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Author Topic: Fuel and Twin Commanders (Part 2) - an important read...  (Read 5759 times)

Russell Legg

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Fuel and Twin Commanders (Part 2) - an important read...
« on: October 10, 2015, 12:32:02 am »
I tell ya, if there is any one thing I find a mystery in the Commanders, it's
the EXACT volume of each bladder.

In all my years of asking, looking, calculating, I never got a straight-up
answer on what each bladder's capacity is.

I asked TCAC. I asked two fuel cell overhaulers.  Nothing ever came of it.

What I do know is that there is a "range" of fuel load on a Commander and to
be up to 10 gallons light is not out of that range.  (I think it is, but I'm
just a pilot, so my opinion does not count.)

I do not think there is a 10 gallon bladder in the standard 156 gallon
configuration.   A 685 or 690 has lots of little bladders (22, if memory
serves me) but not your 680-E.

OK, here's my set of bad answers to your good question, in order of
probability:

The size of the bladders does vary, especially if overhauled or
reconditioned.  This will account for some loss of volume.

Placement of the bladder can effect volume -- meaning if a wing bladder is
put in and all its corners are not "square" you'll lose some capacity.

Also remember that the temperature of the fuel does affect volume and if your
fuel is other than standard (59 F)  your volume will be off from book value. 
There are charts available (but don't ask me where -- Tylor Hall may have
these) that correct for this and it usually has an impact on BIG fuel loads;
not 156 gallons, but it's worth mentioning.

Also, this phenomena is important when you place warm fuel in your tanks, go
to altitude and cold-soak the fuel system.  Your fuel volume will shrink. 
Most piston aircraft don't get high enough to get that cold, but this, and a
clogged vent system evidently led to a fuel starvation incident in a
Commander a few years ago. 

Remember, I'm NOT a mechanic (I don't have the patients nor the intelligence
to be one) but have been around two or three Commanders and have seen these
situations.

I hope some of the A&Ps and "hands-on" type of members speak up on this.
Hey Gang, From trial and error, I found that if I use a routine on my ole
bird(6819 Q)a 680 I can add 15 gallons at a top off.Having recent bladders,I
have been told that they stretch when filled.What I do is to top off the
center,go to the right side,then to the left.Once each one is topped off,I
repeat the routine,and usually get about 15 gallons more than just a
straight fill up.Hope my limited experience helps. Big Al!

The size of the bladders does vary, especially if overhauled or
>reconditioned.  This will account for some loss of volume.
I wouldn't think this would effect over 10-20 gallons though.
(That's a large change in bladder size)
Unless you noticed a large discrepancy between the bladder and the wing
close-out area
when you installed the new fuel cell, this would be unlikely.

If you have had the plane for awhile, you will instinctively know what
your fuel loads should be with various temperature ranges.

>Placement of the bladder can effect volume -- meaning if a wing bladder is
>put in and all its corners are not "square" you'll lose some capacity.
This would be more likely.

I recently replaced 3 fuel cell bladders in my 500B (after the AD
inspection)
and it took me longer to position the bladders than it did to tie them
off.
The positioning of the laces and proper security of the tie downs
can decrease the amount of fuel that is distributed within the set.
(You can refer to the maintenance manual and it will show you the proper
fuel
lacing pattern to use on the fuel cells)

The interconnect tubes allow fuel to evenly disperse between the bladders.
If one bladder is not properly tied up into the upper attach points then
a decrease
in volume can occur. (Gravity can work against fuel distribution,
especially
if the top or corner of the bladder is pushing down or in on the incoming
fuel)

I've seen were a fuel bladder was incorrectly tied in, causing one corner
to displace
a large portion of the fuel cell bladder inward, this resulted in a
folded in bladder.
This could easily displace a good portion of your fuel capacity.
(volumetric displacement)

But after some time, the bladder may find it's natural position and
hopefully
allow more fuel to enter, thus reducing your fuel discrepancy.
(This could also cause leaks in the interconnect tube connections later
on)

>I did manage to squeak another 7.5 (148.5 total) gal after it sat over night,
>but that is still a bit short??
>Harry Merritt, they guy who sold it to me, suggested it was missing a blatter.
Looking at my illustrated parts catalog, I only show the typical 5 cell
system
exactly like my 500B. The catalog refers to 500 series, 560, 680F and FP.
Is there any difference between your 680E and the 680F?
Chris's chart says 223 gallons for the 680E, if this is true then you
might have
a completely different fuel system than the diagram that I'm looking at.

My maintenance manual says the fuel system is comprised of 5 cells which
hold a
total of 150-159 gallons. If you're on the lower end then your close.
When I refueled after the fuel cell installation, I was on the higher end
of 157 gallons.
(I made sure the laces were tight and properly positioned).

I'm sure Morris can fill in the details.
Hope this helps.
Randy

The 680E does have two additional "outboard" tanks with a capacity of about
33.5 gallons each, right?

The "Main" fuel system is identical to a 500 series; 5 bladders holding
around 156 gallons.

Just wanted to clarify that point for everyone watching this thread.

This happens so frequently, that when I was giving Commander initial
training, I admonished my clients to always stay with the airplane during
fueling.

The Cessna products set themselves up with the word "Turbo" on the nacelles
in the mid 70s, which you will notice is no longer part of the paint
scheme.

The Commanders suffer because to a young line service professional, they
look like a Turbo Commander.

The GAMA (and FAA) placards, nozzle restrictors, etc. are helpful, but I
know of an instance in the Kansas City area where the fueler worked extra
hard to get a jet nozzle into a piston model fuel port.  There's no
stopping yankee ingenuity nor poor reading comprehension.

The bottom line is education and if you don't know that the FBO and fuel
person  knows your piston Commander, STAY WITH THE AIRPLANE DURING FUELING.
 Besides, as Big Al and others have mentioned, the only way to get a true
top off is to let the tanks appear to be full, then wait a period of time
and fill again.

By being there, you not only assure the proper fuel, but that you're
getting the last 30 minutes of fuel you're planning on.

For one of the earlier TCFG fly-ins, I created a training aid for the host
FBO.  If there's enough interest, I'll upload it to the website and
individuals can download and print it.

It's a rather large Word document with too many color graphics, but it's
already created, so why reinvent the wheel?

Keith S. Gordon
CLOUD CRAFT
Very early on in flying my 560 I got a lot of comments from line KIDS about how come mine looks different than other commanders and had trouble convincing them that it did not take jet A.
As a result I have started telling line personell everywhere I fill up. In spite of this recently I had to stop an attendent in Punta Gorda Florida as he climbed the ladder to top off my plane with a nozzle connected to a truck that said "jet A".
This happened even though I have a HUGE sign next to the filler cap that says av gas only and clearly my orifice was too small for the monster schnoz this guy was about to stick in it. Now I never let anyone fuel it unless I am watching.

Speaking of fuel gauges,

Morris informed me when he was calibrating my gas gauge that regulations demand that the only acceptable error is that the gauge reads lower than the actual quantity of gasoline. He also says (and I've discovered, after de-fueling the airplane to fix a dead fuel boost pump) that at the upper ranges of the gas gauge, it's very accurate; but at the lower range, they're calibrated to generally read 8-10 gal. less than what's in the tank. Part of Morris' standard annual is to calibrate the gauge, and he strives for 10 gal. in the tank when it reads empty. He also provides a calibration sheet (actual vs. read) in 10-gallon increments.

I still write down my burns, though ;-).
I didn't offer up the obvious reason a Commander could end up 10 + gallons
low because I didn't want to treat anyone -- let alone our fearless leader --
like a fool.

That happened to me last week when I told a very expensive avionics tech
about an air data computer failure on one of our Falcons.  He had the poor
manners to ask me if was turned on.  So with that in mind, I didn't want to
perpetuate that kind of attitude.  But ...

... Last night, in a dream, the ancient Chinese Commander pilot, Wan Hung Lo
came to me and told me to pass this on to the new and "shopping-for"  list
members.

Remember, when you fuel a Commander, you're filling 5 interconnected bladders
from one single point on the right wing. 

If the right wing is low, either due to a steeply sloped ramp or a low strut,
you cannot fill the bladders in the left (high) wing all the way.

Further, he instructs, when topping off, politely ask the line service
professional (nozzle jockey, ramp rat) to clean the windows or check the oil
after the first "top off."    If you do this, the fuel level will settle down
and you'll get as much as 12 more gallons in.

That should keep Tyler Hall's deafening silence away.

      YEP,    The wings were level (kinda) and I did let it settle and got
and additional 7.5 gals  That put the total at 148.5 - 156 = 7.5 gal short. 
It was about 50 degrees and the airplane was setting slightly tail low.
       The Commander manual gives a range allowing for as little as 155 gal
so it is pretty close Now that I know what it really holds I will plan
accordingly.  At 40 gph we are really only talking about 10 minutes or so.  I
suppose the antique pump at the little FBO I live at could also be in error??
       Thanks for all the advice.  Jb

>The size of the bladders does vary, especially if overhauled or
>reconditioned.  This will account for some loss of volume.
I wouldn't think this would effect over 10-20 gallons though.
(That's a large change in bladder size)
Unless you noticed a large discrepancy between the bladder and the wing
close-out area
when you installed the new fuel cell, this would be unlikely.

If you have had the plane for awhile, you will instinctively know what
your fuel loads should be with various temperature ranges.

>Placement of the bladder can effect volume -- meaning if a wing bladder is
>put in and all its corners are not "square" you'll lose some capacity.
This would be more likely.

I recently replaced 3 fuel cell bladders in my 500B (after the AD
inspection)
and it took me longer to position the bladders than it did to tie them
off.
The positioning of the laces and proper security of the tie downs
can decrease the amount of fuel that is distributed within the set.
(You can refer to the maintenance manual and it will show you the proper
fuel
lacing pattern to use on the fuel cells)

The interconnect tubes allow fuel to evenly disperse between the bladders.
If one bladder is not properly tied up into the upper attach points then
a decrease
in volume can occur. (Gravity can work against fuel distribution,
especially
if the top or corner of the bladder is pushing down or in on the incoming
fuel)

I've seen were a fuel bladder was incorrectly tied in, causing one corner
to displace
a large portion of the fuel cell bladder inward, this resulted in a
folded in bladder.
This could easily displace a good portion of your fuel capacity.
(volumetric displacement)

But after some time, the bladder may find it's natural position and
hopefully
allow more fuel to enter, thus reducing your fuel discrepancy.
(This could also cause leaks in the interconnect tube connections later
on)