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FI spider and fuel pump bleed

Blain

Well Known Member
Bendix FI spider has bleed port that should be routed overboard along with the mechanical fuel pump bleed port. Any reason these cant be tied together for 1 exterior drain?
 
Bendix FI spider has bleed port that should be routed overboard along with the mechanical fuel pump bleed port. Any reason these cant be tied together for 1 exterior drain?

The fuel pump does not have a bleed port. It has an evacuation/overflow port for the fuel to flow in the case of a failed diaphragm. This keeps it from going into your oil sump. The bendix spider has no such port to my knowledge; At least mine doesn't have one. Not surprised it has a bleed port, just like your brake caliper has one. It is opened only to bleed the system. By definition, a "bleeder" or bleed port is normally sealed and opened only for the puprose of bleeding a system of air or fluid.
 
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Yes some do. You can tie them togethert, only downside is you can't tell from outside which is the leaker. Big deal, you need to pull cowl and look anyway. Make sure it is really a vent port and not a gage port. The ones without it still have the boss in the top cap, but they're not drilled/tapped for the hose barb and they have a tiny vent hole in the top center of the cap.
 
caution here.. If the flow divider vent line becomes plugged because the fuel pump is leaking and there is a restriction to the flow divider vent because the common outlet is full of fuel or oil from the fuel pump; you could get a air pressure build up in the vent chamber of the flow divider, that could cause downward force on the flow divider diaphragm restricting or even cutting off fuel to the engine as you increase altitude.
If you block or restrict the vent in the top of the flow divider as you climb the air trapped there expands and pushes down on the flow divider piston restricting or even shutting off the fuel. remember that piston is there for the sole purpose of shutting down fuel to the engine as you pull the mixture control. so anything that will allow it to close will shut off the fuel to the engine.
Have seen a mud dauber cause a similar issue. engine quit during climb, due to air pressure build up in the vent chamber of the flow divider causing the piston to move toward the closed position and restarts during engine out decent as the pressure in the vent chamber decreases.
It might not be possible to get the pressure increase necessary with just fuel and or oil in the common vent outlet to shut the flow divider off but personally I wouldn't want to find out.
Good Luck,
Mahlon
 
Terminology..

I need to verify that the flow divider port is not a pressure port. I got this engine off another aircraft and it was plumbed as a drain so I assumed.....

Sounds like either scenario would be safer to isolate them from one another.
 
There are three possible 1/8 pipe ports on the flow divider. Two are in the body and labeled inlet and gauge. The inlet's use is obvious. The gauge port is used for a metered pressure type fuel flow gauge as it connects to metered nozzle pressure. This port is not used and should be plugged if using transducer type fuel flow indicating gauges as most experimental aircraft use. The third possibility is in the cover(the top part held on by 4 screws) and it is not labeled but is a vent. It may be just a boss with a small hole in it or it may be an actual 1/8 pipe port with a small hole at the bottom of the port bore. This port should never be plugged or blocked, but always vented to atmosphere. Either left open to the nacelle or hooked to an OBD line. And btw that vent port if left open to the nacelle, should not face the front of the aircraft but either sideways or aft.
Really old flow dividers had the vent hole in the center of the cover facing up or down depending on the mounting direction of the flow divider and there was no boss or threaded port, just a small hole in the middle of the cover.
Good Luck,
Mahlon
 
Hmm. That was follow the leader I think. Bendix/lyc calls it "manifold valve" and TCM call it "flow divider". Did I get it right now?
 
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