modifications of kit design/intent

[klovning]

We have a couple of DAR's around here so I thought I'd ask... Forgive me if this is a dumb question (yes I can be an inquisitive idiot) from a new guy but here goes....

A lot of people do modifications on Van's original plans (From what I gather a full electrical system with lights was not even part of the original design) A lot of these get posted here for others to consider/improve on/talk about.

How much do modifications on a kit's plan matter when issuing the airworthiness certificate?

For example some of the things I've seen pictures of:

-Showplane's fastback mod

-Putting a IO-390 (210hp) engine in a RV-8 instead of the suggested 200hp limit

-Putting a subaru engine in

-Adding retractable gear

-adding tip/rear seat tanks

-having a full electric ignition

Do these type of things affect weather or not a plane will be given permission to enter phase 1??

I'm guessing that there's quite a bit of leeway, but what is considered "crazy" or dangerous? What would be considered a "no go" modification?

-thanks

P.S. any suggestions as to where I can put the flux capacitor?

[Mel]

Good question Jason. But not so easily answered. The inspector can deny the certificate for anything he considers unsafe. Most common modifications can be easily evaluated. Others can not. If the inspector wants, he may require engineering support data.
Modifications involving changes to the fuel system design and/or an increase in gross weight will usually trigger a request for engineering support data.

[klovning]

Would "somebody else did it and lived" be considered adequate engineering support data?

That is - empiric testing done on a previous airplane without known failure/catastrophe - would likley be considered adequate ... correct?

[X18]

Jason,
Now, before you get started, is the time to find a DAR and get them involved in the project. The DAR will left you know what they expect and what documentation would be required for the engineering changes or deviations from accepted aviation standards. Having a plan also allows the DAR to informally examine the project during construction. Also, a good working relationship with a DAR allows for continuing advice about the project even after it is flying. (Thanks for the email advice last weekend Mel)

Jim Lewman

[Aviator168]

Originally Posted by klovning

Would "somebody else did it and lived" be considered adequate engineering support data?

That is - empiric testing done on a previous airplane without known failure/catastrophe - would likley be considered adequate ... correct?

You cracked me up. This is as saying the Wright brother's aircraft is air worthy.

Seriously, I really don't think so. Making it a retractable definitely will add some weight to the aircraft.

[Mel]

Like Jim says, get with your inspector early if you are planning drastic mods. Ask what kind of documentation he requires and neither of you will have surprises.

[SteinAir]

If you're doing any major changes to an airframe or major system, then my opinion is that you don't just need a DAR, you should look for a DER. No disrespect to the DAR's on this forum, but the engineering knowlege requirements/experience to be an AB-DAR are relatively lax compared to the requirements for a DER. Most DAR's are very smart people that know a bit about a lot of things, but aren't necessarily the guys who I'd use for significant structural changes and their relative impact on airworthiness/structural integrity.

Like I said, no disrespect to the DAR's. Most of them are extremely sharp and intelligent folks - but - we all know some that are, well.....less than superbly qualified. I mean, the sad truth is if you look hard enough, you can find a DAR to sign off a ham sandwich!

If you do choose to use a DAR, make sure they are well respected (Like Mel or Vic or Tim or whomever) and know their stuff.

That's just my 2 cents!

Cheers,
Stein

[osxuser]

Remember though, these airplanes are experimental. You don't really need engineering approval to do anything. The whole point of experimentals is you DON'T need a DER to do stuff. Mind as well buy a grumman tiger if you are gonna go that route.

[SteinAir]

Originally Posted by osxuser

Remember though, these airplanes are experimental. You don't really need engineering approval to do anything. The whole point of experimentals is you DON'T need a DER to do stuff. Mind as well buy a grumman tiger if you are gonna go that route.

True....but, I have dead friends to prove that not getting any qualified engineering assistance (doesn't necessarily have to be a DER) to look over their "experimental" modifications is really a dumb idea. VERY few homebuilders are really qualified to decide what is good and what is bad when it comes to major modifications.

Just because you can doesn't mean you should. While many of the certified rules are outdated and dumb, remember that almost all of them are written in blood.

Cheers,
Stein

[David-aviator]

At this url

http://rgl.faa.gov/Regulatory_and_Gu...2?OpenDocument

resides FAA AC20-27F, CERTIFICATION AND OPERATION OF AMATEUR-BUILT AIRCRAFT. While it is not regulatory in nature, it is an excellent guide, written approved by the FAA. Follow it and you can do just about anything you want within reason.