Fuel pump placement

[D&CJohnson]

I have seen fuel pumps mounted on the lower firewall, under seat, and aft of baggage in build logs posted here.
Where should I mount my pumps and why?

For reference, I am building a model 4 with a 912iS. I have the can shaped header tank that mounts high just aft of the turtledeck windows. My battery will be in the factory plate/ tray also aft. I am using a VP-X sport with mounting to be determined. I will most likely need to keep most equipment aft for balance.

Are pressurized fuel lines in the cabin less safe than ahead of the firewall? Is there any concern about adequate feed in either position? I also am considering whether both pumps should be powered by the VP-X or if the aux should be independent?

let me know what you think.

[Kitfox Pilot]

I have seen fuel pumps mounted on the lower firewall, under seat, and aft of baggage in build logs posted here.
Where should I mount my pumps and why?

For reference, I am building a model 4 with a 912iS. I have the can shaped header tank that mounts high just aft of the turtledeck windows. My battery will be in the factory plate/ tray also aft. I am using a VP-X sport with mounting to be determined. I will most likely need to keep most equipment aft for balance.

Are pressurized fuel lines in the cabin less safe than ahead of the firewall? Is there any concern about adequate feed in either position? I also am considering whether both pumps should be powered by the VP-X or if the aux should be independent?

let me know what you think.

I am sure there is a little more danger having the pumps making pressure in the cockpit but I wouldn't worry about it if you want to mount them inside out of the heat.
You need to run a fuel flow test no matter where you mount the pumps to make sure you have adequate flow.
You will not want to run the pumps through the v power, the pumps should be run from the fuse box that the engine has so the engine alt provides the power to run the engine no matter what happens to the ship power. This makes the engine self-sufficient as far as running.
Harlan

[Eric Page]

There's plenty of head pressure in a high wing airplane to feed fuel pumps mounted low on the firewall. That's where mine are going to go. Harlan is probably right that's it's unlikely to be a problem, but I still don't want pressurized fuel lines in the cabin. In the event of an accident that cracks a fuel line, or even just a simple fitting leak, you could have a lot of atomized fuel in the cabin in a very short time.

As Harlan said, the Rotax iS Fuse Box already provides power for the pumps that's sourced from the engine's independent electrical system, with an automatic fail-back to the airframe electrical system (more on this below), so it would be difficult to improve on that by switching it through the VP-X.

In any case, it appears from my cursory glance at the VP-X Installation & Operating Manual that the VP-X Sport has a single source of power, while the Pro has two sources and dual "banks" of outputs under independent control. If that's correct, then I definitely wouldn't power or switch both fuel pumps through a VP-X Sport.

I saw one caution in the manual (Sec 4.11, page 21) that would tend to support this: in their discussion of ignition wiring, where they talk about electronic ignition systems that replace magnetos (Lightspeed, P-Mags, E-Mags, Lasar), they warn: "Make sure that at least one ignition source is wired directly to the battery as per the ignition manufacturer's instructions if you are running dual Els that require external power!"

Surely the same admonition would apply to high pressure fuel pumps for a fuel injected engine. However, the manual only uses the word "fuel" once, and it only describes the installation of what they call "boost pumps" for Lycoming and Continental engines. There's no mention of fuel injection pumps.

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One unrelated item in the VP-X manual caught my eye: On page 34, they show a wiring diagram for Rotax engines that inserts a relay into the voltage regulator output to provide pilot or VP-X control of the charging system. Please don't do this. Cutting a regulator's connection to the battery while it's under load creates a condition called "load dump," which is a nearly instantaneous rise in voltage, easily exceeding 100V. It can create an impressive arc across the relay contacts and can destroy the regulator. Rotax is very clear in their installation manuals that this must not happen.

If you want control of a Rotax charging system, interrupt the AC feed from the stator, not the regulator output. It's much easier to break an AC circuit than high-current DC. In a single phase system (912UL/ULS, 914UL), interrupt one of the two stator wires. In a 3-phase system (91_iS), interrupt two of the three stator wires (System B only; do not interrupt System A). In the 3-phase case, use two SPST or SPDT relays, not one DPST. If the DPST relay fails open, you lose all of System B. If one of two SPST relays fail, you'll still have two phases and some limited charging capability.

Why would you want to do this in the first place? One good reason -- and one that Rotax has ignored -- is overvoltage protection. It appears that the VP-X can do this for you (see Sec 4.8a Over-voltage Planning on page 21, and Sec 5.9 Alternator Wiring on page 30). Keep in mind, however, that the Rotax ECU and Fuse Box will have no idea what's going on and will be unable to automatically fail-back to System B if it's turned off by the VP-X or the pilot. If System B is off and System A fails, the engine will die (you can restart using the Backup Battery switch and operate on battery only). Be sure to set an EFIS alarm to notify you of low voltage (anything below ~13V) so that you'll know immediately if System B goes offline. It looks like the VP-X has an "alternator off" output as well, so you're double covered.

This dual charging system failure scenario is the reason that Rotax specifies a relatively large 16Ah minimum battery capacity for the iS engines. I've already ranted at length on this forum about this issue so I won't repeat it here...

[D&CJohnson]

Thank you for the thoughtful replies. I am mainly trying to get any mounts and wiring in place that would be difficult after covering the fuselage. I am going to lean towards mounting the pumps low on the firewall. I have to build my own mount for the 912iS so some of the placement of equipment in front of the firewall are tbd.

Regarding the VP-X. Thanks for the tutorial. I am learning that the Rotax is pretty well managed without help from the VP-X.

[Shadowrider]

Good info here. Personal experience with the VPX. Its been rock solid. When deciding to use the VPX Sport, and speaking with vertical power, they claimed they have not had one failure with the VPX Sport. It may not have enough circuits to have each individual item on one circuit, but I felt that was not necessary. I just paired couple items like headset power and ipad power on the same circuit. The vpx sport you can wire both fuel pumps to it. Yes it has a single dc power source, the battery, to the VPX, but two different circuits to each fuel pump. Each fuel pump is powered by a different "bus" on the VPX so you have some redundancy if that bus fails you should be able to power the alternate fuel pump from the other bus on the VPX. Yes you only have one power source to the VPX because you only have one battery. If you want to make a truly redundant system then you would probably want to have two separate batteries powering two separate VPX's. I have seen more failures with batterys and would be more concerned about battery than the VPX. With that being said I will be replacing my battery at 7 years based on the recommendation from the manufacturer. I am running a Edge912STI Rotax, so may be different on the fuel pump installation. After running a Rotax i could never go back to legacy aircraft and after using VPX I could never go back to legacy breakers....yes they are that good!

[Eric Page]

The vpx sport you can wire both fuel pumps to it. Yes it has a single dc power source, the battery, to the VPX, but two different circuits to each fuel pump. Each fuel pump is powered by a different "bus" on the VPX so you have some redundancy if that bus fails you should be able to power the alternate fuel pump from the other bus on the VPX.

I think this dual bus scheme (Vertical Power call it "Dual Bank Technology") is only a feature of the VP-X Pro (see Sec 4.4 on page 15 of the manual). It appears that the Sport has one bus run by one supervisory processor that is shared across all outputs.