Technology: Tiltrotor hits turbulence as second plane crashes

Technology 15 August 1992

The future of a revolutionary ’tiltrotor’ aircraft being developed for
the US Navy is in the balance after a fatal crash of a prototype. A fierce
political battle over funding for the programme could be decided for good
if investigators find the cause was a fundamental flaw in design.

The V-22, manufactured jointly by Bell Helicopters and Boeing, can take
off like a helicopter, with its engines pointing upwards, and then swivel
them to fly like a fixed-wing plane.

Ever since the accident on 20 July, the two sides in the funding debate
have been trying to decide the long-term future of the aircraft. Supporters
in the US Congress re-emphasised their call for a new study of the benefits
of a civil version of the V-22. But Bell-Boeing has repeatedly stated that
it cannot develop a civil version unless the Pentagon pays for development
of a military aircraft first.

The Department of Defense is split into a faction which wants the navy
to get the aircraft, and possibly the army and air force as well, and the
opposition camp, headed by Richard Cheney, the defence secretary, which
wants the programme killed off.


The opponents have requested proposals for a cheaper version of the
V-22, or alternatives such as conventional helicopters, to fulfil the navy’s
requirements. The navy desperately needs new ‘medium lift’ aircraft, but
the argument is over how sophisticated the aircraft need to be and whether
the other services can justify a machine as expensive as the V-22.

The prototype V-22 – the fourth in the series – suddenly plummeted into
the Potomac river while on final approach to a Marine Corps base at Quantico
in Virginia. Observers say the engines were in the process of swivelling
from horizontal to vertical and had reached an angle of about 60 degrees.
Some base personnel reported seeing ‘flashes’ or ‘explosions’ immediately
before the descent.

It took some days to retrieve the wreckage from the riverbed and it
was found to be further damaged by strong river currents. The cause of the
accident was not immediately apparent.

Now investigators are considering the possibility that it was something
as simple as running out of fuel. The aircraft is known to have had less
than 30 minutes of fuel left. Some fuel is carried in the engine compartments
themselves and one suggestion is that this fuel may be made unusable by
the swivelling of the engines. The theory is unproven, however.

The V-22’s two engines are cross-coupled so a single engine failure
should not prove catastrophic. But if both engines failed at low speed and
low altitude, it is unlikely that the aircraft could successfully land.

Last year, the fifth prototype crashed, but this was due to miswiring
of the flight control system, which has been discounted in the crash of
the fourth. Work on the sixth aircraft, which is about three-quarters com-plete,
had already been suspended while the navy and the manufacturers considered
a series of weight-saving modifications.

Weight is the biggest technical challenge on the programme and Bell-Boeing
plan to cut 900 kilograms from the aircraft’s weight when empty. These plans
include a simpler wing stowage system for use on aircraft carriers, lighter
wiring, thinner composite fuselage panels and an improved vibration suppression

The accident aside, Bell-Boeing appears set to move into the final development
phase. This follows a recent ruling from the US General Accounting Office
that Cheney must begin spending funds allocated to him by Congress for 1992,
but which he has refused to release.