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FUEL LINE OBSTRUCTIONS(3 Articles)
Well remembered statement Dick Rutan always preached at RAF
when he worked here. Many, many times we have found it to be so very true. The problem is
to recognize and act on the information.
A classic case in point occured a few months ago with Burt's Defiant. N78RA had always had lower fuel pressure on the front engine than on the back, at least as long ago as any of us could remember, even after we installed the 180 hp engines and constant speed props. Lately though, it seemed the pressure was even lower. On the way to Oshkosh 1987, Burt said he had only 2 psi on the front and 6 psi on the rear. Must be the gauge, right? Wrong! On the approach into Oshkosh, the pressure dropped to 1 psi. Mike and Sally moved into very close formation, looking for any sign of a fuel leak - nothing.
On the trip back from Oshkosh, the fuel pressure hung between 1 & 2 psi. The engine seemed okay though, so Burt pressed on. A few weeks after the return from Oshkosh, Burt and Tonya decided to take two friends to Big Bear for lunch. The takeoff and climb to 300 feet were normal. Then, suddenly, the front engine began to die. Burt was frozen for a second trying to determine if he should turn back and land - should he shut it down and feather it? What?
He happened to glance at the two fuel pressure gauges the rear was at 6 psi , the front was showing ZERO! He reached down and cross fed the front engine to@t e rear engine fuel tank - instantly, the front engine recovered and returned to full power! This airplane had been trying to tell us for a couple of years that something was wrong, but no one was listening.
We knew now that it was in the left (front) fuel system. We checked all the screens and filters - nothing. Finally we pulled out the fuel lines themselves and there we found a blockage of foam chips, small pieces of fiberglass and tiny fragments of micro and epoxy. This blockage was fully 4 inches long in the fuel line from the left tank to the fuel valve, right at the fuel valve. We replaced all the fuel line in the airplane and now we have 6 psi, front and rear, at all times.
The moral of the story is this: If you notice anything unusual, pay attention, the airplane my be trying to tell you something. A new noise, a "different" vibration, any change in fuel or oil pressure, don't ignore these things remember Dick's teachings, "The airplane will always try to tell you, before it zaps you!"
P.S. The accumulation of debris was caused when we had to replace two low-level light switches in the aft sump tank in Burt's Defiant. Apparently, we were not careful enough when cleaning out the tank before closing it. Burt's sump tanks do not have screens in them, the assumption being that the screen in the main tank should do the job.
Similar Problems in a Long-EZ
Marc Borom, N966EZ, writes that he had had many engine
hesitations, slight rough running periods, some requiring the use of the boost pump to
make it run smooth. All of this was during Marc's first 25 hours in his test area.
Needless to say, Marc was rapidly losing confidence in his new Long-EZ. How would he ever
be able to fly cross country in this thing?
He called us here at RAF several times and we had long discussions about his problem. Finally, one day he decided to make a short cross country to visit a fellow Long-EZ builder.
During this flight, the engine literally quit each time he shut off the boost pump. He asked himself, "Am I having fun yet?" The answer was an obvious - NO!
Safely back on the ground, he once more broke down the fuel lines aft of the fireman. Same results, no problems downstream of the gascolator. Then he remembered that when he had done his fuel flow checks, the fuel flow was sluggish at the gascolator (the airplane was trying to tell him!). He mentioned this fact to other pilots who persuaded him that it was due to low fuel "head" pressure with the nose down. He put that important data point aside as probably not being pertinent.
With no other clues, it was time to cher-k the fuel lines forward of the fireman and back to the sumps. He disassembled the gascolator and found he could blow through both lines from the valve to each sump with very little effort. While he had the system apart, he decided to check the line from the fuel valve to the gascolator. To his amazement and horror, he could not blow through this section of fuel line. He had, at last, found the source of his problems.
He called RAF to discuss this problem and we suggested he use shop air to blow the line clear. The blockage cleared itself with a loud "POP". What he found was a 1" long plug of foam and fiberglass chips that had backed up behind a needle of epoxy coated fiberglass that had lodged in the first sharp bend in the aluminum tube.
This problem was very similar to Burt's problem in the Defiant, and it re-enforces the necessity to "listen" to your airplane. When she tries to tell you something, don't ignore her, check it out and you will become more confident in this machine you have built. In time, you will come to trust her and, therefore, enjoy her and to get more utility out of her. Remember, she will always try to tell you ....
Suggested Method of Checking Static Fuel Flow
VariEze, Long-EZ and Defiant - Before first flight, and if you
are now flying but have never done this check, we strongly recommend a fuel flow check.
Disconnect the fuel line at the carburetor and hold the airplane in the normal level
flight attitude of approximately 1-1120 nose up (a 24" level with a 5/8" block
under the rear end of the level on the top longeron will give you this attitude). Now,
using a stop watch and a bucket, turn the fuel valve on for two minutes. Weigh the bucket
of fuel, then weigh the bucket empty. The result is the weight of fuel that flowed in two
minutes. Since a minimum of 10 gph for a VariEze is required, you should have at least 1/3
gallon (2 lbs.) of fuel in the bucket after a 2 minute run.
For a Long-EZ, you need a minimum of 12 gph, so you should have .4 of a gallon or 2.4 lbs. (without the electric boost pump running). This should increase-to a minimum of 16 gph with the boost pump running, or 1/2 of a gallon (3.2 lbs.) in the bucket after 2 minutes. Remember to check both tanks in a Long-EZ, left and right.
For a Defiant you need a minimum of 14 gph (NO boost pump), 0.46 gallons or 2.8 lbs. in 2 minutes. With-The boost pump running, you should see a minimum flow of 18 gph, or 0.6 gallons or 3.6 lbs. in two minutes. Don't forget to test both tanks as well as cross feed on both tanks.
These flows are fairly arbitrary, but are flows we have tested for and measured on each of the above aircraft. You should get at least, and probably better than, these numbers when you test your own airplane. If you are way down on these numbers, you should disassemble the fuel lines and blow through them to check for a blockage. Use caution blowing through lines that go into fuel tanks. High pressure shop air might rupture a fuel tank even with the fuel cap removed.
This fuel flow test should be conducted on any new airplane and it would not hurt at al I to retest at each annual . Keep in mind that foam chips tend to float on the surface of the fuel and may not get into the fuel lines for a long time or, at least, until you run that tank very low or all the way empty.