Antenna Relocation Approval 1

[kontiki99]

Anyone know of any FAA guidelines for substantiating the relocation of an antenna?

I'm installing a lower TCAS Omnidirectional antenna per an STC. We want to move the antenna installation one frame aft to use the previous operators structural mounting provisions.

Ideally I'd like to be able to treat it as a minor alteration.

I think the STC holder willing to give us a letter stating they have no technical objection.

If we have to treat it as a major, I guess we'll need to generate an 8130. Is approval by our DER enough?

 

[rb1957]

i'm a structures guy, but 'round here moving TCAS antennas causes all sorts of worried looks on the avionics guys, but i guess the original STC holder is saying this is ok.

i think your biggest problem is going to be how you reinforce the cabin skin (i'm assuming your plane is pressurised). i expect you'll be using some sort of doubler and you need to account for this (on a FAR25 plane it'll affect the inspection requirements, on a FAR23 plane it'll affect the fatigue life). take into account the load transfer into the doubler, see Michael Niu. i would also consider secondary bending concerns if you don't pickup on the stringer rivets, allowing the stringer to support the edge of the doubler ... there are prretty much two schools of thought on this, picking up the stringer rivets or not, but that's another thread !

you can sidestep alot of these concerns if you mimic the original installation, and justify that your installation can be considered equivalent (how different would the fatigue loads be in the new position compared with the original ?)

 

[kontiki99]

Thanks rb1957,

We’re talking large commercial jet for a part 121 operator.

As an avionics guy. I wouldn't touch the structural issues. In fact had already requested assistance from our structures group.

The good news with this project is that one of the guys on site did find a drawing note in a portion of the STC data package we had stopped issuing to the hangar. The note allows us to make an exception and use existing provisions one frame aft when we find them.

I think our structures people are still going to have to evaluate the integrity and maintainability of the doubler buildup, but from an antenna effectiveness standpoint I consider the note sufficient.

I originally posted looking for the avionics feedback. There’s no Avionics Engineering tips group.

NTOs only carry so much weight with the FAA. Where the STC holder is the OEM, their NTO would have been an excellent supporting basis for classifying it as a minor. As a minor we could approve it under our part 121 authority.

Where ownership of STCs can be transferred, I don’t believe you can always rely on an STC holders technical opinion.

The way I understand it, an STC holder has to amend their STC to make any changes major or minor and they are all approved by the FAA. STC holders don’t have authority to approve any minor changes the way part 121 holder does.

For the benefit of the group, I was considering development of a report noting the aft ward movement is only 20,”discussing physical relationship to nearby antennas (and their operating frequency), pattern shadowing with regard to engine nacelles, landing gear and landing gear doors etc.

TCAS operates on 1030Mhz, and there’s usually other L-band antennas down there (transponder, DME), but these systems are usually linked with suppression cabling to preclude any interference issues, by design.

These were just my thoughts though, I’d really like to see some FAA guidance out there.

 

[berkshire]

kontiki99 (Electrical)
There is a section in AC 43.13-2A Chapter 3 Antenna installations. However I suspect that you already have that.
AC 43.13-2B is in the comment stage. Now is a good time to send in comments, so people can have guide lines for approved data to avoid your type of dilemma.
B.E.

 

[planedr]

This largely hinges on your local FAA in my opinion. You can come from either extreme.

The NTO is largely irrelevant other than for possible support of the broadcasting issues. Structurally it has no use unless the TCAS OEM has data to support the structures change.

If you assume the antenna broadcasting is unaffected (use NTO here) which would be reasonable, that is one down.

Using the STC holders installation you could retain a DER to provide FAA form 8110-3 approval for the structural configuration. The argument being that the flight loads are not significantly different from the basic installation. Therefore the structural provisions are acceptable for the change.

You no longer have the STC installation and depending on your relationship with the TCAS OEM will determine their level of support. You will be on the hook for any future AD's and the appropriate documentation for this one off configuration difference.

Or it's a minor and you write a 1 line Engineering order to move it back 1 bay.

The other extreme being under TCAA you will be recertifying the installation and obtaining an LSTC with the appropriate documentation, fatigue, and or DTA analysis.

Talk to your PMI he may even accept a FAA form 337 who knows.

 

[Sparweb]

If this is a Ryan/Avidyne TCAS, then heed this warning:

Some DME and transponder designs permit high-energy Continuous Wave (CW)
emissions at the TAS reception frequency, which can affect TAS performance. In
order to reduce the possibility of interference, the DME and transponder antenna
cables and terminations must be effectively shielded. The three-foot antenna
separation requirement is essential to minimize interference from DMEs and
transponders with high CW levels. Failure to heed these requirements can result in
reduced reception range of the TAS.

I took that from their installation manual (available from company website).

As for the approval, I'm surprised (and pleased) that you're asking. Apparently some of the message is getting through that antenna installations affect the integrity of the skin. Bad holes degrade the value of the entire airplane (I've seen it that bad).

I concur with most of the advice given above, but take exception on one detail: joggling the edge of a doubler onto a stringer is a useless move. A flat sheet is very stiff when you pull on the edges; a joggled sheet is like a spring when you pull on the edges. No load will transfer through the rivets beyond the joggle.

By trying to pick up on adjacent stringers and frames, in fact, you could interfere with normal structural load transfers by creating "stiff spots". On small antennas, it's not worth it.


Steven Fahey, CET

 

[Sparweb]

Berkshire,

I went looking for info on an update AC 43.13-2A, but the FAA page doesn't show it:

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgDAC.nsf/MainFrame?OpenFrameSet

http://www2.airweb.faa.gov/Regulato...rgAdvisoryCircular.nsf/MainFrame?OpenFrameSet

Do you have another link where I might see the progress on that?



Steven Fahey, CET

 

[berkshire]

Sparweb.

Check out an article in aircraft maintenance technician

http://www.amtonline.com/publication/article.jsp?pubId=1&id=1940

and also

http://www.amtonline.com/publication/article.jsp?pubId=1&id=2300

I though this revision was official, however, it appears to be behind the scene work.
B.E.

 

[debodine]

Kontiki99, you said:

"The way I understand it, an STC holder has to amend their STC to make any changes major or minor and they are all approved by the FAA. STC holders don’t have authority to approve any minor changes the way part 121 holder does."

To enhance your understanding, allow me to rephrase your statement just slightly:

[The way I understand it, an STC holder has to amend their STC to make any major changes. Minor changes do not require an STC amendment but they still must be submitted to the FAA. ALL changes to an STC, major and minor, must be approved by the FAA. STC holders don’t have authority to approve any minor changes the way part 121 holder does.]

The bottom line is you are correct that all changes to an STC must be approved by the FAA, but only major changes require an STC amendment.

debodine

 

[kontiki99]

Thanks debodine

It all helps.

I find my self working almost constantly to develop a logical working model for understanding the FAA’s regulatory processes.

I don’t know anyplace I could go take a course that would fill in some of the gaps specifically designed for someone working as an Avionics Engineer at a part 121 operator.

We have two systems DERs. They are considered the local SMEs for FAA questions. Both are overloaded with work. One is a manager too, so he has very little time, period.

The other guy is technically very good, but is extremely imprecise when it comes to explanations. A lot of that seems to be the spotlight factor. The greater the size of the listening audience, the wilder and more exciting explanations become. They often wind up becoming wild rants about one man fighting to save the company from the: FAA, the management, the OEM, whoever etc. I just shake my head and head in the other direction when I see him coming.

 

[debodine]

kontiki99:

I should clarify for you that I am not a DER, though I have been qualfied to be submitted as a candidate DER for some years now...but have never been in the right place at the right time knowing the right person.

However, I work in an engineering office for a DAS that has half a dozen DERs on staff and at least a dozen under contract for consultation, covering about all the disciplines that exist. So I can usually get some experienced answers for our designs.

I share your frustration at there not being a comprehensive course, book or program you can study to figure out the FAA system. The FAA requirements are spread out over so many individual regulations, it takes years of reading, asking questions, and having your designs rejected and reworked to comply to start figuring out how it works. And to add to the complexity, I am sure you are aware each FAA region can have major differences in regulation interpretation.

I suppose that is why the DER candidate program requires a mentor be assigned. No way to learn what you need without one! And even as much time and experience as I have working directly with many DERS, it is all on Part 25 aircraft so if someone asked me to help on a Part 23 aircraft, I probably would not know where to start except to read Part 23 and look for differences.

I have attended DER conferences in the past hoping to network and find a mentor willing to sponsor me, but no luck. And here where I work I am not one of the "good ole been here for ages" folks who get chosen to be sponsored.

Good luck,

debodine

 

[Stache]

There is three AC’s that should help you as follows:

AC 120-55B Air Carrier Operational Approval and Use of TCAS II
AC 20-151 Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II) Version 7.0 and Associated Mode S TranspondersAC 20-131A Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II) and Mode S Transponders

These AC’s is available on the FAA web:

http://www.airweb.faa.gov/Regulator...rgAdvisoryCircular.nsf/MainFrame?OpenFrameSet

Stache