Re Build of N94978 BC12-d

[astjp2]

Re: Re Build of N94978 BC12-d

Following are the FAA guidelines for mounting a TSO-C91a ELT, per RTCA
DO-183 paragraph 3.1.8:
1. “The ELT shall be mounted to primary aircraft load carrying structures such as
trusses bullheads, longerons, spars, or floor beams.”

2. “The mounts shall have a maximum static local deflection no greater than 2.5
mm (0.1 in) when a force of 451 Newtons (100lbs) is applied to the mount in the most
flexible direction. Deflection measurements shall be made with reference to another
part of the airframe not less than 0.3 meters (3 feet) from the mounting location.”
In addition, RTCA Document number DO-182 recommends that “all ELT system
components which must survive a crash intact, should be attached to the airframe in
such a manner that the attachment system can support a 100g load…in the plus and
minus directions of the three principal axes of the aircraft.”

The ELT must be mounted with the arrow which is printed on the battery case
pointing in the direction of flight. The ELT should be mounted with its longitudinal
axis aligned within 10 degrees of the longitudinal axis of the aircraft fuselage. Avoid
mounting the ELT near sources of strong EMI/RFI radiation. (See Fig. 1)

If this is a new installation or if the current installation is unacceptable, find a location
per the following:
RTCA suggests the aft section of the fuselage. Statistically, this is the least likely
section of the aircraft to receive damage in a crash. It is also near the antenna
connection, minimizing cable length between the transmitter and antenna. Maintain
access for maintenance. If possible, avoid locating the ELT where it will be subjected
to chemical fluids such as deicing compounds, cleaning fluids, etc. Over time, these
may attack the plastic and metal components.

The mounting location must conform to the requirements of RTCA DO-204A and AC
43.13-2A. DO-204A Sec 3.1.8 states:
"The ELT shall be mounted to primary aircraft load carrying structures such as
trusses, bulkheads, longerons, spars, or floor beams (not aircraft skin). The mounts
shall have a maximum static local deflection no greater than 2.5 mm (0.1 in.) when a
force of 450 Newton's (100 Ibs) is applied to the mount in the most flexible direction.
Deflection measurements shall be made with reference to another part of the airframe
not less than 0.3 meters (1 foot) nor more than 1.0 m (three feet) from the mounting
location."

[Guest]

Re: Re Build of N94978 BC12-d

So what uses plywood (shown in figure 2-10) besides a floor? I think its passing through the plywood floor and attaching to the structural tubing....but it still does not qualify the elt mounting requirements.

Tim,

I don't read it that way, here's why; the plywood is labeled "plywood", then the plywood does not lay over top of the tube and floors do, also the title says "TYPICAL ATTACHMENT OF SUPPORT STRUCTURE... the only thing that is attached to the tube is the plywood so I think the plywood is the the support structure being referred to.

Having said all that and holding that opinion about the figure I have to say that after re-reading the text I am wondering if that is merely a support for avionics in general and not necessarily for an ELT. Although I suppose that could be debated.

Dave

[astjp2]

Re: Re Build of N94978 BC12-d

Dave, see above, its the RTC DO requirement for the elt, 43.13-b just tells you how to meet the requirement for the load test on any avionic shelf, an adel clamp wont meet the spec for an elt. Tim

I will try to download the spec when I get to work and upload it to the tech reference section. It seems like a lot of people on here push back if I don't provide all of the supporting documents and addendums...it only took me 2 hours to find what the installation requirements are, now to get them posted

Maybe I should charge by the hour for research????? even though it makes me a better IA

[Guest]

Re: Re Build of N94978 BC12-d

Tim,

Good find on the RTCA doc thanks, Dave.

[astjp2]

Re: Re Build of N94978 BC12-d

Well my cost is $68 for a non member to get the document....maybe I wont post it....but from reading the requirement posted from ACK, or was it AmeriKing, the 100 pound .1" deflection is the stiffness required to provide adequate shock to trigger the automatic function, the 100 G is the requirement to keep the ELT intact. My EBC weighs like 3 lbs, that means that I have to load test to 300 lbs in all directions. I might have to make a fixture to load test my installation...maybe it would be a good STC?

[Leenicklas]

Re: Re Build of N94978 BC12-d

I'm in process of riveting firewall to boot cowl. Should I be using stainless steel rivets as opposed to aluminum.

[Ragwing nut]

Re: Re Build of N94978 BC12-d

I'm in process of riveting firewall to boot cowl. Should I be using stainless steel rivets as opposed to aluminum.

factory rivets were steel. I would not use aluminum

[Guest]

Re: Re Build of N94978 BC12-d

I can look to verify tomorrow but I recall the originals were steel. But you are fastening to an aluminum boot cowl so that joint is probably limited by the boot cowl material either way.

The original rivets had shallow heads as I recall and maybe the only way to reproduce that is with steel rivets. I assume you want to reproduce the shallow head since the back of the cowl will sit there.

Dave

[Scott]

Re: Re Build of N94978 BC12-d

I used AN470 rivets on mine where needed. After bucking the shop head 80% I used a flat snap on the rivet heads to reduce the profile by about 30%, so the cowling chafe tape fairs better. I have not seen this method written up anywhere but Cessna frequently used flat snaps on universal head rivets. I never understood why, but they did, including on primary structure.

The steel rivet thing seems to be a Taylorcraft standard. Most firewalls use aluminum rivets and of course the boot cowl/firewall is not structure on the Tcraft.

[astjp2]

Re: Re Build of N94978 BC12-d

If I remember the rivets you want are AN455, I would also use a soft rivet, the boot cowl is 3003h14 and very soft

http://rivetsonline.com/additional-information/ms-and-an-rivet-data.html

[astjp2]

Re: Re Build of N94978 BC12-d

So were the wing rib rivets on a prewar, it was cheap and a machine punched them in, not anything special on the outside of your firewall

factory rivets were steel. I would not use aluminum

[Ragwing nut]

Re: Re Build of N94978 BC12-d

So were the wing rib rivets on a prewar, it was cheap and a machine punched them in, not anything special on the outside of your firewall

nothing special but not correct since we are going kookoo over regs in this thread.

[astjp2]

Re: Re Build of N94978 BC12-d

substitution of hardware is a minor

[Leenicklas]

Re: Re Build of N94978 BC12-d

Thanks for all the info on rivets. When I look for steel rivets, all I find on A/C Spruce is A (soft alum) and AD (hard Alum). any suggestions for a steel rivet.

[astjp2]

Re: Re Build of N94978 BC12-d

So you didn't explore the link I gave you? And it takes special tools to crimp tublar rivets. You don't want solids, it will crush the boot cowl aluminum if you squeeze or buck them. That is why I said to use A series rivets. Tim

Thanks for all the info on rivets. When I look for steel rivets, all I find on A/C Spruce is A (soft alum) and AD (hard Alum). any suggestions for a steel rivet.