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of Aer Lingus fleet was 4.4 years old.
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The success story of the company goes back to the
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David Waters, Operations Director with GE Aviation, in Cheltenham, UK, discusses
how the switch to 5,000psi hydraulic pressure for landing gear systems presents
certain component manufacturing challenges
16 AEROSPACEMANUFACTURING
PRESSURE
TAKING THE
As most readers will be aware, the
two commercial aircraft which
have been jostling most for the
media headlines during recent years
are of course the Airbus A380 and the
Boeing 787.
While there are significant differences
between the two aircraft one thing they
have in common is their fuel-efficiency,
which is largely down to reduced weight.
Both aircraft have realised reductions
through increasing their hydraulic
systems pressure from the (commercial)
industry norm of 3,000 to 5,000psi.
The higher pressure allows hydraulic
pipes to have reduced diameters. For
example, the diameter of a hydraulic inlet
hose on a 3,000psi system might be three
inches. This diameter could be reduced
to two inches on a 5000-psi system. In
short, higher pressure means reduced
system volume (hydraulic fluid), and a
reduced system volume means smaller
fittings/brackets.
Both the Airbus A380 and Boeing 787
use 5,000psi pressure for their landing
gear systems but, unlike most systems on
an aircraft, landing gear systems are not
in constant use. Indeed, they may go 12
or more hours between cycles.
Further, one must consider the masses
involved. For instance, the Airbus A380’s
maximum operating weight is more
than 600 tonnes, and the brunt of this
mass is felt on landing by its four main
body landing gears (which themselves
weigh about 20 tonnes). Accordingly, the
extension and retraction of the landing
gear (within a few seconds and in sync’
with the opening or closing of bay doors)
are not without challenges.
In addition to narrow gauge pipes, the
switch to 5,000psi has also allowed for
a reduction in the operating area of the
system’s actuators – again helping reduce
the overall size and therefore weight
of the hydraulic system. However, the
high pressure has introduced a number
of component challenges, including the
need to address seal wear characteristics
and devise new component sealing
solutions.
During the development of the A380
landing gear extension and retraction
system (LGERS), GE Aviation (formerly
Smiths Aerospace) did much work with
one of its seal suppliers and Imperial
College, London. Of particular interest
were the effects of ‘high frequency
dither’ on seals working at 5,000psi.
As mentioned, with increased pressure
the operating areas of the actuators can
generally be reduced. Fluid flow can also
(normally) be reduced rate but a landing
gear system is possibly the exception to
the rule.
600 tonne operating weight | The mass taken by
the A380’s four main body landing gears
Photo: Airbus S.A.S.
Nearing completion | Building a valve assembly
LANDING GEAR I HYDRAULIC SYSTEMS
For example, the retraction time from
cockpit ‘landing gear up’ signal to having,
in some cases, more than 20 tonnes of
landing gear raised and stowed, plus
bay doors closed needs to be as short
as possible to achieve a clean airframe:
done to reduce drag and noise signature
on the ground. Large valves are therefore
needed to accommodate the flow rates.
Traditionally, valves are made of steel
or aluminium, with the former heavy (for
a large valve) and the latter susceptible
to fatigue at high pressures. GE Aviation
has experience of designing in aluminium
up to 4,000psi and has tested up to
5,000psi. However, 5,000psi aluminium
　
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