Boeing CH-46: Time running out for ageing workhorse

The Boeing CH-46 is living on borrowed time. First brought into action in 1964 and used extensively in Vietnam by the US Navy and Marine Corps, it has been earmarked for phasing out, to be replaced by newer, tougher machines – the Boeing V-22 for the marines and the Sikorsky MH-60S Nighthawk for the Navy.

But both of those have fallen behind their expected introduction, meaning the ageing CH-46 has had to be pressed into service far longer than envisaged by its designers. The V-22 was meant to be operational in 1999; the MH-60S at about the same time. Instead, costs of the V-22 are spiralling and the MH-60S only began to come into service last year. The US military now expects it will still need to use the CH-46 until September 2004.

The most likely cause for the crash in the desert is sand entering the engine, which poses a problem for any machine. Unlike all the other helicopters used by the Navy and marines, the CH-46 has a twin-rotor design – giving sand twice as many places to cause problems. Sand in the rotor bearings, for instance, could make the engine tear itself apart; and both rotors are needed to stay airborne and in control.

Yet if the crash had happened over water those on board might have fared better: the CH-46 is designed to be able to land and float on water.

Its primary role is for troop and cargo transport, and it was used in the 1991 Gulf War and in operations in Afghanis-tan. Over its lifetime, each aircraft was available 87 per cent of the time – a respectable figure for a helicopter.

Since 1992, more than $700m (£450m) has been spent upgrading all the airframes in service for night flying and extended operation.

Last August, the entire fleet was grounded when a crack was discovered in the rotor assembly of one of the helicopters at the Cherry Point Marine Corps Air Station in North Carolina.

Inspections of all of the rest of the aircraft found a similar crack in a CH-46 aboard an amphibious assault ship in the Gulf. But detailed checks of the whole fleet found only one with the same crack, and, after replacement, they were returned to service. Among the improvements that have also been made during its life are the replacement of its rotor blades with stronger carbon-fibre versions.

Its primary mission, according to the US military, is "troop assault transport" – being in the first wave of troops, supplies and equipment during amphibious or on-shore operations. As such, it is a completely logical choice for the assault on the port of Umm Qasr.

The CH-46's twin rotor design means it can perform remarkable maneouvres, flying sideways or even backwards. It can carry three tons of equipment – which is why the Navy has favoured it for carrying supplies between ships.

Yet the original design dates back to 1958, when the first prototype, built by Vertol, flew in April. The Vertol Corporation (later acquired by Boeing) also designed the Chinook –the CH-47 – indicating it followed the CH-46 in the design.

Also a twin-rotor design, the Chinook was taken up after its first flight in 1961 by the American, British and Australian military for similar operations as the Sea Knight – special forces operations and "medium lift" of cargoes.