Fuel Gauge Inop VFR Day

Ice blonde wrote:
It can be fun getting the gauges on some airplanes to read anywhere near
accurately. Even replacing gauges or senders or both will often not get
them any more accurate.

Why?

Aircraft fuel tanks tend to be fairly short, compared to auto tanks. They also
tend to be wider and longer than a comparably-sized fuel tank in a car. It's not
economical to manufacture special sending units for aircraft, however, so
outfits like Cessna and Piper use units made for cars.

An old-fashioned sending unit is basically a rheostat with a steel wire attached
to it and a float attached to the other end of the wire. Usually they are set up
so that current is sent to the gauge and the sending unit is attached to the
ground ("earth" in GB) wire. When the float is at the top, the gauge is fully
grounded and it reads full -- float at the bottom, no current flows and the
gauge reads empty.

There are several problems with this, at least two of which are caused by the
use of automotive components. First, there's no requirement for accuracy in auto
gauges, and, in fact, the manufacturers deliberately make the systems so that
they still read 1/2 tank when the tanks are actually 3/8 full (this supposedly
gives the impression of better fuel economy). Second, a sending unit that's
designed to have the float travel over a range of perhaps 12" in an auto tank
does not perform as precisely when the wire is shortened and the float travels
over only 7".

You also may have had occasion to drive autos in which the gauges read higher or
lower if you are going uphill? This is caused by the sending unit being located
towards the front or rear of the tank. The same sort of thing occurs to a lesser
extent in some aircraft. Lastly, aircraft tend to bounce around more than autos.
Gasoline also tends to slosh back and forth more in the flatter tanks. This
tends to bounce the floats of the sending units, making the gauges more erratic
than autos.

George Patterson
Drink is the curse of the land. It makes you quarrel with your neighbor.
It makes you shoot at your landlord. And it makes you miss him.

George Patterson wrote:
Ice blonde wrote:
It can be fun getting the gauges on some airplanes to read anywhere near
accurately. Even replacing gauges or senders or both will often not get
them any more accurate.

Why?

Aircraft fuel tanks tend to be fairly short, compared to auto tanks. They also
tend to be wider and longer than a comparably-sized fuel tank in a car. It's not
economical to manufacture special sending units for aircraft, however, so
outfits like Cessna and Piper use units made for cars.


There are other reasons they're inaccurate, too. The Cessna spec is
for the float to not touch the top of the tank (the float's tapping on
an aluminum tank can wear a hole in it), and so the thing reads full,
or even overfull, when the float reaches the upper limit of its travel.
That's before another significant amount of fuel goes into the tank and
submerges the float. The tank will therefore read full (or overfull)
for some time before it begins to drop.
The float is usually hinged to the sender near the top of the
tank. The movement of the rheostat in the sender is linear but the
hinge's location creates a sine function to the indication, with the
top travel being rather slow and the lower travel moving much more
quickly. It implies that the tank has more volume in the top than in
the bottom, which is usually the opposite of the actual tank shape.
Odd-shaped tanks create their own accuracy problems. The fuel
level's descent in the tank is not linear, while consumption is, and
the gauges reflect level drop, not volume drop.
Senders are often located in the inboard end of wing tanks, and
the dihedral keeps the sender up until the fuel is well down. More
inaccuracies.
And we pay big bucks for this sort of thing!

Dan

It's not economical to manufacture special sending units for aircraft, however, so outfits like Cessna and Piper use units made for cars.

Just curious - how much is a sender from Piper? How much is a sender
from Chrysler? When a four dollar microsowitch costs $750 from Piper
(we had to replace one) the economics of manufacturing special sending
units doesn't make me feel sorry for the aicraft companies.

Jose
--
Money: what you need when you run out of brains.
for Email, make the obvious change in the address.

wrote:




No arguement from me. It just seems that a fuel gauge is not as important
as the attitude indicator yet the attitude indicator is not required.
Though fuel is a major part of keeping you in the air as opposed to in the
ground.


Why is an attitude indicator even desirable in day VFR?

No arguement from me. It just seems that a fuel gauge is not as important
as the attitude indicator yet the attitude indicator is not required.
Though fuel is a major part of keeping you in the air as opposed to in the
ground.


Why is an attitude indicator even desirable in day VFR?

Just in case I guess. Not like I really even use it except during
practice of steep truns to get the visual picture of the proper bank
angle.

Mike Alexander
PP-ASEL
Temecula, CA
See my online aerial photo album at
http://flying.4alexanders.com

"Mike 'Flyin'8'" wrote

Not like I really even use it except during
practice of steep truns to get the visual picture of the proper bank
angle.

Is that a more advanced maneuver? I'll bet your instructor taught that
right after steep turns, right? ggg
--
Jim in NC

Usenet rule: always make light of another's typos, especially when they are
funny!

"Mike 'Flyin'8'" wrote

Not like I really even use it except during
practice of steep truns to get the visual picture of the proper bank
angle.

Is that a more advanced maneuver? I'll bet your instructor taught that
right after steep turns, right? ggg


Ah... Yeah... HaHa
Fat Fingers I guess.

Mike Alexander
PP-ASEL
Temecula, CA
See my online aerial photo album at
http://flying.4alexanders.com

It's a "minor" thing because no one should take off without having measured
the fuel (high wing or low wing) using a stick of some sort to determine as
close as possible just how much fuel is in a given tank. (Unless you just
gassed up and saw the fuel level at full.)

Then one should be keeping track, in some fashion, just how long one has
been in the air and how much fuel is being consumed.

Anyone who believes a fuel gage, no matter how many FAA rules say they have
to work, it a fool waiting to run out of gas. And it sure happens a lot.

My 2 cents worth ...


"Ice blonde" wrote in message
oups.com...
Something as seemingly minor as a fuel gauge leaving an otherwise air
worthy plane on the ground... sigh...

Little things can become big things in a hurry.

Forgive me if I'm being really stupid, but I would say flying with a
broken fuel gauge is more than a little thing? :-/

If you run out of petrol in a car, most likely you stall and get stuck
somewhere, if you run out of fuel when flying, the possibilities are
far worse.