Bad fuel gauges?

On 2008-02-23 18:47:30 -0800, "RST Engineering" said:

Gee, the world famous aviator is now telling us that the FAA doesn't know
what it is talking about when it writes documents.

Are you claiming that there are no errors in those documents? That the
regs, handbooks, advisory circulars and all the rest are more inerrant
than the Bible? I was not aware that the FAA publications were written
by God.


Idiot. Get a little real world sense. Whoever wrote the handbook has
dozens of years, thousands of hours, and an understanding of how the world
works.

I have dozens of years, thousands of hours, and a pretty good
understanding of the how the world works, too.


Rod Machado has not made an error of fact or interpretation as long as I've
known him, and that was probably while you were still in liquid form.

Jim

In fact, when Rod Machado repeated this myth in AOPA Flight Training
Magazine I wrote to him and asked him about it, pointing out the
regulations. He wrote back and admitted that my interpretation was
probably correct.

Unlike you, Rod is capable of admitting he made a mistake and I have a
great deal of respect for him.

I am not sure which of us is older.



--
Waddling Eagle
World Famous Flight Instructor

In article ,
Peter Clark wrote:

In a low wing Piper, you look over and see the fuel flowing out of the hole.

You don't need a gauge to detect that hazard. (Perhaps this is another
advantage of flying a cherokee)

And at night?

Flashlight

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

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?

On Feb 24, 7:18 am, Bob Noel
wrote:
In article ,
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?

In a low wing Piper, you look over and see the fuel flowing out of the hole.
You don't need a gauge to detect that hazard. (Perhaps this is another
advantage of flying a cherokee)

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

I have flown airplanes with a fuel sump drain that would not close
properly. This always happened when I took a fuel sample from the
sump, so it was was easy to notice it and fix it. But if it can happen
on the ground, it could also happen during flight.

An accurate fuel gauge is long overdue in aviation. It doesn't have to
be fancy gadgetry, fuel flow integrators or capacitive sensors. One of
the experimental guys had installed a simple pressure sensor under the
fuel tank which measured the total weight of the fuel tank. While not
perfect, it was far better than anything else I have seen, including
sight gauges.

I wonder if a pressure sensor placed inside the bottom of the tank could be
made sensitive enough to "weigh" a column of fuel inside the tank above the
sensor. That would only work for regular sized tanks (no triangles) but
could be integrated over a long enough time to take care of any slosh.

Just a random thought, mindya...and I haven't run the numbers.

Another thought is a string of LEDs separated from photosensors with, say,
10 or 15 of them inside the tank mounted vertically.

Hmmm...any other thoughts for liquid level measurements? Sonar a la
Polaroid?

Jim




An accurate fuel gauge is long overdue in aviation. It doesn't have to
be fancy gadgetry, fuel flow integrators or capacitive sensors. One of
the experimental guys had installed a simple pressure sensor under the
fuel tank which measured the total weight of the fuel tank. While not
perfect, it was far better than anything else I have seen, including
sight gauges.

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 Sun, 24 Feb 2008 12:50:38 -0800, "RST Engineering"
wrote in
:

I wonder if a pressure sensor placed inside the bottom of the tank could be
made sensitive enough to "weigh" a column of fuel inside the tank above the
sensor.

It should be pretty easy to test by connecting the pressure sensor to
the tank drain. I don't see why such a system would be any worse than
a float-based system.

That would only work for regular sized tanks (no triangles)

I don't know what makes you say that. The pressure sensing system
just needs to be calibrated like one would do with a dip stick, or am
I overlooking something?

but could be integrated over a long enough time to take care of any slosh.

Just a random thought, mindya...and I haven't run the numbers.

Another thought is a string of LEDs separated from photosensors with, say,
10 or 15 of them inside the tank mounted vertically.

Placing electrical conductors within fuel tanks always makes me
nervous. I know Boeing does it, but there have been problems.

Hmmm...any other thoughts for liquid level measurements? Sonar a la
Polaroid?


I'll bet you can get some ideas he
http://www.sparkfun.com/commerce/categories.php


http://www.sparkfun.com/commerce/pro...oducts_id=7918
Breakout board for the Analog Devices 7746 capacitance sensor.

http://www.sparkfun.com/commerce/pro...oducts_id=8257
This is a simple breakout board for the SHT15 humidity sensor from
Sensirion. The SHT15 digital humidity and temperature sensor is
fully calibrated and offers high precision and excellent long-term
stability at low cost.

http://www.sparkfun.com/commerce/pro...oducts_id=8161
The SCP1000 is the very first absolute pressure sensor on the
market to use MEMs technology to grant 17-bit resolution. Under
ideal conditions, this sensor can detect the pressure difference
within a 9cm column of air.

http://www.sparkfun.com/commerce/cat...hp?cPath=23_84
http://www.sparkfun.com/commerce/pro...oducts_id=8502
Ultrasonic Range Finder - Maxbotix LV-EZ0

If you're not already familiar with this vendor, I think you'll find
the breadth of their products surprising.


An accurate fuel gauge is long overdue in aviation. It doesn't have to
be fancy gadgetry, fuel flow integrators or capacitive sensors. One of
the experimental guys had installed a simple pressure sensor under the
fuel tank which measured the total weight of the fuel tank. While not
perfect, it was far better than anything else I have seen, including
sight gauges.

On Sun, 24 Feb 2008 12:15:07 -0800 (PST), Andrew Sarangan
wrote:


I have flown airplanes with a fuel sump drain that would not close
properly. This always happened when I took a fuel sample from the
sump, so it was was easy to notice it and fix it. But if it can happen
on the ground, it could also happen during flight.

Your sump re-opened itself?

There's a difference between not closed, as in you just removed the
jar, and a sump opening by itself.

I'm not saying it can't happen, but the two conditions are very
different.

The reason for the weight sensor was because he did not want to
penetrate the fuel tank to add his sensors. Once inside the fuel tank,
I do not see the benefit of an optical sensor vs a float sensor. In
fact, I don't quite understand why float sensors are so inaccurate in
the first place. It is just a variable resistor. The shape of the fuel
tank can be easily calibrated out. Averaging the sloshing is equally
easy to do. Anyone know what makes them so notoriously inaccurate?



On Feb 24, 3:50 pm, "RST Engineering" wrote:
I wonder if a pressure sensor placed inside the bottom of the tank could be
made sensitive enough to "weigh" a column of fuel inside the tank above the
sensor. That would only work for regular sized tanks (no triangles) but
could be integrated over a long enough time to take care of any slosh.

Just a random thought, mindya...and I haven't run the numbers.

Another thought is a string of LEDs separated from photosensors with, say,
10 or 15 of them inside the tank mounted vertically.

Hmmm...any other thoughts for liquid level measurements? Sonar a la
Polaroid?

Jim

An accurate fuel gauge is long overdue in aviation. It doesn't have to
be fancy gadgetry, fuel flow integrators or capacitive sensors. One of
the experimental guys had installed a simple pressure sensor under the
fuel tank which measured the total weight of the fuel tank. While not
perfect, it was far better than anything else I have seen, including
sight gauges.

On Sun, 24 Feb 2008 14:45:58 -0800 (PST), Andrew Sarangan
wrote in
:

Anyone know what makes them so notoriously inaccurate?

A lack of federal regulations mandating their accuracy throughout
their range? :-)

Perhaps inaccuracy is induced over time, because of drift of the
resistance element. But I'd sooner believe, it's because there is no
regulation for accuracy.