the entire fleet after discovering a crack in a rotor system.

Engineers found a similar crack on two more CH-46s but the rest of the 230-strong fleet was given the all-clear.