Fuel pump switch over

[StanFoster]

First of all...upfront I want to thank Heather Poe for sending me a wiring diagram for a circuit that automatically kicks a relay in to switch to the second fuel pump in the event the primary pump fails.

I took a break the other day and studied her diagram.....and then started realizing that I could easily adapt my fuel pumps to do the same thing.

If I have my main pump fail either by mechanical or electrical...or that one time back in May when I inadvertantly shut mine off....had I had this installed my second pump would have come on almost instantly.


MY modified plan takes some of what Heather sent me and uses what I already have in my system.

One of these is a fuel pressure switch that comes on when my fuel pressure drops to a preset level. It operates an idiot light in my pod. However...that light is about worthless as when it comes on...the motor will be be dying within a second.

I am planning on having my fuel pressure switch also operate a relay that is installed in series with my second fuel pump. If the fuel pressure drops...this relay kicks in and runs the second pump. I will have it wired so that it will self excite itself and keep the relay on when the fuel pressure comes back on and switches the fuel pressure switch off. I am thinking I could have another light indicating that the second fuel pump is activated and that something has happened to the primary pump.

This relay would then be simply reset when I throw the switched breaker that is already wired to it. I could still test both pumps on preflight....by first starting on the main pump....switching its circuit breaker off...this would test the backup switching system and the second fuel pump. Then I would just switch the main pump back on...and shut the second pump off thus breaking the relay and resetting it for inflight use.


Note: I do have each individual pump on its own switched circuit and can alternate use between the pumps. I feel that if this works out...I will just let my main pump do all the work and have a fresh pump ready at all times to be instantly switched on when this relay is activated by low fuel pressure.

I think what Heather sent me and the way I want to modify it to simply adapt to what I have is a very simple system. I just need to research the right relay so that it switches the way I want....handles the amps...and can be self energized.



Any thoughts or modifications?

Stan

 

[Mike Schallmann]

Stan

About two years ago I was cruising over some pretty rugged terrain in Southern Az when I got to thinking about dependability--at that time Iwas running only one fuel pump.

After that I gave a lot of though to dual pumps and their setup- I went with the KISS method---I simply run dual pumps in parallel with seperate switches(and seperate fused circuits) I start and warm up the engine on one -then swiitch to the other while taxing--immediately prior to turning on to take off they both go on and STAY on for the entire flight...

To me this is the safest way-I dont have to worry about an automatic kick on circuit to work and the possibility of both pumps failing are nil--

 

[Mike Schallmann]

Stan

I didnt pick up on it --but I now realize that you have FI --it looks like a relay circuit may be the only way--

 

[dragonflyerthom]

Stan with carbed system you had some fuel in the float area. Pressure was 3-4 lbs and you would have some time. The FI system has a 60 lb system with no reserve. I think your backup system would work but I also feel the redundancey of a second pump would be better. You might have to enlarge your return to tank hose inorder to eliminate the lower rpm.

Most cars only have a single pump. But you can just pull over if the pumb goes out. No such luck if you are in the air.

Keep us posted

Thom

 

[route66]

I think what Heather sent me and the way I want to modify it to simply adapt to what I have is a very simple system. I just need to research the right relay so that it switches the way I want....handles the amps...and can be self energized.

If 20 amps is large enough you might look at these!
http://www.firgelliauto.com/product_info.php?cPath=101&products_id=28

 

[Mike Schallmann]

Stan I worked out a simple circuit using a headlight relay, an LED and a diode--=as soon as I get my scanner hooked up I'll send you the schematic--probally later today or tomorrow

 

[Timchick]

Stan,
Would a fuel setup like Birdy added onto his gyro give you a little more time when one pump failed?

 

[PW_Plack]

Stan, here's a simple relay circuit. The +12V on the left should come from the breaker or panel switch for fuel pump #2. If you leave it connected to +12 all the time, you won't be able to switch off pump #2 without switching off the master.

This circuit assumes you can get by using the same indicator for "Pump 1 Fail" and "Pump 2 On". If you connect the panel indicator as shown, either the pressure failure switch or the relay will cause it to light.

Always use a silicon rectifier diode across the coil of a relay as shown, polarized so no current normally flows through the diode. It will shunt the transients created by energizing the relay coil. Otherwise, you'll create spikes which may confuse or damage other avionics.

The relay shown is a single-pole, double-throw (SPDT) relay. The "normally on" set of contacts is unused in this scheme, but it could be used to power a green "Pump 1" panel indicator. I guess that would let you tell the difference between normal operation and a burned-out bulb for "Pump 2". If you don't need the second indicator, you could use a SPST relay.

Just make sure the relay is rated to switch the load of Pump 2 plus the indicator.

 

[Mike Schallmann]

Just as I was getting ready to scan in my drawing --I - saw Pauls -mine looked a little different (hand drawn)--but electrically it is identicle it had the diode ,Panel light (LED) and relay--BTW a headlight relay will easily handle the load...

 

[Aussie_Paul]

Stan I worked out a simple circuit using a headlight relay, an LED and a diode--=as soon as I get my scanner hooked up I'll send you the schematic--probally later today or tomorrow

Mike and Heather. Please post the info so we can all learn. Thanks.

Aussie Paul.

 

[Heather Poe]

Switch-over Circuit

Switch-over Circuit

This is what I sent Stan Foster for review.

When David Overman heard that a fuel pressure sensor switch (Hobbs type, connected when no pressure is present) was to be added to the Randy Coplen's aircraft, he requested that the fuel pumps be automatically switched during primary pump failure.

During normal operations the DPDT switch is in the up or normal position. In this case, the primary left fuel pump and pump relay receives power.

At the beginning of the preflight the switch is flipped downward, to the test position. At this time power stops to the primary left fuel pump and relay. Power flows to the secondary right fuel pump and its operation is checked by ear by the pilot. At this time the sensor latch circuit is reset when it has its power removed. The next step for the pilot is to restore the switch to the normal position. The fuel system pressurized by the secondary right pump does not trigger the sensor or its latch circuit while the primary left fuel pump spins up and the secondary right fuel pump spins down. Operation of the primary left pump is checked by ear by the pilot.

In flight, if the primary left fuel pump fails, the sensor conducts, the sensor latch relay latches, the secondary pump and relay automatically begin running, and the indicator light illuminates. Power to the failed primary left fuel may be switched off, if desired, by flipping its breaker. Also, at any time, power to the secondary right fuel pump and latch may be turned off by flipping its breaker.

 

[gab76]

Fuel pump select switch wiring and auto-failover relay wiring

Fuel pump select switch wiring and auto-failover relay wiring

To make it a little easier, I found the diagrams from Jan Eggenfellner's site to of great help.

I am installing this system on my Air Command Tandem with Soob power at this moment.

It is easy to adapt it to your present wiring and will shorten your search for parts. I believe I ordered everything from Mouser electronics under diagram part numbers.

see http://eggenfellneraircraft.com/Emotor6of7.jpg

here see page 5 of 7 and page 6 of 7.

Tongue tied and twisted, just a earth-bound misfit, I Pink Floyd

 

[gab76]

continued....

continued....

Sorry to take you to just one page diagram.

go to http://eggenfellneraircraft.com/EggenfellnerManual.htm

move to the bottom of the page to "go to index"

next page click on "Chapter 11".

Enjoy!


Tongue tied and twisted, just a earth-bound misfit I

Pink Floyd

 

[gab76]

fuel pump crossover

fuel pump crossover

What's nice about the 3 way switch is, in the "Aux" position, the pilot can take that bad pump "off line" and isolate it from the circuit.

You only have one more item on your start checklist. That is place the fuel pump position switch on "main" so you don't get the blue (or any color you choose)light. This is because your fuel pressure sensor records a below pressure reading and the relay turns on your "Aux" pump instead.

 

[PW_Plack]

It didn't occur to me when doing the simple one-relay diagram, but you'd need to start the engine with "Pump 1" on, "Pump 2" (which also feeds the relay circuit) off, or the relay would switch to the backup before the first pump got the pressure up.

 

[Heather Poe]

Remember it is important to be able to easily hear each pump's operation individually during preflight. This makes the circuit more interesting.

 

[PW_Plack]

Heather, in the simple circuit, pre-flight procedure would be:

(1) Switch on pump 1, listen for pump.
(2) Switch on pump 2 enable, listen for pump 1 still running, indicator light should be dark.
(3) Switch off pump 1, indicator should light, listen to confirm pump 2 start.
(4) Switch off pump 2 enable, neither pump should be running, indicator should be dark.
(5) Switch on pump 1, start engine, switch on pump 2 enable.

Mike, I was thinking horn relays, cheap and sturdy, but you're right - the headlight relays are intended for continuous duty, and to hold good contact closures through vibration. Either should be cheap, and rated for more than enough current.

 

[Heather Poe]

Paul,
In my circuit, switch it to test, confirm auxillary pump is running, return switch to normal, confirm main pump is running, start engine.
Heather

 

[PW_Plack]

On second thought, a cleaner sequence on the one-relay circuit would be...

(1) Switch Pump 2 enable ON, check for indicator ON and listen for pump 2.
(2) Switch Pump 2 OFF, Pump 1 ON, check for indicator OFF and listen for pump 1.
(3) Switch Pump 2 enable ON, start and go.

Heather, you've left me to wrestle with more complexity but a simpler pre-flight, or an extra pre-flight step to get rid of any relays in the failure chain for the main pump. I'm tempted to vote for fewer parts, but then, modern relays fail less often than modern pilot checklist compliance! Hmmm...

 

[M. Pearce]

Stan,

I'm just wondering something about this system. Maybe I missed something in the drawings. The fuel lines comming from both the pumps, do you use check valves so the fuel won't backwash through the non-working pump???? Do these pumps allow fuel to flow backwards when the pump isn't on???? I'm going to be in this situation in about a year from now but my system is going to be used on a carbed engine and not an injected engine. So, my pumps will be lower pressure electric pumps. I'm feeling a check valve will be needed in the non-functioning fuel line to maintain pressure in the active line.

In your situation I'm thinking if you have the check valves your light will go back to the off mode due to the line being pressurized again by the backup pump. I'm also thinking if this happens your seconday pump will be cycling on and then off quite a bit and quite rapidly while running. This unwanted on/off cycling would be very hard to check in a engine off situation. This is probably what will happen if you hook your second pump up to you low pressure light!!!! Why wouldn't your secondary pump turn the light back out if it's doing it's job of repressurizing the syastem????


Have I lost anyone yet??????

Maybe I'm the one lost??????