And Other Operating Tips

© E. R. Martin*
(See Disclaimer and Release below)

On August 28, 2004 a Cessna 337C was approaching an airport in visual meteorological conditions. Suddenly, the pilot alerted air traffic control that he had experienced a total loss of engine power. In the subsequent crash, the pilot was seriously injured and a passenger was killed. Examination of the wreckage revealed that the main wing tanks were empty, while the auxiliary tanks were full of fuel.

Sadly, this is not an isolated incident. Skymasters, which should have superior accident statistics due to their forgiving center-line thrust design, have a record comparable to other twins with retractable gear, partly because of frequent fuel-starvation accidents.

Some contend that it's precisely because of the aircraft's simplicity that problems arise. Too many pilots think they can get in and fly, and you find that the unusually large number of accidents due to simple fuel starvation occur because the pilot didn't check fuel or because he/she didn't know how the four tanks feed the engines. You see, because of its reputation as a bullet-proof aircraft, some pilots treat the 337 as if it was a simple single-engine aircraft. But it isn't. You don't just get in and fly. If you treat it for what it is -- a complex, high-performance, retractable-gear, variable-pitch-prop twin -- and if you maintain it properly, and if you do thorough pre-flights, and if you stay proficient, then it is indeed a very safe aircraft.

In addition to the high percentage of fuel-starvation accidents, a flight test report on the aircraft states categorically that "fuel system requires familiarity". Therefore, this paper provides that familiarization and tells you the four things you need to do to minimize the chances of problems. It's followed by a Summary and then a Quiz.

Pre-flight & In-flight Checks

As indicated above, you need to give the aircraft the respect any complex twin deserves. If we assume that your Skymaster has been properly maintained, then your first task is to do the things you learned as a student pilot. So you start with a thorough pre-flight as set forth in the Pilot's Operating Handbook (POH), including climbing on the wings to visually check fuel quantity on each tank, draining fuel from each tank until it's water-free (and remember that you do these two tasks after each re-fueling, not just before the day's first flight), draining the engine fuel sumps, and confirming that there's adequate oil. Nothing new here, just the old routine stuff, except it's important -- the Skymaster is not bullet-proof. And remember, per the POH, to fuel slowly so that tanks are completely full and to wait several minutes after filling each tank and then toping it off.

Applying what you learned as a student pilot also means monitoring your instruments after engine start-up and frequently during flight. Pay special attention to engine temperatures, oil pressures and fuel gauges. You should make an effort to mentally "calibrate" the gauges, so you have a feel for the range of standard parameters; if you then see a variation from what you're accustomed to, be alert during the rest of your flight and bring it to your mechanic's attention. By the way, the gauges don't always tell you everything, so be alert to other indicators; a strange smell over several minutes once led me to realize that my landing gear doors had not closed -- I was smelling overheating hydraulic fluid from a continuously running pump.

Understanding the System

Your second task is to learn the fuel system of your Skymaster. The system described here is that of a normally aspirated, unpressurized 337, as was built in the late 1960's. Other models of Skymasters may have significantly different configurations, and the descriptions and operating quirks described here may be inaccurate, so be sure to check your POH to check relevancy (for instance, here the auxiliary fuel pumps only work on main tanks, but in other Skymaster models these pumps work with any tank, including auxiliary tanks). Even if the configuration described here differs from yours, you might find that many of the points made here will help you be more proficient in managing the fuel in your Skymaster.

Many Skymasters have, on each wing, a main tank and a separate auxiliary tank. The tanks on the left wing are typically used to feed the front engine and the tanks on the right wing are normally for the rear engine. However, there is a cross-connect capability which allows you to feed the front engine from the right main tank and the rear engine from the left main tank. This overall configuration is shown below, in a diagram simpler than shown in your POH. Again, verify that it corresponds to the configuration on your airplane and study it. It seems complicated but it isn't (if you find it hard, there's an even simpler rendering available later).

Focus first on just the front engine. Notice that it has a mechanical fuel pump which draws fuel from a selector switch (the green circle with the S) and into the engine's fuel injection system (not shown). This fuel selector switch can be set to draw from the left main, from the left auxiliary or from the right main. For the front engine, the left main tank is referred to as the "principal" tank; this is the baseline tank used during take-off and landings (notice the larger/darker arrow). This tank receives excess fuel returning from the engine (even when the engine is being fed from another tank) and has an auxiliary electric fuel pump. (Don't let the term "auxiliary" confuse you into relating it to the auxiliary tank; it works only to push fuel from the main tank.)

If you now focus on the rear engine, you will see that it has a similar arrangement, with the right main tank being the principal tank.

If you had difficulty understanding the system, click here to go see a simpler drawing where the front engine half of the fuel system is divided from the rear engine half; after studying that, return here and you will find the full drawing above easier to understand.

It's very important to realize that the auxiliary tanks cannot be cross-connected. The left auxiliary only feeds the front engine and the right auxiliary only feeds the rear engine. If you lose an engine, the remaining engine can never access the fuel in the auxiliary tank related to the dead engine.

An intriguing aspect is that the fuel flowing into an engine (at point A) is roughly twice what the engine uses. Regardless of which tank was the source of the fuel, the excess fuel and vapor (at point B) always goes to that engine's principal tank (the left main for the front engine and the right main for the rear engine). This leads to some important considerations which will be covered later. Look again at the drawing and make sure you understand it.

Normal Operations

Your third task is to review again normal operations in your POH. Take-off and landing configuration is always with each engine connected to its principal tank. To start an engine you run the electric pump of its principal tank to "prime" the engine, turn it off and then key in the starter. These electric auxiliary pumps are not used for take-off, cruise or landing, but only for special cases that we'll cover later (that's why it's called "auxiliary", because it is seldom used, with the engine's mechanical pump doing virtually all of the work to draw fuel from the selected tank to the engine).

Once at cruising altitude, you can plan to switch to the auxiliary tanks. If your mains were full when you took off, you must fly for at least one hour before switching to the auxiliary tanks. (The one hour is necessary to make space in the mains for the returning excess fuel.) You first switch one engine to its auxiliary tank, confirm proper flow while monitoring EGT and fuel-flow gauges, and 10-15 minutes later switch the second engine to its auxiliary tank (the delay is there so the auxiliaries run dry 10-15 minutes apart, so that switching back to the mains can be done sequentially at your leisure).

This issue of returning excess fuel becomes clearer with an example. Say the front engine is using 9 gph at cruise and, after you are in flight over one hour, you switch it from the left main to the left auxiliary which has 18 gal of fuel. You would figure that you can run for two hours on the auxiliary tank (a consumption of 9 gph from an 18 gal tank). And you'd be wrong, with the auxiliary running dry in about one hour.

Why? Because remember that the engine is drawing from the auxiliary tank roughly twice what it needs, with the excess going to the left main (which will end up with about 9 gallons more and it's why you had to fly an hour to make room for this fuel). By the way, this will also happen if you cross-feed from the main tank on the right wing, which will run dry about twice as fast as you'd expect.

When the auxiliary runs dry, the engine starts stumbling, you switch back to the left main and -- surprise -- nothing happens, the engine won't start! When you first switched from left main to left auxiliary everything worked fine, so how come you can't now switch back to the left main? Because there are voids in the fuel line. When you initially switched to the auxiliary, you had uninterrupted flow, you went smoothly from the main to the auxiliary. Now, you ran the auxiliary tank dry, the engine is stumbling for lack of fuel, there are pockets of air in the line, and the mechanical engine fuel pump can't draw fuel with these voids in the line. Here's the second use of the electric pump. Just turn it on until the engine recovers power, then turn it off.

But wait, something ought to be bothering you right around now. We just learned that if you run a tank dry, then you must turn on the auxiliary pump to re-start the engine when you switch tanks. But we learned earlier that this pump doesn't work when you switch to the auxiliaries. So, it follows that you must switch to the auxiliary while there is still fuel in the main tank, before draining it. Put differently, you need to employ the fuel in the auxiliary tank somewhere in the middle of your cruise, not near the end after exhausting the fuel in the main tank.

Let's highlights the 4 important points we've learned:

  • you switch one tank at a time, confirming proper flow before switching another,
  • don't switch to the auxiliaries until you've got space in the mains,
  • switch to the auxiliaries in the middle of your flight while the mains still have fuel (never at the end, after you run the main dry, because the electric pump doesn't work on the auxiliary tanks), and
  • the auxiliary tanks cannot be cross-connected (e.g., the right auxiliary can't feed the front engine) and will run dry in half the time you expect.

An important element of flight operations is to know when the fuel gauge of each main tank is showing 1/2 hour and 1 hour left of fuel. The safe way to do this is to take a trip with one main tank full and the other main tank with enough fuel for climb and one hour of cruise; as the fuel on this tank drops, write down the times at which the gauge reads 7/8, 3/4, 5/8, 1/2, 3/8, 1/4, 1/8 and when it runs dry. Recover by cross-connecting to the opposite main tank, but make sure you land long before that tank runs out, desirably at the closest airport, and remember that if it does run out, there will now be fuel in the other main that had run dry, because of the return fuel. (This procedure requires you to fly for a time with both engines drawing fuel from the same tank, an arrangement which is OK on most Skymasters, but may not be allowed in your model 337; so check your POH.) On the return trip, calibrate the other tank by putting the smaller fuel load in it.

As you approach the end of your flight, don't forget that for landing each engine should be on its principal main tank. Make sure to check this before you enter the landing pattern.

Emergency Operations

Finally, your fourth task is to be prepared for emergency operations. The auxiliary electric pump may be used to recover an engine which suffered a failure of its mechanical fuel pump. In a twin we're trained to quickly feather a failed engine, but depending on circumstances (adequate altitude and speed, for instance) it may be useful, if the engine is connected to its principal main tank, to run the auxiliary electric pump for a moment to see if the engine recovers.

Several Skymaster accidents may have started as fuel-starvation problems, but were ultimately caused by the pilot feathering the wrong engine. So make sure, if you truly have only one working engine and either can't get the other one started -- or it's on take-off or landing, and you simply don't have the time -- that you identify and verify which is indeed the bad engine before feathering it.

One scenario which employs the auxiliary electric pump and cross-connection is starting an engine with a failed auxiliary electric pump. Suppose the front engine starts but, when you try to start the rear engine, the right-tank auxiliary electric pump fails and you can't prime the engine to start it. You can switch the running front engine to its auxiliary tank (or cross-connect to the right main), switch the rear engine to the left main (cross-connect) and run the left main auxiliary electric pump to prime the rear engine (I could have left the front engine connected to this tank, but I like the electric pump to push fuel only to the engine which needs it). Incidentally, although you can get the engine started, it's not clear that it's safe enough to fly with the failed auxiliary electric pump, since this pump is a back-up in the event that the mechanical fuel pump fails and critical to re-starting an engine after you run the auxiliary tank dry.

Rather than cover all possible non-standard operations here, you need to figure out for each case what the best approach is, based on your knowledge of the system. To aid in this, there are several scenarios presented below (including the most important, scenario 3, where you inadvertently run out of fuel in one of your main tanks and the other one is about to follow). See if you can figure out what you would do in each (answers are provided).


  • Do thorough pre-flight checks. Climb on wings to visually check fuel.
  • Learn your fuel system. Electric pumps only feed from main tanks.
  • You can cross-connect only to the opposite main tank, not the opposite auxiliary.
  • Don't use the electric pump on take-offs unless the mechanical pump fails.
  • Switching between tanks is for cruise, never at low altitude/low speed.
  • Switch to aux tank when main has been emptied 10 gal but has at least 10 gal.
  • Aux tanks last half what you expect (half the fuel ends up in main).
  • When aux runs dry, switch to main and run electric pump momentarily.
  • Land on main tanks (just like take-offs).
  • Know how to solve scenario/question 3 below.


The foregoing will hopefully help you understand your aircraft. See how well you do on the following scenarios/questions. And don't let all the details of the fuel system confuse the main point of this paper: give your airplane the attention and respect you would give any complex twin and it will reward you with exceptional service.

1. Suppose you had a failed right auxiliary electric pump, started the engines as described above, put both engines back to their principal main tanks for take-off, and later during the flight the rear-engine mechanical fuel pump fails. How can you keep running on both engines? Click here to check your answer.

2. Suppose you've lost an engine in flight and have feathered its propeller. You've got 3 hours to the closest airport (perhaps you're over water), with the aircraft having no trouble maintaining the desired altitude on one engine. Because all of your fuel is in the two main tanks, you decide to start cycling between the two main tanks to draw fuel evenly and keep the aircraft left/right weight in balance. In each one-hour period, how much time do you spend connected to the main principal tank and how much time cross-connected to the opposite main tank? Click here to check your answer.

3. You always keep at least some fuel in your auxiliary tanks. Today, as you approach the 2-hour point of a 3-hour flight, you are not planning on using them because 3 days ago you told your FBO to top-off your mains, and the 46 gallons in each main will give you a range of roughly 4.5 hours. Suddenly, however, the rear engine starts sputtering with the tell-tale signs of fuel starvation. Your initial thought is that you've lost the engine's mechanical fuel pump, requiring you to activate the right main tank electric pump, but a quick glance at the fuel gauges -- both at or near "E" -- tell you that you're out of fuel. Aside from kicking yourself for not having climbed on the wings to visually check the fuel during the pre-flight and for not monitoring the fuel gauges in flight, what do you do? Click here to check your answer.

If you fly over moderately large bodies of water, the author's Ditching Page contains preparations for such flights, a ditching checklist, and recommended procedures for splash-down.

* E. R. Martin has a Bachelor of Science in Mechanical Engineering (High Honors) from the University of Florida and a Master of Science in Mechanical Engineering (Jet Propulsion Option) from the California Institute of Technology (Caltech). He spent nearly 20 years in satellite communications and 10 years in the jet engine business, and has consulted for various companies, including GE. Based in Miami, he frequently flies his '73 337G over Caribbean waters. He has flown his previous Skymaster (a '69 Cessna 337D) on search-and-rescue (SAR) missions over the Florida Straits, and on a round-trip from Miami to Boise, ID, using an unorthodox (and not recommended) fuel management technique which he believes safely extends the range of each leg. He may be reached at .

DISCLAIMER AND RELEASE: The author believes that the information above is accurate but it is provided without any warranty. The information is presented for reference only and should not be relied upon. Elements of it may not be applicable to your aircraft. If the information on this page is different from the information in your POH, your POH governs and the information here should be ignored. Finally, you use this information solely at your risk and, by using it, you agree to release the author from any liability.