FlyPast 03.2018

March 2018 FLYPAST 55

TRAINING 100 YEARS OF THE ROYAL AIR FORCE

DECREASED DECELERATION AND ENGINE

NOISE HERALDED A MAJOR FAILURE IN A GNAT

TRAINER. HEAVY ON THE UNDERSTATEMENT,

PILOT ROY GAMBLIN RECALLS: “XP540 WAS

GOING TO HAVE A VERY BAD DAY,” WHILE

RECOUNTING HOW HE LIVED TO

TELL THIS TALE

Above left

Gnat XP540 after

it came to rest at

Llanbedr, with a

rescue Land Rover

and the SAR Wessex

in the background.

VIA AUTHOR

Left

A trio of 4 FTS Gnats

over North Wales,

circa 1971. They are

wearing the red/

white/grey colours

adopted in the late

1960s. KEY COLLECTION

Below left

Gnat XP532 fl ying

along the Menai

Strait, the body of

water that separated

Anglesey from

mainland Wales, in

the 1960s.

KEY COLLECTION

initial actions, had taken place in only

a very short span of about 15 to 20

seconds from the first symptom. We

now began to take in the conflicting

evidence and tried to fathom out what

was going on.

REVERT TO MANUAL

Within a few seconds there was

a HYD (hydraulics low pressure)

warning, which appeared to confirm

seizure. If the engine had simply

flamed-out and was windmilling

the idling hydraulic pump would

maintain enough pressure to provide

power to the tailplane.

We now had a more serious

condition to confront. Not only

did we have a dead engine, we had

no flaps available for landing and

we would have to revert to manual

control for ailerons and elevators,

using electrical movement of the

tailplane, backed up by the small

elevators, for pitch control.

At this point, I should explain briefly

how the three pitch control modes

worked in the Gnat. Normally, the

elevators were mechanically locked

to the tailplane so that the control

stick moved the whole assembly

hydraulically as a slab. Pitch trim was

electrical.

In the second mode, the elevators

were unlocked from the tailplane via

a lever in either cockpit. They would

move directly in response to stick

movement, with the hydraulically

powered tailplane ‘following up’.

This mode was much more sensitive

and so the Gnat could be flown

without the need for pitch trim due

to the very light control forces. This

also conserved electrical power, for

example after an alternator failure or

flame-out.

The third mode was manual,

where the stick moved only the small

elevators but to maintain full control,

especially for landing, the tailplane

had to be moved electrically via

turned to gain altitude on a northwest
heading, decelerating to the gliding
speed of 180kts. Simultaneously, Mike
and I noticed that the RPM gauge was
registering zero. Now this didn’t tally.
Both of us had believed that the
Orpheus had flamed out, rather
than seized, since there had been no
vibration and it had run down in the
same way as one starved of fuel or
flame. We had practised enough in the
simulator to know the difference.
In a flame-out condition, the engine
would ‘windmill’ at low RPM. In
a seizure, there would normally be
vibration and the RPM would be at or
near to zero. Our reasoning, and the

“The Gnat was an absolute pleasure to fl y,

especially in close formation. I felt very

comfortable in it – particularly in the front

seat with its superb view”