Backup for dropping the gear

"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

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altitudes," relates one owner, "the gear sometimes drops after it has been
retracted.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

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

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" 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

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

"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

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

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)

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