This Awesome Human Machine: Part Two

We left off last month comparing some of the functions of the awesome 777 flying machine with our own physical capabilities. This time let’s talk about the innards of the two machines and see how it is all done.  As I wrote in previous posts about our circulatory system, it takes about 60,000 miles of “fuel lines” to supply nutrients to all the far-reaching corners of our anatomy.  The airplane has a much simpler fuel system as the huge fuel tanks sit inside the wings right over the engines they supply.  As we saw last month, the 777 carries about 50,000 gallons of fuel depending on the mission, and to get that from the fuel tanks to the engines uses only about 400 feet of fuel lines. Our “fuel system” is a lot more complex, and with the redundancy of multiple arteries going to our many end organs, ours is more fragile. I think I’d rather have the airplane's simple system, there’s a lot less to worry about!

Speaking of things to worry about, we have only one pump, our heart.  That too is an incredibly engineered and efficient little machine, but care and maintenance is a life-long requirement for us.  The heart of the average adult is the size of a rolled up fist and weighs about 12 ounces.  That average adult has a total of about 5 liters of blood (only 1.3 gallons) and we don’t burn it up like the 777.  The heart circulates 100% of our blood volume every minute and all of that 5 liters of blood pass through each of those arteries and veins pretty much every minute.  With a heart rate of about 60 beats per minute, it means that each stroke of the heart pumps about 85cc of blood.  The marvelous flying machine has 6 fuel pumps that is quite an advantage over our single one. 

The 777 fuel pumps send about 45 gallons per minute to the engines, quite a lot more than our little heart could do. Just think how great it would be to have five extra hearts if one gave you trouble!  Also in the airplane's favor, if one of those 777 fuel pumps breaks down, the mechanics just swap it out for a new one.  Switching our heart for a new one is possible but I can tell you from having done a lot of that kind of surgery, it’s a huge deal!  Again I guess the airplane has us beat on this one.

Driving all those complex moving parts in both our body and the airplane takes a lot of connections.  There are about 250 miles of wires and cables that connect the 777’s electronic brains in the cockpit to the rest of the airplane.  As I have talked about in a previous post on this site, our brain and all of the peripheral nerves running through us comprise an amazing electrical system that neuroscientists estimate would unwind to about 110,000 miles of wires!  Just as an example, there are over 45 miles of nerves just in our skin to provide sensation and warning feedback messages to and from the brain.  Our marvelous human machine gets the ribbon here for complexity and amount of spare parts.

So adding all this up, who won this contest?  I think both the airplane and we humans are pretty well designed to do what we do. But there are a couple of huge advantages the airplane has over us that, in the final scoring, might tip this contest to the mechanized flying machine. 

First, unlike most of us who barely manage to get an annual physical exam or visit our favorite FAA medical examiner, the aircraft receives a complete check-up every 100 hours.  Keep in mind that FAA physical exams are, by intent, not meant to take the place of a thorough yearly exam by your general physician who is looking to find a number of small potential problems so they don’t get to be a major health concern.  But even so, can you imagine going to a doctor every 4 days for an exam? On the airplane, if anything is found to be a problem the broken parts can simply be exchanged for new ones to maintain perfect function.  At any time, pretty much every part of the aircraft is changeable and replaceable.  Of course, flight time accumulates and that wears on the airframe, but blow a pipe, just put in a new one, pop a gasket, just replace it, and on top of all that, the engines are changed out at regular intervals. 

The aircraft has two other important things we humans lack: redundancy and self-diagnostics.  There are two engines, 6 fuel pumps, multiple cross-references for computer guidance and navigation, and most important, two (or more) pilots who cross-check and monitor the aircraft and each other.  The plane is also equipped with multiple sets of monitors and meters that constantly evaluate hundreds of flight and performance metrics that are in continuous communication with the home base.

Except for some pretty sophisticated surgery that is somewhat limited, we pretty much have to make all our parts last for our entire flight through life. Stress on the airplane is fixable. The stresses we humans face from multiple physical, emotional and cognitive sources are cumulative, much more insidious, and also much more difficult to manage.  The negative effects of some stresses can never be undone.  Pilots and surgeons have this problem amplified several fold by the nature of our work.  In general, we are poor at self-evaluations and monitoring of our own physical and mental health.  Without all the advanced monitoring capability of the airborne machine, our physical parts can break down and we don’t recognize the subtleties as this occurs.  That fact alone puts us in much more danger than the airborne machines.  It also means we need to be more diligent with our health maintenance and surveillance.  It seems to me that the 777 mechanics take better care of their airplanes than we do our bodies sometimes.  

As I said at the outset, this attempt to compare us to flying machines would be a little tough.  It has been, but the takeaway from this teaches an important lesson.  We are not machines and if we use our pilot hubris to think we are machines, we will break down and risk our own health and the safety of the passengers we carry through the sky.   Next time we will discuss some of the wear and tear we humans sustain in our routine labors and what we can do to monitor and enhance our physical and cognitive functions to be safe on the ground and in the air.

Kenneth Stahl, MD, FACS
Kenneth Stahl, MD, FACS is an expert in principles of aviation safety and has adapted those lessons to healthcare and industry for maximizing patient safety and minimizing human error. He also writes and teaches pilot and patient safety principles and error avoidance. He is triple board-certified in cardiac surgery, trauma surgery/surgical critical care and general surgery. Dr. Stahl holds an active ATP certification and a 25-year member of the AOPA with thousands of hours as pilot in command in multiple airframes. He serves on the AOPA Board of Aviation Medical Advisors and is a published author with numerous peer reviewed journal and medical textbook contributions. Dr. Stahl practices surgery and is active in writing and industry consulting. He can be reached at [email protected].
Topics: Pilot Health and Medical Certification

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