Harer STC Vent System

[Pearson]

Can someone tell me how the vent system is supposed to be set up with the Harer STC? I received these two pictures when I bought the STC from Mr. Harer before he passed away. In Photo 1 it shows a single vent tube in the center. in Photo 2 there doesn't seem to be anywhere for air to get in to vent the system. What concerns me is Photo 2 shows a "steel sleeve" welded to a tube of the fuselage. I am trying to do all the welding and need to figure out if I need to weld this sleeve to the fuselage. If I set it up as shown in Photo 2, I will need to use the original style fuel caps on the wing tanks. As I understand it, you do away with the original main tank gas cap/gauge. Part of the drawing that contains the part shown in Photo 1 mentions a drawing 170. But the version of the STC that I got from Mr. Harer shows that drawing 170 has been deleted. Anyone?

[N96337]

Re: Harer STC Vent System

I've always done them like the first picture you show. That's what's been in the drawings I gotten in the past. Maybe Terry will weigh in on this??
John

[Pearson]

Re: Harer STC Vent System

I have only seen two planes that have had the Harer/Gilberti STC. But neither of those had a vent tube sticking up in the middle of the upper cross tube. If I am supposed to do it as shown in the first photo, it would seem they should not have deleted drawing 170. Does anyone have a copy of this drawing. I would hope it would show where and how the vent tubes tap into the wing fuel tanks. Does the vertical portion of the vent tube stick up out of the fabric over the cockpit? Or is through the fairing at the upper rear edge of the windshield? I was hoping to go with a skylight, so that might complicate things if the vent tube is supposed to stick up through that area.

[mohawktipi]

Re: Harer STC Vent System

Wouldn't the vents be in the fuel caps?

[Ragwing nut]

Re: Harer STC Vent System

The steel sleeve merely supports the vent tube running up the diagonal. The F-21 vent system was similar but did not tie into the wing tanks. On ours, I just used adel clamps to hold the tube in place since we forgot to weld the sleeve on when we did the mods. The vent tubes attach to the wing tanks in the same manner, at the base of the filler neck.

[Robert Lees]

Re: Harer STC Vent System

Personally I didn't bother with any wing tank vents (apart from the existing vented caps, of course). Nor did I change the wire & cork header tank arrangement.

I've had no trouble with fuel flow.

Remember, the wing tanks only fill the header...they do not directly supply the engine.

The US FAA STC may be a different kettle of fish, however.

[Pearson]

Re: Harer STC Vent System

So John, does your plane have a vent tube sticking up out of the top? If so, where does it mount?

As I understand it, a deviation from an STC requires a field approval. I don't care how it is supposed to be done. I just need some help interpreting the drawings. From what I can tell one drawing is showing one way of venting the tanks and the other drawing is showing something different, but not really showing how the tanks are vented.

[barnstmr]

Re: Harer STC Vent System

Richard,
The second picture is not part of the FAA approved STC data package. Best I can tell, it is a sketch that Mr. Harer may have provided some folks as an alternative method for tying the vent line into the fuselage tank. My suggestion to anyone who decides to use the sketch in the second photo, that they should identify this on their 337 as a deviation from the STC.

As for the reference to drawing 170, no such drawing was transferred to me from the prior STC holder. Clearly this document was deleted from the STC and apparently Mr. Gilberti neglected to remove this reference from drawing G-133.

Remember, the STC is intended as a conversion to the "equivalent" of models BC12D-85, BC12D-4-85, or 19. It does not say the conversion results in an "identical" configuration to those later models. Use caution when deviating from the STC and make certain to obtain appropriate approvals if necessary.

If you consider the deviation to be a minor one, a simple logbook entry by an A&P is all that is required.

[barnstmr]

Re: Harer STC Vent System

Personally I didn't bother with any wing tank vents (apart from the existing vented caps, of course). Nor did I change the wire & cork header tank arrangement.

I've had no trouble with fuel flow.

Remember, the wing tanks only fill the header...they do not directly supply the engine.

The US FAA STC may be a different kettle of fish, however.

Robert,
Your way of venting the tanks individually through the caps is certainly no different from the original BC12-D setup. So it is not a bad setup. The Gilberti STC design, however is considered an improvement from a system safety standpoint, because it allows for redundant vent source in the event of one of the cap-vents becoming clogged. Bees, mud daubers, and such are not unlikely, especially down here in Texas. So for all the trouble it takes to install the Gilberti vent system, it is an improvement over the original design.

[Pearson]

Re: Harer STC Vent System

Terry,

Thanks for your input. I like the idea of just keeping the original vent system. I will see if my IA will consider it a minor deviation. Regarding the "improvement" of the new vent system, I was thinking that by having only one vent all the proverbial eggs would be in one basket. If that one vent gets plugged up, very little wing fuel will be available to the engine.

[Len Petterson]

Re: Harer STC Vent System

I have 2 wing tanks but no Harar STC just a BC 12 D.
The fuel just flows from the wingtanks to the header tank and the needle rises to tell me I can make a bit forther down the road.
Work excelent, it is simple, works, why change it, Ok I forgot FAA approval.
Len

[barnstmr]

Re: Harer STC Vent System

Terry,

Thanks for your input. I like the idea of just keeping the original vent system. I will see if my IA will consider it a minor deviation. Regarding the "improvement" of the new vent system, I was thinking that by having only one vent all the proverbial eggs would be in one basket. If that one vent gets plugged up, very little wing fuel will be available to the engine.

Richard...
I have spent a lot of thought on the Gilberti vent system, because there were several folks who advised me before I acquired the STC that the vent system was a bad design. I looked at the design from an engineered system safety analysis standpoint. I have concluded that it is a good design, but it is cumbersome to install per the approved drawings.

Think on this.... Don't forget, each cap is vented. Consider the original system. If a bee builds a nest in the cap vent hole on one tank (or if in flight this hole ices over), the airspace above the fuel in that tank will become a vacuum as fuel drains. The negative pressure could reach a point that the flow would stop. (I have read of such incidents.) In the Gilberti design, if the same thing happens, the airspace above the fuel in that tank will be sourced to another vented tank. Air flow will equalize as fuel drains and flow would continue... Unless something blocks the vent interconnect tubes or all three vented caps becoming clogged simultaneously. But, the likelihood of either of these scenarios is extremely low (or improbable). Certainly the likelihood is much lower than the single point malfunction scenario of the original tank design.

That's why I have decided to keep the Gilberti design and not change it.

[Pearson]

Re: Harer STC Vent System

I don't know why, but for some reason I thought that when you converted the vent system to comply with the STC, you also did away with the vented fuel caps on the wing tanks. Now it makes sense. I agree, three vents are better than two. Ding! The light bulb is shining brightly now. Thank you!

On another note; I once followed a friend who was delivering an Aeronca Chief that he had sold. I was in my trusty Taylorcraft and we both had handheld radios. It was about a three hour flight one way. When we were about an hour from the destination, he started telling me on the radio that his engine was cutting out. With all of our chatter on the handhelds, my radio battery ran out of juice. Shortly afterward, my friend started a gentle glide toward the earth. I watched as he landed in a cow pasture. I picked what I thought was a slightly better spot and joined him on the ground. By the time I walked up to him, he already had the cowl off and was removing the gascolator. From where I was standing I could see his gas cap for the main tank, in about the same spot as it is on a Taylorcraft. I noticed it was smooth, like a baby moon hub cap. I walked around to the other side of the plane where his auxilary gas tank cap stuck out of the side just behind the seat. Immeadiately I knew what the problem was. I went back up to the front and told him to start putting the gascolator back together, that I was going to fix his airplane in 5 seconds and with no tools. He was stupefied. After he got the gascolator back on, I switched the gas caps and put the one with the little vent tube on the main (front) tank. Once he saw the difference between the two gas caps, he slapped his forehead and said "OH NO". We returned the tools to the rancher we had borrowed them from and tookoff.

[N96337]

Re: Harer STC Vent System

So John, does your plane have a vent tube sticking up out of the top? If so, where does it mount?

Yes, Richard, there's a vent tube sticking out the top, just through the forward skylight fairing. I had one guy that was telling me all about Taylorcrafts one time, and was ranting about that vent system and how terrible it was and how terrible it looked, yadda yadda yadda.... he then proceeded to tell me how he'd NEVER seen one done the way the drawing shows, because it looks so terrible to have a big vent tube sticking out.... all the while pointing to a friend of mine's plane, which had the tube, exactly like the drawing. He just hadn't ever noticed it, because it's not very noticeable up there.... I pointed that out to him and he had to go find someone else to expound his vast knowledge to.....
I don't see why everyone thinks this is such a bad way to go... like Terry said, it's a very redundant vent system, and that's a really good thing in my mind. The designer of the STC had a reason, so why not just comply?
I get the same thing lots of times on the STC I own (aux fuel tanks for a 108 Stinson) even to the point that the last email I got back was from a guy that wanted to get my approval to rivet the tanks together instead of welding... his reasoning was that he liked the looks of rivets better and it was kind of tough to weld the tanks so they wouldn't leak...

John

[Hank Jarrett]

Re: Harer STC Vent System

So what's wrong with riveted fuel tanks? Probably works just as good as his wicker butter dish and cotton condoms. Worked so good for him all 12 of his kids use them too!
Hank