Alwasy been taught never to trust your fuel gauge...

Almost all student pilots are taught "never to trust your fuel gauge..." Why is that the case?

I have been driving for just over 10 years now and the fuel gauge in my car has never let me down.

For the past few years I have been using real world weather addon's and plan my flight using a flight log, and the Headwind/Tailwind makes a huge difference.

So my reasoning is that the fuel gauges in an airplane must be designed/built to a standard that is as good as a car, if not better!

The conclusion I came to is that you should never express the fuel you have as a function of volume, but express it as a function of time by consulting the POH for the power/altitude/temp settings. (of course, never forget to take a peek in the tank for color, contaminants, water)

Is this reasoning correct?

So my reasoning is that the fuel gauges in an airplane must be designed/built to a standard that is as good as a car, if not better!

Unfortunately that isn't the case. So:

Consider where you are in an aircraft, that is, in the air where you can't pull over to the side of the road. Consider how inaccurate aircraft fuel gauges are, and how important fuel quantity is to the safety of flight. Consider the shape of fuel tanks, typically located in the wings (some exceptions, of course) and what dihedral might do as the aircraft sits off-level, as it taxis, as it climbs, descends and turns, perhaps skidding or slipping.

Also, a slight leak in the float driving the gauge and it no longer works at all, plus the effects of turbulence.

So combine all the above with the fact that they often (usually?) are not very accurate, and then consider the large, large number of crashes and forced landings caused over the years by running out of fuel. Add in that a prudent pilot is a belt-and-suspenders type of person, checking and double checking, just in case.

The conclusion I came to is that you should never express the fuel you have as a function of volume, but express it as a function of time by consulting the POH for the power/altitude/temp settings. (of course, never forget to take a peek in the tank for color, contaminants, water)

Unfortunately, before you can do that, you must be sure of the starting point you're working from. You don't always start out with a full tank (so how much is it?), and perhaps your leaning isn't exactly the same as the book is based on, and maybe your particular engine drinks a tad more fuel per hour than the book claims.

And "taking a peek in the tank" doesn't do much good for the factors you mention. You must drain a sample from each tank's low point for that. and that peek might make you think you've got a full tank when you're actually 5 or 6 gallons short, especially on the high-dihedral low wing aircraft, where the outboard part of the tank is higher (AGL) than the inboard part, where the tank fill port is often located.

Perhaps this helps?

Unfortunately that isn't the case. So:

And "taking a peek in the tank" doesn't do much good for the factors you mention. You must drain a sample from each tank's low point for that. and that peek might make you think you've got a full tank when you're actually 5 or 6 gallons short, especially on the high-dihedral low wing aircraft, where the outboard part of the tank is higher (AGL) than the inboard part, where the tank fill port is often located.

 

Perhaps this helps?

Yup, so true. Check the fuel level in the tanks every time with the fuel dip stick for every flight. To be honest, I rarely look or depend on what the fuel gauge reads after dipping the tanks. I know what my very safe flight times are based on average consumption (7-8 Gals/hour for my RV9A) plus leaving enough in the tanks for an extra hour. The gauges are more of a back up reference after seeing the wet line on the dip stick.

They had those plexiglass tubes back in the day when I was flying where you stuck them to the bottom of the tank and held your finger over them while you pulled them out, and then they were graduated according to the aircraft you were flying, pretty much you just checked to make sure the gauges more or less coincided with the dip tube within a tolerance of a few gallons to ensure the gauge hadn't gone completely wonky. They weren't very precise, I never left the ground without an extra hour of fuel beyond what I calculated I would use.

I don't think I ever made a flight long enough that I couldn't carry an hour's reserve. Had to buy fuel at KTTD once though for the return trip back to 65S. About all I remember about that flight was she climbed like a SOB departing at basically sea level with just me and some fuel onboard, and a Mooney from the dealership that left 30 min after I did made it to 65S and back to Troutdale with a signed check for 101LL, (a C185 the FBO I was working for was trying to buy) before I even made it home lol. I was flying a 160 HP Skyhawk, N3857Q.

Almost all student pilots are taught "never to trust your fuel gauge..." Why is that the case?

 

I have been driving for just over 10 years now and the fuel gauge in my car has never let me down.

 

How many different vehicles? I've been driving a while myself. My motto is - "In your main ride, the one you trust, you gotta know where "E" really is." It is sufficiently different across vehicles to warrant attention. I don't say car gauges are bad. Just that the sum of all factors in any specific vehicle results in a different "E."

As for airplanes, I'm a newbie Sim-Only flyer, who does not yet feel I can call myself a pilot. I am amazed at all there is to know. I have gained a special appreciation for pilots. It is a bigger accomplishment than I think is generally understood by the public.

I suspect all my thinking will change as I continue to learn. For now though, regarding airplane fuel - My motto is, "fill the dang thing up."

In my newbie mind, that makes most sense. Eventually I'm sure I will discover the motivation for all the time spent being concerned about it.

How many different vehicles? I've been driving a while myself. My motto is - "In your main ride, the one you trust, you gotta know where "E" really is." It is sufficiently different across vehicles to warrant attention. I don't say car gauges are bad. Just that the sum of all factors in any specific vehicle results in a different "E."

 

As for airplanes, I'm a newbie Sim-Only flyer, who does not yet feel I can call myself a pilot. I am amazed at all there is to know. I have gained a special appreciation for pilots. It is a bigger accomplishment than I think is generally understood by the public.

I suspect all my thinking will change as I continue to learn. For now though, regarding airplane fuel - My motto is, "fill the dang thing up."

 

In my newbie mind, that makes most sense. Eventually I'm sure I will discover the motivation for all the time spent being concerned about it.

Doesn't make ANY sense, unless you're also considering the CG and loadout... Weight and Balance calculations should be considered and calculated before EVERY flight. Otherwise how would you know you're within limits?

regarding airplane fuel - My motto is, "fill the dang thing up."

In the sim that works fine. In real life, though, weight and balance are very important, and you truly do NOT want to exceed max gross weight for the airplane, since performance suffers and overstressing becomes a potential concern. Perhaps you should be aware that just because an airplane has four seats does not mean that it can always carry four people. Think of four people who weigh around 140 lbs each, vs four people who weigh 225 lbs each. Then consider cargo weight. Then consider that avgas is about 6 lbs per gallon.

An owner's manual I have on hand is for the 1968 Cessna 172, so I'll do an example for it. It has a fuel capacity of 42 gallons, and a table in the book shows its range at three different altitudes using "optimum cruise performance." Note that this is statute miles per hour and statute miles of range.

[TABLE=width: 500, align: left] [TR] [TD]Altitude[/TD] [TD]RPM[/TD] [TD]TAS[/TD] [TD]Range[/TD] [/TR] [TR] [TD]Sea Level[/TD] [TD]2490[/TD] [TD]123[/TD] [TD]575[/TD] [/TR] [TR] [TD]5000 ft.[/TD] [TD]2600[/TD] [TD]128[/TD] [TD]600[/TD] [/TR] [TR] [TD]9000 ft.[/TD] [TD]full throttle[/TD] [TD]132[/TD] [TD]620[/TD] [/TR] [/TABLE]

"All figures are based on lean mixture, 38 gallons of fuel (no reserve), zero wind, standard atmospheric conditions and 2300 lbs gross weight."

[TABLE=width: 500, align: left] [TR] [TD]Empty weight[/TD] [TD]1306 lbs[/TD] [TD]moment 47.6[/TD] [/TR] [TR] [TD]Oil, 8 qts[/TD] [TD]15 lbs[/TD] [TD]moment -0.2[/TD] [/TR] [TR] [TD]Fuel, 38 gal usable[/TD] [TD] 228 lbs[/TD] [TD]moment 10.9[/TD] [/TR] [TR] [TD]Pilot & front pass[/TD] [TD]340 lbs[/TD] [TD]moment 12.2[/TD] [/TR] [TR] [TD]Rear pass[/TD] [TD]340 lbs[/TD] [TD] moment 23.8[/TD] [/TR] [TR] [TD]Baggage[/TD] [TD] 71 lbs[/TD] [TD]moment 6.7[/TD] [/TR] [TR] [TD]Total[/TD] [TD]2300 lbs[/TD] [TD]moment 100.9[/TD] [/TR] [/TABLE]

Looking at the graph in the book for these figures puts the weight right at max allowed and the moment just forward of the center of the allowable range.

But what if, instead of 340 lbs of people in front and 340 in back (the FAA's 170 lb person), you had 440 lbs in front? That's 100 lbs overweight, so you'd need to either leave someone behind or offload some fuel (or not fill it up), so you wouldn't get those range figures above.

Another consideration is density altitude, that is, if you are at a higher elevation than sea level and/or a hotter day than the standard 15º C (59º F). Think of an 80º F day at 5,000 feet. Up here we consider the C-172 to usually be a two place aircraft, especially on a hot day. Perhaps if you had 35º F and enough runway (takeoff distance increases a lot in the high country) four people might work.

And how about at Leadville, CO, where the elevation is 9927 ft. You don't fly the C-172 if it's hot, period.

And finally, there's not a gas station on every corner for aircraft, so you have to carefully plan your flight to have enough fuel for the flight, plus 30 minutes' additional fuel if VFR. If IFR you need fuel to the destination, to your planned alternate destination (required) plus 45 minutes additional fuel at cruise speed. Those are the legal minima. A prudent pilot will plan additional fuel in reserve, depending on weather both enroute and at the destination airport.

There are more considerations, but hopefully this gives you a little bit of an idea why "just fill'er up" isn't a good idea in an airplane.