Jet Pumps

Sat Sep 23, 2000 3:27 am

I work on Honeywell TFE-731 engines and P&W 305A's. On the Pratts the jet pump is controlled by the motive flow valve located just outside of the aft fuselage tank in a Lear 60. On the 731's found on Lear 35, 45 and 55's the motive flow valve is located internally on the engine driven fuel pump.

My question is this: Do tranport cat. aircraft (A320, 737, 747 etc.) have these jet pumps located in the wings to supply the engines, and if so how are they controlled, with the fuel pump or a seperate valve.

RE: Jet Pumps

Sat Sep 23, 2000 6:28 am

Here's how it works on the EMB-120:
(had to dig out my systems manual)
The main jet pump is located in each wing collector tank and supply the required fuel flow and pressure to their respective engine under any condition, except engine starting and crossfeed operations (two electric boost pumps per side provide this pressure). The pump transfers fuel from the collector tank to the engine supply line. The primary fuel flow to operate the main jet pumps is provided by bypass fuel from the HMU (fuel unit on the engine).

That's about as detailed as I can go.
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RE: Jet Pumps

Sat Sep 23, 2000 5:00 pm

No large airliner uses jet pumps for supplying the engines as their thirst for fuel is to great. They all use elect boost bumps.

Jet pumps are used however, in some fuel transfer systems. The 747-400, A321 & the BAC1-11 500 series all have them to transfer fuel from the Centre Tank.
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RE: L-1011 Jet Pumps

Sat Sep 23, 2000 9:32 pm

I accidentally put some fuel in a center tank of an L-1011-250 last night and had to use the jet pumps (Lockheed calls it an ejector) to get it out. The center tanks feed the inboard wing tanks by using four ejectors with the wing tank boost pumps providing motive flow. The L-1011 also uses several ejectors in the fuel tank scavenge system. These jet pumps keep the fuel circulating anytime the boost pumps are on, which continously scavenges water from the tanks and minimizes algae growth. The scavenge system also provides an adequate supply of fuel to the boost pumps in all aircraft attitudes.
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RE: L-1011 Jet Pumps

Sun Sep 24, 2000 11:13 am

On the 767-400s there are two types of jet pumps. One type is used as a water scavenge. The pick-ups are in the tank sump area and the output is immediately adjacent to the boost pump inlet. As DLMECH stated, this reduces water and helps prevent Mike Robe and his friend Fun Gus from any extended stays.

The other type operates the same way but for a different reason. They are called 'transfer jet pumps' and they work to reduce the unusable fuel inside of the center wing box portion of the center tank. When the center tank overrride pumps are shut down for lack of fuel in the tank (normal operation) there is still up to a few hundred pounds of fuel in the tank which the override pumps can't get. When the main tanks are down below 1/2 capacity, the transfer jet pumps begin to operate. They act to scavenge any fuel left in the center back into the main tanks. They can transfer about 400lbs per hour.

BTW DLMECH, what was 06 doing transferring fuel????      
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RE: Jet Pumps

Sun Sep 24, 2000 12:20 pm

Just as Jim pointed out, jet pumps are used to prevent water accumulation in the fuel tank by pumping water-laden fuel from tank low-points and discharging it to a boost pump inlet.

As I recall, there is another application where fuel schedule is maintained with the aid of float-triggered jet pumps to put fuel into the outboard sections of wing fuel tanks to keep the weight out there and minimize wing flex. I remember a McDonnell Douglas engineer telling me once that they underestimated the amount of wing flex in the design of the DC10 wing. As a result, the wingtips rise more than desired.

Due to the construction of the wing, the tip rise also twists the wing box, increases the angle of incidence and hurts fuel efficiency.

Having more weight out there served two purposes; a smoother ride, and increased efficiency. At least that is what I recall of the DC10/MD11, but it has been a few years.
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RE: Jet Pumps

Sun Sep 24, 2000 1:06 pm

You're recollection is correct Boomer. The DC10/MD11 retains approx 5700 lbs of fuel in the outboard compartments of #1 and #3 tanks until the main part of the tank reaches approx 6000 lbs. Then through use of float valves it directs boost pump pressure to operate the jet pumps to redistribute the outboard (tip) fuel to the main compartment to maintain approx 6000 lbs in this compartment. Should for any reason a loss of boost pump pressure occur which will disable the jet pump in that tank, their is another means to transfer fuel to the main compartment. A transfer valve controlled by a float valve which when opened allows the fuel to flow from tip to main compartment using gravity to maintain the desired 6000 lbs. The main fuel tanks on the DC10/MD11 are quite dynamic in their operation with even more aspects to discuss regarding this topic.
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RE: Jet Pumps

Tue Sep 26, 2000 1:40 am

The F28 does not have jet pumps, but the engine driven pumps draw the fuel from the wings. There are electric boost pumps, but they are not critical , the engines run fine without them. One of the advantages to jet pumps is the ability to recirculate some of the fuel to maintain a minimum fuel tank temperature to avoid "gelling" of the fuel in extreme low temperatures. The dash 8 does this through the fuel/oil heat exchanger.

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