Bad fuel gauges?

Look at the Pilot's Handbook of Aeronautical Knowledge, too.

FUEL GAUGES

The fuel quantity gauges indicate the amount of fuel measured by a sensing
unit in each fuel tank and is displayed in gallons or pounds. Aircraft
certification rules only require accuracy in fuel gauges when they read
"empty." Any reading other than "empty" should be verified. Do not depend
solely on the accuracy of the fuel quantity gauges. Always visually check
the fuel level in each tank during the preflight inspection, and then
compare it with the corresponding fuel quantity indication.

Bob Gardner

wrote in message
...
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.

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

Let me just quote these.

91.205, b9:
"(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."
....
(9) Fuel gauge indicating the quantity of fuel in each tank.

I did my primary training in a plane that had a right wing tank fuel
gauge that always showed less than "F" when the tank was completely
full. If the interpretation of the FAR is STRICT, I guess that means
my training was done illegally, because the gauge DID NOT indicate the
quantity of fuel in the right wing tank (ie, FULL). Rather, it
indicated less than full.

But the writing of the reg is not explicit (they get explicit in
1337b). To me it comes down to ramp checks, FAA enforcement, and case
law. I have never flown that aircraft to the end of its usable fuel,
so I don't if the aircraft I did my primary training in adheres to
1337b:

"(b) Fuel quantity indicator. There must be a means to indicate to the
flightcrew members the quantity of usable fuel in each tank during
flight. An indicator calibrated in appropriate units and clearly
marked to indicate those units must be used. In addition--
(1) Each fuel quantity indicator must be calibrated to read "zero"
during level flight when the quantity of fuel remaining in the tank is
equal to the unusable fuel supply determined under [Sec. 23.959(a);]"

So, what's the case history? How many pilots / mechanics have been
fined, had their certification suspended or even revoked because a
fuel gauge was shown to not indicate "the quantity of fuel in each
tank."

The FAA regs listed only speak to calibration when talking about zero
useable fuel. Nothing else.

If that's the only unambiguous calibration statement in the FARs then
I think that's because the FAA realizes fuel gauge accuracy is subject
to reasonable limitations. Why would they otherwise have us spend so
much time understanding calculated vs real fuel usage, especially
during cross country training for the private certificate? If gas
gauges were really accurate then the FAA wouldn't bug us so much about
learning to calculate and cross-check, and cruise charts wouldn't be
such a critical part of the POH. The recommended way and the way we
train is to KNOW YOUR AIRCRAFT through repeated measurement and cross-
checking the perf charts.

No FAA or other training manual says "Just check you gas gauge real
quick to see if you need to pull over for gas -- and make sure you
land and refuel when the idiot light comes on!"

Gas gauges provide solid value as indicators of possible leak
situations and when you are almost out (again, calibration is mandated
to be correct only at zero).

Lawyers specialize in writing that is crystal clear when they want it
to be, and subject to court judgement otherwise. I think the fuzzy
language the government lawyers used in crafting the regs around fuel
gauges was done on purpose.

wrote:
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).

Well, even there you have significant challenges; you have to account
for the fuel in takeoff/climb (according to power/leaning), as well as
in descent, and you have to somehow figure out how much you've used in
taxi and runup. There are still large areas where the best you can do is
guesstimate at actual fuel usage.

Even then, unless you have a penchant for wingwalking, it's exceedingly
difficult to measure actual fuel usage in cruise without the fuel
gauges, which we have already established as really "best guess"
instruments anyway.

TheSmokingGnu

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

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:

Horse****.




(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

...
(9) Fuel gauge indicating the quantity of fuel in each tank.


To what precision and accuracy and where defined? More than half but less
than full? More than empty but less than a gallon? More than ten gallons
and less than 11? Cite accuracy regs, please.

Jim

On 2008-02-23 18:41:51 -0800, "RST Engineering" said:


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:

Horse****.




(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

...
(9) Fuel gauge indicating the quantity of fuel in each tank.


To what precision and accuracy and where defined? More than half but less
than full? More than empty but less than a gallon? More than ten gallons
and less than 11? Cite accuracy regs, please.

Jim

I have had this issue come up in a ramp check. The inspector insisted
that the plane was not airworthy because the gauges read half full and
the tanks were full.

If there is a regulation specifying the tolerances for accuracy, I am
unaware of it. However, the regulations do specify that you must have
fuel gauges indicating the quantity of fuel in each tank and that the
gauges must be calibrated so that "0" means 0 usable fuel. I guess it
depends on how far you think you can get away with it if you are ramp
checked.
--
Waddling Eagle
World Famous Flight Instructor

In article
,
says...


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?


What kind of car do you drive?
I've never yet driven a car with an accurate fuel gauge. Usually they go
down very slowly for the first half, then much more quickly - exactly the
opposite of the response you would want if your life were to depend on
it.

Fuel gauges in small airplanes often have problems, and should not be
relied upon. There are better methods for soiuynd fuel management.

This said, thje later-generation (post GARA) airplanes have made huge
strides in this respect, and I definitely have a much higher level of
copnfidence in the gauges of a 172-R or a 182-S than the older models.

On Feb 22, 11:24*pm, WingFlaps wrote:
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.

The admonition not to rely on fuel gauges is half-right. The half
that's right is important; the half that's wrong is dangerous.

The half that's right is that you need another way to know how much
fuel there is, because the gauges have limited accuracy, and because a
previously working gauge can always fail suddenly. For the GA planes
I'm familiar with, a visual preflight inspection of the tanks is
required by the POH. From there, you calculate your fuel consumption.
If the gauges say you've got more fuel than you expect, trust your
calculations instead (double-checking if necessary). Aviation safety
is about redundancy.

The half that's wrong is that the admonition ignores a vital
possibility: the gauges may show way LESS fuel than you expect. (Say
you're supposed to have 3/4 of a tank left, but the gauge shows 1/8.)
In that case (if the gauges are working well enough to meet the
airworthiness requirements) you know you've got a problem--perhaps a
fuel leak, which your consumption calculations can't warn you about
without the gauges. A leak may be a very infrequent event, but
aviation safety is about being prepared for unlikely problems, rather
than unnecessarily relying on the gamble that it will never happen to
you.

The Part 91 airworthiness regs (which pilots are required to know
before being allowed to solo) mandate a gauge that indicates the fuel
level in each tank. No specific accuracy is mandated, either in Part
91 or in the aircraft-certification regs in Part 23. So it becomes a
matter of common sense: a working fuel gauge has to be accurate enough
to serve its intended purpose, which (in familiar light GA planes
anyway) is to provide a rough cross-check of the consumption
calculations, to warn of a leak or other problem.

The half that's wrong is that the admonition ignores a vital
possibility: the gauges may show way LESS fuel than you expect. (Say
you're supposed to have 3/4 of a tank left, but the gauge shows 1/8.)
In that case (if the gauges are working well enough to meet the
airworthiness requirements) you know you've got a problem--perhaps a
fuel leak, which your consumption calculations can't warn you about
without the gauges. A leak may be a very infrequent event, but
aviation safety is about being prepared for unlikely problems, rather
than unnecessarily relying on the gamble that it will never happen to
you.

Very good point.

A fuel guage rapidly dropping toward E would cause me to go for the
closest airport (hopefully with gas -- but I wouldn't insist on that!).

In article ,
wrote:

A leak may be a very infrequent event, but
aviation safety is about being prepared for unlikely problems, rather
than unnecessarily relying on the gamble that it will never happen to
you.

But aviation safety is not about being prepared for every single problem
no matter how unlikely. Aviation safety is about reducing the residual
risk to an acceptable level. These levels are defined in AC 23.1309
and AC 25.1309.


The Part 91 airworthiness regs (which pilots are required to know
before being allowed to solo) mandate a gauge that indicates the fuel
level in each tank. No specific accuracy is mandated, either in Part
91 or in the aircraft-certification regs in Part 23. So it becomes a
matter of common sense: a working fuel gauge has to be accurate enough
to serve its intended purpose, which (in familiar light GA planes
anyway) is to provide a rough cross-check of the consumption
calculations, to warn of a leak or other problem.

You've just added an "intended purpose" with the claim that the gauge
is there to warn of a leak, etc. The reg states the intended purpose,
that is, to indicate the fuel level. If a aircraft manufacturer wants to
add another intended purpose, then it is allowed to do that. In that
case, it would also be obligated to include that in the Failure Hazard Analysis.

--
Bob Noel
(goodness, please trim replies!!!)