Chasing the issue - low fuel pressure on takeoff

[BigJohn]

Originally Posted by Piper J3

Did Rotax maintenance instructor say why he recommended shutting off electric fuel pump? Seems to me that he would have a good reason to tell someone to alter a safety feature designed by mfgr. I don't think I would want that liability...

Jim, I realize you are a strong proponent of running the electric pump at all times, for valid safety reasons. However there are valid reasons for not running the pump at cruise in some airplanes. One of those is to prevent false high fuel flow indications and accompanying alarms when cruising at high altitudes. This was a known issue some years ago with certain airplanes. The cause was never definitively identified, and the solution employed by quite a few of us was simply to shut off the pump in cruise flight. Just adds one item to the before takeoff and landing checklists.

[Piper J3]

Originally Posted by BigJohn

One of those is to prevent false high fuel flow indications and accompanying alarms when cruising at high altitudes. This was a known issue some years ago with certain airplanes. The cause was never definitively identified, and the solution employed by quite a few of us was simply to shut off the pump in cruise flight. Just adds one item to the before takeoff and landing checklists.

Why not just raise the upper alarm limit and let the pump run all the time? I'd rather have fuel pressure/fuel flow slightly above spec rather than no fuel flow because mechanical pump failed and electric pump was switched off...

[rvbuilder2002]

Originally Posted by Charlie12

But for now my guess is that there is a possibility of inconsistent engine-driven pump operation due to placement of the tank unusually far aft, which would explain why the kit does not provide a switch for the electric pump.

As has been explained here in the forum previously, the reason the electric pump on the RV-12 is operated full time with no switch available is to mitigate vapor lock.

This is the same reason the fuel pump is located at the tank in cars and trucks.

Vapor lock is much more likely when fuel is being pulled through the system. With a pump located near the tank (and operating), it is pushing fuel through the system and holding it at a pressure higher than atmospheric, making vapor lock much less likely.

[Charlie12]

The instructor's recommendation for an on/off switch was a general observation on Rotax 912 operating requirements. He did not know why Van's chose not to include a switch on the RV-12 but said the decision puzzled him. I have always wondered about it and think I now understand why.

[NinerBikes]

Originally Posted by rvbuilder2002

As has been explained here in the forum previously, the reason the electric pump on the RV-12 is operated full time with no switch available is to mitigate vapor lock.

This is the same reason the fuel pump is located at the tank in cars and trucks.

Vapor lock is much more likely when fuel is being pulled through the system. With a pump located near the tank (and operating), it is pushing fuel through the system and holding it at a pressure higher than atmospheric, making vapor lock much less likely.

Any ideas on if that additional fuel pressure is a consideration in Carb floats on the Bing Carbs turning into sinkers instead of floats? Does that fuel pressure cease to exist once the fuel enters the float bowl, since at that point, the carb jets are now creating lower pressure due to air stream through the throttle bore of the carb.

[DHeal]

The carb bowls are continually vented to the atmosphere via the short clear plastic vent tubes. I suspect that the fuel pressure has nothing to do with sinking floats.

[Piper J3]

Originally Posted by NinerBikes

Any ideas on if that additional fuel pressure is a consideration in Carb floats on the Bing Carbs turning into sinkers instead of floats? Does that fuel pressure cease to exist once the fuel enters the float bowl, since at that point, the carb jets are now creating lower pressure due to air stream through the throttle bore of the carb.

The float bowls are vented to atmosphere with a small clear plastic vent tube. Fuel pump pressure is not "seen" by the floats...

DHeal and I typed at same time

[Piper J3]

What Scott says in post #23 above regarding vapor lock may be contributing factor in Nov 17, 2018 RV-12 accident in Florida? http://www.kathrynsreport.com/2018/1...-accident.html

?The owner stated that the problem was likely "vapor lock," which the airplane had experienced in the past?.

Need to wait for final NTSB report?

[BigJohn]

Originally Posted by Piper J3

Why not just raise the upper alarm limit and let the pump run all the time? I'd rather have fuel pressure/fuel flow slightly above spec rather than no fuel flow because mechanical pump failed and electric pump was switched off...

Well, Jim, that?s the beauty of this wonderful thing called experimental aviation. We all get to make the decisions that provide the best tradeoffs in our individual situations. No right or wrong, just choices. I chose the path that preserved the accuracy of the fuel totalizer function in the D-180. Even at my advanced age I can remember to run the pump at low altitudes. Checklists help!

In your case, if running the pump continuously causes no problems, by all means let her run!

Also - a word of caution - your last post could be interpreted as speculation and it would be a shame to get this thread shut down.

[RFSchaller]

There are definitely two opposing strategies on whether or not to put a switch on the electric fuel pump. I doubt either camp will convince the other. We should all just agree to disagree and move on.