Fuel pressure snubber

Ed Haywood

Ed Haywood

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Big Ed
I bought a snubber from American Champion to dampen the pulses to the mechanical fuel pressure gauge in my Decathlon. The instructions are lousy, the part is not shown on my parts manual, and I haven't been able to get the factory on the phone.
Should the snubber be installed on the engine end of the line, or the gauge end? Gauge end would be easier, but I wonder if the snubber would provide some mitigation for a line break if installed at the engine side.
PXL_20211017_221238216.jpg PXL_20211017_221442903.jpg PXL_20210718_211000996.jpg

Does the orientation make a difference? Eg male end towards flow source or away?
PXL_20211214_034819752.jpg
 
I'm not familiar with that one, but just for general knowledge until someone more familiar chimes in.....Look up Pulsation Dampeners.
I used to apply them in industrial pumping applications. That was a very long time ago but as I recall it was consider good practice to install them as close to the pump's discharge as possible. The types we used were installed in a vertical orientation...but those were a diaphragm type so perhaps different. I'm guessing this might be a piston
 
I use a restrictor fitting for both fuel and oil pressure, mounted at the location I'm measuring, not at the sensor. Orientation is such that it requires the minimum number of fittings, so in both cases it's male NPT and male AN flare.

Nauga,
pinched
 
Do not use teflon tape on pipe fittings, especially in a fuel application. Fuel lube excluding the first two threads.
 
Bell206 said:
Can you post the instructions?
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Not really anything worth posting. Just an insert from the manufacturer with guidance on chemical compatibility and cleaning. It is a standard industrial fitting, not an aviation-specific part.

Bell206 said:
Most get installed at the instrument. But there are exceptions.
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Thanks, that is helpful. After looking at the various fittings, I think that makes the most sense. Looks like I can install it at the gauge, on either side of the blue F2F coupling, so either direction is equally doable. Thinking about it, there is no "flow" other than due to pressure fluctuations, which go in both directions, so the snubber should be non-directional. Easiest install would be between the flare adapter and coupling. That would be about 2 minutes of work.

PXL_20210718_211000996 (3).jpg

Bell206 said:
No. That is what the orifice fitting at the carb is for.
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I had to see for myself. Yup, you are right (no surprise). The snubber can't go there. Installing it between the orifice fitting and the flare line would require a ridiculous series of adapters. So the gauge side is the obvious choice.

PXL_20211219_202605072.jpg

More pics:

PXL_20211219_202541354.jpg PXL_20211219_202654609.jpg PXL_20211017_221442903.jpg Screenshot_20211016-124324_3.png Screenshot_20211016-124130_2.png
 
Ed Haywood said:
Not really anything worth posting. Just an insert from the manufacturer with guidance on chemical compatibility and cleaning. It is a standard industrial fitting, not an aviation-specific part.


Thanks, that is helpful. After looking at the various fittings, I think that makes the most sense. Looks like I can install it at the gauge, on either side of the blue F2F coupling, so either direction is equally doable. Thinking about it, there is no "flow" other than due to pressure fluctuations, which go in both directions, so the snubber should be non-directional. Easiest install would be between the flare adapter and coupling. That would be about 2 minutes of work.

View attachment 102943



I had to see for myself. Yup, you are right (no surprise). The snubber can't go there. Installing it between the orifice fitting and the flare line would require a ridiculous series of adapters. So the gauge side is the obvious choice.

View attachment 102945

More pics:

View attachment 102944 View attachment 102946 View attachment 102947 View attachment 102948 View attachment 102949
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That sure looks exactly like an AN4022 fuel primer nozzle.

upload_2021-12-20_12-5-12.jpeg

Hanging more stuff off the carb, fittings is not advisable. That carb shakes a lot with engine vibration, and more or longer overhung loads often results in fitting breakage.
 
Ed Haywood said:
so the snubber should be non-directional. Easiest install would be between the flare adapter and coupling.
Click to expand...
Easiest place will definitely work. If it is a mesh type snubber there is no specific direction to install. Some of the larger/expensive piston-type snubbers can have a specific direction and are usually marked.
 
Bell206 said:
Easiest place will definitely work. If it is a mesh type snubber there is no specific direction to install. Some of the larger/expensive piston-type snubbers can have a specific direction and are usually marked.
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It is a mesh snubber. Thanks!!!
 
Ed Haywood said:
Any idea what the indentations are for on the recessed face? Staking the spring to the fitting?
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Staking the orifice plate in place. The AN4022 has another plate behind it with tangential grooves to cause the fuel to spin as it exits, forming a conical spray cone.
 
Dan Thomas said:
Staking the orifice plate in place. The AN4022 has another plate behind it with tangential grooves to cause the fuel to spin as it exits, forming a conical spray cone.
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Cool, so not a factor in this application, then?
 
It has been kindly pointed out to me that the snubber, of course, is the silver fitting in post #1, not the primer nozzle lookalike. The latter is the fuel leak reducer, having the same function as the restricted orifice fitting used at the engine oil pressure port.
 
So I finally got through to the factory. ACA says it goes at the fuel servo, in place of the old restrictive orifice. Here is what a new one looks like.

image_67502337.JPG
 
Ed Haywood said:
ACA says it goes at the fuel servo, in place of the old restrictive orifice.
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Interesting. So the aircraft OEM states to replace the specific P/N special fitting with a standard part fitting without any factory documentation for the alteration?
 
Bell206 said:
Interesting. So the aircraft OEM states to replace the specific P/N special fitting with a standard part fitting without any factory documentation for the alteration?
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Not sure. Per your advice, I'm going to press the factory for documentation. SB's are posted on the ACA website and it is not addressed in those. I would assume the parts manual drawings have been updated with the new fitting configuration, which would provide a factory P/N for the new fitting.

The parts guy is very helpful and hard working, but he is not a tech.

Odd question that popped up in my head: do you have to be an A&P to work on aircraft in a factory?
 
Ed Haywood said:
The parts guy is very helpful and hard working, but he is not a tech.
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A good parts person is always helpful. But direct calls to the tech support side are where you want to start especially when swapping parts around.
Ed Haywood said:
do you have to be an A&P to work on aircraft in a factory?
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No. And neither in a CRS. Even a lot of the desk based tech support people are not APs with a number being some sort of engineer. The field support guys will usually have an AP as they sometimes perform work on an aircraft depending on OEM.
 
Bell206 said:
A good parts person is always helpful. But direct calls to the tech support side are where you want to start especially when swapping parts around.

No. And neither in a CRS. Even a lot of the desk based tech support people are not APs with a number being some sort of engineer. The field support guys will usually have an AP as they sometimes perform work on an aircraft depending on OEM.
Click to expand...

Parts guy is pushing me more info now. Here are the two choices for the elbow that goes outboard of the snubber. The one on the left is the one that fits my application. I am trying to get some sort of documentation for my IA such as a revised parts drawing to support doing as a minor.

fittings - small.jpg
 
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A manufacturer (TC/PC holder) is limited on what they can do to an aircraft after it is built, and a airworthiness certificate is issued. From 43.3.,.

(j) A manufacturer may -

(1) Rebuild or alter any aircraft, aircraft engine, propeller, or appliancemanufactured by him under a type or production certificate;

(2) Rebuild or alter any appliance or part of aircraft, aircraft engines, propellers, or appliances manufactured by him under a Technical Standard Order Authorization, an FAA-Parts Manufacturer Approval, or Product and Process Specification issued by the Administrator; and

(3) Perform any inspection required by part 91 or part 125 of this chapter on aircraft it manufactured under a type certificate, or currently manufactures under a production certificate.
 
I installed the snubber. Works fantastic. Zero flutter in the fuel gauge needle.

I have been obsessed with replacing my mechanical fuel gauge with an electric gauge and sender, to get the pressurized line out of the cockpit. I might rethink that now that I have redundant restrictive flow fittings at the engine. The mesh snubber is designed to be permeable by gasses only, so flow of avgas would be nearly nonexistent. Even a complete break of the line inside the cockpit would result in barely a fraction of an ounce of fuel being spilled, just the contents of the 1/8" heavy walled SS tube from the break to the gauge.
 
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