Systems Knowledge is VIP

Why You Need to Know Your Aircraft Systems.

Providing initial and recurrent training (or any transition training) requires the instructor to decide what items to emphasize. Because time is always limited an instructor cannot always cover every system in detail down to every screw, wire, resistor, solenoid, etc.  Not to mention the fact that going too far into detail and the student will get the glazed over look and be off in their daydreams cruising along in their new airplane.   As an instructor the decision has to be made as to what items to cover in more depth.  This is where the experience of the instructor can really aid the learner in knowing what systems are more troublesome for a particular airframe As an instructor I have been on both sides of the training table. (In addition to being an ATP/CFII I am also an A&P Mechanic).

Occasionally when instructing I am asked the question, “If there is nothing I can do about this system why do I need to know about it?”  While that is a perfectly valid question the truth of the matter is that you never know when systems knowledge will benefit a pilot.  The benefit of knowing a system will expose itself when dealing with either an in flight problem or when assisting a mechanic in dealing with maintenance problem.  Being able to accurately describe a problem and lead the mechanic in the right direction can reduce downtime and costs.  The problem is that you need to be able to speak the language.  Clients have told me, “I just tell the mechanic that it is broke.”  While that might seem an acceptable answer, you can save yourself aircraft downtime and money by accurately describing how the system is not functioning properly or perhaps you can streamline the troubleshooting process.

Let me give you an example from my own experience. I had flown to an area that had received about 4-5 inches of snow.  The airport was in the Southeastern U.S.  and not accustomed to getting snow in the winter and therefore did not have snow removal equipment.  The snow was melting quickly and the runway was 90% clear except for some patches of slush on the runway.  We elected to depart and had fairly normal take off except that we ran through some of those patches of slush.  I believe that slush was thrown up into the nose gear well by the nose wheel.  After retracting the gear we began our climb up t0 10,000 ft our initial assigned altitude.  I began working through the climb checklist stored in the Garmin 430.  When I reached the pressurization check item I noticed that the cabin was climbing at 1,000 fpm, and that I still had no pressure differential reading on the gauge.  At this point I began normal checks to make sure that controls were in the correct position for the aircraft to pressurize.  I checked the Cabin Pressurization Knob, Cabin Dump Valve Switch, Pressurization Controller Setting, and the Cabin Rate Knob on the controller.  All of these controls were positioned in the proper position.  The Cabin Press Knob was all the way in, Dump Switch was in the normal position, and the controller was set for a cruise altitude of FL190.  This is where I began to process my systems knowledge to think of why the aircraft would not pressurize.  This is when the proverbial light bulb went on.  I started thinking about taking off and going through the slush.  It was possible that the squat switch had frozen in position due to some slush getting onto it and since the temps was hovering right a freezing a little cold air mixing with slush/water and the switch could be frozen in place.

The squat switch for the PA46 Malibu is located on the left main gear.   The reason that the squat switch is tied to the pressurization system is to prevent the aircraft from landing pressurized.  In the event that the pilot has improperly set the pressurization controls (another article for a later date) the squat switch will be activated  upon landing which will power to the dump solenoid.  When the dump solenoid is activated aircraft system vacuum will open the safety valve which will immediately dump the cabin pressure.  This is basically the same procedure that happens when the cabin dump valve switch is activated, except the ground for the system comes through the dump switch rather than the squat switch.

In my case since the squat switch was frozen closed the safety valve was still wide open, and therefore I could not pressurize.  Not being able to pressurize on this flight was a problem, as I needed to climb about 15,000 ft. to be able to get above the icing conditions, not to mention at FL190 I have a generous tailwind which factored into my fuel planning.  So I began thinking through the system.  One thing that did not occur to me until much later is why I had to move the little lever by the gear handle to get it to retract.  On the Gar Kenyon Malibu’s ( pre late 86 models previous to the installation of the Parker Hannifin system with electric flaps) there is a gear handle safety solenoid which helps prevent the inadvertent retraction of the gear when the squat switch is in the ground position.  On the Malibu Mirage the sytemsis different, so the gear would not have retracted at all. The reason was that the squat switch was frozen in the ground position, and this is exactly the same reason I could not pressurize.

Now I was at a crossroads, if I could not pressurize I really did not feel comfortable continuing on the flight.  The thought dawned on me (I must admit that the schematic was not popping up in my mind as an indelible picture) what would happen if I took the squat switch out of the loop. Well only one way to find out exactly what would happen.  I pulled the gear warning circuit breaker and of course the safety valve closed and the cabin began to pressurize (albeit at an uncomfortable rate).  So now I had pressurized the airplane and I could continue.  Two things HAD to happen before landing. I had to depressurize the cabin, and also the gear warning circuit breaker needed to be reset so that I actually had a gear warning system.  Since I was cruising at FL190 that would give me a cabin altitude of 4,000 ft I would leave the pressurization controller set where it was which would allow the airplane to be completely depressurized slightly above 4,000 feet MSL.  Upon reaching 0 PSID on the pressurization gauge I simply reset the gear warning circuit breaker and everything was back to normal.

The point of this whole story was to say that had I not had a fairly good idea of how the system worked (gear warning/pressurization) I would have probably had to abort the flight and spend time on the ground waiting to resolve the simple problem. It was my systems knowledge that allowed me to safely complete the flight.  The knowledge of aircraft systems can and will assist you in being a better pilot and manager of the maintenance of your aircraft.



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