Backup for dropping the gear

[Andrew Gideon]

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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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[Paul Tomblin]

In a previous article, Andrew Gideon > said:
>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?

The P32R Lance that I'm currently checking out in uses hydraulic pressure
to hold the gear *up* against springs and gravity - lose the hydraulic
pressure and the gear goes down. As a matter of fact, sometimes in
turbulence you get a gear unsafe light and you have to quickly cycle the
gear lever to repressurize the hydraulics.


--
Paul Tomblin > http://xcski.com/blogs/pt/
"Maybe if your vcr is still blinking 12:00 you shouldn't be using Linux."
-- Slashdot poster

[H. Adam Stevens]

Piper gear PIREP

The nose gear on the Seminole and many other similar Pipers has a coaxial
pair of springs.
In my case the inner spring failed, jamming the outer spring and resulting
in a nose idiot light not coming on.
Expecting a collapse, I landed the Seminole like a tail dragger and walked
away.

The springs are a few dollars. Replace them periodically.
My Seminole was on leaseback,
I was scheduled between two renters.
Better me than them.

Old pilots have paid attention to detail.
So said one.

Blue skies.
H.



"Paul Tomblin" > wrote in message
...
> In a previous article, Andrew Gideon > said:
> >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?
>
> The P32R Lance that I'm currently checking out in uses hydraulic pressure
> to hold the gear *up* against springs and gravity - lose the hydraulic
> pressure and the gear goes down. As a matter of fact, sometimes in
> turbulence you get a gear unsafe light and you have to quickly cycle the
> gear lever to repressurize the hydraulics.
>
>
> --
> Paul Tomblin > http://xcski.com/blogs/pt/
> "Maybe if your vcr is still blinking 12:00 you shouldn't be using Linux."
> -- Slashdot poster

[Peter Duniho]

"Andrew Gideon" > wrote in message
online.com...
> 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.

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.

Occasionally you might find a "fail safe" system like the one on the Lance
that Paul mentions, but as he even points out, those systems come with their
own issues. Aircraft designers often come to the conclusion that the extra
complexity, cost, and weight isn't worth the marginal increase in safety.

Especially when one considers just how dangerous a gear-up landing *isn't*,
it's not hard to see why that conclusion is reached so often. As far as I
know, such "insufficiently redundant" systems are more common than
"sufficiently redundant" ones.

Pete

[C J Campbell]

"Andrew Gideon" > wrote in message
online.com...
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> 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]

"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. :-)

[John Harper]

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...
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> 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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[Mark Astley]

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...
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> 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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[Dale]

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

[d b]

There are a lot of things that are not redundant.

The lesson to learn is to know the systems of your plane very, very, well.
This is not an easy task, nor is it common between planes.

You should know all the systems, and the what-if scenarios that go with
them. This includes the mechanical systems. Like how the cables and
pushrods are run through the plane.

For example: What if you pump down the gear but do not get a green
light? Do you cycle the gear? Answer - it depends. You need to give
thought to the possibility that cycling the gear may lose what little
hydraulic fluid you have left. When the hydraulic system runs more
than one thing, you need to worry about what else you might lose.
There is no one pat answer that fits all planes. Reading the emergency
procedures is great - except the emergency procedures have difficulty
determining which combination of failures has happened. You really
need to understand the systems.




In article e.com>, Andrew
Gideon > wrote:
>-----BEGIN PGP SIGNED MESSAGE-----
>Hash: SHA1
>
>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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[Jim]

The hand pump gives redundancy to the hydrolic system only in the event that
the electric power pack fails. In the event of an electrical failure, the
hydrolic system itself would still be intact, but the hand pump is a one way
pump, only allowing you to pump the gear down, not up. The 182RG's gear
system is a closed hydrolic system, as it only operates the gear. To have a
complete hydrolic system (gear) failure such as a blown hydrolic line or
fitting causing the system to loose it's entire quantity of hydraulic fluid
would be quite rare I would think. I'd much rather have a backup system to
an electrical system malfunction than have a back up to an hydrolic system.

Jim

[Kyler Laird]

(Paul Tomblin) writes:

>The P32R Lance that I'm currently checking out in uses hydraulic pressure
>to hold the gear *up* against springs and gravity - lose the hydraulic
>pressure and the gear goes down.

Cool! So if you lose your engine the plane helps you get back to the
ground quickly by extending the air brakes? Is there an override (for
those who fly over inhospitable terrain/water)?

Is there a placard that says "Do not lose engine power in excess of max.
gear extension speed."?

--kyler

[Peter Duniho]

"Kyler Laird" > wrote in message
...
> (Paul Tomblin) writes:
>
> >The P32R Lance that I'm currently checking out in uses hydraulic pressure
> >to hold the gear *up* against springs and gravity - lose the hydraulic
> >pressure and the gear goes down.
>
> Cool! So if you lose your engine the plane helps you get back to the
> ground quickly by extending the air brakes? Is there an override (for
> those who fly over inhospitable terrain/water)?

The hydraulic pressure is probably provided by an electric pump (I don't
know for sure, not being familiar with that specific airplane).
Furthermore, loss of pressure should only happen if there's a leak in the
system somewhere; theoretically, once the system has pressurized with the
gear up, the hydraulic pump wouldn't need to operate at all. Even if there
was a slightly leaky o-ring or valve somewhere, requiring the occasional
operation of the pump, it seems likely that the battery could handle the
load for the brief period of time it would take to glide to a landing.

Bottom line: there's no reason an engine failure would cause the gear to
extend.

> Is there a placard that says "Do not lose engine power in excess of max.
> gear extension speed."?

There would be no need for such a placard.

Pete

[Peter Duniho]

"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

[Peter Duniho]

"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

[john smith]

Mark Astley wrote:
> Yet other planes have an emergency
> tank for blowing the gear down (beech maybe?).

Bonanza's: 50 turns of the little crank, located behind the little door
on the back of the main spar below the copilot's seat. The crank is
attached to a gear that meshes with another gear on the shaft attached
to the gear motor and landing gear.

[C J Campbell]

"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.

[Peter Duniho]

"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

[John Galban]

"Mark Astley" > wrote in message >...
<snip>
> There are
> various anecdotes about pilots reaching out the door with the towbar to pull
> the gear all the way down.
>

I read one in a flying mag several years ago. Under the direction
of a ground based A&P, the passnger accessed the empty hydraulic
reservoir from inside the cabin and replenished it with a biologically
manufactured fluid. It was enough to get the gear locked and they
landed safely.

I'll bet the A&P that had to work on that system was ****ed :-)

John Galban=====>N4BQ (PA28-180)

[Michael]

"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

[Buff5200]

All this discussion of alternate landing gear extension methods, and not one
person mentioned Fred Flinstone's landing gear.

[Rick Durden]

Peter,

You have got to expand your horizons <g>. In a number of the Piper
singles the gear does come down if the engine takes the day off. The
automatic extension system, if not disconnected or overridden, will
extend the gear if the engine quits. It's why I override the system
on takeoff as I have no burning desire for the gear to extend should
the engine quit.

It's also a fun system when you are descending at high speed through a
layer of stratus clouds full of rime ice and the pitot heat decides
that it doesn't want to heat the little pitot tube on the left side of
the fuselage that provides the input for the automatic gear system, it
freezes up and extends the gear suddenly when you are whistling along
in the yellow arc. Doesn't do anything to improve your outlook on
life at all.

All the best,
Rick

"Peter Duniho" > wrote in message >...
> "Kyler Laird" > wrote in message
> ...
> > (Paul Tomblin) writes:
> >
> > >The P32R Lance that I'm currently checking out in uses hydraulic pressure
> > >to hold the gear *up* against springs and gravity - lose the hydraulic
> > >pressure and the gear goes down.
> >
> > Cool! So if you lose your engine the plane helps you get back to the
> > ground quickly by extending the air brakes? Is there an override (for
> > those who fly over inhospitable terrain/water)?
>
> The hydraulic pressure is probably provided by an electric pump (I don't
> know for sure, not being familiar with that specific airplane).
> Furthermore, loss of pressure should only happen if there's a leak in the
> system somewhere; theoretically, once the system has pressurized with the
> gear up, the hydraulic pump wouldn't need to operate at all. Even if there
> was a slightly leaky o-ring or valve somewhere, requiring the occasional
> operation of the pump, it seems likely that the battery could handle the
> load for the brief period of time it would take to glide to a landing.
>
> Bottom line: there's no reason an engine failure would cause the gear to
> extend.
>
> > Is there a placard that says "Do not lose engine power in excess of max.
> > gear extension speed."?
>
> There would be no need for such a placard.
>
> Pete

[Peter Duniho]

"Rick Durden" > wrote in message
m...
> You have got to expand your horizons <g>.

That's always been true.

> In a number of the Piper
> singles the gear does come down if the engine takes the day off.

But not because of the failure of the hydraulic system. Kyler's comment
clearly was based on an assumption that without engine power, there would be
no hydraulic pressure, and that without pressure, the gear would drop (the
first assumption being the incorrect one, but the second is incorrect in
other airplanes as well).

And actually, while I haven't flown the Pipers you're referring to, my
understanding is that the gear extension was based on airspeed, not engine
power. Your comment about the behavior of such systems in icing conditions
would seem to reinforce this understanding. Perhaps you could elaborate on
why it is you say that engine failure alone will result in the gear
extending.

Or perhaps you meant that the drop in airspeed that normally results after
an engine failure (to achieve best glide) is enough to cause the gear to
extend. In which case I'd argue that it wasn't the engine failure, but the
airspeed change that caused the gear to extend (I'd also wonder why the
system was designed such that the activation airspeed was at or above best
glide...seems pretty non-optimal to me).

Pete

[Dennis O'Connor]

<snip>

Safety features sometimes spawn new hazards while eliminating old ones. The
automatic gear extension system is a good example. "At high density
altitudes," relates one owner, "the gear sometimes drops after it has been
retracted. This, of course, nullifies any climb!" Indeed, there have been
incidents in which the airplane might have been able to climb out safely had
the gear not dropped at the wrong moment, causing a stall/mush into the
terrain.

Then there are Arrow pilots who lose their engines and decide to ditch with
the gear up. Unfortunately, some forget to override the automatic extension
system. The gear plops out seconds before splash down-sending the Arrow head
over heels.

Such mishaps are rare-we only counted a few (none fatal) in our five-year
survey. But in mid-1987 Piper, then owned by Lear-Siegler, ordered the
system deactivated because of concern over liability suits. It sold kits to
do so, and told customers it wouldn't provide parts to repair the existing
system. Piper sold 1,400 kits.

One year later, Piper-then owned by M. Stuart Millar-withdrew its order to
deactivate the automatic extension system, provided that pilots "take the
necessary actions to assure that any pilot flying these aircraft are fully
advised of the system and its proper operation." In part, Piper was
responding to the complaints of irate owners who believed the system worked
often enough to be desirable.

<snip>

***************************************

Throttle coming back to a certain point actuates a microswitch and/or the
airspeed dropping to 95 knots drops the gear... There was an override, but
you had to remember to actuate it... Every Arrow that I was involved with
had the autoextension deactivated, period... Can't have the airplane killing
me, I can do a fine job of that all on my own...

denny

[Paul Tomblin]

In a previous article, "Peter Duniho" > said:
>And actually, while I haven't flown the Pipers you're referring to, my
>understanding is that the gear extension was based on airspeed, not engine
>power. Your comment about the behavior of such systems in icing conditions

The POH for the Lance isn't 100% clear on this, but my understanding is
that with the emergency gear extension, the gear will come down if you go
below a certain manifold pressure at one speed, or will come down
regardless of the manifold pressure at a lower speed.

The POH says "Some aircraft also incorporate a pressure sensing device in
the system which lowers the gear regardless of gear selector position,
depending upon airspeed and engine power (propellor slipstream). Gear
extension is designed to occur, even if the selector is in the up
position, at airspeeds below approximately 103 KIAS with power off. The
extension speeds will vary from approximately 91 kts to approximately 103
KIAS depending on power settings and altitude."

Actually, reading that again, it's possible that what is happening isn't
that it comes on at a lower speed if you have power on, but that the
propellor slipstream holds the gear up against the springs if you have
power on.

Sometimes I learn more answering other people's questions than I do
getting answers to my own.

--
Paul Tomblin > http://xcski.com/blogs/pt/
If you're not part of the solution, you're part of the precipitate.

[Paul Tomblin]

In a previous article, "Dennis O'Connor" > said:
>Then there are Arrow pilots who lose their engines and decide to ditch with
>the gear up. Unfortunately, some forget to override the automatic extension
>system. The gear plops out seconds before splash down-sending the Arrow head
>over heels.

First item in the emergency checklist is to put the gear override on.


--
Paul Tomblin > http://xcski.com/blogs/pt/
Microsoft - Where quality is job 1.0.1

[Paul Tomblin]

In a previous article, Kyler Laird > said:
>>> Is there a placard that says "Do not lose engine power in excess of max.
>>> gear extension speed."?
>
>>There would be no need for such a placard.
>
>Yeah, it looks like it would need to be "Do not leak hydraulic fluid in
>excess of max. gear extension speed."

On the Lance (which is the only plane I have any experience with), even if
you lost hydraulic fluid, air pressure would hold the gear up against the
springs until you dropped down to max gear extension speed. I suspect
you'd get a bit more drag from the gear drooping slightly into the
airstream, though.



--
Paul Tomblin > http://xcski.com/blogs/pt/
You can be jailed for lying about being good in bed.
-- Lionel, paraphrasing the Criminal Code of Canada, 159(3)(b)(i)

[Kyler Laird]

"Peter Duniho" > writes:

>> Cool! So if you lose your engine the plane helps you get back to the
>> ground quickly by extending the air brakes? Is there an override (for
>> those who fly over inhospitable terrain/water)?

>The hydraulic pressure is probably provided by an electric pump

Ah...that makes a difference.

>Furthermore, loss of pressure should only happen if there's a leak in the
>system somewhere;

I can buy that (although my gear commonly "droops" after many checks) with
the emphasis on "should". (On farm equipment, the typical culprit is the
controlling valve.)

>> Is there a placard that says "Do not lose engine power in excess of max.
>> gear extension speed."?

>There would be no need for such a placard.

Yeah, it looks like it would need to be "Do not leak hydraulic fluid in
excess of max. gear extension speed."

--kyler

[john smith]

Paul Tomblin wrote:
> On the Lance (which is the only plane I have any experience with), even if
> you lost hydraulic fluid, air pressure would hold the gear up against the
> springs until you dropped down to max gear extension speed. I suspect
> you'd get a bit more drag from the gear drooping slightly into the
> airstream, though.

Okay Paul, I have to ask you to explain those two statements.

[Paul Tomblin]

In a previous article, john smith > said:
>Paul Tomblin wrote:
>> On the Lance (which is the only plane I have any experience with), even if
>> you lost hydraulic fluid, air pressure would hold the gear up against the
>> springs until you dropped down to max gear extension speed. I suspect
>> you'd get a bit more drag from the gear drooping slightly into the
>> airstream, though.
>
>Okay Paul, I have to ask you to explain those two statements.

What don't you understand? In the Lance, the hydraulic system is there to
raise the gear. Besides manually lowering the gear, there is an automatic
gear extension system and a manual emergency gear extension system.

If you activate the manual emergency gear extension system, springs and
gravity bring the gear down - I'm not entirely clear if hydraulic pressure
helps bring them down if you use the normal gear extension. The
description of the emergency gear extension system in the POH says that
the speed the gear will come down is lower if the plane is developing
power because the prop slipstream holds it up.

I'm told (I haven't verified it yet) that if you activate the emergency
gear extension when you're above max gear speed, the gear won't come down,
but will come down slightly into the airstream developing drag.

--
Paul Tomblin > http://xcski.com/blogs/pt/
"The way I see it, unless we each conform, unless we obey orders, unless
we follow our leaders blindly, there is no possible way we can remain
free." - John Ashcroft^W^WFrank Burns

[Kyler Laird]

(Paul Tomblin) writes:

>>Yeah, it looks like it would need to be "Do not leak hydraulic fluid in
>>excess of max. gear extension speed."

>On the Lance (which is the only plane I have any experience with), even if
>you lost hydraulic fluid, air pressure would hold the gear up against the
>springs until you dropped down to max gear extension speed. I suspect
>you'd get a bit more drag from the gear drooping slightly into the
>airstream, though.

Now *that* makes sense. I caught that there was a sensor connected to
the pitot tube that interacted with the gear, but it's clever to use
airflow to keep it from dropping on its own too soon. Thank you for
explaining it.

(BTW, some of my confusion with hydraulics is because of my background
with farm machinery where the oil flows through the valve when "off" -
not because the explanations in this thread were misleading.)

--kyler

[Paul Tomblin]

In a previous article, "Peter Duniho" > said:
>"Paul Tomblin" > wrote in message
...
>> [...]
>> Actually, reading that again, it's possible that what is happening isn't
>> that it comes on at a lower speed if you have power on, but that the
>> propellor slipstream holds the gear up against the springs if you have
>> power on.
>
>I don't know how the "propeller slipstream" could hold the gear up. The
>only "propeller slipstream" I'm aware of is the helical movement of the air
>around the fuselage, and compared to the other forces involved I doubt it's
>all that strong.

The air coming off the propellor is going faster relative to you than your
air speed. If it wasn't, your propellor wouldn't be producing power.
It makes perfect sense to me. Being behind the propellor disk, the nose
gear (and possibly the mains) is experiencing more dynamic air pressure
when the engine is producing power than if you were gliding at the same
speed.


--
Paul Tomblin > http://xcski.com/blogs/pt/
Can I LART an aol'r for attempting to subscribe to a majordomo list
with their street address, or should I wait for a second offence?
-- Allan Stojanovic

[Peter Duniho]

"Paul Tomblin" > wrote in message
...
> The POH for the Lance isn't 100% clear on this, but my understanding is
> that with the emergency gear extension, the gear will come down if you go
> below a certain manifold pressure at one speed, or will come down
> regardless of the manifold pressure at a lower speed.

Well, if it's manifold pressure based, then an engine failure would only
cause the gear to come down if the failure was caused by a blocked air
intake, or if the pilot later pulled the throttle to idle. Again, an engine
failure would not, in and of itself, necessarily cause the gear to extend.

> [...]
> Actually, reading that again, it's possible that what is happening isn't
> that it comes on at a lower speed if you have power on, but that the
> propellor slipstream holds the gear up against the springs if you have
> power on.

I don't know how the "propeller slipstream" could hold the gear up. The
only "propeller slipstream" I'm aware of is the helical movement of the air
around the fuselage, and compared to the other forces involved I doubt it's
all that strong.

I think it's interesting that the POH puts "propeller slipstream" in
parentheses after "engine power", because to me it's not at all clear what
they mean. The two aren't exactly equivalent, so it's odd they would use
them as if they were. The parenthetical comment ought to add information,
but to me it just confuses things.

Pete

[Peter Duniho]

"Paul Tomblin" > wrote in message
...
> The air coming off the propellor is going faster relative to you than your
> air speed. If it wasn't, your propellor wouldn't be producing power.
> It makes perfect sense to me. Being behind the propellor disk, the nose
> gear (and possibly the mains) is experiencing more dynamic air pressure
> when the engine is producing power than if you were gliding at the same
> speed.

Hmmm...I suppose so. It still seems to me like the gear enjoys such a small
portion of the overall prop thrust, and that the difference in prop thrust
airspeed and airframe airspeed is greatly reduced at or near cruise
airspeeds. But I admit, I can't imagine what else the POH could be talking
about.

Still, it doesn't explain why an engine failure would in and of itself cause
the gear to extend. Presumably the hydraulic pressure is still holding the
gear up; the prop thrust would be a backup for that, not the sole mechanism
for holding the gear up. The extension mechanism still would need to be
triggered by airspeed or MP changes.

Right?

Pete

[Paul Tomblin]

In a previous article, "Peter Duniho" > said:
>Still, it doesn't explain why an engine failure would in and of itself cause
>the gear to extend. Presumably the hydraulic pressure is still holding the
>gear up; the prop thrust would be a backup for that, not the sole mechanism
>for holding the gear up. The extension mechanism still would need to be
>triggered by airspeed or MP changes.
>
>Right?

If the hydraulic system was still holding pressure, then I think you're
right.


--
Paul Tomblin > http://xcski.com/blogs/pt/
What happens if a big asteroid hits the Earth? Judging from realistic
simulations involving a sledge hammer and a common laboratory frog, we
can assume it will be pretty bad. -- Dave Barry

[john smith]

Paul Tomblin wrote:
> In a previous article, john smith > said:
>
>>Paul Tomblin wrote:
>>
>>>On the Lance (which is the only plane I have any experience with), even if
>>>you lost hydraulic fluid, air pressure would hold the gear up against the
>>>springs until you dropped down to max gear extension speed. I suspect
>>>you'd get a bit more drag from the gear drooping slightly into the
>>>airstream, though.
>>
>>Okay Paul, I have to ask you to explain those two statements.
>
>
> What don't you understand? In the Lance, the hydraulic system is there to
> raise the gear. Besides manually lowering the gear, there is an automatic
> gear extension system and a manual emergency gear extension system.
>
> If you activate the manual emergency gear extension system, springs and
> gravity bring the gear down - I'm not entirely clear if hydraulic pressure
> helps bring them down if you use the normal gear extension. The
> description of the emergency gear extension system in the POH says that
> the speed the gear will come down is lower if the plane is developing
> power because the prop slipstream holds it up.
>
> I'm told (I haven't verified it yet) that if you activate the emergency
> gear extension when you're above max gear speed, the gear won't come down,
> but will come down slightly into the airstream developing drag.

I do not believe the air flowing under the wings will hold the gear up.
If the hydraulic pressure is released, the gear will come down.
If you are flying fast, the air may grab the gear doors and possibly rip
them off or otherwise torque/twist the gear (whichever comes first).
Look up stories about the P-51B (or C?) and the redesign of the gear
uplock. Granted, you do not have the Mustang's cruise speed, but the
principle is the same if you exceed Vge/Vgo.

The override prevents the gear from automatically deploying.
You still have to push the lever to release the hydraulic pressure to
enable the gear to drop. The POH further says to kick the rudder right
and left to create a sideward airload on the gear to lock it into
position. In some cases, you must stall the aircraft to get the nosegear
to lock by penduluming forward as the nose drops and airload is decreased.

[[email protected]]

On Thu, 26 Feb 2004 12:34:18 +0000 (UTC),
(Paul Tomblin) wrote:

big snip

>Actually, reading that again, it's possible that what is happening isn't
>that it comes on at a lower speed if you have power on, but that the
>propellor slipstream holds the gear up against the springs if you have
>power on.

little snip

A crude "pitot" head (located within the propeller slipstream)
provides the combined airspeed/prop stream pressure input to the
"super high tech auto extension device"-which consists of a crude
diaphram balanced against a pair of ground adjustable springs attached
to a parking brake valve plumbed between the "up" and "down" hydraulic
lines.

The diaphram sees lower airspeed + higher slipstream the same as it
sees higher airspeed + lower slipstream. If the combined "sense"
pressure is higher than the spring pressure, the gear stay up. If the
sense pressure drops below the spring pressure, the mechanism trips,
and the park brake valve opens, allowing hydraulic fluid trapped in
the "up" line to flow into the "down" line. The mains fall out via
gravity, the nose does the same with a spring assist.

As others have mentioned, the trapped "up" pressure is what holds the
gear in the wells, there are no mechanical up-locks. There are
strictly mechanical spring-loaded down-locks that hold the drag/side
braces in an over-center position. With normal or emergency extension,
these mechanical downlocks are all that is holding the gear down,
there is no "down" pressure in the system with the gear
down-and-locked. The switches that turn the green lights on (and turn
the electro-hydraulic pump off) are driven directly off of these
mechanical down-locks.

Turning the auto extension "off" mechanically pins the trip linkage
into the park brake valve closed position, irrespective of "sensed"
diaphram pressure.

The SB to remove the auto extension feature removes the diaphram from
the loop, and the valve stays in the closed position during normal
operation. The lever that useta be used for auto extension, is now
used to mechanically open the valve to allow emergency extension if
desired.

The really, really neat part is adjusting the balance springs standing
on your head in the back of the airplane while the test pilot
repeatedly accelerates and decelerates (airspeed pressure), and varies
power setting (prop stream pressure) until you get the durn thing
adjusted so it works...

Hope some of this helps;

TC

P.S. the "smallest" hydraulic gear airplane that I am aware of with a
totally independent back-up emergency extension system is the Cheyenne
II XL. It has a secondary set of pneumatic actuators that un-up-lock
and assist/extend the gear pneumatically using 4 cute little CO2
bottles (Mr. Laird very likely has a similiar bottle)

[Paul Tomblin]

In a previous article, said:
>On Thu, 26 Feb 2004 12:34:18 +0000 (UTC),
>(Paul Tomblin) wrote:
>
>big snip
>
>>Actually, reading that again, it's possible that what is happening isn't
>>that it comes on at a lower speed if you have power on, but that the
>>propellor slipstream holds the gear up against the springs if you have
>>power on.
>
>little snip
>
>A crude "pitot" head (located within the propeller slipstream)

Oh duh. Of course. I forgot the pitot was in the slipstream as well.


--
Paul Tomblin > http://xcski.com/blogs/pt/
"It is my prayer that other Americans will fully realize that to condone the
whittling away of the rights of any one minority group is to pave the way for
us all to lose the guarantees of the Constitution" - Harold L. Ickes

[Peter Duniho]

> wrote in message
...
> [...]
> Hope some of this helps

Sure, thanks. Nice to hear the full description, so we can finally stop
guessing. :)

[Ron Natalie]

"Paul Tomblin" > wrote in message ...
> In a previous article, "Peter Duniho" > said:
> >And actually, while I haven't flown the Pipers you're referring to, my
> >understanding is that the gear extension was based on airspeed, not engine
> >power. Your comment about the behavior of such systems in icing conditions
>
> The POH for the Lance isn't 100% clear on this, but my understanding is
> that with the emergency gear extension, the gear will come down if you go
> below a certain manifold pressure at one speed, or will come down
> regardless of the manifold pressure at a lower speed.

The Arrow auto extend is simply a mechanical belows on the second pitot (just
behind the clear prop window) that dumps hydraulic pressure when you get slow.

[Julian Scarfe]

"d b" > wrote in message
ink.net...

> You should know all the systems, and the what-if scenarios that go with
> them. This includes the mechanical systems. Like how the cables and
> pushrods are run through the plane.

Reminds me of a story about the Aztec, on which I did my inital MEP
training.

The procedures go something like this:

To extend the gear, you move the gear selector lever to "down".

In case of failure of the engine-driven hydraulic pump:
1. move the gear selector lever to "down"
2. Extend handle of manual backup pump
3. Pump <mumble> times to extend gear
4. Stow handle

In case of failure of the hydraulic system
1. move the gear selector lever to "down"
2. actuate the emergency gear extension system
(AFAIR that releases something and the gear drops under gravity)

Not just one backup, but two! But I've only heard of one undercarriage
incident with the aircraft: As the pilot retracted the gear, the gear
selector lever came off in his hand, detaching at a point behind the
panel...

Julian Scarfe

[Kyler Laird]

"Julian Scarfe" > writes:

>In case of failure of the hydraulic system
>1. move the gear selector lever to "down"
>2. actuate the emergency gear extension system
>(AFAIR that releases something and the gear drops under gravity)

"actuate" here means "reach under the left seat, flip up the door, and
yank the ring(?)". It causes CO2 to be released from a small tank into
the hydraulic system, making a mess and eliminating further worries
about what to do with the gear or flaps. Gravity plays a minor role.

Realize also that the Aztec is somewhat unique in that it can land and
roll out on retracted gear. Sure, you'll be replacing lots of stuff as
a result but at least it'll roll instead of skid.

--kyler

[Julian Scarfe]

> "Julian Scarfe" > writes:
>
> >In case of failure of the hydraulic system
> >1. move the gear selector lever to "down"
> >2. actuate the emergency gear extension system
> >(AFAIR that releases something and the gear drops under gravity)

"Kyler Laird" > wrote in message
...
>
> "actuate" here means "reach under the left seat, flip up the door, and
> yank the ring(?)". It causes CO2 to be released from a small tank into
> the hydraulic system, making a mess and eliminating further worries
> about what to do with the gear or flaps. Gravity plays a minor role.

That's the one. It did sound like a desperation measure.

Julian