Subject: Fuel Flow and Pressure deviations in RV-10, N541RV

Background: My RV-10 has been flying since January 2012, over the past 10 months and 100 flight hours I

would notice an occasional drop in fuel pressure and flow while in cruise flight. There has never been any

noticeable change in engine performance during these events. The airplane has a stock Lycoming IO-540-

E4B5 engine with Airflow Performance Fuel injection. The engine had 22 hours SMOH when I installed it in the

aircraft.

The fuel delivery system utilizes the stock location and routing for fuel lines, electric fuel pump and fuel flow

meter. The actual equipment installed is as follows. Andair Gascolators in each wing root are utilized as

primary filter, standard 3/8 aluminum lines run to an Andair fuel selector valve mounted in the stock location,

this is followed by an Andair electronic boost pump, from the boost pump, the line continued through the EI

fuel flow transducer, then to the firewall bulkhead fitting in the stock location. Note the Andair Fuel filters

utilize the same mesh screen size as recommend by Air Flow Performance.

The tunnel and all floor areas in the aircraft are insulated with the 1”thick black foam insulation sold by ACS.

My tunnel has always run cool enough that you can easily keep your hand on it.

Observations: On cross country flights in the cruise configuration I would occasionally notice my fuel

pressure drop from about 26 psi to about 15 psi. This event would last about 3-5 seconds and would occur once

every 10 minutes or so. Other flights I would not see an issue. The pressure drop would always be associated

with a drop in fuel flow followed by a recovery to higher than normal flow rate as it settled back to normal.

During this event the engine would run perfectly smoothly. After downloading data files from my EFIS I did

not detect the deviations I was observing in flight, finally I recognized that the sampling rate on my EFIS was set

too long to capture these 3-5 second long events. I changed the collection rate to 1 sample per second and

finally had real data to look at. As seen in the next two charts the fuel pressure was truly falling with an

associated bobble in fuel flow.

Chart 1 is from two flights I made on Oct 14th

from Allentown to Meadville PA. The first flight was at 6000 msl

and 15 C OAT, the second trip home was at 11,000 msl and 8 C. You can see the periodic drop in pressure and

flow. For each flight segment you can see when the electric boost pump is used and the pressure rises to

about 28-29 psi and is rock steady.

Chart 2 is a zoomed in view of one event with the associated pressure drop and fuel flow bobble.

Other notable observations, I never have been satisfied with my ability to run lean of peak with my aircraft, at

about 10 degrees LOP my engine would begin to run rough. Additionally, this whole fuel pressure and flow

event thing would completely go away if I raised the fuel flow rate to about 13 gph from 12.2 to 12.5 gph,

thereby running about 50 ROP.

Chart #1, fuel pressure on two flight segments, each flight is about 90 minutes long.

Chart #2 fuel pressure and flow during event

0

5

10

15

20

25

30

35

T

I

M

E

Axis Title

Fuel Pressure

Fuel Pressure

0

5

10

15

20

25

30

16:52:52

16:52:53

16:52:54

16:52:55

16:52:56

16:52:57

16:52:58

16:52:59

16:53:00

16:53:01

16:53:02

16:53:03

16:53:04

16:53:05

16:53:06

16:53:07

16:53:08

16:53:09

16:53:10

16:53:11

16:53:12

16:53:13

16:53:14

16:53:15

16:53:16

16:53:17

16:53:18

16:53:19

16:53:20

N541RV fuel flow @ 11k, 56% pwr

fuel pressure(psi)

fuel flow (gph)

The Problem: After reviewing this data with Air Flow Performance I was politely informed that I was on the verge of vapor lock

and that I was getting bubbles forming in my fuel line. NOT GOOD! Their recommendation was to get the fuel

flow transducer out of the tunnel where it warm and is in the suction side of the fuel system and move it to the

pressure side of the fuel system, preferably between the fuel controller and the fuel spider. Now there has

been plenty of email discussions about the benefits to moving the fuel flow sender to this location, however, all

that I have read has been about eliminating the fuel flow fluctuations seen when the electric boost pump is

running, not about vapor lock. With this hard data in hand, I decided that I must move the fuel flow sender,

for those that have seen my plane, you’ll appreciate that getting into the tunnel to access the sender and change

out the fuel lines from the pump to the firewall would be no small task. My finished interior and center

console was about to cause me pain and time (10 hours’ worth to be exact) .

Deed done.

I moved the fuel flow transducer to the fuel line between the fuel controller and the spider as recommended.

Also I fabricated an insulated fuel line to go between the fuel pump and the firewall bulkhead fitting inside the

tunnel.

Flight results have shown the all deviations in fuel flow and pressure have gone away. I’d chart the data, but its

boring, a completely flat line at 26 psi.

The really excellent result however is that I can now run as far LOP as I want, I did a couple of test flights and I

can easily run the engine 100 degrees or more LOP if I want, even at this point the engine was running very

smooth, just making a lot less power. I ran fuel flow rates as low as 9.5 gph without any difficulty, I could

never accomplish this prior.

Epilog:

I called Van’s about my observations, quite unbelievably, they actually have heard of this, they even

see it in their demo aircraft, however since the engine has always ran well regardless of fuel pressure

fluctuations they felt it was best to leave the fuel flow transducer in the tunnel. They told me they

weren’t thrilled with mounting the transducer firewall forward as it exposes it to higher temperatures

(maybe) and exposes it to more vibration. They felt the best compromise is to keep it in the tunnel.

I disagree. It does affect the way the engine runs, in this last chart you’ll see the egts recorded for one

of the above mentioned flights. You can see that after I was established in cruise flight, each bobble in

fuel flow was real, the egt for each cylinder responded to the fuel deviation just as expected.

For me it’s clear that the pressure drops in the suction side of the fuel system must be absolutely

minimized and the way to best do this is to insulate the fuel lines and mount the flow transducer in the

mentioned pressure line.

Bob Newman

Chart #3 EGT showing deviation/spikes when fuel flow is interrupted due to impending vapor lock.

1100

1150

1200

1250

1300

1350

1400

1450

1500

16:23:32

16:26:03

16:28:34

16:31:06

16:33:37

16:36:08

16:38:39

16:41:11

16:43:42

16:46:13

16:48:45

16:51:16

16:53:47

16:56:18

16:58:49

17:01:22

17:03:53

17:06:24

17:08:56

17:11:27

17:13:58

17:16:31

17:19:02

17:21:34

17:24:05

17:26:36

17:29:07

17:31:38

17:34:10

17:36:41

17:39:12

17:41:45

17:44:18

Series1

Series2