This question is for all you CL65 drivers and AMTs. If you lose both jet pumps(motive flow) and both electric fuel pumps, will your engs still keep running? why/ why not?
Thanks ahead, I have a debate with some wanna be CL65 capts.
Not a likely scenario, but if motive flow and electric pump pressure are unavailable, the engine will not operate. There is no cross flow system that allows one tank to feed the cross side engine, although fuel may be transferred between the two wing tanks.
The motive flow pumps are located in the collector tank, and without them, there is no pressure to the engine driven pump. I don't believe that suction feed is a circumstance that was discussed in class. Correct me if I am wrong, it has been a while.
Off the top of my head...I'm going to say no, the engine will not run.
When the boost pumps switchlights pressed in (armed) they provide back up to the main ejectors (motive flow) when the fuel flow drops below the minimum required for normal operation.
An engine lesson (a subject close to my heart...)!!!
Let me see if I understand.
The motive flow is an electric pump in the collector tank (reminds me of a king air..) and it supplies low pressure fuel to the engine driven pump (tower shaft?) which supplies high pressure fuel to the nozzles via the FCU.
Is that right, and are the boost pumps between the motive flow and the engine driven pumps?
DISCLAIMER: I HAVE NEVER FLOWN A CRJ AND KNOW NOTHING ABOUT THEM.
The fuel systems that I've seen, usually have some combination of pumps that provide low pressure-motive flow pumps, electric boost pumps, low pressure engine driven pumps. They also usually have engine driven pumps which are high pressure. The only ones you usually have direct control(switched) over are usually the boost pumps. The low pressure pumps usually provide head pressure for the high pressure pump, and sometimes keep that pump cooler, but if they are lost the high pressure engine driven pump keeps running as well as the engine. If the engine driven HP pump fails, then bye-bye engine. Again, I've never flown a CRJ but it would surprise me if the system is drastically different from what I'm describing-it just wouldn't make a ton of sense to have a plane where the engine would fail if only the boost pumps failed.
In my experience loss of motive flow and electric pumps would be extremely rare.
Motive flow pumps are powered simply by a small amount of high pressure fuel from the high-pressure engine driven pumps. They use a venturi setup to move a larger volume of fuel at a lower pressure. They have no moving parts, and hence are very reliable. Generally, you have to lose that high pressure feed, have a clog or have no gas in the tank for a motive flow jet pump to fail.
Electric pumps are less reliable because they have moving parts. However, on the airplanes I have flown, there are multiple pumps in each tank.
However, if for some reason you happen to lose ALL of those low pressure pumps, the engines on the airplanes I've flown will usually continue to run.
In the Sabre, when you lose the low pressure pumps (all of which are electric if I remember right, correct me if I'm wrong) there is actually a chart to tell you if the engine will still run. I think below about 25,000ft the high pressure engine driven pump will be able to suck enough gas from the tank to keep it going. Above that, the first indication you have of a failure will be the engine spooling down. Once you get down into the suction feed envelope, you can generally get a re-light.
In the E120, the engines will also keep running if you lose that low pressure feed. We actually check this on the airplane once everyday sort of indirectly during the first flight of the day checks. When we open the x-feed, motive flow is automatically shut off, and the electric pumps are used to feed the HP pumps. Well, at a couple of points, all the electric pumps and motive flow pumps are not working, and the engines keep on running.
Now, I don't know specifically about the CRJ, but I'd be willing to bet that the engines will keep running if you are not up too high. However, those low pressure pumps are there for a reason and having them all fail would be a bad thing, as the posibility for a flame out will be higher.
All that being said, remember that if you lose the HP engine driven pump, the first thing you'll notice will be the engine spooling down, regardless of how many other pumps are running, and you'll never get it to re-light. The HP pump boosts pressure by a very large amount in order to force enough fuel through the atomizers in the right spray pattern to keep the engine running.
Thanks guys and girls for your responds. The fuel system pumps consist of a jet pump (pri), AC pump (sec), and engine driven pump. My feeling/guess was the engines would continue to run below a max altitude. The EDP by nature should be able to pull fuel out of the fuel lines. I also suspect the eng driven fuel pumps are altitude rated in the event of a motive flow pump failure. While I understand how motive flow (jet pump) works, its purposes in life is just to provide positive fuel VOLUME in the fuel lines. Head pressure to the EDP is the result. The EDP is what regulates fuel pressure to the FCU and VSA.
I knew this was the right forum to ask.
Thanks again and for your help.
I agree with everyone that this situation, should it occur, would be beyond rare. However, I have heard of a situation in which the motive flow(Jet Pump, Venturi Pump, whatever you want to call them) has failed. Again, very rare! In this case(don't remember the Aircraft), someone had dropped a piece of cloth, towel or something in the tank. The fuel somehow separated the threads of the cloth and it ended up clogging the motive flow.
Should probably be in the General posts, but good Thread!
Here are some numbers which may explain things better...
The engine driven fuel pump delivers fuel at 800+ psi. The electric boost pumps go to standby when the engines are operating. They come on line when the fuel line pressure drops below 9 psi. As you can see, there's no way the electric boost pumps can sustain engine operation when the engine driven pump fails.