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From: Robert R Pastusek <rpastusek@htii.com>
Subject: RE: [LML] Re: LML roots
Thread-Topic: [LML] Re: LML roots
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Bill Wade wrote:

I agree. I've never used an oxygen system before but would like to install =
an emergency system in the IV-P I'm building. For starters, if there was a =
rapid decompression at altitude how quickly can a system be activated? Mask=
 or cannula- should they be worn around your neck and plugged in just in ca=
se? Flow adjustments- I don't know enough yet to even ask intelligent quest=
ions. Thanks- Bill Wade


Bill,

For the IV-P, I'd recommend that you test/certify the aircraft to no higher=
 than FL 250. The airplane will climb and fly higher, especially if lightly=
 loaded, but the FARs require annual equipment and aircrew checks if it's o=
perated above FL250. The other reason to make this your operational ceiling=
 is that the "time of useful consciousness (TUC) at FL 250 is a couple of m=
inutes under normal situations (several factors/conditions can reduce this =
significantly), but reduces rapidly to seconds as you go higher. Even consi=
dering the reduced TUC from illness, rapid decompression--loss of a window-=
-etc), you have time to get the masks on if you do this as the first action=
 after a de-compression/loss of cabin pressure at FL 250. Above FL 250, a r=
apid de-compression is a traumatic/attention -getting event. The most commo=
n scenario for IV-P's is loss of a window or part of the door, where the ve=
ry rapid loss of pressure will cause you to forcibly exhale, significantly =
reducing your TUC...above FL 250, this time quickly reduces to seconds. Hav=
ing experienced the loss of a canopy in an F-4 many years ago at FL 280, I'=
m pretty sure my wife would not get back in the airplane if we blew a windo=
w or door up there.

I mounted my Mountain High (emergency) O2 bottle to the front of the main s=
par box, basically under the front edge of the pilot's seat. Wherever you m=
ount the bottle, it needs to be in a place you can turn on the main tank va=
lve easily from your pilot seat when needed, AND can check the pressure/qua=
ntity during pre-flight. You'll need a mask that you can slip over your mou=
th/nose. You'll use this for emergencies only, so a small bottle that suppl=
ies 20 minutes of O2 for you and your passengers is more than sufficient. A=
ny of the O2 system suppliers can help you with sizing the pressure bottle =
to your needs--be sure they know you have a pressurized cockpit or they wil=
l really press you to buy a more elaborate system. I installed the very sim=
ple/basic O2 masks from Mountain High in pockets in the sidewall beside the=
 pilot/copilot and each back seat passenger. The front masks are always con=
nected and flow O2 as soon as the main valve is turned on. The back masks a=
re not normally connected and have shut-off valves in the connectors; I con=
nect them only if taking passengers above 15,000--which is  not normal for =
me. All masks are very quick and easy to access. Use should be part of your=
 crew/passenger pre-flight briefing if you're taking them above 15,000'. Yo=
u and your passengers need to practice donning/activating this equipment wi=
th the same commitment as you practice engine-out landings.  The need for t=
he emergency O2 system is so infrequent that without practice and conscious=
 thought about it, you won't do the right things, in the right sequence dur=
ing a rapid decompression.

I'd recommend a cabin pressure sensor connected to a warning light/annuncia=
tor or similar instrumentation. Mine is connected to my Electronics Interna=
tional MVP-50 engine/system monitor, and is set to alarm any time the cabin=
 pressure goes above 10,000'. It's performed flawlessly for 5+ years now. (=
I also have an identical sensor connected to an outside pressure source tha=
t's wired in series with my boost pump so it turns a warning light on if th=
e airplane is above 10,000' and the boost pump is not on. It also lights be=
low 10,000' if the boost pump is on... This system is my personal preferenc=
e...but handy.

In the past five years, I've lost cabin pressure twice while at altitude. B=
oth were caused by door seal failures, once the seal itself failed, and onc=
e the pump motor failed. Both resulted in fairly rapid loss of cabin pressu=
re, but not "explosive decompression" as will occur with loss of a window. =
Turning on the O2 and donning masks, then asking for a descent to a lower a=
ltitude were "routine" and caused nothing but a bit of anxiety in my co-pil=
ot/wife.=20

One last caution. If you're using the big Continental, its turbochargers wi=
ll maintain cabin pressure at/above about 25" of manifold pressure. If you =
pull the power back below 25" to descend from the flight levels, you'll rap=
idly dump the cabin pressure back through the engine. I considered installi=
ng an "anti-backflow" valve in the system, but then realized that it would =
only slow the loss of cabin pressure a little, as the cabin leaks a good bi=
t (they all do) and need a pretty steady flow of compressed air to maintain=
 pressurization. So I abandoned this idea, and am careful with the throttle=
 on descents...which is a good engine management practice anyway. The other=
 result of this operating limitation is that you'll need to start down from=
 the flight levels long before you can see your destination...60-90 miles o=
ut, depending on altitude and winds. This takes some getting used to, and s=
peed brakes can help a little. I have them, but almost never use them; it s=
eems such a waste to burn up the energy/fuel you spend climbing to altitude=
 in aero drag coming back down... My standard/much preferred procedure is t=
o plan a descent well out from destination...you usually have to ask ATC to=
 start down.

In summary, you will need only a basic emergency O2 system in your pressuri=
zed cockpit. A continuous-flow system with the most basic masks is adequate=
; and don't waste money on pulse flow regulators. I'd also stay away from r=
emote mounting of the bottle, but in any case, do be sure to check the O2 s=
upply on pre-flight if you plan to go up into the flight levels. I consider=
 the cabin pressure warning system a critical part of my airplane; as impor=
tant as the AOA system and accurate fuel indicators. All of these are avail=
able in relatively inexpensive and reliable instrumentation these days.

Write or call if questions/issues.

Bob Pastusek