An Existential Threat…

One of my good customers, Dave Boyle, had an unusual occurrence recently, and I thought it might help the PA46 community to understand the situation.  Dave is an excellent pilot, is the self-assigned “maintenance manager” in the 3-person partnership that owns a very nice 1987 Malibu, and is a good friend to our PA46 Community.  Here’s his story…

I’m a relatively new Malibu owner/pilot.  Recently, while preparing for a long cross country at the flight levels, I decided to practice the actions I would need to perform in a loss-of-pressurization emergency.  My partners and I have the pilot’s emergency oxygen bottle mounted on top of the cabinet behind the co-pilot’s seat, and the mask and hoses are also stowed there in a clear plastic bag, all readily accessible to the pilot.  In the hangar, sitting in the pilot’s seat, I began running through the procedures as realistically as I could.  Step one: don the O2 mask and turn on the oxygen flow.  I reached back, grabbed the plastic bag, extracted and donned the mask, and fully opened the valve on the O2 bottle.  With the all-important mask in place, I calmly read through and simulated the relevant checklist items.  Great job, Dave!  I took one more well earned sniff of that delicious 100-percent oxygen, reached over and shut off the oxygen flow valve, and began to gather up the O2 delivery hardware to restore the system to readiness.  Uh-oh…the tygon tubing which I had assumed was connecting my mask to the oxygen source was just lying there on the cabin floor connected to…nothing!  Here’s what had happened:

The clear plastic tygon tubing on typical Malibu/Mirage emergency oxygen systems runs from the O2 bottle, through a flow meter/verifier, and then to the face mask.  The flow meter/verifier can be either a rotameter (i.e., a floating plastic “BB” that rises in a vertically held tube as the O2 flow rate increases) or a “turbine wheel” that spins to give a visual indication when O2 is flowing.  We have the turbine wheel option shown in the picture below.

During my “test” of the system, the tubing had slipped off one of its connection points (designated by maroon arrow in picture).  If this had been a real in-flight emergency, you might argue that I should have looked initially at the turbine wheel to confirm oxygen was flowing.  And you would be correct.  However, in this case, the “failed” connection and related tubing were lying so far back on the cabin floor, that I would have had to undo my safety belt and climb back into the cabin to fix the problem.  This step is not something I want to perform during an in-flight depressurization emergency.  You probably don’t either.

The tygon tubing is held onto the short plastic nipples on the turbine wheel by the elasticity of the tubing.  You must exert a significant force to push the tubing onto the nipple whose outer diameter is larger than the tubing inside diameter.  This action stretches the tubing radially, and the tubing’s natural elasticity provides for a strong radial holding force.  This approach works well until time and possibly sun-heated cabin temperatures slowly erode the elasticity of the stretched tubing on the nipple.  In my case I could see that the short section of tubing which had been on the turbine wheel nipple for many years now maintained a much larger inside diameter than the rest of the tubing.  To solve this problem, all I had to do was snip off the quarter inch or so of “hardened,” no-longer-elastic tubing and force the end of the remaining “fresh” tubing over the nipple.

I then got back in the pilot’s seat and repeated the procedure to ensure everything was working properly.  Guess what?  When I pulled the tygon tubing to bring the turbine wheel into view to ensure oxygen was flowing to the mask, the connection on the other side of the turbine wheel came off!  It came off for exactly the same reason the other side had failed, long term loss of tygon tubing elasticity.  And, again, to repair this problem required leaving the pilot’s seat and moving into the cabin to get hold of all the loose ends.  Had this been a real in-flight emergency, things would probably not have ended well

The Message:  This experience convinced me that those of us who fly pressurized airplanes need to check these press-on connections on a regular basis (every six months?).  A firm tug on the tubing that stresses the connections will do the job.  If something pops off, it’s an easy fix with a pocket knife to slice off the quarter inch of “non-elastic” tubing and then push the freshly cut end of the tubing back on the nipple.  A firm tug on this new connection will give you a good feeling.  Moreover, it may save your life and the lives of your passengers in the future.

Dave Boyle
Commercial Pilot ASEL/AMEL, Instrument
Chemical and Nuclear Engineer, Texas A&M University (Retired)
US Air Force (Retired)

About Joe Casey

ATP, CFI, CFII, MEI, CFI-G Commercial Pilot - SE, ME, Rotorcraft, Glider US Army AH-64 Pilot and UH-60 Instructor Pilot and Instrument Flight Examiner
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