Langewiesche’s “Silver Chain”

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Langewiesche’s “Silver Chain”

I have been working my way through Stick and Rudder: An Explanation of the Art of Flying by Wolfgang Langewiesche and it has been a very interesting read.  There are multiple places where Langewiesche talks about making planes safer and easier to fly and he also talks about the idea of a “silver chain” that would keep an airplane from stalling.  The thought experiment is that if you put a physical chain on the stick it would never let the pilot pull back far enough to stall the airplane.  He debunks the idea because while it would provide a level of safety in some situations, the problem is too complex for this simple design to deliver a plane that wouldn’t stall.  I am too much of an airplane novice to understand all of advances in aircraft design that have occurred since the book was published, but I think a lot of the concepts discussed in the book have gone in to making general aviation airplanes easier to fly and more forgiving than the planes he was referencing in the book.

As I thought more about the safety chain concept I realized the envelope protection feature on our DFC90 and DFC100 autopilots is something that Langewiesche would be really happy about.  When our autopilot is engaged it acts like the silver chain that Langewiesche realized couldn’t exist as a purely mechanical system.  As Mark Krebs explained in his very technical blogs the Avidyne autopilot is constantly running calculations to determine the lift available in the wings of the airplane.  This works with any power setting or flap configuration and overcomes the mechanical challenges that hampered Langewiesche’s chain to keep pilots from stalling the airplane during training or in stressful situations.

Envelope protection as a tool during an engine out emergency.

During my primary training last year I spent a lot of time with my instructor practicing engine out emergencies.  I have been trying to take to heart the instruction to “fly the airplane” but I found that I barely had enough bandwidth to fly at Vg, pick a landing spot (no CAPS in a 172) and then get set up to put the airplane on that spot.  When I was in training mode I did get better at using the GPS to find the nearest airport and I effectively ran through the checklist and “communicated” my emergency.  Granted I am still a new pilot, but just flying the aircraft at Vg took most of my bandwidth.

One of the very interesting applications of envelope protection is managing an engine out situation.  While cruising the autopilot will most likely be in GPSS mode and holding an altitude.  If the engine quits and you do nothing the autopilot will lose airspeed as it tries to maintain altitude.  It will do this until the aircraft reaches 1.2Vs and then envelope protection will take over and sacrifice altitude for airspeed.  The pilot will have both visual and aural alerts while this is occurring.  You can then use the GPS to see if one of the nearest airports is within gliding range and if it is the autopilot can fly direct to the airport.  If an airport is not within gliding range then the pilot can use the heading mode for lateral guidance and focus on picking an optimal CAPS deployment or off airport landing spot while the autopilot is doing its job of flying at approximately Vg, not stalling the plane and giving you the most time possible to make the right decisions.  Our product management team will work with the instructor community to determine how best to utilize this technology in an emergency situation.

As a new pilot that is still working on my stick and rudder skills I like having this kind of help in the cockpit.  The DFC90 and DFC100 can really act like a co-pilot in this situation and allow me to focus on managing the entire emergency beyond just flying the airplane.