A380 hydraulic pressure for landing gear systems

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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
valves would be bulky and production
would be critically reliant on machining
practices.
The Airbus A380’s and Boeing 787’s
hydraulic valves are therefore made from
titanium. However, titanium is much
harder to machine than aluminium.
Cutting speed is often constrained, heat
builds up quickly and vibration can also
be a problem: not good news for any
manufacturing process.
Once again, GE Aviation is partnering
with other specialists, in this case the
machine and tool manufacturers and
AMRC (Advanced Manufacturing
Research Centre – part of Sheffield
University). The partnership has created
specialist monitoring and control systems
that allow running speeds to be set
higher than normal. In addition this
is further enabled through the use of
specific machine upgrades and choice
of coolants. The result: cycle times are
significantly improved upon previously
understood limits.
Also aiding manufacture of the valves
is GE Aviation’s use of Single Minute
Exchange of Die (SMED) – which
minimises changeover time as it allows
operators to change tooling whilst
machines are running.
SMED also allows batch sizes and
inventory to be reduced, in keeping with
lean practices. A number of strategies
were used which, in combination, provide
set-up and change-over times dramatically
lower than those previously realised.
Cycle time
Billets are prepared and loaded outside
of machine cycle time. Finished parts
are also dealt with outside of cycle, and
all elements of the change process are
facilitated with rapid action systems and
mechanisms to further reduce time and
manual interactions.
An intelligent tooling system and
features within the machines and tool
suites ensure maximum tool swaps within
machine time cycles. In addition this
system provides error proofing (‘poke
yoke’) in that the incorrect tools cannot
be fitted.
Titanium provides other advantages
for product concepts and as a result
GE Aviation has created multi-function
hydraulic valve concepts bringing
further challenges to the manufacturing
environment. These products have such
high value that the whole approach to
manufacturing must ensure very capable
processes and thus very high right-firsttime
figures.
As a result the whole extended
GE project team applied latest
methodologies of design for manufacture
(DFM) and advanced quality planning to
ensure this outcome. In support of this
approach the team ensured that the inprocess
and CMM-based technologies
available were fully embedded into
the control plans for the products to
maintain capability.
The latest system requirements of the
aircraft manufacturers and the flow down
of these into the landing gear systems in
which GE specialises created significant
challenges to the manufacturing
programme teams.
Through close-working partnerships,
the leverage of the latest technologies
and the application of the latest
management methodologies, GE
Aviation has taken these manufacturing
challenges and turned them into
competitive advantages. The result is
highly optimised functional products
providing optimum weight with the
ability to supply customer requirements
in a true lean manufacturing
environment. ❙
www.geae.com
Supporting manufacturing | Automated CMM technologies
Acquisition
GE Aviation acquired UK-based supplier
of integrated systems for aircraft
manufacturers and components for
engine builders earlier this year for
$4.8 billion. The acquisition broadens
GE’s offerings for aviation customers
by adding Smiths innovative flight
management systems, electrical power
management, mechanical actuation
systems and airborne platform
computing systems to GE’s growing
commercial and military aircraft engines
and services.
AEROSPACEMANUFACTURING 17

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