A couple answers from my experience (we all have slightly
different styles):
The altitude adjustment knob has both cabin altitude and outside
(ambient) altitude on the clear dial (inner and outer ring, adjacent
to each other, incremental) with a 5.5psi difference in the markings.
Setting the dial to field elevation (outer numbers) is equivalent to a
certain ambient altitude (inner numbers) when 5.5psi difference is
obtained (ex, FL230 outside ambient, 8000' inside cabin).
Being mostly on the east coast (local field 370' MSL), I set my
dial to ~500 ft above my field elevation and never touch it again.
Even if I land at an elevation much higher (example, Smokey Mountains
or Rocky Mountains or Cascade Mountains), and even if I hop there with
a fuel stop in between, the gauge is not adjusted. Once you are
on final at your higher elevation airport and power has been reduced
to something less than 15", the cabin will not maintain any
pressure differential with the outside, so the setting on the gauge is
no longer controlling (Duke's is fully closed). Essential, any
turbo PSI previously given to the cabin will vent via porous outlets
and even back flow into the engine's upper deck when MAP is reduced
below a fairly high threshold.
When taking off from a high altitude airport, but the gauge is
still on the lower field elevation setting (missed that check list
item), the turbo air will pressurize the cabin and the Duke's valve
will try to pressurize the cabin to the knob setting, giving you some
ear pressurization during your high power take off roll, and your
cabin altimeter will descend. The spouse will not be
happy.
The cabin dump switch is (in my opinion) not worth having on the
panel; if you have smoke in the cabin, the smoke and heat (and CO)
will vent past your nose/face as it moves towards the rear seat.
One only needs to hit the door seal switch, deflating the seal, to
effectively vent everything out laterally before it reaches you,
probably also improving your visibility by keeping the ventilation low
and out of your field of view. The air driven switch on the door
is an excellent example of non-electrically dependent control of a
critical system (I worry when some want an electric toggle, not the
pneumatic switch).
And the auto-dump squat switch off the gear down position is also
not sensible, a point of pressurization failure due to squat
malfunction that adds nothing to the functionality of the system,
since this cabin is never pressurized above ambient prior to gear down
in the pattern.
The rate knob is the smaller unlabled (white dot only) knurled
knob adjacent to the larger pressure dial. Rotated fully
counter-clockwise is slowest cabin rate, and rotated fully clockwise
is the fastest cabin rate change setting.
Hi Pete,
Thanks for your detailed email. I have a couple questions...
1) which is the rate knob? The only knob with any description on it is
the one where I set the altitude. There is another knob beside it,
however it doesn't have a label on it (see attached photo).
2) If I am FL200 and my destination airport is still 150nm away... at
what point would I turn the pressurization to the field elevation? As
soon as I start my decent?
Thanks,
-dr
lml@lancaironline.net
On Tue, Sep 6, 2011 at 1:21 PM, pete@leapfrogventures.com
<pete@leapfrogventures.com>
wrote:
The rate knob sets how fast the cabin pressure will change
after moving the dial. If you are in test mode, turn the rate fully
clockwise so the rate will change more noticeably.
The dukes should be set such that it pressurizes the cabin
to about 700 feet above the field elevation on the dial, until it
reaches 5.5 psi, at which point the cabin altitude will rise in lock
step with increasing airplane altitude.
I fly out of a sea level airport. I typically put the dial
at 3000 feet airport elevation. This gives me a 3700 foot minimum
cabin pressure. I do this so that, when on a precision approach, which
typically start at about 3000 feet, my pulling the power to capture
glideslope will not pop the ears of the plane occupants due to a loss
of residual pressurization. If I am flying to a high altitude airport,
about half way through the flight I will reset the dial to the
destination altitude plus 3000 feet. If I know for sure I am not going
to be doing a precision approach and can descend to pattern altitude
with at least 50% power, then I will only add 1000 feet.
I set the rate knob at the 500 feet/minute rate and leave
it there unless I am doing testing. You can determine the proper
position on your unit during testing by changing the dial and timing
how long it takes to change the cabin altitude 500 feet, then
loosening and repositioning the know so that, in the 500 foot
position, the work RATE is lined up horizontally.
If I am cruising in the flight levels where 5.5 psi will
not maintain a 3700 foot cabin, I will set the dial at 2000 feet BELOW
the flight level I will be cruising at. This setting gives me a 5.5
psi cabin every time.
I always do my descents at 65-70% power so as to have
enough boost to maintain the 5.5 psi differential.
Note these comments assume the brass plunger on the dukes
has been adjusted to the correct maximum pressure differential (mine
is set to about 5.6-5.8 psi - this can be done while flying high -
note it is very sensitive) and your dial and pressure head are
properly matched (you can correct this yourself, but it takes a few
flights as it can only be changed on the ground), as the connection
between the two is not based on actual values but rather pre-set
positions.
Pete
--
Regards,
Dico Reijers
InternetWorks Ltd.
300 University Avenue
Charlottetown
PE, C1A 4M4
902-892-4671 (T)
888-368-9484 (F)
www.internetworks.ca
www.apartmentspei.com
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