Additional hole in spar carry-through

[Flying Scotsman]

I'm finishing up installing the smoke system, and I need to run the smoke oil line from the baggage area to the firewall. To do this, I need to drill a hole in each of the two spar carry-through box parts (forward and aft pieces). Naturally, this makes me slightly nervous, so...what's the spec and/or advice on max hole sizes and placement relative to anything else that I can do here?

Normally, I'd just drill it, deburr it and be done with it...but hey, it's the *spar*, so I want to proceed cautiously here...

TIA!

Steve

[LettersFromFlyoverCountry]

I asked Van's about this last year. I needed an additional hole to run a pitot line through. Their advice was it wouldn't be a problem. Obviously you have to observe all the edge distance rules etc, and stay away from the doublers etc.

[Buggsy2]

Be sure to ask Vans again though, and tell them where you want to put it and what diameter you want. Get the OK from The Man, or at least his company.

[terrye]

There are already a number of holes in the vertical web of the spar carry through. These were provided by Van's as electrical routings. I would take your cue for your additional hole from these holes regarding size and location. A small hole, say up to 3/4" to allow for a grommet in the web near the neutral axis (equidistant from the top and bottom flange) should be no problem. Do not put any additional holes through the top or bottom flange. By all means talk to Van's about it.

[Andrew M]

Here a a few things to look at and think about before you commit. Generally speaking, in the certified world, "negligible damage" can be cleaned up to a circle up to a few inches in diameter, as long as it is two diameters away from any edge or fastener hole. Like a 5/8" hole 1.125" from the edge or another hole. What existing non fastener holes are all ready in the spar webs? What is the ratio of diameters to edge or fastener hole? Applying the "rule of thumb" the designer used to plan a new hole location should make it easier to get approval from the designer when you call them.
Also note, "negligible damage" usually includes "allowable if not specifically mentioned elsewhere" and spars are almost all ways get specific limits. We don't have that available to us. It's definitly worth a call or better yet E-mail (documentation for plans deviation in a critical area, resale value).

[Flying Scotsman]

Thanks, all...based on advice here, and looking at the plans and the existing hole locations (for wiring pass-throughs, etc.), I put the hole inboard one bay and at approximately the same height as the holes called out in the plans (allowing for a little more clearance from the stiffeners because it's slightly larger).

Plenty of clearance, edge distance, deburred well, etc....smoke system lines installed, moving on!

Thanks much...

Steve