lml@lancaironline.net Mailing List Archive

I had to drain a tank on a IV to repair a fuel leak. I plugged the vent while I removed the drain valve from the bottom of the wing, with the hope that it would slow the flow of fuel down my arm. I only got about a half gallon from the tank until the flow slowed to a drip. I don't think the engine driven pump or boost pump could overcome the vacuum caused by a clogged vent line. So a clogged vent would stop the engine I'm guessing, not cause it to surge.

I did the calculation one time to determine the air flow that has to come in the vents to replace the fuel going into the engine. I have 3/8" vent lines on my ES and that air is moving through that vent line a couple feet per second if I remember correctly.

Mike Easley

Colorado Springs

In a message dated 5/18/2011 6:42:57 A.M. Mountain Daylight Time, n5zq@verizon.net writes:

Good point, Bill. The check valves supplied by Lancair for the vent system on my IV are free flowing inward and “somewhat” restricted flowing outward. This prevents venting a lot of fuel overboard while taxiing with full wing tanks (no winglets) yet allows air to escape at a relatively slow rate to avoid the problem that you describe. Certainly, a normal one way valve (free flow one direction – no flow in the other) should NOT be used on a vent system.

Bill Harrelson

N5ZQ 320 1,900 hrs

N6ZQ IV under construction

From: Bill Hannahan

Sent: Wednesday, May 18, 2011 7:43 AM

To: lml@lancaironline.net

Subject: [LML] Fuel restriction or air?

{Did you verify the fuel vents are working properly? ....and that the check valves on the vents are oriented correctly?}

There should be no check valves on vent lines. If you take off at sea level with a wing tank at atmospheric pressure, 14.7 psia trapped by a check valve, and climb to 18,000 feet, static pressure 7.35 psia, the differential pressure on the tank will be 7.35 psi, 1,058 pounds per square foot trying to rip the skins off the spar and ribs.

Vent lines must be free flowing both directions.

Regards,

Bill Hannahan

wfhannahan@yahoo.com