Walking
on air
A
thorough understanding of hydraulic system architecture of an aircraft
can sort out malfunctions or failures
The
crew of a twin-engine corporate jet on a passenger-carrying flight was
cruising over Minnesota when something began to go wrong. The crew noticed
that the hydraulic pressure was dropping slowly through 1,200 per square
inch (per square inch). Approximately 10 minutes later, the pressure
had dropped to 1,000 psi, and the “low hydraulic pressure” light illuminated.
As the Captain wrote in his Nasa report, “We reviewed the abnormal checklist
and discussed the options. It was decided to make a precautionary landing
at Lincoln, 100 miles ahead, where there was a 12,900-ft runway and
a light turbine transport service center.”
During
descent, the hydraulic pressure continued to deteriorate. “Ten miles
from Lincoln” the report continues, “flaps were selected to 20 degrees
and the pressure fell to 300 psi. We attempted to lower the landing
gear via the normal system, resulting in both main landing gear down
and locked and the nose gear unsafe.” The gear blowdown lever was selected
and subsequently all gear indicated down and locked. “Some pressure
was regained after the blowdown,” the Captain wrote, “and it was used
to get the flaps nearly full down. We declared an emergency with approach
control and requested crash and rescue equipment. The north 3,600 ft
of the 12,900-ft Runway-17R was closed for survey, but at our request
the full length was made available to us after a short delay.”
The
aircraft touched down on speed 1,000 ft from the threshold of Runway-17R
at a maximum gross landing weight of 17,000 pounds. The thrust reversers
and ground spoilers deployed normally, and the aircraft tracked straight
for about 2,500 ft. Then the aircraft started to drift to the right.
When the toe brakes were applied to correct the drift, nothing happened.
There was no pressure remaining in the system to operate the brakes.
Despite their best efforts, the crew was just along for the ride. They
were just “going with the flow”.
Hydraulic
systems basics
The
operation of a hydraulic system is based on the fact that fluids are
incompressible. If we apply pressure to a fluid, that pressure is instantaneously
transmitted throughout the fluid in all directions. Pressurised fluid
traveling through a tube or “line” can be reconverted to mechanical
motion through an actuating cylinder that drives a pushrod.
Like
other mechanical systems, a hydraulic system can be used to develop
considerable mechanical advantage, much the same way that a lever or
pulley can multiply force. By selecting the proper size actuator cylinders,
we can adjust the ratio between the force applied and the force delivered.
A
number of design advantages endear hydraulics to aircraft designers.
First, hydraulic systems are both lighter and more reliable than other
mechanical systems, such as electrically driven screw-drive or cable
and pulley systems. Unlike cable systems that can stretch and suffer
from excess play, hydraulic systems offer a distinct lack of sloppiness.
The use of an engine-driven hydraulic pump to operate landing gear,
flaps and other high-energy applications reduces the demands placed
on the electrical system. Hydraulics are relatively easy to maintain
and can provide almost unlimited force – an important consideration
as aircraft evolve to higher speeds and weights.
A
basic aircraft hydraulic system consists of a hydraulic pump, fluid
reservoir, hydraulic lines, control valves, actuator(s), accumulator(s),
filters, and a variety of relief and check valves. The hydraulic pump
provides the pressure to operate the system, and can be engine driven,
electric motor driven or manual. The hydraulic lines - small aluminum
or stainless-steel tubes and reinforced flexible hoses - transmit the
pressure from the power source to the actuator(s). Control or selector
valves are used to direct the pressure to the proper actuator(s) at
the correct time to operate them.
Hydraulic
actuators on aircraft can operate anything; from landing gear and brakes
to primary flight controls, flaps and slats, speed brakes, pitch change
mechanisms, doors and thrust reversers. In some cases, a hydraulic motor
is used rather than an actuator to convert hydraulic pressure back to
rotary motion.
Although
they come in a variety of sizes and designs, accumulators share a variety
of functions within the hydraulic system. First, they allow for the
expansion of the hydraulic fluid with changes in temperature, and they
help to dampen transient pressure excursions during normal operations.
In addition, accumulators can provide an emergency pressure source to
operate the system. In this respect, an accumulator can be considered
analogous to a battery. The accumulator provides only a limited energy
supply, and pilots must be frugal when it comes to using that energy
in an emergency.
Hydraulic
system architecture
As
with aircraft electrical systems, hydraulic systems are designed with
a significant degree of redundancy to ensure the operation of systems
and components critical to safety. This redundancy can be provided in
any number of ways and at various levels in the system architecture.
In general, the complexity of the hydraulic system increases as aircraft
size and speed increase.
Most
small jets and turboprops utilise two engine-driven hydraulic pumps,
either one of which is capable of powering the entire hydraulic system.
In addition, the system may incorporate an electrically driven pump
for emergency use. Further redundancy for systems such as landing gear
and brakes may be provided by an accumulator, which provides a final
“one shot” capability. For example....
....CONTD
