Need guidance for Bell 412 (or similar) fuselage access for new cables. 2

[Chris843]

Hi,

I just got put on a project working with a Bell 412 helicopter(s). The aircraft has external stores mounted to the standard hard points under the door and we will be mounting sensors there. I've never worked with these helicopters and I will be going on site to look at one to determine how to route cables from the exterior equipment into an interior rack. I've got the service manual and see all the access locations but they all are required for flight. Not sure if they all are structural or mainly protecting the service areas under the skin. My time is very limited looking at this helicopter so my question is dose anybody have some good insight for routing external equipment cables through the fuselage? We want to avoid modifying putting any new holes in the fuselage in order not to get a DER involved into it. Maybe I will get a better perspective when I see the helicopter, but it would be nice to have a game plan when I get there to reduce my time. Also, I've search the net and I was also wondering if any companies sell STC approved modifications kits for cable access points?

Thanks in advance for any help...

 

[Sparweb]

I can't be sure, but your question implies that none of the external or internal equipment is approved for installation in a 412, or maybe the system has a patchwork of approvals that completely ignored the umbilical between the two major components. This gets forgotten when "approved" equipment gets carried across the field to helis it wasn't approved to be fitted to, and bolted on despite these issues. It happens. In cases like this the operator may remove the sliding door, or pop out a window. I don't condone this when a non-aircrew member must operate the rack of equipment in the back - it makes for a hazardous work environment for a person untrained or unprepared for it. It doesn't sound like you've been asked to stick your nose in to that particular issue, but be aware of it.

It will be hard to convince members of the group here that you won't need an approval for your externally- or internally-mounted equipment. Convincing an FAA inspector that you don't need an approval is even more crucial, of course, but I won't comment on that rather obvious point. Do make sure to alert the customer that once the rotorcraft is significantly modified, the insurance could be invalidated, depending on the nature of the equipment, if they haven't sought an airworthiness approval. What about its effect on flight characteristics, any significant effect on the weight or electrical loads on the aircraft? Also consider electrical interference issues, components of the equipment becoming susceptible to fluttering about in the airstream, etc etc. Your game plan should include a determination that this is a "major" or "minor" modification with respect to the facts of the equipment involved, and the law, and not what is expedient. Also note that the 412 is certified in the transport category and thus subject to more stringent airworthiness rules than a smaller heli; among them, hazard assessment, emergency exit access, flammability of the wire jacketing of the "mystery meat" bundles of wires, plus a dozen others that may or may not affect you.

That's what a DER can do for you.


STF

 

[Chris843]

Thanks sparweb for the reply,

There are approved external modifications on the helicopter... sort of. The current aircraft has an external multipurpose pylon system that was originally used as external fuel tank stores. We are removing these fuel tanks and putting our own custom sensor there instead. This new sensor has quite a few cables that need to feed into the interior. I may get lucky and see some pre existing locations I can do this, but from just looking at the MX documentation the conservative assumption is I will not. So I'm trying to keep any modifications to the helicopter out of the picture. Worst case we install something that can be easily replaced back to the original configuration we found it.

Unfortunately, I only have 1-1/2 days to get my hands on the helicopter figure out how to get them inside. This is why I'm trying to get a good idea in place before hand.

 

[MikeHalloran]

Given a little external power, it should be possible to transmit all of your signals optically through an existing transparency without making a new penetration.

Just a possibility, more affordable than it used to be...



Mike Halloran
Pembroke Pines, FL, USA

 

[Sparweb]

Hi Chris,
Watch out for other things that get done in rush jobs:
makeshift "fairings" that are really no more than large spans of unsupported and unreinforced sheet metal or polycarbonate, prone to flapping about in the airstream,
unsecured cables that flow all over the floor, and dangle about externally, also conducting rain water into the airframe through unsealed penetrations,
unfused electrical power wires,
commercial fasteners holding critical structures together, etc.

If I've guessed correctly about your situation, upon arrival, your customer is going to give you a tour of the equipment installation "so far", then put a drill in your hand, or have some guy with a drill in his hand ready to let loose, waiting on your word where it should go. Do not let the next obvious step happen. Instead, ask them where THEY think the hole should go, and then tell them either briefly or in explicit detail (whatever they need) that you need time to consider the ramifications of the modification, the effect on the structure, the reinforcements needed, and that you will have a response asap. Then seriously get yourself a minute of calm to really think about it with a clear head. Having that service manual handy again will help a lot. I still cannot guess how much authority you have, to authorize or participate in the work, but if you're arriving at the hangar sight-unseen, then there's other stuff to do first before you start making promises. The early part of your 1.5 days should be spent standing at a distance of 5 meters away from the rotorcraft, without the customer talking to you, considering each of the parts you see, thinking of ways it can/will fail, and thinking through the consequences of each failure. Get your head around all that, first, then decide if the threats you see can be solved in the time frame. If not then inform people of your opinion. If they disagree, so be it, you have satisfied your professional duty. But I've also been in that situation, done those things, and once the advice of caution was heard and understood, time was made available to produce an installation that would satisfy us! The job got done better and we got paid.

I still don't know how you are going to bridge the "approval" gap in your situation. I just know that if you give in to temptation to get your hands dirty (we're all engineers, but deep down we also want to play with the toys) then you're accepting even more responsibility for changes to the aircraft than you otherwise should.

I have had good customers who have contracted for airworthiness approval engineering on even simple things like utility plugs, just to be sure. People like that knew that even simple electrical mods can touch on so many airworthiness issues, and must meet the design standard, and furthermore the PROOF inherent in the approval meant that the insurance company can't stick it to you, if there ever was an accident.

Face it, helicopters have accidents. They usually fly low and slow, and photo missions like yours just make them lower and slower.

Mike:... possible to transmit all of your signals optically...

True, but probably not feasible to learn how, buy the necessary equipment, and implement in the time frame. The second post from Chris hints that this is a IR/video camera ball. If so, the cables are "ready made" by the camera manufacturer, and probably the best way to ruin the schedule is to change any of those wires.


STF