G/A Light Aircraft ditches off Leighton Beach, WA
Quite a few pilots who flew AWS got a nasty fright when the fuel in the auxes ran out before they were supposed to... She also had a quirky port aux tank too - you could fill it to the brim then stand there & watch the fuel drop then top it off again. Nobody could ever figure that one out!
DF.
DF.
The need to top up after a pause had me intrigued. At first I thought perhaps due to trapped air not allowing fuel to fully fill the tanks initially. But found an archived Pprune thread on "C402B Fuel System" where a reasonable explanation is offered (assuming it can also be applied to VH-AWS):
"Towering Q
20th Jan 2008, 17:32
When refuelling...fill aux first then start on mains. By the time the main tank is full the level in the aux should have dropped. The fuel takes a little longer to flow through the pipe work and into the second aux tank."
Return of excess fuel to main tanks and risk of then venting it overboard is also described in some of the posts on that old thread.
The Cessna 310 & 402s had bladder type Aux’s. They took a little while to expand which is why if you wanted ‘full tanks’, you filled the aux’s first, then the mains and then topped up the aux’s. Most operators also left a small amount of fuel in the aux’s so the rubber bladders didn’t dry out, as they weren’t always used that often.
I have flown AWS. Don’t recall it being any better or worse than other 402’s of that vintage.
I have flown AWS. Don’t recall it being any better or worse than other 402’s of that vintage.
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I like having Left, Right, Both, and Off selections on the fuel valve. However, some valves of that type allow cross feed when Off which means Left or Right must be selected if parked on a significant slope. Forget to do that and the high tank fills the low tank and fuel vents overboard.
I’m not aware of a low wing (certified) GA aircraft with a ‘both’ selection on the fuel selector.
There's a few reasons for a good fuel log and regularly swapping tanks. First and foremost is to crosscheck that your usage vs the gauges makes sense. Used 30 minutes in a Pa28, the gauges should be down 12-18 liters (or USG equiv), swap tanks and run the other one and repeat the check, have a log to show you verified each time. Then you have in-flight evidence you are complying with whatever fuel burn you planned on, even the latest gadgets with fuel flow gauges showing used fuel/fuel remaining can be wrong and only show feed into the engine or what the engine has taken in, not accounting for things like leaks and so on. You also have verification your fuel gauges are remotely accurate, and some protection against identifying fuel leaks early on. In aircraft with bladder tanks and inter-tank connections/ baffles etc, it's important to check that what you expect to see on the gauge is verified, sudden or unexpected changes (or no change at all, ie using fuel but the gauge stays still) in indicated fuel load could be indications of blockages and problems inside the tanks, that is your tank is not capable of supplying the fuel it's supposed to have in it, if you are not regularly checking the fuel vs a log you will probably miss these things. Then there's the chance of stuff in the tanks blocking ports/filters/breathers and whatever other holes, regular swapping of tanks will mean you catch this problem with fuel still in the other tank, hopefully, if you ran one dry and then the other had a blockage at half capacity, you now have no fuel and it's paddock time.
Lateral balance goes without saying, if you enjoy driving with the brakes on then fly with an imbalance, obviously you don't mind paying for extra fuel and sector lengths.
Lateral balance goes without saying, if you enjoy driving with the brakes on then fly with an imbalance, obviously you don't mind paying for extra fuel and sector lengths.
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PA28 with it's 2 tanks can be a nightmare to manage! Why do they make it so hard? 
I have never found it hard to manage and I like being able to run on one tank or the other. If I think I'm going to be tight on fuel I run one tank down to 5 gals indicated then run the other to zero but not to exhaustion. I know I have 5 gals left.
I’m not aware of a low wing (certified) GA aircraft with a ‘both’ selection on the fuel selector.
Thanks RG! I’ve learned something.
I’ll have to do some research into why it’s OK on that type.
I’ll have to do some research into why it’s OK on that type.
14 CFR 23.993 Fuel valves and Controls does not mention wing position.
High wings typically gravity feed with no pump unless injected. Low wings need a pump (actually at least two) whether injected or not. Why would the wing position influence the available fuel valve selections? If not specified in 23.993 where would such a requirement be specified? Isn't it just a designer choice.
High wings typically gravity feed with no pump unless injected. Low wings need a pump (actually at least two) whether injected or not. Why would the wing position influence the available fuel valve selections? If not specified in 23.993 where would such a requirement be specified? Isn't it just a designer choice.
14 CFR 23.993 Fuel valves and Controls does not mention wing position.
High wings typically gravity feed with no pump unless injected. Low wings need a pump (actually at least two) whether injected or not. Why would the wing position influence the available fuel valve selections? If not specified in 23.993 where would such a requirement be specified? Isn't it just a designer choice.
High wings typically gravity feed with no pump unless injected. Low wings need a pump (actually at least two) whether injected or not. Why would the wing position influence the available fuel valve selections? If not specified in 23.993 where would such a requirement be specified? Isn't it just a designer choice.
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High positioned fuel tanks will provide a natural head of pressured fuel to the selector, via gravity. Fuel drawn through the system via pumps can result in air being drawn into the lines, so in order to have a both selector on say a PA-28, you would have to introduce some active protections in each fuel tank to prevent air being drawn in, or pumps in the tanks, therefore extra cost, complexity and weight.
What special preventive measures are included in the Rockwell Commander design?
Sec. 23.951 — General.
(a) Each fuel system must be constructed and arranged to ensure fuel flow at a rate and pressure established for proper engine and auxiliary power unit functioning under each likely operating condition, including any maneuver for which certification is requested and during which the engine or auxiliary power unit is permitted to be in operation.(b) Each fuel system must be arranged so that—
(1) No fuel pump can draw fuel from more than one tank at a time; or
(2) There are means to prevent introducing air into the system.
(c) Each fuel system for a turbine engine must be capable of sustained operation throughout its flow and pressure range with fuel initially saturated with water at 80 °F and having 0.75cc of free water per gallon added and cooled to the most critical condition for icing likely to be encountered in operation.
(d) Each fuel system for a turbine engine powered airplane must meet the applicable fuel venting requirements of part 34 of this chapter.
I can't answer why the Rockwell design is allowed, it may have floated tank outlet covers that prevent air (simple method) or something else. Gravity pressure is the means to prevent air in the 'high wing' system, and they don't need fuel pumps so both (1) and (2) are satisfied. On more advanced systems then you might have an interconnect system or hopper tanks within the fuel tank etc...
A simple way to think about it, is that with gravity the fuller tank will provide higher pressure, so that will drain first, keeping balance. When you are 'sucking' the fuel through, the line that provides least resistance will get priority, so any air in the system will be sucked first.
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That confirms my understanding, 43.
Interestingly, the report of this ATSB investigation, following a fuel-related forced/precautionary landing involving a Rockwell International 114 at Phillip Island on 30 October 1994, says among other things:
One wonders how the type made it through certification. The ATSB report also says this:
But why have the BOTH position in the type in the first place, if it demonstrably results in air being introduced into the fuel system?
Interestingly, the report of this ATSB investigation, following a fuel-related forced/precautionary landing involving a Rockwell International 114 at Phillip Island on 30 October 1994, says among other things:
A study of the fuel system suggests that if one tank is empty and the other contains fuel, then it is possible that the engine will be supplied with a "fuel air cocktail" through the fuel selector if the selector is in the BOTH position. Tests done on another Rockwell 114 confirmed that this was probably the cause of the engine malfunction.
n the Emergency Procedures section of the Pilot's Operating Handbook, for the Engine Failure in Flight checklist, item 5 of the checklist says "Fuel Selector - FULLEST TANK (check other two positions)". In the Airstart checklist, item 2 of the checklist says "Fuel Selector - FULLER TANK". This item is followed by a note which says "To minimise restart time, select the fuller tank. Do not use the BOTH position". Although the pilot was not attempting an airstart, he did leave the selector in the BOTH position.
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I'm still not understanding the high wing / low wing distinction here. Any high wing aircraft with a fuel injected engine will have at least two fuel pumps in series. How is the risk of air ingestion any different for such a high wing aircraft with a Both selection than it is for a low wing aircraft with a Both selection.
I'm still not understanding the high wing / low wing distinction here. Any high wing aircraft with a fuel injected engine will have at least two fuel pumps in series. How is the risk of air ingestion any different for such a high wing aircraft with a Both selection than it is for a low wing aircraft with a Both selection.
Precisely!
Warning: Adult supervision required.
One fuel tank is the cup and the other could be an empty cup but is the open air because the other tank has no usable fuel. Note that both tanks are below the inlet to the 'fuel pump'.
Now imagine the cups were above the guy's head and the straws were pushed through a hole drilled in the bottom of each.
One fuel tank is the cup and the other could be an empty cup but is the open air because the other tank has no usable fuel. Note that both tanks are below the inlet to the 'fuel pump'.
Now imagine the cups were above the guy's head and the straws were pushed through a hole drilled in the bottom of each.
Last edited by Clinton McKenzie; 2nd Dec 2023 at 05:55.
Gravity pushes fuel into the system with high wings, meaning weight of fuel stops air from entering, so if one tank gets close to dry the full tank will still be pushing fuel to the selector. Without gravity, the fuel pump sucks fuel through the system, and most simple light singles will have both pumps forward of the firewall, not pushing from each tank. Thst means if one tank gets close to empty it will favor sucking air over fuel as nothing is pushing the fuel towards the selector from the fuller tank. Try take a drink from a cup using two straws, one in the liquid one outside in the air. You wont be sucking much fluid, unless you turn the cup so that the liquid flows into the straw.
Are you saying that this regulation is not applicable to high wing tanks because the pump does not "draw" fuel but always has a gravity feed to the input port?