Pelican’s Perch #7:
Run That Fuel Tank Dry!

AVweb's John Deakin takes aim at yet another OWT (Old Wive's Tale). While running a fuel tank dry in your recip powered plane may serve to increase your heart rate, John explains why it's not such a bad thing at all, and it is probably a really good idea for most of us. In fact, John explains why it's one of the first things you ought to do with a new plane and how it could save your life someday.

0

Pelican's PerchIf you’ve been reading any of thisstuff, you might have figured out by now that the headlines arewritten to grab some attention, in the hopes you’ll read on.These “leaders” or “teasers” are written bysomeone else, I’m too lazy. They may or may not resemble theactual gist of the article, if there even is a gist. So, sueme. Better, sue AVweb, they’ve got more money [Editor’s note: We do? Where’d they hide it?] If you want really serious, goread William F. Buckley, we’re trying to have a little fun here,poke a few holes in some egos, puncture a few OWTs (Old WivesTales), and with luck, maybe exercise a few brain cells.

Fuel Management

I’d like to take a look at fuel management,and since my method sometimes calls for running a tank dry, let’sget that out of the way first.

I hate OWTs. There are new ones,and there are old ones, and they seem to have a life of theirown, with no way to kill them. “Beware the downwind turn,”never turn into the dead engine,” and “don’t runlean of peak” are all so silly, yet they persist, even amongpeople who should know better. Other stupidities are “Theonly time you have too much fuel is when you’re on fire,”the three most useless things in aviation are runway behindyou, fuel in the truck, and altitude above you” (or variationson that theme), and the endless inane argument over whether pitchcontrols speed or altitude. I hope to shoot at some of thosein future columns.

How about Running a Tank Dry?

“Never run a fuel tank dry”is one of the newer ones, because it used to be a standard methodof fuel management. We used to do it on virtually EVERY flight,before the jets came along! I still do it regularly, in non-turbineaircraft, and it works great. What happened to change this?

Now please, folks. I’m not talkingabout running a tank dry when it’s the only tank in the airplane! Nor when it’s the last tank with fuel in it! Some readers haveblasted me on the checklist columns about single-pilot operations,saying “Well, how about the A-26, or the single-pilot Citation?” Folks, there are exceptions to every rule, and if you haven’tgot the smarts to figure that out, you not only shouldn’t be readingthis column, you probably shouldn’t be flying (or even walking). As I tried to point out in “What Really Counts,” youought to read this, think about it, figure out what applies toyou and what doesn’t, and act accordingly. You don’t like something? Argue about it! Find some data to prove I’m wrong! Please! I want to know more than anyone.

Who Started This Insanity?

In my opinion, the change in attitudewas mostly due to turbines (jets and turboprops), in which evena tiny bit of air in the fuel supply may kill the engine. Turbines,as wonderful as they are, can be miserable or impossible to restartin the air, often requiring a descent into “thicker”air, specific airspeeds and windmilling RPM, and several systemshave to function properly to get a re-light. The turbine is likea campfire on a rainy day. Keep it going, and it’s fine, butlet it go out, it can be really tough to get it going again.I suspect that as many pilots started flying turbines, the ideadeveloped that running a tank dry was “a bad thing,”and this attitude permeated down to the recips, as well.

This is simply not true of recips. When a recip runs out of fuel, nothing else has changed. Thespark is still there on every power stroke, the piston is stillpumping air, driven by the prop, which is nearly impossible tostop, inflight even when you want to. Two of the “threenecessities” (fuel, air, spark) remain, totally unaffectedby the lack of fuel.

Stopping a Prop

Some pilots seem scared the propwill stop. It won’t, it generally takes a lot of effortto stop the prop, though there is some variation between airplanes.

For example, in order to stop theprop on my Bonanza (IO-550, three bladed prop), I have to killthe mags, or stop the fuel (mixture or fuel valve), pull the propcontrol all the way back, extend full flaps, and fly very nearthe stall for many seconds with the throttle closed (makes theengine have to suck a little harder). Once the prop stops, Ican fly at about 95 knots without the prop starting to windmillagain. It will slowly work its way past the compression stroke,then come up against the next one, and stop again. Anything above95, it will spin up past that next compression stroke, and thenext, and then increase to whatever RPM I’ve set with the propcontrol (about 1200, on mine).

So, even if the fuel stops, the “suck,squeeze, bang, blow” of the normal internal combustion processcontinues. All we have to do is introduce fuel again, and theengine simply has no choice, it MUST start producing poweragain.

Will It Start Again?

Is there a problem restoring thefuel flow? In a word, NO. The FAA certification rules requiretesting in this area. For example:

FAR 23.955(e) Multiple fuel tanks.For reciprocating engines that are supplied with fuel from morethan one tank, if engine power loss becomes apparent due to fueldepletion from the tank selected, it must be possible after switchingto any full tank, in level flight, to obtain 75 percent maximumcontinuous power on that engine in not more than –

(1) 10 seconds for naturallyaspirated single engine airplanes;

(2) 20 seconds for turbochargedsingle engine airplanes, provided that 75 percent maximum continuousnaturally aspirated power is regained within 10 seconds; or

(3) 20 seconds for multiengineairplanes.

FAR 25.951(a) Each fuel systemmust be constructed and arranged to ensure a flow of fuel at arate and pressure established for proper engine and auxiliarypower unit functioning under each likely operating condition,including any maneuver for which certification is requested andduring which the engine or auxiliary power unit is permitted tobe in operation.


(b) Each fuel system must bearranged so that any air which is introduced into the system willnot result in –

(1) Power interruption formore than 20 seconds for reciprocating engines; or

(2) Flameout for turbineengines.

I don’t have a copy of the old CARs(CAR 3, and 4b) here, but I’m pretty sure that language is liftedright out of those early, common-sense rules.

I know of no accidents that haveoccurred because an engine would not restart when supplied withfuel in flight. I have personally done this literally thousandsof times myself, and never seen more than a couple of secondsof interruption, even when I was completely unaware the enginewas about to quit. If we count all the people I know who routinelydid it, there are literally millions of such events.

The Passenger Factor

There is also the perception thatrunning a tank dry scares passengers. Maybe, but I don’t thinkso. I’ve done it many times with all types of passengers watching,and if it’s treated matter-of-factly, with a few words in advance,a chuckle when it does happen, none have ever expressed anythingbut curiosity. In other words, no big deal. Some of the less-awaredon’t even realize anything happened. A frantic scramble to turnthe fuel valve, swearing, praying, screaming, or sweating doesnot induce confidence, however.

The Creeping Crud OWT

Then there is the secondary OWT thatsays something like “But what if some crud gets sucked intothe system from the tank bottom?” Give me a break! Thinkabout this, for a moment. There are three areas where “crud”might be a concern. Crud lying on the bottom, crud suspendedin the fuel, and crud floating on the surface. When we fuel theairplane, fuel is injected rather violently, stirring up the wholetank. When we fly in turbulence, fuel sloshes rather violentlyaround the tank. Do you really think anything will be peacefullylying on the bottom, year after year? If it were, why would runningthe tank dry stir it up, and if it’s that tenacious, how on earthis running the tank dry going to magically pick it up?

How about suspended crud? It isno more, or less likely to be sucked into the fuel lines at anyfuel level. Floating crud, on the surface? Well, maybe, butcan you name me something that will do that? And if there is,well, how much of it will you allow, before you rip the tank outfor “cleaning,” or how WILL you get it out, someday? Just how, and when, will you identify it, detect it, and getrid of it? And, how much fuel do you want as a “buffer”below the floating crud, to keep from sucking it in? In fact,if there is a little something floating on the surface, I WANTit to be sucked into the fuel lines, preferably a little at atime, so that the strainers and filters can catch it, and alertme that something is going on in there. The likelihood of therebeing enough to cause a problem is remote, at best, and if runninga tank dry will pick up a little crud, then running a tank dryoften is a very good thing, because you’ll catch it a little ata time, and drain it out the strainer.

Of course, if you keep the tank fullmost of the time, and the cap on, and drain the sumps often, thereisn’t any way for crud to get into, or stay in the tank in thefirst place.

Shock Cooling?

I’m beginning to think this businessof “shock cooling” is perhaps the worst of the “modern”OWTs around, but it’s one that sounds very reasonable at firstglance. I’m still not utterly certain myself. But consider,which airplanes get the longest TBO? Why, it’s TRAINERS! TheyCONSTANTLY jam the power on, and yank it off, many times an hour,all day, every day. No regard or thought of temperature control,at all. If shock cooling is so bad, why do they last so long? Does anyone know how many hours Bob Hoover gets from his Shrikeengines? They go from full power to a full stop (feathered) withNO interval, and right back to full power again. I’ve tried toget to him to ask, but he’s always mobbed. Does anyone know?

Now please, I’m not advocating abuseof any precision machinery, here! I prefer gentle engine management,it’s just good airmanship. But “shock cooling” maybe a non-issue.

In any event, if properly done, runninga tank dry does NOT cause “shock cooling” at all! Oncethe pilot knows what he is doing, he should know precisely whenthe tank will run dry, and will detect the early signs well beforethe engine is affected (fuel pressure, mostly). Even after theengine begins to falter, there is time to change tanks and getthings back to normal before the CHTs drop. We now know this,with the modern digital engine monitors. The EGTs do drop a coupleof units, but they recover very quickly, so even turbochargedengines shouldn’t have a problem. I would be more careful witha turbo, however, because other issues arise, like surging, bootstrapping,etc.

The first time any pilot runs a tankdry, the heart rate does increase, but it recovers quickly, too! As experience is gained, it becomes a non-event.

Broken Fuel Selectors

Ok, this one may be a slightly morevalid concern. If you are concerned about this, you need to haveyour mechanic check the fuel tank selector out thoroughly, inorder to make sure it moves freely, and that it is not worn.Once this is done, the possibility of it breaking becomes so remoteit’s hardly worth considering. If you want to consider it anyway, carry a small pair of pliers, and think of glider pilots. EVERYlanding in a glider is a deadstick landing! If you are not utterlyconfident of your ability to deadstick your airplane into anyreasonable airport, allow me to point out the fact that you needsome training, or practice. Then you can plan to run that tankdry when within gliding range of an airport. In most parts ofthis country, it is fairly unusual to be too far from any airportsat all. Do I need to point out that if you don’t know the glidecapability of your aircraft, it’s time to hit the books, and confirmthat with some training? Most GA aircraft can do 10:1, so foreach thousand feet above the airport you’re hoping to reach, youcan be almost two nautical miles away (give me a little room here,this is not an exact science).

Just How Much Fuel Is Safe?

The FARs require a minimum of 45minutes fuel remaining at night, 30 minutes in the daytime, and20 minutes for helicopters. I think most of us can agree thoseare NOT conservative figures! Only under very unusual circumstanceswill I approach those limits deliberately. Using the techniquesI am about to discuss, I have pushed to those limits, when approachingmy home airport (Arlington Wash.). But only in very good weather,maintaining a high enough altitude to deadstick it into that airport,with good airports within gliding range along the way, and knowingthere are several runways available, including the grass alongsidethe runway. Take away any of those, and my personal minimum fuelstarts going up.

You may never want to push this. You need to make up your mind before flying just what yourpersonal “minimum fuel” is, under ideal conditions,and how much should be added for less than ideal conditions.These should be positive calculations, based on reality, not “GrandmotherFuel,” or “Feel Good” fuel. Additions should bebased on hard data, not “feel.”

I frequently hear “Aw, I’vegot a six-hour airplane, but only a two-hour bladder, I don’treally care about fuel.” That’s a cop-out, and extremelyunprofessional. What about the case where you arrive somewhereand there is no fuel available? Just HOW MUCH fuel DID you havewhen you took off? Do you KNOW? HOW MUCH fuel did you use duringthe flight (or flights)? Do you KNOW? All this leads to theall-important question, HOW MUCH do you have left, and how farcan you go with it? If you cannot reliably answer those questionswithin about 3% of the fuel capacity of your airplane, you’renot doing your job as a pilot.

How Many Ways to Cheat?

Then there is the Bonanza pilot whosays “Well, I’ve got two 40-gallon tanks, with 37 usableeach, so I’ll call that 35 (70 total), to be safe. I burn about12 gph, so I’ll call that 15, to be safe. It probably takes 5more gallons to taxi out, takeoff, runup and climb, so I’ll callit 10, to be safe.” That brings our “safe” aviatordown to 4 hours of total fuel for flight! Now he starts in again,saying “I don’t think an hour’s reserve is enough, so I’lladd one, leaving me two hours of planning fuel, or 320nm in stillair.” Now he can’t find a fuel stop right at 320nm, so helooks for anything short of that, how far it’ll be is anyone’sguess. Finally, he’ll go out and fly it, and if he gets a hold,or a diversion, he starts sweating, because he knows he planned”minimum fuel”! Is he safe? Well, yeah, but he’s notmaking effective use of his tools, and he isn’t very professional.

So, How Much Fuel Do I Have?

I have deliberately used the numbersabove, because they apply to my airplane, or so it might seem. The POH, and the fuel tank placards for my Bonanza all statethere is only 37 usable gallons per side, of 40 total. The veryfirst long flight I made, I ran the left tank an hour, then ranthe right tank dry, “maneuvering” while burning thelast few gallons. With an aggressive slip to the right, I couldunport the tank, and get a fuel pressure fluctuation, and thisis probably the reason for the “37 usable.” I ran itdry, and when I filled it, I found it actually took 41.2 gallons. The next flight, I reversed the pattern, and found the left tankwas identical. Does this meet FAA standards for “usable”fuel? No, but the fact remains that I can reliably get 41.2 gallonsout of either tank in cruise, where unporting is no issue at all,and if there no turbulence, or low-altitude maneuvering, the othertank is fully usable, too. Of course, I can’t count on “noturbulence,” so some allowance for this is needed.

At the very least I have 78.0 gallonsavailable (41.2 + 37.0) up to 82.4 (41.2 x 2).

Note this cannot be determined withoutrunning a tank dry, in flight. I consider it so important toKNOW this figure, that if I regularly flew a single-tank airplane,I would probably fly it to fuel exhaustion over a quiet airport,and deadstick it in, like a glider, then tow it to the gas pump. I know, the FAA probably wouldn’t like that! Draining the fuelwon’t be accurate enough, some tanks drain above or below thefuel outlet. Even disconnecting the fuel line to drain it isn’ttotally accurate, because you can’t maneuver the airplane in thehangar. Better than doing nothing at all, of course.

Exactly How Much Do We Burn?

On those same early flights, I madesure that for the entire time I was running on the tank I wastesting, I ran at a constant power, and constant altitude, andconstant fuel flow. By timing that from “full” to “drytanks,” I knew EXACTLY how accurate my fuel flow indicatorwas. By noting the fuel quantity indicators during the burn,and noting the time at which they indicated “3/4,” “1/2,”and “1/4,” I calibrated the fuel quantity indicators,too. While I’m lucky to have dead accurate instrumentation, itdoesn’t really matter, as long as you know what the instrumentsare really telling you, at any given reading. Make some notes,so that you can use the data later.

On subsequent flights of more thanan hour, but less than about three, I used just one tank for start,taxi, runup, takeoff and climb, and for exactly one hour fromliftoff, then switched to the full tank for the remainder of theflight. By noting the fuel used from that “takeoff tank,”I now know with great accuracy that all that requires only about4 gallons additional fuel, over the normal hourly rate.

At normal altitudes and power settings,I find 12.7 gph to be a reasonable figure. Now, let’s look atthose numbers, again. Use a conservative 78.0 gallons. Thatcomes to 6.17 hours of usable fuel. NOW take off your “neveruse” fuel, perhaps an hour’s worth, leaving 5.17 hours forplanning, including fuel for alternates. At 160 knots (140 thefirst hour), that’s 787nm in still air, more than double thatfor our “nervous nellie” above.

All these tests were duplicated atdifferent power settings and altitudes, during normal flights,over the course of the first few months I had the airplane. Infact, I continue to do them, as a systems check. For example,there is no guarantee that a bladder tank will always hold thesame fuel. Perhaps it will partially collapse, and take less,leaving you less fuel than you think. If I run the left tankdry today, and it takes 41.2 gallons, I know it’s still ok, andas a side benefit, I’m also checking the accuracy of the FBO’sfuel pump! Next week, I’ll reverse my usage pattern, and checkthe other tank.

How to Use All This

Normal power settings in my airplanewill range from about 9 gph to 15 gph, depending on altitude,and whether I want speed or non-stop distance. But a very commonfigure keeps popping up, flight after flight, about 12.7 gph,full throttle, 2500 RPM, leaned for “best power” somewherebetween 10,000′ and 15,000′. Lower altitudes will be well leanof peak EGT, still full throttle and 2500.

I take off, and shortly after takeoff,I set a timer to alert me at exactly the one-hour point. Whenthat timer beeps, I KNOW I’ve used 12.7 + 4 gallons, and thushave 24.5 gallons remaining in that tank. I know my fuel quantityshould show about 2/3 of that “takeoff tank” remaining.

I’m now on the full tank, and ifI’m burning 12.7 gph, I know it will take 3+14 to run that tankdry. I set the timer to 3+10 or 3+12, so it will warn me a fewminutes before that happens. 48 minutes later (at 1:48 into theflight), that fuel quantity indicator should show “3/4,”at 2:36 into the flight it should show “1/2,” and at3:24 elapsed, it should show “1/4.” When the timerdings, I have a couple minutes to keep a wary eye on the fuelpressure. When it flickers, I switch back to “The TakeoffTank.” I have re-validated my instrumentation, my actualfuel capacity and burn, and most important, I know precisely howmuch fuel remains, 24.5 gallons, or 1+55 to dry tanks. For afinal check, when I pull up to the pump, I run the final calculation,and “predict” how much fuel the tanks will take. Inseveral years, I’ve never missed it by more than about 2 gallonson the “safe side,” which I consider an acceptable error.

Know Thine Own Airplane

Some airplanes lend themselves tothis sort of thing better than others. My Bonanza is nearly idealfor this, having two tanks that can be filled quickly to the topwith great accuracy, and with the vapor vent return line returningto the tank being used. Some older Bonanzas have all the vaporvent return going back to just the left tank, which makes allthis a bit more difficult, or a bit less accurate. High-wingCessnas are particularly difficult, because it’s very hard toknow with certainty how much fuel is aboard, that last few gallonsgoes in very slowly, if at all, and there is often a cross-transferthat happens in flight. Still, these procedures can be modifiedand used to determine the numbers for almost any airplane. Testflights are seldom necessary, you just need to log the data onnormal flights, and within a month or three, you’ll have good,useful data that may just save your life, someday.

Be careful up there!