Left? Right? Or BOTH?

I've been involved in some discussions lately regarding which fuel selector setting to use on a Cessna 172.  Most 172s have four settings:  Left, Right, Both, and Off.  This seems like a brain-dead question, doesn't it?   Maybe not!

MY STORY
During flight training in a C172, I, like most student pilots, was taught to leave the fuel tank selector on "Both" unless there was a reason to change it. That setting worked well for about 200 hours.  

But on March 26th, 2016, I was cruising at 7,500 ft (~6,500 ft AGL) when I noticed a drop in RPM.  At first, I thought it was just a throttle setting issue.  So I made an adjustment.  A minute later, the needle on the tach was back below the bottom of the green arc.  That's when I felt my heart crawl up into my throat.  Something was wrong.  Very wrong.  I was losing power.

​TROUBLE
When the RPM drops in a carbureted 172, my first suspicion is always carb ice.  No big deal.  Pull the carb heat and expect a sputter.  All good.   

Not in my case. 

​I pulled the carb heat and the RPM increased!  What?  That shouldn't happen.  ​​At that point I could hear the voice of my flight instructor in my head, resonating from years ago:  "TURN TOWARD THE NEAREST AIRPORT, NOW!" So that's what I did.

Nice cruise at 7,500ft - what could go wrong?

Next, of course, out came the emergency checklist.  Standard stuff.  Switch tanks.  Left mag.  Right mag, etc., etc.  Nothing I did helped the situation.  The engine at this point was beginning to sputter, and things got real.

"OK city approach, Cessna 1234Echo, seven south-west of three-Oscar-three at seven thousand five hundred, losing engine power, inbound Purcell, one on board."  Let me assure you that even before declaring an emergency, once you say "one on board" you have ATC's full attention and everyone else on the radio gets really, really quiet.

Schematic of the fuel system on a Cessna 172.

With ATC watching for traffic and aware of my position and situation, I could concentrate more on the problem at hand.  What was I missing?  "It must be fuel," I thought.  "Tanks are full.  Fuel selector on 'Both.'  Mixture rich....  Nothing is helping.  So what now?" 

"Fly the plane.  Fly the plane.  Fly the plane.  Trim for best glide.  Set a direct heading to the airport.  You get one shot.  What are the winds?  Manage energy... Uhm.  Oh sh*t, there's a damn lake at the end of the runway!  You must be kidding." 

Left Base - Purcell, OK, Runway 17 - Not a nice view with an engine failure.

Let's just say that I approached high and dropped the flaps at the last minute.  I may have landed long. But I wasn't wet! Or worse!

Right before touching down (yes, on the runway) I remember thinking about how I was going to have to PUSH the plane from the runway to get it clear.   But, that wasn't the case.  As soon as the wheels touched the runway, the engine started and ran perfectly.  Unbelievable.  I taxied to parking.  Fifteen minutes later, I was able to think straight, get my legs under me and exit the airplane.

THE CAUSE
If you carefully read the POH for early Cessna 172s, the POH cautions you about flying on both tanks when cruising at altitudes over 5000 ft.  Doing so may cause "power irregularities" (A.K.A. engine failure).  But I guess this is okay if it is in the POH.  Just switch to one tank, ignore the sputtering, glide for 60 (very long) seconds, then switch to the other tank and hope the vapor has cleared and the engine restarts.  That is why many early Cessna 172s have a placard on the fuel selector (between the seats, out of sight) that says something to the effect of "Use single tank above 5000 ft."  Yes, on those models, it matters.

LESSON LEARNED
The decision to fly on Left, Right, or Both may not be a simple judgement call.   The decision shouldn't be based on the consensus opinion of you and your buddies.  And the proper option may not even seem logical.   You must read the POH, ADs, and ACs for your aircraft and follow their guidance.   Your life may depend on it.

RESOURCES
FAA: https://www.faa.gov/regulations_policies/airworthiness_directives
Example C172 POH: ​http://rob.com/matt/manuals/172_poh_59.pdf

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Actual Fuel Management Procedure - 1959 Cessna 172