Bad fuel gauges?

Hi all,
A recurring theme seems to be that one should should not rely in fuel
gauges. I can understand that from a safety point of view (I always
dip the tank before start), but I get the feeling that they are
considered just plain inaccurate. Why is that, every car I've driven
has a fuel gauge that seems accurate. Am I missing something?

A connected point is that I was taught that if you start to worry
about low fuel you switch to the lowest tank (2 tanks) and note the
time. When the engine splutters you know what time/range you have left
to find a good landing spot (after switching tanks). But if a gauge
can't be trusted is that the best thing to do?

Cheers

WingFlaps wrote in news:63ec40c3-352b-450a-973d-
:

Hi all,
A recurring theme seems to be that one should should not rely in fuel
gauges. I can understand that from a safety point of view (I always
dip the tank before start), but I get the feeling that they are
considered just plain inaccurate. Why is that, every car I've driven
has a fuel gauge that seems accurate. Am I missing something?

A connected point is that I was taught that if you start to worry
about low fuel you switch to the lowest tank (2 tanks) and note the
time. When the engine splutters you know what time/range you have left
to find a good landing spot (after switching tanks). But if a gauge
can't be trusted is that the best thing to do?


It would depend on the type, and I would only do that if I were really
looking for max range and wanted to ascertain I had a decent head of fuel
in the tank I was using for approach. They really are mostly crap, though,
especially the electric ones found in most airplanes. We have a lot of
problems with them in jets too, believe it or not.



Bertie

WingFlaps wrote:
Why is that, every car I've driven
has a fuel gauge that seems accurate. Am I missing something?

Ah, grasshopper. But how often do you get to actually look at the fuel
in your car's gas tank? How many cars do you think are on the road that
have bunk gas gauges (I have one, for example; my friend has another)?

Many older aircraft use float-type gauges, and no matter how hard you
try, most floats will develop a slow leak and indicate improperly (or
worse, get stuck or bent and refuse to indicate at all.

Newer electric gauges rely on the fuel to act as a dielectric between
two charged plates (like in a capacitor); except that fuel at different
temperatures, in different climates, from different sources, or
different formulations all have varied dielectric constants, meaning
that the gauge could indicate 22 gallons one day and 18 the next; do you
want to bet your life that there's four extra gallons in the tank?

TheSmokingGnu

We have a lot of
problems with them in jets too, believe it or not.

Bertie

Doesn't that mean there's a certain fudge factor in landing weight
calculations?

Given the lack of apparent crashes due to inadequate flap settings I
guess that ya'll pretty much have the innacuracies under control.

different formulations all have varied dielectric constants, meaning
that the gauge could indicate 22 gallons one day and 18 the next; do you
want to bet your life that there's four extra gallons in the tank?

TheSmokingGnu

Ye Olde E6B and zeroing in on real fuel consumption at cruise gleaned
from actual flight should save the day (older planes not up to chart
performance, usually).

IIRC on similar thread(s) the idea was put forth that the regs only
require the indicator to indicate properly when full and when
empty ... don't know where that would be in the regs (if in fact it
is); maybe part 23?

On Feb 23, 9:49*am, wrote:
IIRC on similar thread(s) the idea was put forth that the regs only
require the indicator to indicate properly when full and when
empty ... don't know where that would be in the regs (if in fact it
is); maybe part 23?

That's an unfortunate myth. (The usual form of the myth is to say that
the gauges only have to be correct when the tanks are empty.) Pilots
who belive it are likely to underestimate the importance of the fuel-
gauge airworthiness requirement. The myth was just debunked on the
other fuel-gauge thread:

There's an urban legend that the fuel gauge is only required to be
correct for an empty tank. The legend apparently arises from a
bizarre
misreading of 23.1337b1. What 23.1337b1 actually says is just
clarifying that the 'empty' reading must correspond to zero USABLE
fuel, as opposed to zero TOTAL fuel. There is nothing whatsoever to
suggest that non-empty readings needn't be correct--that would be
absurd. (If it were true, a gauge that ALWAYS says 'empty' would be
legal! You could just write 'empty' on a piece of paper and call that
your fuel gauge!)

The requirement for indications of a tank's fuel level (not just on
empty) is stated in 91.205b9, 23.1305a1, and 23.1337b.

wrote in news:38ce9894-6517-473f-b36e-602d70fa1a94
@z17g2000hsg.googlegroups.com:

We have a lot of
problems with them in jets too, believe it or not.

Bertie

Doesn't that mean there's a certain fudge factor in landing weight
calculations?



Not really. We have procedures to know what the actual fuel on board is
before departure ( we compare the indicated against the arrival and the
uplift) and we have fuel used based on FF as we go to cross check. When the
gauges work, they work well, problem is when there is any moisture in the
tank they go nuts, though. The senders are capicitor type and just a drop
renders then next to useless. That's th ebiggest bugaboo with them, but the
computers can also give difficulites. If things don;'t add up we "drip" the
tanks. There are little tubes on the bottom of the wing we can pull down
and get an accurate reading of the fuel in the tank.

Given the lack of apparent crashes due to inadequate flap settings I
guess that ya'll pretty much have the innacuracies under control.

We pretty much always land at the same flap setting. Just the approach
speed varies. A ton of weight would only mean about a knot in the
difference.


Bertie

On 2008-02-22 20:24:47 -0800, WingFlaps said:

Hi all,
A recurring theme seems to be that one should should not rely in fuel
gauges. I can understand that from a safety point of view (I always
dip the tank before start), but I get the feeling that they are
considered just plain inaccurate.

If an airplane does not have accurate fuel gauges then it is not
airworthy. Knowingly climbing into an aircraft that you know has
inaccurate fuel gauges is both dangerous and illegal. That is why you
check them during preflight. From FAR 91.205:

(a) General. Except as provided in paragraphs (c)(3) and (e) of this
section, no person may operate a powered civil aircraft with a standard
category U.S. airworthiness certificate in any operation described in
paragraphs (b) through (f) of this section unless that aircraft
contains the instruments and equipment specified in those paragraphs
(or FAA-approved equivalents) for that type of operation, and those
instruments and items of equipment are in operable condition.
(b) Visual-flight rules (day). For VFR flight during the day, the
following instruments and equipment are required:
(1) Airspeed indicator.
....
(9) Fuel gauge indicating the quantity of fuel in each tank.
--
Waddling Eagle
World Famous Flight Instructor

On 2008-02-23 07:57:38 -0800, said:

On Feb 23, 9:49*am, wrote:
IIRC on similar thread(s) the idea was put forth that the regs only
require the indicator to indicate properly when full and when
empty ... don't know where that would be in the regs (if in fact it
is); maybe part 23?

That's an unfortunate myth. (The usual form of the myth is to say that
the gauges only have to be correct when the tanks are empty.) Pilots
who belive it are likely to underestimate the importance of the fuel-
gauge airworthiness requirement. The myth was just debunked on the
other fuel-gauge thread:

There's an urban legend that the fuel gauge is only required to be
correct for an empty tank. The legend apparently arises from a
bizarre
misreading of 23.1337b1. What 23.1337b1 actually says is just
clarifying that the 'empty' reading must correspond to zero USABLE
fuel, as opposed to zero TOTAL fuel. There is nothing whatsoever to
suggest that non-empty readings needn't be correct--that would be
absurd. (If it were true, a gauge that ALWAYS says 'empty' would be
legal! You could just write 'empty' on a piece of paper and call that
your fuel gauge!)

The requirement for indications of a tank's fuel level (not just on
empty) is stated in 91.205b9, 23.1305a1, and 23.1337b.

Despite my years of fighting ignorance, many pilots still seem to
believe that the fuel gauge only has to be accurate when it reads 0. In
fact, I have seen both Rod Machado and the Kings assert this. We have
an uphill fight, my brother.
--
Waddling Eagle
World Famous Flight Instructor

There's an urban legend that the fuel gauge is only required to be
correct for an empty tank. The legend apparently arises from a
bizarre
misreading of 23.1337b1. What 23.1337b1 actually says is just
clarifying that the 'empty' reading must correspond to zero USABLE
fuel, as opposed to zero TOTAL fuel. There is nothing whatsoever to
suggest that non-empty readings needn't be correct--that would be
absurd. (If it were true, a gauge that ALWAYS says 'empty' would be
legal! You could just write 'empty' on a piece of paper and call that
your fuel gauge!)

The requirement for indications of a tank's fuel level (not just on
empty) is stated in 91.205b9, 23.1305a1, and 23.1337b.

Thank you!

What you say makes perfect sense. I'll go look at the regs too.