Twin Commander

General Category => Fuel Systems => Topic started by: donv on October 20, 2015, 11:33:42 pm

Title: 980 Won't Fill to Capacity
Post by: donv on October 20, 2015, 11:33:42 pm
My 980 has the long range tanks. I know this because, on the pre-buy, we opened up the panels and looked at the bladders.

Yet, for the past 4 years, I cannot get more than 3,000 pounds in the airplane. Lately, it's been more like 2,900 (but that might be due to warm fuel). We calibrated the fuel gauges, so I know they're right, and in that process took it down to completely empty and filled it up-- 3,000 pounds.

Anyone ever seen this before?
Title: Re: 980 Won't Fill to Capacity
Post by: Adam Frisch on October 21, 2015, 02:30:43 am
Could the bladders be wrinkled or not connecting properly?
Title: Re: 980 Won't Fill to Capacity
Post by: ghancock on October 21, 2015, 08:49:13 am
First thought would be whoever put them in last didn't get them all straightened out and they can't fill up completely.
Title: Re: 980 Won't Fill to Capacity
Post by: donv on October 21, 2015, 02:58:48 pm
Wrinkled bladder could be an issue. We have looked at them from the outside, and they appear to be fine, but I should follow up on that.
Title: Re: 980 Won't Fill to Capacity
Post by: donv on October 27, 2015, 05:37:56 pm
I was chatting with Norm Ralston yesterday, and he suggested a vent problem. I'm going to follow up on that, as I think he might well be on to something.
Title: Re: 980 Won't Fill to Capacity
Post by: Adam Frisch on October 27, 2015, 05:47:04 pm
Could be.

I had a cockroach nest completely blocking my fuel vent on the old 520 once. Pure chance I found it. After that I was always adamantly clearing fuel vents on pre-flight.
Title: Re: 980 Won't Fill to Capacity
Post by: donv on July 07, 2016, 12:33:09 pm
At the 150 hour, we went all through the fuel system. Everything seems to be as it should be, yet going from empty to completely full (during the calibration of the fuel gauges), it took only 420 gallons.

No one, including Geoff at Twin Commander, can figure out where the missing fuel is.

Vents are all good, long range tanks are there and fill with fuel, foam in the bottom of the bladders seems to be correct. They even measured the distance to the fuel fillers, to see if maybe they were placed incorrectly at the factory... nope. They're in the proper place.
Title: Re: 980 Won't Fill to Capacity
Post by: Adam Frisch on July 07, 2016, 01:42:27 pm
Could you send in a probe camera and see if there's creases somewhere or something?
Title: Re: 980 Won't Fill to Capacity
Post by: donv on July 07, 2016, 02:36:54 pm
They did that. All looked good.

Could you send in a probe camera and see if there's creases somewhere or something?
Title: Re: 980 Won't Fill to Capacity
Post by: Adam Frisch on July 07, 2016, 04:23:39 pm
Bizarre.

Checked center tank? Problem could be there maybe.
Title: Re: 980 Won't Fill to Capacity
Post by: donv on July 07, 2016, 06:43:52 pm
The 980 fuel system is a little bit different. It has a wet wing outboard, and four bladders on each side, inboard. They looked at all the bladders, as well as all the tubes connecting them. It's also odd in that the missing fuel is symmetrical-- you might expect to have an issue on one side or the other, but not both together.

Bizarre.

Checked center tank? Problem could be there maybe.
Title: Re: 980 Won't Fill to Capacity
Post by: Bruce Byerly on July 07, 2016, 09:44:10 pm
54 gallons missing ??  :o

I've had one that was indication and another now which I don't know what the facts are yet.  But 50+ is huge - I'd guess the tanks are missing.
Title: Re: 980 Won't Fill to Capacity
Post by: donv on July 07, 2016, 11:57:20 pm
Nope. That was the first thing we checked. They are definitely there, and definitely filling with fuel.

54 gallons missing ??  :o

I've had one that was indication and another now which I don't know what the facts are yet.  But 50+ is huge - I'd guess the tanks are missing.
Title: Re: 980 Won't Fill to Capacity
Post by: Bruce Byerly on July 09, 2016, 07:26:50 pm
Nope. That was the first thing we checked. They are definitely there, and definitely filling with fuel.

54 gallons missing ??  :o

I've had one that was indication and another now which I don't know what the facts are yet.  But 50+ is huge - I'd guess the tanks are missing.

Yeah where is the air hiding? Wrong part number fuel cells could be somewhere in the system.  Full, complete and total defuel followed by refuel by weight (checking quantity while you do it) with temp density noted would be the only thing to give me confidence in it.  54 gallons is 100% of reserve in some cases!  :o
Title: Re: 980 Won't Fill to Capacity
Post by: Bruce Byerly on July 25, 2016, 11:50:26 pm
Don, I was taking to Mike G in Indy and he recalled planes being short of capacity when new. In fact the plane I was picking up, a 1000, has one wing that differs much more than I would have expected from the other.  I can get the numbers for you.  Not a great answer but makes me want to ask Geoffrey P at Twin if he recalls any solutions. 
Title: Re: 980 Won't Fill to Capacity
Post by: ghancock on July 26, 2016, 07:56:46 am
I'm of the opinion that unless you have perfectly installed bladders you're never going to get the full load specified in the manual, and I carry that as far as thinking you'll never get perfectly installed bladders for numerous reasons.  If you consider how they are installed and what they actually are, I bet only a hand full of planes actually got the listed amount of gas when filled.

My airplane has 33.5gal aux tanks and if I fill them completely to the top I get 30gal even in both of them.  I still don't know exactly how much fits into my main tank and hoping to figure that out soon.  I can guarantee it is not 156gal though. :-)

Glenn
Title: Re: 980 Won't Fill to Capacity
Post by: JimC on July 26, 2016, 10:15:36 am
This should be a fixable problem. I can fill aux tanks in my Cessna to stated capacity - and it's a similar complicated arrangement of two connected bladder tanks with one filler.

Up to 1-2% short, sure. But people are talking about being 10% short on capacity - can't that be fixed?
Title: Re: 980 Won't Fill to Capacity
Post by: donv on July 26, 2016, 03:03:57 pm
Bruce-- That's what I think as well. We looked at the flight logs the original German owners kept, and they logged every flight. After the tanks were installed, they started flying about 20-30 minutes more, so that did make a difference.

Don, I was taking to Mike G in Indy and he recalled planes being short of capacity when new. In fact the plane I was picking up, a 1000, has one wing that differs much more than I would have expected from the other.  I can get the numbers for you.  Not a great answer but makes me want to ask Geoffrey P at Twin if he recalls any solutions.
Title: Re: 980 Won't Fill to Capacity
Post by: Bruce Byerly on July 26, 2016, 04:12:36 pm
This should be a fixable problem. I can fill aux tanks in my Cessna to stated capacity - and it's a similar complicated arrangement of two connected bladder tanks with one filler.

Up to 1-2% short, sure. But people are talking about being 10% short on capacity - can't that be fixed?

You'd think - same as me -  but it's a bit more complicated unfortunately.  Funny thing is I've seen well over 400 gallons go into a 690A/B vs the 389 they are supposed to hold in 22 bladders. 
Title: Re: 980 Won't Fill to Capacity
Post by: ghancock on July 27, 2016, 08:10:40 am
I'm sure you could fix them.  Remove all bladders and try to square the corners as best as possible.  The question:  Will this actually fix the issue are or you throwing money at a completely unknown?  I personally just figure out what I can carry exactly and deal with it.  It would be nice to have the extra gas for sure but knowing that I can only fly 4.5 hours instead of 5 hours is sufficient enough for me.

Glenn
Title: Re: 980 Won't Fill to Capacity
Post by: SKYFLYER on July 27, 2016, 10:06:37 am
If I recall correctly when I had all the bladders replaced in the 680FL, several years back, they hooked up a compressor attached to special fuel caps and inflated them like a balloon to make sure the bladders filled all the cavities flush. Not sure how they blocked the overflows but you could hear them "pop" into the corners of the cells. Clearly they knew the max air pressure to apply before any damage could be done.
Not sure if you could get that done on yours or not but it seems that maybe a few corners of one or more cells are not sitting in the corners and bunched up. Also if there had ever been an issue with the vents you could get negative pressure in a cell and cause it to not retake the same position it had been in before especially the corners.
Title: Re: 980 Won't Fill to Capacity
Post by: ghancock on July 27, 2016, 01:05:35 pm
That sounds very interesting, but would have no idea how they did it.  I guess if you had rubber plugs you could shove into the holes of each feed and vent line you could pull that off and sounds like an awesome way to make sure things were pushed out.

Thanks for sharing,

Glenn