Regulatory question: Two tail position lights

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addaon

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Hi folks, this is a regulatory question, in the form of "is there a document / clause / paragraph somewhere that would change my interpretation of the rules"?

23.1385 requires (para (c)) either one rear position light "as far aft as practicable" on the tail, or two rear position lights "on each wingtip." It is unclear if the "as far aft as practicable" clause applies to wingtip mounts as well, and AC 20-30B doesn't clear this up, because it's definition of "as far aft as practicable" in para 7a refers to "the rear position light", singular.

I am currently trying to decide between wing-mounted rear position lights for my design, and aft-mounted. The wing-mounted ones are well understood and would meet my needs, and are the default choice.

For an aft-mounted position light, "as far aft as possible" is about 8" short of the aft-most point, due to constraints on mounting surfaces. The obvious fix here, which would look quite good and would meet all the angular requirements of Part 23 and AC 20-30B, would be to have two aft position lights, separated by the ~6" width of the fuselage at this point, each covering a bit more than half the required viewing angle (there is some overlap directly behind the aircraft where both would be visible).

However, I can't find anything in the regulations that would actually permit this type of installation. The rules seem to be "one if by aft, two if by wing", with no allowance for dual aft-mounted.

Obviously I could just build this and ask for a variance, since it seems that dual aft lights meet the intent of the rule if not the letter, but I'd rather not design something where that kind of after-the-fact negotiation is needed, so… does anyone have a different reading of this than me? What's your interpretation? Two aft lights clearly banned, clearly permitted (why), or fuzzy?
 

Tiger Tim

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There’s a transport category jet that has two lights on the tail (actually probably four because they’re often doubled up so a burned out bulb isn’t a show stopper). It might be the 737MAX, or possibly the Dreamliner. I’m pretty sure it’s not any Airbus and the bigger Embraers have their white lights on the wing tips.

I would think as long as a white light can be seen from all the angles it’s supposed to be seen from you ought to be good.
 

addaon

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I would think as long as a white light can be seen from all the angles it’s supposed to be seen from you ought to be good.
This would be my mental model too, but I can definitely see a stickler saying that it could be confusing to a viewer (like a motorcycle with two nearly-adjacent headlights in your rear view mirror looking like a car much further away, rather than just a motorcycle), and I'd love something to point to to say "this is clearly fine."

I'll look at some Boeing pictures, if I can find an existing case that would be useful. Do you have a memory of how those two lights are arranged? Are they on alternate sides of the rudder or something?
 

Marc Zeitlin

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Obviously I could just build this and ask for a variance, since it seems that dual aft lights meet the intent of the rule if not the letter, but I'd rather not design something where that kind of after-the-fact negotiation is needed, so… does anyone have a different reading of this than me? What's your interpretation? Two aft lights clearly banned, clearly permitted (why), or fuzzy?
So here's MY recommendation in situations like this, having had experiences where DAR's made up their own BS about what the lighting rules say or don't say.

Build your airplane. Register it as E-AB (which is, I assume, the category you're shooting for). Do NOT install ANY strobe or position lights on it for the DAR/FSDO AWC inspection. Your OL's will state that the aircraft is limited to Day VFR unless equipped per 14 CFR Part 91.205 for night/IFR flight. You will need to fly off the first 25 or 40 hour Phase I period to show compliance with 14 CFR Part 91.319 under day VFR conditions in any case, so you don't need lights for that time period.

Once the inspection is completed and the DAR/FSDO has gone home, you can then install whatever lights you want in whatever location you want, and as long as you can prove to yourself that you meet the regulatory requirements, you're good to go. There are no lighting police out there, and you'll be meeting the intent of the regulations. This way, you avoid the possibility of some rogue inspector deciding that your lighting doesn't meet the regs, for whatever absurd reason they make up based on their incorrect interpretation of the regs. Obviously, don't tape flashlights to the wingtip and hope that fools anyone, but it sounds like you're trying to do the right thing, so that's not an option.

I've had way stupider crap happen - this just avoids the issue entirely (and legally, since you are allowed to modify the aircraft in any way you like after the AWC is issued, with no subsequent inspection).
 

Tiger Tim

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I'll look at some Boeing pictures, if I can find an existing case that would be useful. Do you have a memory of how those two lights are arranged? Are they on alternate sides of the rudder or something?
Either side of the APU exhaust. After a brief search I found a pic that shows one. Just imagine the other side looking the same.
1669138998882.jpeg
 

Tiger Tim

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I've seen some RVs with Aveo ZipTips which have a white, aft facing light in each. No rear white light in the tail. I suppose they were legal...
The Seminole I used to fly was like that. I guess between the stabilator up top and full-height rudder beneath it there was just nowhere to put a light on the tail. IIRC those tip installations had similar 'blinders' to what you see on the red and green lights to limit viewing angles, ie. you shouldn't be able to see the left tip white light from the right rear quarter and vice versa.
 

ScaleBirdsScott

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We put a pair of white rear position lights on the vertical stab of the P-36, one on each side, as well as a pair of colored nav lights on each of the wingtips, one top and one bottom. All to replicate the original's lighting, as was the custom of the time.

I suppose if trying this on a modern design there could be someone that would want to argue against using such an antiquated and frankly superfluous lighting method, as opposed to using literally half the lights. But there is something to be said for how cool it looks, and how much the doubled-up lights pop when seen from an angle where both are exposed.

_1660733.JPG
 

djmcfall

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If you look at the Bell 47 helicopter you will see that they used two rear facing white navigation lights mounted just in from the rear most position of the tail boom. The main thing is you read the navigation lighting requirements, and make sure you are in compliance there. I have seen several RV homebuilt aircraft which mount the old Grimes style nav lights inboard the wing tips on the sloped surface of the wing tips. Those do not comply with the proper degrees the light must be visible. Also, Xeon strobe tube and LED anti-collision flashing lights typically have a Fresnel lens which are designed to be mounted aligned with the horizon for the light output to meet or exceed lumen requirements, so they should not be mounted at an angle. The short story is your two lights mounted near the rear of the aircraft that meet the degree of visibility requirements indeed appear to be accepted, at least by interpretation by the FAA.
 

Toobuilder

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Either side of the APU exhaust. After a brief search I found a pic that shows one. Just imagine the other side looking the same.
View attachment 132013


Lockheed U-2 tail position lights are actually standard wingtip navigation lights with clear lenses, facing backwards. They are mounted on the vertical stab, about mid chord, and significantly distant from the tip. This requres two assemblies - one on each side of the surface to provide the required coverage.
 
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