In the question about if it's OK to change your own map light, why did JScarry say "don't you dare replace [your map light] with an LED"? Or was he kidding?
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1$\begingroup$ When I made the comment a lot of people were replacing their GE 4509 lights with LEDs. The approved (STCd or PMAd) versions were $ 235 & the versions sold at Tractor Supply were $ 12. People were claiming that you didn't need an STC since they were similar & cheap ones were legal. I disagree. I had an LED in the house overheat & start smoking because the heat generated by the base was trapped in a rubber gasket. So there is a risk that a random LED won’t be safe in your airplane. They sell aircraft versions of map light LEDs now, so even though they are more expensive, that’s what I would use. $\endgroup$– JScarryMay 28, 2021 at 18:59
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4 Answers
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White LED spectra contains quite a bit of blue range wavelength, which is bad for night vision. Red light is much better.
The eyes have cone receptors for day time and rod receptors for night vision. The rods contain rhodopsin, which enables night vision. It takes about 1/2 hour for enough to build up to have full night vision.
Exposure to shorter wavelengths quickly breaks down rhodopsin, which is why blue light is to be avoided.
Natural sunlight is strong in blue (and UV) spectra. Incandescent tungsten filaments have much less blue/green spectra, and a lot more orange/red. So it may be a bit "old school" to shun LEDs.
Newer OLEDs have spectra closer to candles and incandescent.
One interesting measure is how much the light suppresses melatonin, which helps one sleep. Newer OLEDs were found to be only fractional of blue light, so it would be important to be sure any replacement map light is appropriate for the task.
answered May 26, 2021 at 4:40
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36$\begingroup$ Red LEDs are monochromatic. You can read a text fine under one, but not a color-coded map (so it's ok for IFR plates, but not VFR sectionals). $\endgroup$ May 26, 2021 at 5:56
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6$\begingroup$ Many white LEDs have a high colour temperature, which means they have relatively more blue light than typical filament lamps. 6500K is equivalent to daylight, 4000K or higher is often used because it’s more efficient than lower colour temperatures, and gives better colour rendition if you’re not worried about night vision. It is possible to get lower colour temperatures, but they aren’t easy to find in smaller lights. 2400K would be good if you can find any. Meanwhile, potential issues with blue light are being addressed; this is an active area of research in the lighting industry. $\endgroup$– FrogMay 26, 2021 at 6:45
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2$\begingroup$ @Jpe61 that's true for almost all but white (from a single emitter); even RGB white can be a problem as you've found. The phosphor in a decent white LED has a broadband emission like fluorescent lamps have, but ensuring you have a decent LED means looking up the datasheet for the actual emitter and that's not done for general-purpose bulb. A bulb used for aviation would of course be expected to have the relevant data available, though this could just be the colour rendering index and not the spectrum $\endgroup$– Chris HMay 27, 2021 at 8:13
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8$\begingroup$ Why the quotes around "organic"? It doesn't mean anything like "free range" or "no pesticides", it literally means that the active layers are made of organic molecules instead of an inorganic semiconductor crystal. Also light bulbs based on OLEDs aren't really common. $\endgroup$– TooTeaMay 27, 2021 at 10:32
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Quite simply because a random LED bulb is not certified for operation in an aircraft, and the light holder is not certified to operate with a LED bulb.
In practice it would probably work fine, but it is not out of the question that a LED bulb (sometimes containing internal SMPS power supply) might emit electrical noise, or perhaps even fail in ways that a normal light bulb wouldn't.
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2$\begingroup$ @Kenn It doesn't seem like "manufacturing parts for" would include a change of technology. $\endgroup$ May 26, 2021 at 18:38
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3$\begingroup$ This is the answer, it's not certified. All fun and games until the EMI from the cheapo LED driver interferes with your instruments! $\endgroup$ May 26, 2021 at 18:42
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4$\begingroup$ @Harper-ReinstateMonica, LEDs themselves do not make EMI noise, which I think you recognize when you refer to the LED driver. Not all LEDs require drivers, and not all LED drivers make noise. In the case of a map light, where brightness and efficiency are not primordial and the bulb circuit is properly fused, then a driverless 12V led would be a fine substitute. The FAA is very clear on these kind of minor repairs/alterations, and on an owner's authority to sign off on them. And this is before we even question whether the plane is Experimental or Type Certificated. $\endgroup$ May 26, 2021 at 19:18
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4$\begingroup$ @KennSebesta While your technology grasp is solid, it depends on something the vast majority of aviators simply do not have: a clear understanding of the difference between a bare LED semiconductor emitter, and an "LED Product" consisting of LED emitters and some sort of electronics. $\endgroup$ May 26, 2021 at 19:34
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6$\begingroup$ Well I'm still not entirely bought into the idea that the "manufacture parts" allowance applies to busting into new technology zones. Can you replace an approved nickel cadmium starting battery with a homebrew lithium job built out of 18650s and nickel strips? $\endgroup$ May 26, 2021 at 20:13
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My interpretation of the comment was that replacing the bulb with an LED is going to draw attention to the fact that you have broken the rules. If you are going to go rogue on a minor thing like this, give the inspector some plausible deniability.
Sort of like if you buy some custom aftermarket taillights for your car that maybe aren't officially DOT approved. If you buy red ones the cop probably won't notice and wouldn't care much anyway (unless you're a jerk or have a dozen other equipment violations). If you buy green ones, the cop won't be able to overlook it or let you get away with it.
The other answers here about night vision etc. I think are good advice also.
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1$\begingroup$ Unless a specific type of part is governed by the CFRs, this is a gray area of the law. Your example of a green auto taillight is a violation of the law (that expressly says "red" lights on the rear of the vehicle), but not everything is so clear-cut or a "fact". $\endgroup$ May 27, 2021 at 20:37
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2$\begingroup$ @Suncat2000 you may very well be correct, but Pondlife has the top rated answer on the linked question and they disagree with you. That being said, I really don't have the expertise to take sides between the two of you. As far as my answer goes, what i was trying to get at with my red and green examples was "superficially appears to be legal even if not" vs. "obviously not legal without even looking closely." $\endgroup$ May 28, 2021 at 2:40
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Physical reasons for not changing to LED include
- Lower heat output - which sounds good unless there's something else dependent on the warmth. LED traffic signals in cold locations require heaters to melt ice, whereas the old 420V incandescent lamps keep themselves warm and ice-free.
- Lower power draw means less current - again plausibly good, unless there's something measuring the current. Example - indicators (blinkers) in a car may not cycle between on/off if they are LEDs The unit that does the blinking in older cars is thermal, and depends on the current flow to switch.
- Insurance - In the event of a claim, an insurer will take every possible option to reject the claim. If you've got anything out of specification the insurance adjuster will look for it, and potentially not pay out. Doesn't matter if its your insurer or the other person's insurer. And they know what to look for, its their job to find ways to not pay on claims.
