Racers22
Topic Author
Posts: 109
Joined: Wed Jul 04, 2001 6:03 am

Pressurized Fuel Tanks

Mon Jun 03, 2002 12:46 pm

Today at work, I fueled a Mitsubishi MU-2 for the first time and was warned to let the fuel tanks vent before I pulled the fuel cap off because the tanks were pressurized. What is the advantage of pressurizing a fuel tank? Is this done on airliners or just something that is unique to the MU-2? Any information that can be provided would be great!
 
cdfmxtech
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Joined: Wed Jul 05, 2000 11:37 am

RE: Pressurized Fuel Tanks

Mon Jun 03, 2002 1:36 pm

I don't know what a Mitsubishi MU-2 is - but all te jets that I work (Boeing type) utilize ram air to keep a slight amount of positive pressure in the fuel tanks.
An advantage is that without fuel pumps, the positive pressure in the tank will still supply fuel to the engines.
 
ThirtyEcho
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RE: Pressurized Fuel Tanks

Mon Jun 03, 2002 1:52 pm

This is a common system, in high-wing A/C especially, for the reasons related above. In a single, like a 172, gravity-feed is sufficient. In a multiengine, the wings and engines are on the same level and pressure is a backup to the fuel pumps. Incidentally, as an Aero Commander 680FP pilot explained it to me, this is why single point refuelling is sometimes used. This provides for a closed system in which pressurization can also serve to drive fuel crossfeed.
 
JETPILOT
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RE: Pressurized Fuel Tanks

Mon Jun 03, 2002 4:30 pm

Without a "fuel pump" a jet engine is dead. Withour "boost" pumps you have a chance up to certain pressure altitudes of the engine siphoning fuel from the fuel tank with the fuel pump.

CDFmxtech.... Can you give me some info on this ram air pressure? I've never heard of it.

Single point refueling has nothing to do with pressurization of fuel tanks. Fuel tanks in transport category planes are ambient pressure systems with a vent box.

JET

 
VC-10
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RE: Pressurized Fuel Tanks

Mon Jun 03, 2002 5:10 pm

My understanding of tank pressurisation is the reduce the evapouration rate of the fuel at altitude.

Jet,

To partially answer your question, on the 747-400 the vent system relief valve opens at a +tive press of 3.5 psi and a -tive press of 1psi.
 
dripstick
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RE: Pressurised Fuel Tanks

Mon Jun 03, 2002 8:54 pm

Racers22,

Always put one hand over the overwing fuel cap and open with the other hand. Even tanks that aren't pressurised can retain some air pressure if the tank hasn't vented.

Another one to watch out for is the Falcon 20.

Doesn't hurt to turn you face away from the cap while you turn the tab either just in case it spit-farts at you.
What's another word for thesaurus?
 
Guest

RE: Pressurised Fuel Tanks

Mon Jun 03, 2002 9:10 pm

One of the things that a MU-2 pilot will check VERY closely after refueling is the security of the fuel caps on the tip tanks. If a cap were to come off, say immediately after takeoff, when that tank was full and the aircraft at a high angle of attack, the pressurised air would force the fuel out of the tank. Sort of a "pressurised" siphoning system, if you will. At FlightSafety they told us that such a scenario would create an out of balance balance condition that would result in the loss of lateral contol (remember the airplane has spoilers for roll control) and would be unrecoverable.
Jetguy
 
PPGMD
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RE: Pressurized Fuel Tanks

Mon Jun 03, 2002 11:26 pm

Well at our local FBO I've heard that there still are a couple of MU-2 fuel caps on the roof. That system is a real pain in the butt, though it is funny to see the geyser that results from a fueller that wasn't properly trained, that is unless you are the pilot of the aircraft or the fueller that got soaked in JA.
At worst, you screw up and die.
 
Racers22
Topic Author
Posts: 109
Joined: Wed Jul 04, 2001 6:03 am

RE: Pressurized Fuel Tanks

Tue Jun 04, 2002 12:49 am

Someone that I work with did not realize that the tanks were pressurized the first time he went to fuel one and ended up with a fuel cap slamming into his chin and he got the pleasure of taking a shower in Jet A. He said the owner just sat there and laughed. Another thing that I find interesting about the MU-2 is that you can't put more than 30 gallons into one tip tank without putting an equal amount into the other because the plane has a tendancy to tip over. I wonder how many times a plane has tipped over due to the fueler not knowing this?
 
FDXmech
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RE: Pressurized Fuel Tanks

Tue Jun 04, 2002 4:01 am

Any large commercial jet (including the 727) with overwing fuel tank caps are vulnerable to the fuel siphoning out at a very rapid clip should the cap be missing or the seal defective. I've seen this on rare occasion.

Further, on the DC10/MD11, without fuel boost pump pressure, #2 engine will flame out.
You're only as good as your last departure.
 
PPGMD
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RE: Pressurized Fuel Tanks

Tue Jun 04, 2002 4:09 am

I know that in the might Cessna 152 it takes exactly two mins to drain both of the fuel tanks dry.
At worst, you screw up and die.
 
JETPILOT
Posts: 3094
Joined: Tue May 18, 1999 6:40 am

VC-10..........

Tue Jun 04, 2002 5:16 am

From what I understand.... the vent system relief valve is only there in case the vent system fails.

The fuel tank might pressurize or vacuumize anytime the aircraft climbs or decends, and the tank not being able to vent.

Fuel tanks don't have pressurization systems on large airplanes, but they do have pressure/vacuum relief systems in case the vent system fails.

JET
 
Spacepope
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RE: Pressurized Fuel Tanks

Tue Jun 04, 2002 7:42 am

Just hope the pressure relief system works, or this may happen:

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T.J.
The last of the famous international playboys
 
cdfmxtech
Posts: 1319
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RE: Pressurized Fuel Tanks

Tue Jun 04, 2002 1:04 pm

JETPILOT

Most large jets (at least those that I've worked) have vent scoops on each wing. It does what it's name implies. It vents and it scoops. It vents any excess fuel pressure (or fuel for that matter) and it scoops ram air for a slight fuel tank head pressure.
-----------------------
Straight from the B737-300/500 Maintenance Training Manual

Fuel Tank Vents

Purpose
The fuel vent system prevents damage to the tank structure by providing positive venting of all fuel tanks, regardless of airplane attitude. During flight, the system also helps to decrease fuel evaporation and assists the fuel boost pumps by providing a small positive pressure head on the fuel.
----------------
By the way, not all aircraft have the pressure relief valves. It is a backup for the vent systenm in case it doesn't vent. But some of our classics don't have them. But the B737 Next gens, and B757, B767, and B777 have them
 
JETPILOT
Posts: 3094
Joined: Tue May 18, 1999 6:40 am

RE: Pressurized Fuel Tanks

Tue Jun 04, 2002 4:56 pm

I never thought of the vent system being used to put head pressure on the fuels system. But it appears it does.

As far as the vent system venting fuel.... if its vents fuel your anti siphon check valve took a vacation.

For all puposes I think it's safe to say that in the real sense of the term Boeing and Airbus fuel systems aren't pressurized in the manner of which the question is being asked.

Agreed?

JET



 
cdfmxtech
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RE: Pressurized Fuel Tanks

Tue Jun 04, 2002 11:45 pm

Absolutely.............



 
JohnM
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RE: Pressurised Fuel Tanks

Wed Jun 05, 2002 10:45 am

The C-5 fuel tanks are pressurised with nitrogen for fire suppression and tank inerting. Something like 1.5 psi. It is a nightmare for the guys who try to keep the system running. The aircraft is serviced with liquid nitrogen, stored in 2 dewars, and converted to a gas for tank inerting. I think each dewar is 750 litres. The hassle to keep valves and seals in the liquid nitrogen system working is a problem, since you are dealing with a cryogenic. This system does not help with fuel feed, gravity can feed the engines from the main tanks, and the engine driven fuel pump can do the rest. The advantage of this setup is no moist air will enter the tank as fuel is used. The vapor space is nitrogen inerted. Almost unheard of to have water in the tanks. Also if something is having a bad day in the tank and is producing an ignition source, it is very hard to make something go boom. I think this airplane is the first and last to have this system. I have been told the C-17 tried something like this by extracting nitro from bleed air, but it doesn't work. If a gravity tank fill cap is taken off, LOTS of air/nitro is expelled. (Enough to mess up your hair).
 
AJ
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RE: Pressurized Fuel Tanks

Wed Jun 05, 2002 2:23 pm

The Mu-2 has five fuel tanks. The main (centre) tank is a three part tank which supplies fuel to both engines (no crossfeed required when operating engine-out). The outboard tanks have an electric pump which transfers fuel to the main tank. The tip tanks are pressurised to tranfer fuelunder pressure to the main tank. This pressure is provided by bleed air from the engines. A float in the main tank disables the transfer systems to stop fuel transfer to a full main tank.
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Bellerophon
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RE: Pressurized Fuel Tanks

Mon Jun 10, 2002 4:36 am

Hi JETPILOT

For all puposes I think it's safe to say that in the real sense of the term Boeing and Airbus fuel systems aren't pressurized in the manner of which the question is being asked.

Yes, I agree with you, however….

Fuel tanks don't have pressurization systems on large airplanes.

....actually one current 4 engined jet transport aircraft has just such a system!

At high altitude, the tank-vent shut off valves close, and ram air from a small intake near the fin leading edge is used to increase the tank pressure.

This facilitates fuel pumping, but more importantly, in view of the wing temperature, helps stop the fuel boiling off!

Regards

Bellerophon
 
galaxy5
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RE: Pressurised Fuel Tanks

Mon Jun 10, 2002 9:46 pm

The C-5 uses a nitrogen fuel vapor inerting system. Using liquid nitrogen that is converted into gaseous form used to scrub the oxygen out of the fuel tank ullage, and pressurizing the fuel tanks to prevent boiling of the fuel at high altitudes. The system enures a safe oxygen depleted atmosphere in the fuel system and prevents tank fires. This type of system would have prevented the TWA800 explosion, however the airlines think its to expensive and would rather not have the system installed.
"damn, I didnt know prince could Ball like that" - Charlie Murphy
 
Guest

RE: Pressurised Fuel Tanks

Tue Jun 11, 2002 12:12 am

Here's a couple of things for you line guys to watch out with the MU-2s...

The MU-2 has a definite refueling sequence when it comes to refueling. Unless you're fueling with 2 hoses, you have to alternate filling the tip tanks to keep the airplane in lateral balance.

When the tips are empty, a standard ladder will fit nicely under the tip tank. If you're not paying attention and place the ladder directly under the tank it will settle on to the top rung of the ladder as it's being fueled. The only option you have at that time is to cut the ladder out from under the tank. Usually, I tried to hang around while the plane was being refueled to monitor the process, but it wasn't always possible. In the three years that I flew the MU-2 line guys ended up having to cut 3 ladders from under my airplane.

The airplane definitely has it quirks, but they are well built and perform well. All in all, I really enjoyed flying them.

Jetguy

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