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(see Graphic)
(see Graphic)
Europe’s new fighter aircraft will be a supreme warplane with great power
and agility in the air. But while huge sums of money are being spent
developing the technology for the Eurofighter 2000 (formerly the European
Fighter Aircraft), little cash has been set aside to investigate the effects
of this formidable flying machine on the people who will pilot it.
According to specialists in aviation medicine, the aircraft could well be
too agile for its own good. As things stand, pilots will become unconscious
long before the fighter reaches the peak of its design performance. Even
more worrying, however, is that there will be no warning of the impending
crisis in the cockpit. One second pilots will be in complete command, the
next they will be slumped at the controls.

Fighter aircraft need to be fast but speed alone is not enough, not even
when you can fly at more than 1500 kilometres per hour. What gives one
fighter the edge over another is the ability to make tighter turns so that a
pilot can outmanoeuvre or tail an opponent with ease. But as pilots twist
their aircraft away from trouble or into a kill, they generate enormous
forces on their bodies that they may not be able to withstand.

As they go into a turn in a modern fighter, their blood is pushed down
their bodies and begins to pool in their legs and abdomens, starving their
brains of oxygen. If the turn tightens, colour vision fades into black and
white, a phenomenon known as ‘greyout’. Then peripheral vision goes, their
sight narrowing into a circle dead ahead. If the pilot doesn’t ease off the
turn, the tunnel vision becomes a ‘blackout’ – pilots are conscious, and
able to hear and talk, but they cannot see. Finally, if they ignore all the
warning signs, they are knocked out, a phenomenon known as G-induced loss of
consciousness, or G-LOC.

These classic and progressive symptoms are well known. What has surprised
specialists in aviation medicine, and is something they realised only after
several fatal accidents, is how severe are the effects on the body of making
tight turns in the latest fighters, such as the F-22 and the Eurofighter
(see figure 1).
There are no warning signs. Pilots do not progress through
the classic symptoms of lack of oxygen in the brain, which they are trained
and equipped to counter. They are simply and suddenly unconscious.

TAKING TURNS

The cause of restricted blood flow to the brain and, potentially, G-LOC, is
the large centrifugal force generated in a turn by the aircraft’s circular
motion. When a fighter turns, it tilts or banks in the direction of the turn
to maintain equal lift on each wing. If it didn’t, and made a flat or
skidded turn, the wing on the outside of the turn would be going faster than
that on the inside, and would naturally develop more lift. If this lift
were resisted and the wing held flat, the aircraft would develop drag and
slow down, ultimately spinning out of control.

So pilots must make banked turns. But whichever way they roll their
aircraft, they face an unpleasant experience that can put their lives at
risk. Usually, pilots bank their aircraft so that they are on the inside of
the turn. This means that they are pointing towards the centre of the turn
and suffer a centrifugal force pushing down on their heads, which drains
their brains of blood – and oxygen. If pilots bank their aircraft so that
they are on the outside of a turn, the opposite happens: more blood is
forced into their brains. But most pilots avoid the experience – it causes
them to see red, literally, and to feel as though their heads are about to
explode.

The greater the rate of turn, increased by higher speed or tighter curvature
or both, the greater the centrifugal force. This force is measured in terms
of the normal gravitational force (G) acting on the body. At 2G, a pilot
feels twice his or her normal weight, three times more at 3G and so on.
Modern fighter aircraft are capable of 9G turns and the next generation will
be capable of more than 12G – the exact figures are classified.

While these forces are clearly well beyond what the body can cope with
naturally, the rate at which its response deteriorates as the G-force
increases very much depends on physical stature. For someone like me, who is
fit with a solid frame of medium height, my limbs begin to feel heavy
between 2G and 3G and it’s hard to keep my head erect. About 4G, I see white
specks and feel my lungs and stomach pressing down towards the pit of my
abdomen. Then my colour vision fades and my peripheral vision narrows so
that I can see only straight ahead. Suddenly everything is black, though I’m
still conscious. So far, I haven’t been knocked out during a flight.

G-LOC is directly related to the amount of oxygen circulating in the brain
and thus to oxygenated blood delivered to it. When the flow is impeded,
which happens when the G-force increases rapidly, the body has two ways of
defending itself. When the increase in G, known as the onset rate, is less
than about 1G per second, the main defence mechanism is the pumping of the
cardiovascular system, which tries to overcome the effect of the G-force. At
greater onset rates, the body must rely on its oxygen reserves in the
brain, though these will last for no more than five seconds. If the
reserves are used up or, at lower onset rates, pilots ignore the warnings
that their cardiovascular systems cannot cope, G-LOC is inevitable.

Researchers reckon that the unconsciousness usually lasts about ten seconds.
If by then the G-force has reduced sufficiently, amnesia follows for a
similar period. They base their estimates on tests in centrifuges, which
subject volunteers to G-forces by spinning them at high speeds, and on the
only video recording of a pilot known to have suffered G-LOC and survived, a
lucky F-16 airman. The experience leaves an individual feeling frightened,
confused and generally incapacitated for between five and thirty seconds
after regaining consciousness. Volunteers in tests have sometimes suffered
rapid and uncontrolled muscle contractions and relaxations, which cause
their limbs and bodies to jerk as though they were having an epileptic fit.
The overall timings depend on an individual’s physical fitness and health.
Paradoxically, those who practice aerobics are more vulnerable because this
form of exercise increases the size of the heart and lowers blood pressure –
two factors that you can do without when you need blood in the brain, not in
an enlarged heart, and more pressure to pump ‘uphill’ against an increasing
G-force.

LETHAL FORCES

Several test pilots and military pilots have flown into the ground and been
killed in suspected G-LOC incidents but it is extremely difficult to
pinpoint the cause. If an autopsy is feasible, then determining if the pilot
was conscious at impact is not. However, investigators can rule out causes
such as engine or aircraft failure, listen to tapes of radio calls, examine
flight data recorders and take testimony from other pilots who were in the
air at the same time.

Two prototypes of the Northrop F-20 Tigershark both flown by very
experienced test pilots crashed while practising displays to demonstrate the
aircraft’s agility to prospective customers. In October 1984, Darrell
Cornell died while practising a high-G air display at Suwon in South Korea.
Just over seven months later, his colleague David Barnes was killed in
similar accident at Goose Bay in Labrador. Official accident reports cited
G-LOC as the most likely cause. Over the past decade, the US Air Force has
attributed 20 fatal crashes to G-LOC; none are believed to have occurred yet
in Britain, although this is hardly surprising. The Royal Air Force’s latest
fighter, the Tornado F3, was designed as a long-range interceptor of
bombers and thus does not need to be very agile, while its older fighters,
such as the Jaguar and Harrier, are simply not up to it.

But the prospect of the Eurofighter 2000 coming into service at the turn of
the century has convinced researchers at the Royal Air Force Institute of
Aviation Medicine (IAM), Farnborough, of the need to investigate G-LOC
thoroughly. Eurofighter will be so agile that it could generate as much as
12G in just over a second if pushed to the limits of its performance.

Researchers at Farnborough are considering pressurising the oxygen that
pilots breathe as a way of reducing the influence of G-forces. This could be
done automatically by modifying the equipment that controls the supply of
oxygen to the pilot so that it responded as the G-forces increased. Higher
pressure in the chest cavity would squeeze the heart and so raise the
pressure of blood flowing out of it. The trouble with positive pressure
breathing for G-protection (PBG), as the technique is called, is that it
hampers the return of blood to the heart. But the researchers are confident
that this could be overcome by improving the design of the traditional
G-suit, which squeezes the lower half of a pilot’s body to restrict blood
flow into it.

Most air forces started using G-suits in the mid-1950s and their basic
design has changed little since then. The suits worn by modern crews
consist of five inflatable bladders; one across the stomach and a pair on
each leg located on the thigh and calf. These G-suits are extremely close
fitting, even before automatic and very rapid inflation by compressed air
that occurs when G-forces rise. However, the Farnborough institute is
convinced that a better G-suit could provide more protection by increasing
blood pressure even more. ‘We developed a one-piece set of full-coverage
inflatable trousers which, when used in combination with PBG, is very
effective,’ says Wing Commander Andy Prior, who is leading the institute’s
research team.

On balance, PBG raises the blood pressure slightly and gives between 0.5
and 1G improvement in an individual’s tolerance to G-force. It also helps
pilots to perform an exercise designed to counter the effects of high
G-forces. In the exercise, known as anti-G straining manoeuvres, or AGSMs,
pilots must tense their limbs and strain against a closed or partially
closed glottis by inhaling and exhaling rapidly every three seconds. The
timing of the breathing is important: straining for longer impedes the
return of blood to the heart while longer periods of inhalation reduce blood
pressure in the chest cavity and thus in the brain too.

In centrifuge runs at the IAM, subjects were taken from 4G to 9G and back
repeatedly, while using PBG and the standard AGSM, until exhausted. These
tests showed that although the techniques increase pilots’ tolerance to
G-forces only slightly, they do improve pilots’ endurance of G-forces by
more than 50 per cent. This means that pilots do not tire as quickly under
the influence of high G-forces. Full-coverage anti-G trousers, which
increase blood pressure considerably in the upper half of the body, also
enhance a pilot’s tolerance to G-forces.

Prior has begun to study the long-term effects of these techniques, and
particularly whether they cause damage to the heart and lungs. So far, RAF
pilots who regularly experience high G-forces have reported only minor
complaints, such as cricked necks and pulled neck muscles, says Prior. But
he admits that the physical traumas could be much more severe for aircrews
when the Eurofighter is introduced in 1999.

Another avenue of thought on G-LOC is to accept that it will occur, detect
that the pilot is unconscious and have the aircraft automatically fly
straight and level until the pilot revives and is able to resume control.
Several methods of monitoring the pilot’s level of consciousness are being
studied. Are the pilot’s control inputs rational or not? Is the pilot
slumped or just looking down at a cockpit display? Are the pilot’s eyes
open? Warning devices that measure the level of blood oxygen at eye level
are being studied. All are in the experimental stage and perhaps the best
solution would be a combination of these, voting for or against taking
control from the pilot.

The USAF has developed such a device but Prior foresees problems: ‘It is
only useful for a combat aircraft in peacetime, especially if the other side
knows you have it. In war you can’t afford to fly straight and level. You’d
never wake up because the other guy would shoot you down. Also, how do you
detect G-LOC in a noninvasive, unobtrusive and fail-safe way with no false
alarms? It would only need to mistakenly take control once or twice before
pilots distrusted it and simply switched it off.’

Prior goes further: ‘But is it really the right approach – to accept that
your pilots will become unconscious? At Farnborough we are concentrating on
G-LOC prevention by giving aircrew better protection. We do not want our
pilots unconscious in the air in peacetime and in air combat, we want our
pilots to win.’

The USAF Armstrong Laboratory at Wright-Patterson air force base in Ohio is
the centre of US G-LOC research, led by Bill Albery. The USAF is taking a
two-pronged approach to G-LOC based on detecting it and lessening its
effects. The air force is also investigating ways of taking control of
aircraft from incapacitated pilots until they recover.

American military researchers have reviewed several systems for monitoring
the consciousness of pilots, but is currently only developing one, a device
that is inserted in the pilot’s helmet earcups or oxygen mask. ‘It allows
you to monitor heart rate, or whether you’ve lost the pulse, which tells you
that you have no eye-level blood pressure. It also measures the blood’s
oxygen level, which is a good indicator of the physiological state of the
pilot,’ says Albery. The device uses two different wavelengths of light to
monitor oxygen levels in the blood; deoxygenated haemoglobin in the blood
absorbs one and oxygenated haemoglobin the other. By comparing the level of
light absorbed, the oxygenation level of the bloodstream is determined. The
device is still being tested in the centrifuge.

The USAF is examining two other G-LOC sensors, which would be mounted on the
pilot’s helmet. These would consist of electroencephalography arrays to
monitor the electrical activity of the brain and deduce the level of
consciousness. One is under test, the other is not yet ready for evaluation.

To lessen the effects of high G-force, the USAF is putting into service the
PBG technique and a fuller-coverage Advanced Technology Anti-G Suit. With
the ATAGS and PBG system, the USAF is finding a four to five-fold increase
in G-force tolerance compared with a pilot wearing a standard G-suit and
having no PBG assistance. Where the pilot is tired and not able to withstand
as much G-force as earlier in the flight, PBG combined with ATAGS gives
more endurance. However, even these measures may not be enough to protect a
pilot against a rapid onset rate, admits Albery. The USAF has evaluated a
progressive arterial occlusion suit that pinches the body at various points
to cut off blood flow temporarily. Though it enabled pilots to sustain an
additional 3G, it was a painful solution and one that has not been
developed further.

Because of the deaths from G-LOC, the USAF has been experimenting, as a
near-term solution, with a Ground Collision Avoidance System (GCAS) in a
General Dynamics F-16 fighter. The GCAS, which uses topographical data
derived from satellites, is linked to an aircraft’s navigation and flight
control networks. If a pilot becomes unconscious, GCAS takes over and
prevents the aircraft from descending below a predetermined height. Though
the system cannot yet avoid constructions such as tall buildings, the USAF
is optimistic that it should keep aircrews out of danger until they recover
and can regain control. The GCAS-test F-16 has been deliberately dived
‘hands off’ about 100 times to date and has recovered safely each time.

Mike Gaines is a freelance journalist. He is the former military editor of
Flight International and has flown many times in high-performance military
aircraft.