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 case? Flow adjustments- I don't know enough yet to even ask intelligent questions. 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 operated 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 minutes under normal situations (several factors/conditions can reduce this significantly), but reduces rapidly to seconds as you go higher. Even considering 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 rapid de-compression is a traumatic/attention -getting event. The most common scenario for IV-P's is loss of a window or part of the door, where the very rapid loss of pressure will cause you to forcibly exhale, significantly reducing your TUC...above FL 250, this time quickly reduces to seconds. Having 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 window or door up there.

I mounted my Mountain High (emergency) O2 bottle to the front of the main spar box, basically under the front edge of the pilot's seat. Wherever you mount the bottle, it needs to be in a place you can turn on the main tank valve easily from your pilot seat when needed, AND can check the pressure/quantity during pre-flight. You'll need a mask that you can slip over your mouth/nose. You'll use this for emergencies only, so a small bottle that supplies 20 minutes of O2 for you and your passengers is more than sufficient. Any 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 will really press you to buy a more elaborate system. I installed the very simple/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 connected and flow O2 as soon as the main valve is turned on. The back masks are not normally connected and have shut-off valves in the connectors; I connect 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'. You and your passengers need to practice donning/activating this equipment with the same commitment as you practice engine-out landings.  The need for the 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 during a rapid decompression.

I'd recommend a cabin pressure sensor connected to a warning light/annunciator or similar instrumentation. Mine is connected to my Electronics International 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 that's wired in series with my boost pump so it turns a warning light on if the airplane is above 10,000' and the boost pump is not on. It also lights below 10,000' if the boost pump is on... This system is my personal preference...but handy.

In the past five years, I've lost cabin pressure twice while at altitude. Both were caused by door seal failures, once the seal itself failed, and once the pump motor failed. Both resulted in fairly rapid loss of cabin pressure, 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 altitude were "routine" and caused nothing but a bit of anxiety in my co-pilot/wife. One last caution. If you're using the big Continental, its turbochargers will 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 rapidly dump the cabin pressure back through the engine. I considered installing 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 bit (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 out, depending on altitude and winds. This takes some getting used to, and speed brakes can help a little. I have them, but almost never use them; it seems 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 to 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 pressurized 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 remote mounting of the bottle, but in any case, do be sure to check the O2 supply 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 important as the AOA system and accurate fuel indicators. All of these are available in relatively inexpensive and reliable instrumentation these days.

Write or call if questions/issues.

Bob Pastusek