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The Fuel System Management Problems
By Tony Bingelis (originally published in EAA Sport Aviation, October 1979)
IN AN AUTO you turn on the key, glance at the gas gage, hit the starter and go. There is no fuel valve to turn on; there are no tanks to switch and check against the gages; no fuel pump, transfer pump or booster pump to operate; no fuel pressure gage to monitor . . . in short, there is no fuel management problem. Pretty hard to screw-up a system like that, isn't it?
Airplanes, on the other hand, do have some . . . maybe all of those puzzling gages, selectors and switches . . . each of them demanding the performance of some ritual before you can even taxi out.
When you think about it, it's easy to see why so many pilots are dissatisfied with the fuel management requirements of many of the airplanes they fly. Dissatisfied, in general, with the workloads which are imposed on them and, dissatisfied in particular, with fuel selector valves, their accessibility, location and logic of operation which is often opposite to the pilot's instinctive actions.
Too many of the airplanes we fly require tank switching in order to properly manage the fuel supply. Why should this be? Why can't an aircraft's fuel management system be as simple as an automobile's? Granted, airplanes are more complicated than autos and aircraft have balance problems due to the installation of fuel tanks in the wings, etc. But, why should the pilot have to do everything? If complex systems are necessary, why can't they be made to work automatically . . . without the assistance (or interference) of the pilot. After all, isn't it the pilot who switches to an empty tank, misreads fuel gages and does all sorts of other pilot error things?
The fuel management problem results primarily from the lack of a standardized fuel system. The kind of fuel system we have now usually consists of a fuel selector (with the "proper" markings), one or two fuel quantity indicators and perhaps a booster pump switch. With only these few elements to cope with you'd think standardization would be easy. If so, it certainly hasn't been accomplished thus far. The fuel systems now in use do not safeguard against pilot error incidents.
During March of 1978 the FAA published a report, General Aviation (FAR 23) Cockpit Standardization Analysis (AD/A-052 803). It is a good publication and should be of considerable interest to anyone who designs, builds, or flys airplanes. This report, compiled by Robert J. Ontiveros, Roman R. Spangler, and Richard L. Sulzer, in essense outlines a set of recommendations for cockpit standardization.
According to the report many safety experts believe that certain design elements, particularly in the cockpit area, contain features that tend to lure pilots into taking improper action and making bad decisions based on erroneous impressions. This and previous studies, along with the recommendations made by GAMA (General Aviation Manufacturers Association) to the FAA, all point out the need for the standardization of fuel management functions.
In addition, I believe that cockpit fuel management remains complex, and sometimes confusing because we have not been demanding enough, or perhaps, smart enough to devise standardized automatic fuel systems that do not require assistance from the pilot. I am not too sure, however, I would know a standardized fuel system if I saw one, but this thing is certain . . . if the demands on the pilot are kept to minimum there is sure to be less opportunity for his mismanaging its operation.
The consequence of fuel mismanagement, of course, is engine failure. An engine dying for lack of fuel immediately commands your attention, confronting you with a decision-making situation. If you are flying an aircraft with multiple fuel tanks and a non-standardized cockpit layout, you might well have difficulty determining immediately which tank is presently on-line and even determining the quantity of fuel remaining in each tank. Your need for a rapid assessment of the available information is critical. Lacking this, you may make an incorrect decision and take inappropriate action. As far as fuel systems are concerned, there are numerous conditions which invite fuel system mismanagement.
Fuel Quantity Indicators
The lack of standardization in fuel gages is a common complaint among pilots. Some aircraft have a separate gage for each tank. Others, sometimes, share one gage among several tanks by utilizing a switch to obtain the reading for the level of fuel in each tank in turn. Still other aircraft have fuel tank selector controls with a fixed relation between the selector and the fuel quantity indicator, making it necessary to switch fuel flow to a particular tank to obtain a reading of the amount of fuel remaining. Confused? Me too; But then, fuel level indicator problems are not peculiar to just a few models of small aircraft, either.
I have just finished reading a NTSB (National Transportation Safety Board) account of a DC-8 airliner with a similar fuel management problem complexity which contributed to a fatal accident.
For some foggy reason, the airline had changed the fuel quantity indicators on the airliner from the direct reading, digital type to a three-figure indicator that had to be multiplied by a factor to get the actual fuel tank values. And, as if that wasn't enough, the new fuel gage used for displaying the total quantity of fuel on board had an identical read-out display of the same three-figure presentation except that the numbers on this gage had to be multiplied by a different factor to get the actual total fuel quantity on board . . . a perfect set-up for pilot error under stressful conditions. Well, stress there was, and the pilot, apparently confused and still believing he had 1,000 pounds remaining (he really had only 100 pounds at the time), ran out of fuel and crashed his big bird. Pilot error? Of course, that's what they'll say. But was the blame really all his?
The accuracy of the electrical fuel quantity indicators is another problem. When installing your system, calibrate each gage to read zero in a level flight attitude with the amount of fuel remaining down to the unusable level. Remember, in some tanks a lot of fuel is unusable. Fuel gages apparently do become inaccurate over a period of time, so the homebuilder flying an older homebuilt might verify the accuracy of his gages to forestall a future noise abatement surprise during an extra long flight.
Remember those wire and float fuel gages on the old J-3 Cubs? Simple, eh? And, when properly constructed, reliable too because they required no switches, no selectors and no mathematics to mess with. They were totally trouble-free. It seems to me that an improvement is no improvement at all if it imposes additional requirements on the pilot. Let's look at another problem area.
Fuel Shut Off Valve
Each fuel tank should have an individual fuel shut-off. If for no other reason than to be able to shut off the flow of fuel to enable you to repair a line or to replace it. It really serves little other purpose and most pilots have never shut off the valve for as long as they can remember. The shut-off valve, however, might have to be used during an in-flight emergency and should, therefore, be easily reached by the pilot. A shut-off valve may also be necessary to control the flow of fuel from a transfer tank. Avoid confusion in the valve's operation by ensuring that the OFF and ON positions are clearly marked. The fuel valve control handle must have these two positive identifiable positions. Although it may vary with the design, the handle of a fuel shut-off valve should never be positioned in an up attitude for the ON position. Vibration has been known to cause such selectors to drift downward to the OFF position, shutting off the flow of fuel. It should, furthermore, be located where it will not be accidentally activated by the pilot's leg or clothing.
Common belief has it that a selector valve must be used whenever more than a single fuel tank is installed in the fuel system. This is not necessarily true. Some aircraft models do not need a fuel selector, even though two fuel tanks are installed. A simple ON/OFF valve is often sufficient. Such an arrangement is found on many homebuilts and on a few production aircraft like the Lake Amphibian and the Cessna 152, for example. The arrangement is an excellent one and virtually eliminates the possibility of mismanagement of the fuel. Both tanks feed automatically all the time . . . without help from the pilot. Now that is logical, isn't it?
When a fuel selector valve is necessary, isn't it logical, too, to have one with a handle (pointer) that points to the tank being used? With some selectors it is difficult to tell. But, why use that type.
Choosing A Selector Valve
Fuel selectors which are poorly designed and confusing in their mode of operation only add to mismanagement problems in the more complex fuel systems. Fuel selector valves are not normally available from most EAA supply sources and this tends to discourage their use in homebuilts. However, homebuilders who really need one often manage to find a salvage selector. Using a salvaged selector generally means that the homebuilt will have a fuel system quite similar to the one the selector was originally designed for. This may or may not be good depending upon the success record of the original system. If the valve is a complex one and was intended for use with several tanks . . . tanks which the builder doesn't intend to install, the valve will have a vacant position or two and the installation will become a candidate for mismanagement incidents.
If you must have a fuel selector, get one that is no larger than required for the number of tanks to be installed. Here are a few recommendations for its selection:
Pick one with a pointer that looks like a pointer and leaves no doubt as to the tank position selected.
Pick one that has a positive "ident" for each position. This will spare you the trauma resulting from inadvertent misalignment when selecting a particular tank. If it doesn't have a positive click or feel for each position, it is easy, when moving the selector handle (without looking at it), to unwittingly stop at a halfway position. The immediate result is the sudden, terrifying silence of an engine that has just run out of gas . . . even though there is plenty of fuel on board. (The NTSB calls that "fuel starvation due to pilot error".) If you wish to comply with current cockpit standardization practices look for the following natural fuel selector pointer indications:
Turning the valve selector to the right should give you the RIGHT tank . . . to the left, LEFT tank . . . forward for ALL TANKS . . . rear for OFF. All other tank positions (as provided) must be located between the LEFT and RIGHT tank position.
The OFF position should be at least 90 degrees from any tank selection position. Something else to consider. Your fuel selector handle (pointer) should not pass through the OFF position for obvious reasons. Think about these recommendations before buying your selector. Incidentally, in the event your selector valve doubles as a shut-off handle, its OFF position should be RED . . . not black as you will find on some older production line aircraft.
Locating The Fuel Selector Valve
Locate the selector handle at some point directly in front of the pilot so he can see it and reach it without having to move a pile of junk like maps, coffee cups, rags or the seat.
If your aircraft features side-by-side seating, the fuel tank selector should be within reach of both pilots. Such a provision is quite difficult, although not impossible, to make in a dual control aircraft with tandem seating.
Avoid mounting the selector valve on the cockpit sides . . . it is just that a small human quirk may lead to confusion in time of stress. Why? Well, turning the selector to the Right Wing in such an installation would point it to the tail or to the nose (depending on the cockpit side used). Don't laugh, strange occurrences do transpire. You know, Murphy's Law says if anything can go wrong, it will . . . and you can bet your boots that all mechanical objects abide by that eerie edict.
Fuel System Recommendations
If you can get by with it, the single tank gravity flow system with a simple ON/OFF valve installed just beneath the tank, is the best and about the most standardized fuel system installation you can make or buy (Figure 1). A simple fail-proof wire and cork fuel level indicator may be used. Adding embellishments to this clever rig merely detracts from the simplicity of the system. However, an electrical fuel indicator or some other visual fuel level indicator may be substituted, if desired.
The installation of a second tank always increases the complexity of any fuel system and the workload for the pilot. It also increases the potential for pilot error. A good installation is one where both tanks feed into a single Y or T fitting with both tanks feeding the engine simultaneously.
High wing aircraft and biplanes equipped with a tank in each wing can still provide you with a simple gravity-flow system that requires no particular fuel management attention. It is simply ON all the time and the engine will continue to run as long as you have fuel on board.
The only objection to a single BOTH ON arrangement for a two tank installation is the inability to shut off each tank individually. Ordinarily, there is no need to do so except in the remote event where a tank or one of its lines develops a visible fuel leak, draining both tanks simultaneously, or creates a fire hazard. If you were to shut it off the engine would quit anyway. If on the other hand you were able to shut off one tank, the opposite tank would still be available for use.
A two-tank gravity flow fuel installation must have a crossover vent line connecting the two tanks in order for the fuel to flow equally from both tanks. This is essential when only one of the tanks is vented to the outside atmosphere. You can see that when you opt for additional fuel tanks, installation as well as fuel management becomes more demanding and complex.
A two-tank installation in a low wing aircraft introduces complexity simply because the tanks are situated below the carburetor level. In order for the fuel to flow up to the carburetor a fuel pump must be installed. This is usually an engine driven pump. Since fuel will not flow uphill and since the engine cannot continue to operate if the engine driven fuel pump fails, a back-up system for the engine driven pump is required. An electrical fuel boost pump is, therefore, installed in most aircraft that do not have a gravity flow fuel system. In addition to providing the fuel flow needed to start the engine, the electric fuel boost pump could keep the engine running in the event the engine driven pump fails. Of course, all this leads to the need for pilot intervention. The complexity grows as the number of tanks increases as each must be provided with its own fuel level indicator. In addition, a fuel selector is needed for more options . . . and unfortunately, more ways to goof up.
If, as in the case of quite a few single seaters and other homebuilts, the second tank is installed in the fuselage as a transfer tank, you will have to install a shut-off valve, a transfer pump, a separate fuel gage for that tank, the usual plumbing and perhaps a one-way valve and some sort of warning light or device to let you know when the transfer pump is pumping fuel into the main tank. In an installation such as that it is possible to forget that the fuel transfer is going on and the main tank will overflow through the vent and overboard if the pump is not switched off in time. This loss of fuel could be dangerous if you need most of it to complete your flight.
Anyone can install a complex system - all it takes is time and money but not many can come up with a complex system that is not dependent on the pilot.