Why no reserve tanks?

[Dan Thomas]

How would you measure the fuel in a tank that is sloshing around? What about in a nose high attitude or nose low attitude?

For a simple aircraft like the cherokee 140, visually confirming the fuel in the tank and watching the clock is almost fool-proof. In other words, more technology isn't needed to reduce the risk of running out of fuel to an acceptable level. Adding some magic fuel gauge would just mean more dollars not spent on fuel to, ya know, fly the airplane.

There is available to homebuilders a capacitance gauge kit. These things have been used for years on big airplanes. There are no moving parts; the device relies on the difference in the dielectric properties of air and fuel; a couple of concentric tubes reach into the tank and an AC signal is applied and electron movement is gauged and converted into a fuel level reading. In tanks that are subject to varying levels due to sloshing or attitude changes, several probes are used and summed to give an accurate reading in any normal maneuver.

http://www.aircraftspruce.ca/catalog/eppages/skysportfuelsys.php

And it's not really expensive. But Cessna, at least, still uses float-driven potentiometers in its brand-new airplanes. 1950s technology, in airplanes that have complex computing power available. It's silly.

Dan

 

[Everskyward]

Here's another vote for more accurate fuel gauges. Totalizers are great but you need to know what you started out with and they don't account for leaks. Henning's example of airliners many times taking off with less than full fuel is fine but they are also equipped with better fuel gauges so that know what they started with and they know what they have left to a much higher degree of accuracy.

How much would it cost to install capacitance fuel probes in small airplanes. Probably too much...

The B-747-100 FQIS measures fuel volume and density, allowing computation of fuel weight to be displayed on cockpit gages for each fuel tank. A group of capacitive fuel quantity probes are distributed throughout the volume of a given fuel tank. (See Figure 2-3.) Each fuel quantity probe consists of an electrically conductive tube surrounded by an outer tube. (See Figure 2.1-1.) When alternating electrical voltage is applied across the tubes, a displacement current will flow between them. The relationship between the voltage and current depends upon the capacitance between the two conductive tubes. As these probes are immersed in fuel, the capacitance value changes in proportion to the fuel level.

http://www.tpub.com/content/nasa2000/NASA-2000-tp209867/NASA-2000-tp2098670023.htm

 

[flyingcheesehead]

Maybe a dumb question, but I know boats and motorcycles use them, for example, and a no-fuel situation in these are less critical than on airplanes. So why not on airplanes? Is the performance hit too much? I think not since you are required to carry a reserve amount anyway, the extra weight carried makes no difference. Wouldn't it be a good idea to have reserve tanks that hold 1 hour of fuel?

There are plenty of safety features already designed into aircraft that don't relieve the pilot of the responsibility of good planning, so it seems to me that there is really no negative impact of having some reserve system designed into the aircraft in addition to the planning of the pilot. A simple one would be the standpipe feature mentioned above used in some motorcycles (I originally thought it was a separate tank for reserve fuel, but the different pipe lengths is a more elegant solution). At least if you've neglected to plan sufficiently for the flight and run out of fuel, the engine sputtering and stopping can allow you to restart and focus on getting safely to an airport rather than panicking and causing unnecessary harm.

Lots of negative impacts:

1) Even if the fuel weight is the same, the extra tanks/ports/lines/selectors/etc. required are going to add weight.

2) If we can screw up with a fuel system as dirt simple as "BOTH" with a gravity feed, why would adding an extra selector make things better? One more thing to forget. An unexpected engine stoppage is NOT a good way to alert a pilot that he's starting to use his reserves.

3) What happens when you're on final to your destination and your engine quits to alert you that you're running on reserves? You may crash short of the runway before you get the engine running again.

4) If the fuel is in separate tanks, you're creating a situation where the pilot either has to run tanks dry to know where that reserve fuel is, or have the reserve fuel spread among several tanks which can result in unexpected stoppages during a low-fuel situation as well. Bad.

Great intentions - Terrible idea.

It floors me that the most brilliant minds on the planet still can't build an affordable, accurate, fuel gauge for light aircraft that can also make it's way through the Certification process.

The best we've come up with (and mind you, it's good and cheap) is an impeller in the fuel flow and a human to push the fill-up button on the totalizer?

What would you pay for an accurate fuel gauge in your aircraft? Is it an economic problem? Are we all too cheap?

There *are* certified fuel gauges - I think transport category aircraft are probably required to have them, and I think there are some light aircraft that have them as well (Cessna 400-series?).

In terms of light single-engine airplanes, well, again, the technology exists and here I have to point my finger squarely at liability concerns and the certification process.

The certification process for such things is now such a pain in the ass that the "affordable" part of these fuel gauges would go away. How many pilots do you know that would spend a fortune on STC'ed fuel gauges instead of new avionics, paint, interior, etc? After all, it's all those OTHER guys who are dumb enough to run out of fuel. And those guys aren't going to pay for gauges either. So where's the market?

Then, you have to realize that regardless of how good the gauges are, the universe is always building better idiots. There WILL be people that run out of fuel regardless of how good the gauges are, and they know they'll be sued out of existence the first time someone crashes with their gauges aboard.

So, no market and the prospect of inviting repeated legal actions until the company is gone - Again, why should someone do this?

 

[Bob Noel]

[snip]
And it's not really expensive. But Cessna, at least, still uses float-driven potentiometers in its brand-new airplanes. 1950s technology, in airplanes that have complex computing power available. It's silly.

Dan

Interesting, but it is still applying a "solution" to a problem that simply doesn't exist for the really simple aircraft.

Sometimes good enough is good enough.

 

[COFlyBoy]

What are you flying?

'66 PA-32-260. The POH says to use the mains first. I think this is a main spar carry through strength thing. They want fuel weight out at the tips to reduce the bending moment on the carry through.

 

[Henning]

'66 PA-32-260. The POH says to use the mains first. I think this is a main spar carry through strength thing. They want fuel weight out at the tips to reduce the bending moment on the carry through.

Yeah, I used to use the same system. I'd burn half out of each main then burn down the tips in 15 minute increments until empty and then switch back to the mains.

 

[AEROBAT]

The only time I believe the old cessna gauges is when they read low, other wise I depend on my watch and the average fuel burn I have previously experience.

 

[Dan Thomas]

Interesting, but it is still applying a "solution" to a problem that simply doesn't exist for the really simple aircraft.

Sometimes good enough is good enough.

Cessna's "good enough" gauges cost too much. One fuel sender costs much more to replace than it costs to buy the whole non-certified capacitance gauge setup. It's no wonder kitplanes are becoming so popular.

Dan

 

[Bob Noel]

Cessna's "good enough" gauges cost too much. One fuel sender costs much more to replace than it costs to buy the whole non-certified capacitance gauge setup. It's no wonder kitplanes are becoming so popular.

Dan

Note that I've never included the fuel gauge in the good enough box. Visually inspecting the fuel and using the clock is good enough. The fuel gauges in my cherokee 140 take up panel space. I don't use them for anything at all.

 

[flyingcheesehead]

'66 PA-32-260. The POH says to use the mains first. I think this is a main spar carry through strength thing. They want fuel weight out at the tips to reduce the bending moment on the carry through.

I doubt that's it - If it can handle the weight at takeoff, it can handle the weight in flight - After all, it's getting lighter either way.

In fuel-injected airplanes with Continentals, there is a fuel return line that goes back to different tanks on different airplanes - Some go back to whichever tank is selected, some go back to a specific tank, some do something in between. Either way, if you're pumping fuel back into a full tank, it'll go overboard out the vent so you need to burn fuel from that tank first.

Since the PA32-260 isn't fuel injected, I'm not sure what the deal is - Maybe they just want to be feeding from shorter lines for takeoff in case of a fuel starvation problem?

Does the POH say that you must run the mains dry prior to burning from the aux tanks? (Doubt it.) Like many airplanes, the POH probably says to burn from the aux tanks only in cruise or something like that, and many of them also want you to be on the mains for landing. So, the most generally usable solution (that will work for the largest number of airplanes) is to burn from the mains for taxi, takeoff, climb, and the initial portion of cruise, switch to the auxes and run them dry if the flight's long enough, and then descend and land on the mains.

 

[jasc15]

An unexpected engine stoppage is NOT a good way to alert a pilot that he's starting to use his reserves.

I don't mean to suggest this as being a good way of alerting the pilot. What I mean is that it's no worse than running out of fuel with the added benefit of actually having fuel to get yourself on the ground safely.

 

[Fearless Tower]

On my private pilot checkride, I got "yelled" at for not immediately switching tanks AS I was going to best glide airspeed. The reason being, the fuel is a most common reason for engine failures and you might not need to go any further than just switching tanks.

Meanwhile, if it wasn't fuel related, you'd probably be entering a stall spin about that time.

You should have punched that DPE in the throat.

 

[flyingcheesehead]

I don't mean to suggest this as being a good way of alerting the pilot. What I mean is that it's no worse than running out of fuel with the added benefit of actually having fuel to get yourself on the ground safely.

The problem is, to those who will run out of fuel, it's just another tank. They'll run it dry too.

To those who won't run out of fuel, it creates more potential problems with fuel management, additional complexity with the airplane (higher maintenance costs), and you might experience fuel starvation on final approach on an otherwise properly planned flight.

 

[COFlyBoy]

I doubt that's it - If it can handle the weight at takeoff, it can handle the weight in flight - After all, it's getting lighter either way.

In fuel-injected airplanes with Continentals, there is a fuel return line that goes back to different tanks on different airplanes - Some go back to whichever tank is selected, some go back to a specific tank, some do something in between. Either way, if you're pumping fuel back into a full tank, it'll go overboard out the vent so you need to burn fuel from that tank first.

Since the PA32-260 isn't fuel injected, I'm not sure what the deal is - Maybe they just want to be feeding from shorter lines for takeoff in case of a fuel starvation problem?

Does the POH say that you must run the mains dry prior to burning from the aux tanks? (Doubt it.) Like many airplanes, the POH probably says to burn from the aux tanks only in cruise or something like that, and many of them also want you to be on the mains for landing. So, the most generally usable solution (that will work for the largest number of airplanes) is to burn from the mains for taxi, takeoff, climb, and the initial portion of cruise, switch to the auxes and run them dry if the flight's long enough, and then descend and land on the mains.

1- No fuel return issues and there's no complicated venting/fuel cross-feeds like some of the Cessnas
2- The POH only says to "Use the main tanks first", there are no restrictions as to which tank to land on other than the checklist saying select fullest tank
3- The PA-32 does have a maximum zero fuel weight which is usually indicative of main spar carry through limits (most likely the landing loads, not flight loads). Carrying your fuel at the tips lessens the landing loads on the carry through.

 

[COFlyBoy]

To those who won't run out of fuel, it creates more potential problems with fuel management, additional complexity with the airplane (higher maintenance costs), and you might experience fuel starvation on final approach on an otherwise properly planned flight.

Yep, a replacement fuel valve for the PA-28s/32s with 4 tanks is not cheap.

 

[flyingcheesehead]

3- The PA-32 does have a maximum zero fuel weight which is usually indicative of main spar carry through limits (most likely the landing loads, not flight loads). Carrying your fuel at the tips lessens the landing loads on the carry through.

If it's a spar carry-through thing, there could be negative-G issues with landing as well, which fuel in the tips wouldn't help.

What's the exact wording of what the POH says? "Mains first" is much different from "mains dry first". I just can't see why they'd have you burn completely through the mains before using anything from the auxes.

 

[Everskyward]

I don't mean to suggest this as being a good way of alerting the pilot. What I mean is that it's no worse than running out of fuel with the added benefit of actually having fuel to get yourself on the ground safely.

In thinking about it it seems to me it would be much easier to install and idiot light such as there are on cars.

 

[Jaybird180]

Heh. Fuel exhaustion procedure checklist: Initiate steep turn to the left.

Wouldn't help you if the turn were coordinated. Knife Edge flight is more like it.


To everyone that says use time for flight planning: What do you do if your watch stops (serious question)?

 

[brcase]

A Reserve Tank or at least the Reserve selector like many motorcycles use has been done before. I know the F4U Corsairs used this system. Reviewing the check list one selected the Reserve setting for Take off and landing.

I personally like this system. it does add another valve but is still pretty simple and reliable. You plan your flight so you don't need the reserve. But if you have less fuel on board or burn more than you pla, the engine will quit at a safe altitude giving you time to switch to reserve and you know you only have a set amount of fuel (45 minutes for example) available.
The new fuel flow meters are pretty good, but the rely an there being a a know quantity of fuel available.

Brian

 

[Greg Bockelman]

To everyone that says use time for flight planning: What do you do if your watch stops (serious question)?

Well, you have the airplane clock. You also have a clock in you GPS, I think. May be GPS dependent. And you have your flight planning.

What I HAVEN'T seen mentioned yet is what if your tank developed a leak? What if you accidentally left the gas cap off? A watch won't help with that. A fuel totalizer won't help with that.

 

[Fearless Tower]

To everyone that says use time for flight planning: What do you do if your watch stops (serious question)?

I've never been in an airplane without more than one time keeping device:

Watch
GPS (panel mounted or portable)
Panel mounted clock
Portable timer in flight bag
Hobbs meter in most rental aircraft
Cell phone
Tachometer (not 100 percent correct time depending on RPM, but close enough to approximate and get you safely on the ground before your fuel runs out).

 

[steingar]

My motorcycle doesn't have a reserve tank. I don't much miss it either. Doesn't have a fuel gauge either, which is a little silly in this day and age.

 

[denverpilot]

Wouldn't help you if the turn were coordinated. Knife Edge flight is more like it.

Insert "You haven't seen my steep turns!" joke here.

To everyone that says use time for flight planning: What do you do if your watch stops (serious question)?

Well, since in the FAA's infinite wisdom to keep my aircraft certified for IFR, I had to pay 80 bucks for an electric clock, I guess I always have that backup... as well as the three other timekeeping devices on board.

(iPhone clock, iPad clock, wristwatch -- when I bother to wear it, aircraft clock, and ASA flight timer.)

And of course, there's always... "Hey Center... What time is it? My watch just went Tango Uniform here..." Which probably wouldn't get you the time right away, but would probably garner some good jokes from someone on frequency. And some bad ones too, like "Time to buy a watch!"

 

[Fearless Tower]

Wouldn't help you if the turn were coordinated. Knife Edge flight is more like it.

Actually, I think you have it reversed - think about the fuel system - it tends to follow gravity so even in a coordinated turn, the fuel is going to want to flow to the low point which is usually the belly of the plane.

If you knife edge it (basically a skid) and have fuel in the uphill wing, the g-force is going to want to hold the fuel in the tank instead of letting it flow to the low point.

Think of the fuel as the ball - do you want it down in the center where the engine is or do you want to keep it up in the wing tip?

 

[SkyHog]

Reserve tanks for a motorcycle? I have not heard of a specific tank for them. Possibly, but it has been my experience that for motorcycle the most common arrangement was a reserve position on the fuel selector. This really was not a reserve tank but just a valve that would allow one to take fuel from the lowest part of the tank. The idea was that if you were running in the main position and the engine stopped due to fuel starvation then selecting reserve would let you use that last few ounces to pints or gallon to get to the next fuel stop. But the tank itself had no partitions in it.

Of course if you forget to put the selector back to main after refilling and ran out again. You were would have to push your bike to the next stop.

As for airplanes it is an interesting idea. But like the motorcycle idea if one were to become dependent upon it, what would happen when they forgot to set it right at take off? If the pilot is already so lazy as to not be calculating their fuel consumption I cannot help but wonder how well they would be following their check lists.

If there was such a fuel system how much needs to be in reserve? On my Piper Warrior 8 gals would easily give me an hour an a 100NM range, but what if the nearest airport was 150 NM because I was flying in the wilderness? That reserve bought me nothing but the ability to plan my crash. The point is that the reserve amount varies for each flight. Because of that fact having a dedicated tank is not very useful. Instead when planning the flight part of the total fuel load becomes the designated reserve and the pilot monitors fuel consumptions such that if they touch their reserve they take the appropriate and preplanned actions.

My old bike (83 Honda CX650C) had a reserve tank. When running on the main tank, it would die with approximately 30 miles left. When it dies, you reach down, flip to reserve and find a gas station.

That was my method of ensuring maximum endurance from a tank. It worked really well, and would work for an airplane too. Maybe have a checklist item to switch to reserve on final to ensure you don't lose power at a low altitude.