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High indicated fuel flow with fuel pump on

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Bill Strahan

Joined: 20 Aug 2008
Posts: 145

Posted: Wed Nov 25, 2009 6:06 pm Post subject: High indicated fuel flow with fuel pump on

I've been fleshing out my checklist and recently decided to add turning the fuel pump on for takeoff and landing.  I'm used to that with my Grumman, and it felt natural to do it that way.



Today I took off using the new checklist, and once I was a few miles from the airport I noticed my fuel flow was 11GPH!  I've never seen anything near that high and my fuel flow already was reading about 10% too high.



I throttled back to a setting that typically shows 4.5-5 gph and was seeing 7.  For a moment, I pictured a fuel leak and decided to turn the boost pump on and see if the fuel flow surged, which would confirm a leak on the firewall side, and I was going to immediately return to the airport.



Instead I realized I had left the boost pump on since takeoff and then I wondered if that was the cause of the high reading.  Sure enough, with the boost pump off the EIS indicated 5.1 gph.  Boost pump on and it jumped back to 7 or so.



I was pretty sure I didn't have a fuel leak at that point, so left the pump on for a half hour or so while I flew.  I saw a growing disparity between what the totalizer thought I had left and what the tanks were indicating, confirming to me that the indication is all that was changing, I wasn't pumping fuel overboard.



So, anyone else noticed the same thing?  I would have thought the flow sensor would have been independent of fuel pressure, but that appears to not be the case.



Bill

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Clive J

Joined: 03 Nov 2007
Posts: 340
Location: UK

Posted: Thu Nov 26, 2009 2:54 am Post subject: High indicated fuel flow with fuel pump on

The fuel flow meter picks up the pulse of pressure from the pump which

you've no doubt gathered. 

This is what I've seen on several jabs with fuel flow meters, yet

another application where knowing too much causes questions that the old

simple applications wouldn't have brought up!



I did wonder if there was a sensor that wouldn't do this, maybe an

ultrasonic one but I imagine those are either not made yet or too

expensive.



CJ



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wb2ssj(at)frontiernet.net
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Posted: Thu Nov 26, 2009 4:05 am Post subject: High indicated fuel flow with fuel pump on

Bill , I have seen bthe same thing and have not figured out what causes it. 

There is no leak.  Tex



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IFLYSMODEL(at)AOL.COM
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Posted: Thu Nov 26, 2009 4:11 am Post subject: High indicated fuel flow with fuel pump on

Me Too.

 Lynn Nelsen

  

  In a message dated 11/26/2009 7:06:09 A.M. Eastern Standard Time,  wb2ssj(at)frontiernet.net writes:

 [quote]-->    Lightning-List message posted by: "Tex Mantell"    <wb2ssj(at)frontiernet.net>



Bill , I have seen bthe same thing and    have not figured out what causes it. 

There is no leak.     Tex



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Kayberg(at)aol.com
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Posted: Thu Nov 26, 2009 5:46 am Post subject: High indicated fuel flow with fuel pump on

In a message dated 11/25/2009 9:06:51 P.M. Eastern Standard Time,  bill(at)gdsx.com writes:

  	  Quote: 	
		  

I've    been fleshing out my checklist and recently decided to add turning the fuel    pump on for takeoff and landing.  I'm used to that with my Grumman, and    it felt natural to do it that way.



	


 

  

 I want to offer an alternitive thought about when to use the electric fuel  pump.

  

 I use the electric to be sure the carb is full before starting.  Then  I turn it off....  I turn it back on briefly during runup before takeoff to  insure the engine runs with both on.

  

 My emergency procedures for a loss of power call for  1) electric fuel  pump on 2) switch tanks.

  

 If you  lose power on takeoff with both pumps running, you dont know  what went wrong.   If the power is restored with the electric pump on,  it is likely the mechanical one failed.  If it failed it could be pumping  fuel out somewhere and you will want to land immediatly.

  

 If you happened to run a tank dry or had it unport because of low fuel in a  down wing, you will want to be sure the pump is on, so just assuming it was on  for takeoff  ( notice how often we forget to turn it on?)  may not be  the best.  Hence, always: Hit electric pump switch and switch  tanks.   It takes about a 5 count for the engine to pick up  on  an empty tank switch.  Dont ask me how I know!

  

 Switching tanks is a must because the fuel filter could have clogged on the  one being fed...or the fuel guages are wrong.   Of course if the other  tank is dry or too low to reliably feed, shame on you!!   I would  suggest you never take off with either tank that is too low to feed on a  takeoff.

  

 Just my thoughts on use of the boost pump.

  

 Now, The Facet pump has a built in check valve.  I suspect the engine  driven pump has a higher pressure than the Facet we commonly use.    When the facet is on, the check valve is fluttering.  I suspect the higher  presure from the engine driven is pushing some fuel past the check  valve.   Even a very little bit will show up on the fuel flow.

  

 FWIW

  

 Doug Koenigsberg

  [quote][b]

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N1BZRich(at)AOL.COM
Guest

Posted: Fri Nov 27, 2009 6:14 am Post subject: High indicated fuel flow with fuel pump on

Bill and all,

     Sorry for taking so long to respond to this  question, but with the turkey day holiday and finalizing the December newsletter  (should go to Pete and Jim today), I have kept busy.  Anyway, I also have  seen the same increase in fuel flow when the electric fuel pump is  on.  I have always just assumed it was because the fuel flow transducer  (like the fuel pressure sensor) was between the electric fuel pump and the  engine driven fuel pump and not between the engine driven fuel pump and the  carb.  So I think it is probably normal to see a rise in fuel flow (as you  see the fuel pressure go up - if you have the fuel pressure indicator) when you  turn the electric fuel pump on.  

 Buz

  

  

 In a message dated 11/25/2009 9:06:51 P.M. Eastern Standard Time,  bill(at)gdsx.com writes:

   	  Quote: 	
		  -->    Lightning-List message posted by: "Bill Strahan"    <bill(at)gdsx.com>



I've been fleshing out my checklist and recently    decided to add turning the fuel pump on for takeoff and landing.  I'm    used to that with my Grumman, and it felt natural to do it that    way.



Today I took off using the new checklist, and once I was a few    miles from the airport I noticed my fuel flow was 11GPH!  I've never seen    anything near that high and my fuel flow already was reading about 10% too    high.



I throttled back to a setting that typically shows 4.5-5 gph and    was seeing 7.  For a moment, I pictured a fuel leak and decided to turn    the boost pump on and see if the fuel flow surged, which would confirm a leak    on the firewall side, and I was going to immediately return to the    airport.



Instead I realized I had left the boost pump on since takeoff    and then I wondered if that was the cause of the high reading.  Sure    enough, with the boost pump off the EIS indicated 5.1 gph.  Boost pump on    and it jumped back to 7 or so.



I was pretty sure I didn't have a fuel    leak at that point, so left the pump on for a half hour or so while I    flew.  I saw a growing disparity between what the totalizer thought I had    left and what the tanks were indicating, confirming to me that the indication    is all that was changing, I wasn't pumping fuel overboard.



So, anyone    else noticed the same thing?  I would have thought the flow sensor would    have been independent of fuel pressure, but that appears to not be the    case.



Bill





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