Backup for dropping the gear

"Peter Duniho" wrote in message

I'm not sure there's enough standardization in gear retraction/extension
systems to say what's "normal". However, certainly the lack of redundancy
is common enough on light planes. In fact, not only is the gear on my
airplane designed similarly, the flaps and elevator trim use the same
hydraulic system. A failure in the hydraulic system that takes out
certain
lines, and/or results in a loss of fluid would affect all three systems
simultaneously.


I suppose you could also run the brakes off the same hydraulic system as the
gear, too. After all, if the gear fails you won't be needing any brakes. :-)

In article ,
"C J Campbell" wrote:


I suppose you could also run the brakes off the same hydraulic system as the
gear, too. After all, if the gear fails you won't be needing any brakes. :-)

The B-24 I used to fly is like that. Brakes, flaps, landing gear and
bombbay doors all on the same system. There is one engine driven pump
(#3 engine), an electric pump and a hand-pump. There are also 2
accumulators. If you have pressure in the accumulators you will have
one shot at the brakes...release the brakes and you release the
pressure. The copilot will be pumping like crazy on the handpump about
then. G The gear will freefall into position (the nose gear has to be
manually thrown out), and the flaps can be pumped down using the
handpump.

I only had one problem with the hydraulics. The main feed line from the
engine driven pump cracked at an elbow filling the bombbay with
hydraulic fluid...took less than a minute to pump all the fluid out
rendering us helpless. Luckily we had just landed and were taxiing to
parking when it failed. Mixtures to "cutoff" and coasted to a stop.
Had just enough time to say "WHEW!" before the airplane started rolling
backward due to a very slight grade on the taxiway. NOT a good feeling.
G The crewchief was scrambling trying to get out to throw himself
under the wheel as a chock when we came to a stop. If the crack had
opened just a minute or two before I probablyl would've parked the
airplane in the same gas station Southwest did a few years back.

--
Dale L. Falk

There is nothing - absolutely nothing - half so much worth doing
as simply messing around with airplanes.

http://home.gci.net/~sncdfalk/flying.html

"C J Campbell" wrote in message
...
I suppose you could also run the brakes off the same hydraulic system as
the
gear, too. After all, if the gear fails you won't be needing any brakes.
:-)

Well, actually...the brakes do use the same reservoir for their hydraulic
fluid. But all pressure for the brakes comes from the master brake
cylinders, not the hydraulic pump used for the other three systems. Only a
leak in the hydraulic line between the master and slave cylinders for the
brakes would cause any trouble with the brakes (by emptying out the brake
lines, along with the rest of the hydraulic system).

Again speaking only of light planes, I doubt there are any that use a
hydraulic pump to operate the brakes.

As for whether you'd need the brakes, I guess that depends on whether the
gear fails in the up position, or down.

Pete

"Andrew Gideon" wrote in message
online.com...
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I've been reading the POH for my club's 182RG, and I find myself
surprised.
The manual gear extension replies upon the same hydrolic pressure system
as
the powered mechanism.

Isn't that insufficiently redundant?

I'm not sure what I expected - perhaps something purely mechanical. But I
didn't expect a lone pressure system to be a single point of failure.

Is this normal?

The Cessna 172RG is similar.

Hydraulic pressure is used to hold the gear up. The pump runs every few
minutes to maintain hydraulic pressure. Unfortunately, if the pump or
another part of the hydraulic system fails, the gear will come down only
part way, streaming behind the airplane like a duck with broken legs.

The manual system is just another pump, only it is only capable of lowering
the gear. If you lose hydraulic pressure the manual system will provide
enough pressure to lower the gear and lock it in place if there is any fluid
in the system at all. You would have to spring a leak at the bottom of the
sump to lose all your hydraulic fluid.

If someone absolutely cannot get the gear down and locked it is nearly
always because of some fracture at the pivot point. No backup system would
salvage that -- the landing gear is physically broken.

I personally have seen the gear system work with no hydraulic fluid left in
the reservoir -- just a little bit left in the lines. The emergency
extension lever was not even needed.

The landing gear system is not all that critical anyway. If more redundancy
is required, the weight penalty is better applied to other systems.

"C J Campbell" wrote in message
...
Hydraulic pressure is used to hold the gear up. The pump runs every few
minutes to maintain hydraulic pressure.

Assuming the 172RG gear system is similar to the 182RG and 177RG systems, if
the pump is running on a regular basis, you have a leak somewhere. Not
necessarily one venting fluid, but one allowing fluid from the pressurized
side of the system to the non-pressurized side.

(The pressurized side changes, of course, depending on whether you're
raising or lowering the gear)

The manual system is just another pump, only it is only capable of
lowering
the gear. If you lose hydraulic pressure the manual system will provide
enough pressure to lower the gear and lock it in place if there is any
fluid
in the system at all. You would have to spring a leak at the bottom of the
sump to lose all your hydraulic fluid.

A leak on the pressurized side of the system would allow the hand-pump (or
the electric pump) to pump all the fluid out of the system. You don't need
to spring a leak at the bottom of the sump to lose all your fluid.

[...] The landing gear system is not all that critical anyway. If more
redundancy
is required, the weight penalty is better applied to other systems.

True without a doubt!

Pete

"Peter Duniho" wrote in message
...

A leak on the pressurized side of the system would allow the hand-pump (or
the electric pump) to pump all the fluid out of the system. You don't
need
to spring a leak at the bottom of the sump to lose all your fluid.


It actually does not pump it all out, as the intake for the pump is high
enough to leave a small reservoir to be used by the emergency extension
lever.

"C J Campbell" wrote in message
...
It actually does not pump it all out, as the intake for the pump is high
enough to leave a small reservoir to be used by the emergency extension
lever.

Fine. However, as soon as you start using the manual pump, you will still
wind up pumping the fluid out.

My main point is that it is not necessary to have a leak at the bottom of
the sump in order to lose all the hydraulic fluid.

Pete

"C J Campbell" wrote
The Cessna 172RG is similar.

All the Cessna retract singles have the same basic gear design. Only
difference is some of the earlier 210's had an engine driven hydraulic
pump.

Hydraulic pressure is used to hold the gear up. The pump runs every few
minutes to maintain hydraulic pressure.

If the pump is running every few minutes, there's a leak somewhere.

You would have to spring a leak at the bottom of the
sump to lose all your hydraulic fluid.

That's absolutely not correct. All you have to do is blow a high
pressure hose and you will pump all the hydraulic fluid overboard, and
completely lose any ability to lower the gear. Know more than one
person who has had it happen.

Michael

Not sure what you mean by "normal"...? It certainly is for
all the Cessna retractable singles - yours hasn't been singled
out for special treatment. As to whether it's a good idea, well
no, it sure doesn't seem so. But that's the way it is.

I've had to hand-pump mine once, when a switch in the
pump circuit failed. It's a nasty moment when the gear doesn't
go down, and a very pleasant feeling when that green light
comes on. Failure of a seal anywhere in the hydraulics means
you'll be using a lot of power to taxi off the runway. According
to Aviation Consumer it's rare (amongst failures) but not unknown.

John

"Andrew Gideon" wrote in message
online.com...
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I've been reading the POH for my club's 182RG, and I find myself
surprised.
The manual gear extension replies upon the same hydrolic pressure system
as
the powered mechanism.

Isn't that insufficiently redundant?

I'm not sure what I expected - perhaps something purely mechanical. But I
didn't expect a lone pressure system to be a single point of failure.

Is this normal?

- Andrew

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If by "normal" you mean "does everyone else do it this way", then no it's
not normal. Piper arrows have an override which releases hydraulic pressure
so that the gear can free fall into position. Older moonies have a "johnson
bar" which is a purely manual system. Yet other planes have an emergency
tank for blowing the gear down (beech maybe?).

For the 182RG, I believe the hydraulic system provides for "up pressure"
meaning that if you spring a leak the gear should drop. So if only the pump
fails, out comes the handle, otherwise the gear are coming down anyway. And
now for the bad news: because the main gear fold backward into the fuselage,
they likely won't drop all the way on a hydraulic failure. There are
various anecdotes about pilots reaching out the door with the towbar to pull
the gear all the way down.

Folding legs on the high-wing Cessnas have always been a bit of a black eye,
usually due to maintenance issues.

cheers,
mark

"Andrew Gideon" wrote in message
online.com...
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I've been reading the POH for my club's 182RG, and I find myself
surprised.
The manual gear extension replies upon the same hydrolic pressure system
as
the powered mechanism.

Isn't that insufficiently redundant?

I'm not sure what I expected - perhaps something purely mechanical. But I
didn't expect a lone pressure system to be a single point of failure.

Is this normal?

- Andrew

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