Silver hawk Fuel Injection Question

[Taildrgr]

This is from my Precision manual:



Calls for 50 hour inspection of the inlet strainer or per the airframe manufacturer.

So as the builder, I inspect mine at the annual CI.

I also check the boost pump filter annually.

[JonJay]

Originally Posted by Taildrgr

This is from my Precision manual:



Calls for 50 hour inspection of the inlet strainer or per the airframe manufacturer.

So as the builder, I inspect mine at the annual CI.

I also check the boost pump filter annually.

These where what I had as well. However, I think there is a newer instruction set out there that might be causing the confusion. I was "told", don't want to start any rumors, that the manufacturer found people where damaging the inlet and it just wasn't worth the trade off. If that is true, some people just shouldn't do their own maintenance.
Please, this is hear say....so treat it as such.

[RV7A Flyer]

My manual does *not* say to remove and inspect it every X hours; it has the wording which I quoted above, essentially, don't take it out unless you have a reason to do so. So I must have a later version (not at the hangar right now to check the revision date).

From Precision's website:

http://www.precisionairmotive.com/Pu...s/25-020_c.pdf

dated 2/23/15.

[Taildrgr]

The one I posted is dated Nov 8, 1996 and not for the experimental model.
Maybe there are different requirements for the certificated version, or as you said, maybe just updated their documents.

[Captain Avgas]

I found this thread in the archives and I'm resurrecting it. I notice in this thread that several posters have stated that they clean their Precision Silver Hawk fuel servo filters at annuals but none of them have ever found any debris in the filter.

I'm a bit suspicious of this particularly when the majority of builders have the stock standard Van's AirFlow Performance auxiliary pump/filter upstream which is quite coarse at 125 micron. The SilverHawk servo filter on the other hand is considerably finer at approx 70 micron.

My question is: How do you know that you have no debris in the servo filter when it seems virtually impossible to visually inspect the interior due to the extremely narrow dimension of the filter.

And what technique is being adopted to "clean" the servo filter. I don't believe Precision has any specific cleaning technique in their installation manual.

[JonJay]

Good question. I clean with MEK, tap out on end, straining through a paper filter element, then repeat, carefully, with compressed air also through the paper filter. I inspect the paper filter with a 10x magnifier.
Whether or not that process dislodges any particulates, or whether or not I can see them in the paper filter is certainly questionable.
I changed my inspection interval and no longer check at every CI. There is a lot of filter area in that finger screen and the odds of not seeing anything in the 125 micron filter and having it pass enough smaller contaminate to clog the 70 micron filter I would think are pretty slim.
Right, wrong, or indifferent, that is what I do.

[lr172]

Originally Posted by RV7A Flyer

Right..and the manufacturer's instructions say:

This filter does not need to be checked or cleaned unless it is suspected that contaminants have entered the fuel line after the aircraft's fuel filter."

But that is assuming you follow ALL of their instructions, which include a filter DOWNSTREAM of all pumps, which most RV's don't follow. They put the filter upstream of the pump. This means that any debris shed from a deteriorating pump will end up in their screen, which is not sized to handle much debris.

Larry

[Captain Avgas]

Originally Posted by lr172

But that is assuming you follow ALL of their instructions, which include a filter DOWNSTREAM of all pumps, which most RV's don't follow. They put the filter upstream of the pump. This means that any debris shed from a deteriorating pump will end up in their screen, which is not sized to handle much debris.

Larry

Spot on Larry. And Precision's installation manual also calls for a 30 micron filter upstream of the servo. The Airflow Performance boost pump/filter that virtually every RV with fuel injection currently has (as specified and supplied by Vans) has a quite coarse 125 micron filter. And generally there is no gascolator specified by Vans on the fuel injected models. Gascolator filters are typically around 30 micron.

The end result is that most fuel injected RVs have a relatively coarse 125 micron filter upstream of the boost pump and the only other filter is the Precision servo filter which is approx 70 microns. In other words coarse debris will lodge in the AirFlow filter and finer debris will pass through and lodge in the Precision servo filter.

In other words the statement that "the Precision servo filter is only a last chance filter that does not need to be regularly inspected" is simply not true for the fuel system set-up that most RV builders have in their fuel injected Lycomings.

As I've said previously, most builders report that they never find debris in their servo filter (so they tend to stop inspecting it). But that is because a visual inspection of the servo filter is realistically impossible due to the very narrow internal dimension and the fact that the internal bore is the upstream side of the filter. They probably have debris in that filter but they cannot see it.

I flush both my AirFlow boost pump filter and my Precision servo filter with mineral spirits and then drain the fluid through a white paper filter. Then I inspect the paper filter under my microscope. Typically I find microscopic debris that is generally not visible to the naked eye in both the AirFlow and Precision filters. As would be expected the debris in the 125 micron filter is coarser and the debris in the 70 micron filter is finer.

I highly recommend that all builders with the typical Vans fuel injected fuel system and no gascolator thoroughly clean their servo filter at every annual. Just taking it out and looking at it cannot determine if it is actually clean. Actually the same can be said for the AirFlow filter...don't rely on a visual inspection...take it out and clean it even if it visually looks clean.

[Bevan]

Can anybody post a source for a 30 micron cleanable/inspectable filter and suggest a suitable mounting location/method in a FWF (hot) environment?

Bevan

[JonJay]

Well stated Bob and Larry. Your timing is good as I haven't missed any inspection intervals. I will continue as I have before with the inspection and cleaning regime.
I have not heard of a fuel starvation incident. Don't want to be the first.