Bad fuel gauges?

On Feb 24, 7:15*am, Bob Noel
wrote:
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. *

Agreed. Or more precisely, it's about reducing the risk-to-cost ratio
to an acceptable level. Some particular small risk might be deemed
acceptable if it would cost $100,000 to avoid, but unacceptable if it
would cost $100 to avoid.

These levels are defined in AC 23.1309 and AC 25.1309.

The levels are implicitly defined throughout the FARs. For instance,
whenever some item of equipment is deemed necessary for airworthiness,
the FAA is thereby stipulating that the risk of not having that
equipment (in operable condition) is unacceptable, compared to the
cost of having and maintaining that equipment.

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.

Indicating the fuel level is WHAT the gauge is required to do. We need
to consider WHY it's required to do that if we want to draw a common
sense conclusion about what kind of accuracy is required (since the
regs don't specify it quantitatively). If we can infer the gauge's
intended purpose, then common sense tells us the gauge is supposed to
be at least accurate enough to be usable for that purpose.

We all agree that fuel gauges are typically much less accurate than
flow calculations or flow measurements. Yet the FAA requires the
gauges, not just the calculations and flow measurements. And one
obvious reason is that the calculations and flow measurements don't
take into account the possibility of a leak. That's not a
controversial explanation, is it?

In article ,
wrote:

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. *

Agreed. Or more precisely, it's about reducing the risk-to-cost ratio
to an acceptable level.

No. Cost is not part of the equation wrt reducing risk, at least as far
as the FAA is concerned. If you were an applicant and tried to get
an aircraft certified that didn't meet the standards in AC 23.1309 or
AC 25.1309 because it would cost too much, the FAA would deny
the application.



Some particular small risk might be deemed
acceptable if it would cost $100,000 to avoid, but unacceptable if it
would cost $100 to avoid.

More important is the hazard, not the risk.




These levels are defined in AC 23.1309 and AC 25.1309.

The levels are implicitly defined throughout the FARs. For instance,
whenever some item of equipment is deemed necessary for airworthiness,
the FAA is thereby stipulating that the risk of not having that
equipment (in operable condition) is unacceptable, compared to the
cost of having and maintaining that equipment.

Have you read AC 23.1309 or AC 25.1309?

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

On Feb 24, 4:10 pm, Bob Noel
wrote:
In article ,
No. Cost is not part of the equation wrt reducing risk, at least as far
as the FAA is concerned. If you were an applicant and tried to get
an aircraft certified that didn't meet the standards in AC 23.1309 or
AC 25.1309 because it would cost too much, the FAA would deny
the application.

I see what you're getting at. Those ACs indeed specify a maximum
acceptable probability for e.g. a catastrophic failure, regardless of
the cost of keeping the probability within that bound. But that's
still consistent with my point about cost, for three reasons.

First, the decision where to set the acceptability threshold is
already informed by the FAA's knowledge of what threshold is
affordable. The ACs' acceptable probability of catastrophic failure,
especially for the less expensive classes of GA aircraft, is high
enough to allow many fatalities per year across the fleet. If much
higher safety were achievable at a reasonable cost, the FAA would
presumably have set the probability threshold lower.

Second, for the more expensive classes of GA aircraft, that threshold
IS set lower, by two or three orders of magnitude! Presumably, that's
in part because the bigger planes can afford to meet higher safety
standards--standards that would swamp the cost of the smaller planes.

Third, those ACs set a CEILING for acceptable failure probabilities.
Unless I've missed something, there's nothing in the ACs to prevent
the FAA from deciding that a particular item of safety equipment is
required for airworthiness, even if the absence of that equipment
would still leave the catastrophe probabilities within the standards
set by the ACs. And cost is surely a factor in making THOSE decisions.
(For example, if ADS-B technology cost $500,000 per plane, the FAA
would not be proposing to require it.)

The best fuel gauge is a watch. If the book says that your plane burns 7.4
gallons an hour, subtract 8 gallons from your usable fuel figure and use
that number in all further calculations. Establish a conservative
wheels-on-the-ground time...."Let's see, it's noon now, I have fuel for four
hours on board, I will have the wheels on the ground somewhere (maybe not my
destination), at 3:30."

Bob Gardner

"WingFlaps" wrote in message
...
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

On Feb 23, 7:10*pm, "Bob Gardner" wrote:
The best fuel gauge is a watch. If the book says that your plane burns 7.4
gallons an hour, subtract 8 gallons from your usable fuel figure and use
that number in all further calculations. Establish a conservative
wheels-on-the-ground time...."Let's see, it's noon now, I have fuel for four
hours on board, I will have the wheels on the ground somewhere (maybe not my
destination), at 3:30."

Bob Gardner

"WingFlaps" wrote in message

...



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- Hide quoted text -

- Show quoted text -

That's how I'm being taught.

Wil

Hi,

In article ,
William wrote:

That's how I'm being taught.

And if you've left the fuel cap off in your pre flight, and your fuel has
gradually been ****ing all over the wing, how will you know?

Andy

On Feb 23, 7:49*pm, Andy Hawkins wrote:
Hi,

In article ,
* * * * * *William wrote:

That's how I'm being taught.

And if you've left the fuel cap off in your pre flight, and your fuel has
gradually been ****ing all over the wing, how will you know?

Andy

I was taught to use that method to cross check with the gauge.

Wil

On Feb 23, 8:23*pm, William Hung wrote:
On Feb 23, 7:49*pm, Andy Hawkins wrote:
In article ,
* * * * * *William wrote:
And if you've left the fuel cap off in your pre flight, and your fuel has
gradually been ****ing all over the wing, how will you know?

I was taught to use that method to cross check with the gauge.

Wil

Yup, that's absolutely the way to do it.

There is this big blue cloud coming off your flaps and the gas gauge,
admittedly a piece of crap, will SUDDENLY go from full to empty.

Jim


And if you've left the fuel cap off in your pre flight, and your fuel has
gradually been ****ing all over the wing, how will you know?

Andy

On 2008-02-24, Andy Hawkins wrote:
And if you've left the fuel cap off in your pre flight, and your fuel has
gradually been ****ing all over the wing, how will you know?

By the blue streaks back from the cap along the top of the wing?

Yeah, this kinda leaves 172 drivers out in the cold...
--
Jay Maynard, K5ZC http://www.conmicro.com
http://jmaynard.livejournal.com http://www.tronguy.net
http://www.hercules-390.org (Yes, that's me!)
Buy Hercules stuff at http://www.cafepress.com/hercules-390