Fuel pumps

[Lyndon]

I'm new to this but I have a mechanical background. I understand why we have a back up pump. One electric and and mechanical. The electrical backing up the mechanical. I'm building a Savannah. I'm freshening up a 912 uls with 1740 hrs on it. I have priced a new mechanical pump. Outrageous. Why not fit an electric pump. Separate circuit that runs 100 %. This will easily do 2000 hrs. I have a back up. Chances of both failing are no more than electric and mechanical. Engine looses charge rate I'm guessing after shutting down excess power the battery will run it for at least 30 to 60 mins. Battery dies then as long as the engine is running it has power. I'm guessing the efi engine that Rotax make does not have a mechanical pump ???. Like I say I'm new to this and appreciate any input.

 

Lyndon

 

 

[Geoff13]

From experience, if the cabin fills with smoke from an electrical fire and you hit the master switch to try and stop the smoke you might wish for a mechanical pump to keep the fan going.

 

 

[Lyndon]

Yes I get that. Cut all electrical power and hope it stops burning. All being well the engine keeps running as it produces its own power. But on a redundant loom is your back up pump. It still pumps ???

 

 

[Geoff13]

If the backup pump is the problem what then? And I would need to go to the circuits to check but if you turn off the master then the charge circuit may not continue to work. A master switch by its very nature is designed to kill all electrical power so there is not much point having a master switch which still allows things to run. In my situation it was the auxiliary fuel pump that caused the fire, what them?

 

 

[Lyndon]

The master does not turn off the engine. It still fires the plugs. What I'm saying is two separate fuel pumps. Easy to wire on two curcits

 

 

[Lyndon]

More likely a wing to fall off than two circuits fail at the same time

 

 

[Lyndon]

The battery is still alive after a master has been cut

 

 

[Lyndon]

I may be wrong. But you kill every thing. What I do know is the engine still runs if it gets fuel. The battery still has a full charge. On a complete redundant circuit is another pump. Has to have 30 to 60 mins in it. Pumps don't fail and wiring looms don't burn for no reason. I'm very particular in what I do and I would be very surprised if you needed the second backup. Then the third is to land which is easy in a Savannah. But like I say I'm new to this so I really appreciate your comments.

 

Lyndon

 

 

[Lyndon]

Guess really what I'm saying is a mechanical pump any better than an electrical one. I can answer that. No one makes a mechanical pump any more. So two separate circuits two pumps should be fine. Yep no one makes a flat tappet engine either. Unless your Rotax. . They do both because it works. Not great but it works. If they were building car engines they would be broke. I totally understand the litigious society we live in. It's a Shame really.

 

 

[Geoff13]

More likely a wing to fall off than two circuits fail at the same time

Good luck with that belief.

 

Just had a quick look at the wiring diagram in the Rotax installation Manual and the X-Air Hanuman Manual.

 

Both Diagrams have the Master switch Isolating the positive side of the Battery so you are correct turning off the master switch does leave the battery alive. If installed correctly however it should not be connected to anything except the positive side of the Master Switch and as such is not capable of running anything.

 

The master does not turn off the engine. It still fires the plugs. What I'm saying is two separate fuel pumps. Easy to wire on two curcits

I certainly wouldn't want to run my aux fuel pump from the electronic module circuit.

 

 

[maulonir]

Do any fuel injected aircraft motors run mech pumps?

 

 

[Geoff13]

Here is one of those aux fuel pumps that don't fail

 

 

 

[Geoff13]

So two separate circuits two pumps should be fine.

Not if they both go through the master switch, and if they don't then you do not have a master switch.

 

 

[Lyndon]

Your mis understand. Sorry. What I'm saying the battery is not dead. You have killed one circuit with the master switch. But you can have another. Yes I do know the meaning of master. But you get my drift. Believe me your engine still runs even your master turned off

 

 

[Lyndon]

Does the current efi engine run a mechanical pump ???. If it does then I will rethink my fuel arrangements. Because one thing I can assure is the carburetor is dead. Even in my lifetime.

 

 

[Lyndon]

Did you find out why the pump failed ???.

 

 

[SDQDI]

My master switch is an isolator and the engine WILL stop if that gets turned off.

 

The 914 (well mine anyway, am pretty sure this is how the 914 is meant to be set up) has two electrical pumps obviously wired seperately so that you can switch them individually if one fails ect BUT IMO the 912 works well with one mechanical and one elec so if I had a 912 that is how I would run it.

 

I've heard of a couple of mechanical pump failures (in a thread here somewhere) and Geoff has shown us that elec can fail too but I think in either case having two fail would be unusual but if the standard 912 runs with one of each I'd keep it that way.

 

 

[Lyndon]

Current diesel engines run and electric to feed the real pump which is mechanical and does all the work. But these are direct injection and run well ove 20,000 psi. Some to almost 30,000 in a performance diesel. But these are the ignition system with a boosted engine. Thus the pressure. Apart from these I think everything with a spark plug uses and electric pump as it it goes in under no pressure.

 

 

[Lyndon]

As I say I'm new to this. I was thinking you cut the fuel to a Rotax to stop it. I was of the understanding once running turning off the master does not stop it running. My apologies. I thought it had a redundant ignition system.

 

Lyndon

 

 

[Lyndon]

I also to be clear. Was indicating in a failure. The spare pump for take off and landing be run thru the battery. Nothing else. I'm involved in race engines that run a total loss system. As charging equals horse power. If they were a plane you would easily find a spot to land.

 

 

[Geoff13]

Lyndon

 

In the case of a 912 wired in accordance with the Xair wiring cct and from my reading of the diagram it would be the same with it wired to the Rotax circuit if the master is turned of the engine will continue to run.

 

I tested that this was the case after rewiring my plane in accordance with the diagram and have proven it in flight with a Foxbat.

 

The master switch is designed to isolate the battery and all aux circuits. To stop a Rotax 912 you either remove the earth from or supply the earth to the electronic modules I can't remember which way it is. The ignition switches actually switch the earth. I can't remebr where the supply for the modules comes from but is certainly not direct from the battery.

 

 

[planesmaker]

My 914 kept running when my main fuel pump stopped working and the other was off at the time. It did gravity feed( high wing) but at a lower power setting. 914 runs 2 elec pumps.

 

 

[Lyndon]

I haven't got to the plumbing stage yet. If gravity fed I'm sure the bings would work fine. But if they run through an electric fuel pump. Let alone two what I'm contemplating it will not work. You need pressure to run through a dead pump. Not much but it is more that a tank in a high wing

 

 

[planesmaker]

You could run them in parallel. Why not keep the original mech pump ? Is it broken? Going to be 19 registered ?

 

 

[Blueadventures]

I haven't got to the plumbing stage yet. If gravity fed I'm sure the bings would work fine. But if they run through an electric fuel pump. Let alone two what I'm contemplating it will not work. You need pressure to run through a dead pump. Not much but it is more that a tank in a high wing

Hi Lyndon

With the electric pump check the wiring diagram. As well as a switch there should / may be a fuse or circuit breaker of the specified amp rating. Fit a good quality one. Like in the Rotax wiring diagram for 912's, the master switch circuit is protected by a fuse / circuit breaker and also another seperate fuse / circuit breaker protects the engine alternator circuit to the battery positive. Geoffs advice is great as it shows what can occur to that type of electric fuel pump if not protected by a fuse or circuit breaker. (For some unkown reason they were not included in that Foxbat's wiring; aso most Foxbats don't seem to have them. )

 

Cheers

 

Mike