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Knowing your lower limits


Roger Lee

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I've been pondering this question for years.My CT2k is 12 years old, 700 hours on the engine and still on the original fuel pump! No backup electric pump because someone calculated that would put the airframe weight out of the microlight category.No fuel pressure gauge either so when I flew 85 miles from Plymouth England to Cherbourg France over the English Channel I had plenty of time for pondering. The view out the window was great.

Sorry I can't answer your question Roger.

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Without looking anything up most float type carburetors have a minimum around 1/2 PSI.  

 

IIRC Rotax want around 2 PSI as the minimum setting.

 

As long as the fuel pump failure doesn't restrict the flow the head pressure from the overhead wing tank of the CT should provide enough pressure.

 

This is from memory, so the actual values might be a little different.

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I'm with Tom on carb engines. The fuel pump defaults to bypass unless the pump is operating, so unless it catastrophically fails, fuel will still flow.

 

Anyways, the carb itself doesn't work on pressure to put fuel into the airstream, just as long as there's a little pressure to push the fuel into the carb bowl, you'll be fine. That, and the carb bowl needs an atmospheric pressure equal to the intake upstream of the carb. If it's high or low, it will force a rich or lean mixture.

 

As for the injected engine: I'd have to look that up, but it does require considerably higher fuel pressure for the port injectors to work (double digit psi i think).

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I've been pondering this question for years.My CT2k is 12 years old, 700 hours on the engine and still on the original fuel pump! No backup electric pump because someone calculated that would put the airframe weight out of the microlight category.No fuel pressure gauge either so when I flew 85 miles from Plymouth England to Cherbourg France over the English Channel I had plenty of time for pondering. The view out the window was great.

Sorry I can't answer your question Roger.

 

Mac, with the CTs I don't think it's much of a problem, they will gravity feed enough to run the engine very well even with no working fuel pump.  It's more of a problem with low wing airplanes and those high wings with fuel not in the wings like the Remos.

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In a CTLSi, if the fuel pump  quits, the engine quits right now.  It does have 2 fuel pumps.  The second pump should be on in take off and landing conditions or when in low proximity to the ground where you might not have time to turn on the second pump and execute an inflight engine start.

 

Bottom line - no fuel pump on and working - no engine working.

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In a CTLSi, if the fuel pump  quits, the engine quits right now.  It does have 2 fuel pumps.  The second pump should be on in take off and landing conditions or when in low proximity to the ground where you might not have time to turn on the second pump and execute an inflight engine start.

 

Bottom line - no fuel pump on and working - no engine working.

 

Yeah, you fuelies are a different beast, you need high and continuous fuel pressure to run those fancy injectors.  Not to mention power too, which is why you get two batteries.

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I posted this before, but can't find it now. This was another research project for Rotax people.

The CTSW high wing was used.

I took the hoses off the fuel pump and joined them with a piece of tubing. This is now only gravity feed and unobstructed. It will run the engine at full rpm without a hiccup.

The second test was with the fuel hose back on the pump, but the pump was removed from the engine and left to hang. It got nothing through other than gravity feed. The pump has a free floating check valve and opens and closes with fuel flow caused by the diaphragm. It only works when the pump is pumping (more or less). So with the pump off the engine can sustain 5K rpm without a hiccup, but somewhere just above that the engine stumbles and coughs. The check valve does cause some restriction, but still flows adequately. Fuel pumps rarely ever totally fail. It is usually a slow death and leakage occurs and should easily be seen . 

 

The Rotax line in the sand bottom fuel pressure for carbed 912's is 2.2 psi. It's a good pressure cushion and a set point to help cover any liability. You'll never have a fuel flow issue at 2.2 psi, but it does make you feel better when you see it from 3.5-5.5 psi.

Tom pretty much got it. Just give it .5-1 psi and it will run.

Low wings are in a totally different boat and absolutely should have a back up electric pump, but it doesn't need to run all the time if the mechanical pump is doing its job..

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  • 3 months later...

Hi, I was thinking about installing an electric fuel pump in my CTSW 2007. I know many high wing planes that have it (non injected) like Tecnams P92, P2008 and use it when take off and approach for safety but reading this post, any of you will install the electric pump if there was no legal restriction? 

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In the US you can't install an electric pump because of the regulations and would need an LOA which I doubt they would approve. In the CT high wing it isn't needed. 1800+ CT's worldwide and none of them need it. It would really be a waist of time and money. You engine will run with a complete mechanical fuel pump failure. Complete mechanical failures are so rare you might as well play the lottery. 

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I lived this last fall.  Took off from my annual and felt a lot of vibration which I attributed to my wheels.  Once in the air it didn't stop and I could only do 200 fpm climb.  When I throttled back, the engine ran smooth.  Full throttle was rough.  I flew the pattern at 85 kt with a smooth engine and landed.  My mechanic diagnosed a complete fuel pump failure.   Now I know what it's like, there is no reason to panic.  Just fly at 80 kt or so until you can get on the ground.

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Is good to know that the engine can run even a fuel pump failure but after following Jon's Hilton travel around the world in a CT (he started about 3 weeks ago but had to come back due to extreme hot in Middle east) He talked about engine stoppage several times, he had an auxiliar fuel cell in the passenger seat and told that had to squezze it to put more fuel in the engine and can run it again. That was not a fuel pump failure, it seems was vapor lock (using avgas?) 

If he had not that fuel pack to force fuel in, an electric fuel pump could help? what to do in this case?

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