8KCAB Fuel Pressure Snubber

Big Ed

Big Ed

N50247 - '79 Super D
Joined
Jul 20, 2020
Messages
1,938
Location
Tampa, FL
I ordered a snubber from ACA for my fuel pressure gauge. Basically it is a fitting with a mesh restrictive orifice to dampen out spikes and pulses from the engine driven fuel pump.

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Instructions are not so great, and I have not been able to get Chad on the phone to clarify. So, questions for the group:

1. Should it be installed on the engine or gauge end of the line? I have the old mechanical gauge so the line terminates behind my panel.

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2. What is the correct orientation? Part is not labeled for flow. Photos of similar fittings on the internet show them oriented with male end towards source of flow. If I install at engine side, looks like it would fit in between the flare adapter and the fitting that is safety wired, with male end towards the source and away from the line.
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3. Any idea what that brown crud is on the fitting in the picture above?
 
4. I am going to replace the entire line with a flexible hose.

Yeah, that is probably Aviation Permatex.
Did not know they made a snubber. Cost? I shall get one.
 
$35 ... a bargain if it fixes the problem. Chad told me they just started installing it about 10 years ago.
 
where is the point in the fuel line where pressure is measured? i am trying to order an engine monitor and have to specify the adapter for the sensor.
thanks
 
Generally, restrictor fittings are installed at the source, so that in the unfortunate event that a line breaks, it can restrict the hemorrhaging.
 
Bartman said:
where is the point in the fuel line where pressure is measured? i am trying to order an engine monitor and have to specify the adapter for the sensor.
thanks
Click to expand...

At the fuel servo. There is a port on the side of the servo body. A hose or tube runs from the port to the fuel pressure sender, which would be mounted on the firewall or engine mount. See item 23 on the drawing below.
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We did a bit of exploring. The middle fitting on the connection from the gauge line to the fuel servo is a restricted orifice, which serves as a safety device in the event the line is broken. Looks like there is some kind of spring loaded check valve inside. Pressing on the spring stops fuel flow out of the port.

Has anyone ever taken that big wired nut off the servo body? Looks to me like the spring end was staked into the recessed face of the orifice, though for what purpose I am not sure. The spring is under compression when the fitting is installed. Perhaps for ease of installation.

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That could be the safety solution - say you had a light spring on the gauge outlet. Pressure would not close the valve, because it is the same everywhere, but flow could close it!

So if that stainless line breaks, maybe the flow closes the valve?

Just guessing.

I have the electric gauge here, but it is really pretty, so maybe a short flex line to the sender, and an electric gauge for me? I temporarily rescind the offer.971EF222-1B9F-402B-8A07-66739198FE64.webp
 
That spring is in case the fuel screen becomes plugged, the pressure from the pump will push the screen against the spring and open the inlet side so that the fuel flow will not stop, although, no longer filtered either.
 
Tangogawd said:
That spring is in case the fuel screen becomes plugged, the pressure from the pump will push the screen against the spring and open the inlet side so that the fuel flow will not stop, although, no longer filtered either.
Click to expand...

Any idea whether the spring end is supposed to be staked to the orifice face? Or, are those detents around the inner radius designed to prevent the spring from getting stuck in the recessed face of the orifice? I can't see any logic to the former, except for ease of initial assembly. The latter makes more sense, since pulling the fitting off with the spring struck to it could damage the spring.
 
Bob Turner said:
I temporarily rescind the offer.
Click to expand...

Haha!! That IS a great looking gauge, exactly what I am looking for ... except for the Vans stamp. My IA would not sign off on that replacement unless it also has a TSO stamp on the back.

I found an identical one on eBay, except it has a "Chief Aircraft" stamp on front where the Vans goes. I thought about grabbing that one, but it does not come with a sender.

The pressure range on the gauges perplexes me. Our era AFM says the arc redline should be marked at 45 PSI. I can find a ton of gauges and senders made for 30 PSI, but only a few that go high enough to accommodate 45. I also noticed most of the fuel computers and engine monitors that display fuel pressure go up to 30. My actual pressure in flight is between 20 and 25. Is it possible the specification to 45 is unnecessary and and was later revised?
 
A lot of IAs get really uptight about TSO. The original Bellanca Champs came with auto gauges, so one can assume they can be replaced with same. Mine have never failed, so they remain in place, but I did the panel so I can easily bolt in standard 2 1/4” instruments.
I would have to see the regulation on TSO - I am only an expert on non-TSO avionics.
 
The spring is attached to the screen, and it does not attach or lock itself to the fitting. The intends on the inner radius is what holds the orifice plate inside the fitting. Normally that safety-wired piece is simply a plug, and you pull that plug to remove the screen for inspection, usually.

I've seen the fuel pressure fitting threaded to the top of the fuel servo, tapped into the fuel distributor on top of the engine, as well as many other locations. Each airframe manufacturer come up with different ways to make things work.
 
Still trying to get a straight answer on correct installation. I talked to Chad at ACA today, who went and asked the shop floor. They said "at the engine".

That raises a question: does the snubber replace the orifice? Or is it in addition? Either way, I don't think there is room there, because of the throttle linkage arm. So I don't have 100% confidence in that answer. Could it be they meant "in the engine compartment", since they aren't installing mechanical gauges anymore, and all senders are in the engine compartment?

I asked Chad for a photo or some drawings. He said he would get it to me, but I have not heard anything back yet. Very frustrating since I am AOG with cowl and glare shield off trying to fix this. If ANYONE has ANYTHING that might provide insight, please share. In particular, if anyone has a parts manual dated in the last 10 years, after the change, posting the engine installation page would be quite helpful.

I installed it behind the gauge last night. Did not do a test run of the engine, but I did turn on the boost pump. Pressure was under 10 PSI. Usual pressure is around 23 PSI. So I don't think I have it correct. I cleaned out the lines real good so I don't think there is a clog.
 
Got the answer from Chad. It is installed at the fuel servo, and replaces the existing orifice.

I am a bit worried about clearance, as my throttle control rod appears to be closer, but we'll see tomorrow.

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