A question just popped into my mind: Why does an A320 overhead panel have 2 landing lights switches? What's the logic behind that? Why isn’t there only a single switch to turn on both landing lights?
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$\begingroup$ It's not unique to the A320. Older 737s and 747s have 4, a 777 has 3! $\endgroup$– BianfableCommented Jul 26, 2020 at 17:58
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4 Answers
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On a dark morning preflight check, I can verify that the lights all work by turning each one on, one at a time, without leaving the flight deck. Less lifespan on the bulbs & less disruption to the ramp to have several quick flashes of light within a few seconds, than turning everything on, walking outside to observe everything, and then going back inside.
Plus, it's probably a hold-over from days of minimal automation (think, 727 Flight Engineer panel - now entirely replaced by automation & a few switches on the overhead panel) when each {whatever} had its own switch.
I could get creative & invent another scenario or two when I want "this" but not "that" light on, but the preflight is probably the best example of that.
answered Jul 26, 2020 at 20:31
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$\begingroup$ You've got up votes, so this makes sense to some, but I'm not following you or seeing how this answers the question. If you can see the lights from the flight deck, would you not be able to see them if one switch turned them on instead of 2 or more switches? $\endgroup$– FreeManCommented Jul 29, 2020 at 14:24
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$\begingroup$ @FreeMan, if I turn on 'the' switch for 'this' light & things light up, I know it - singular - is on. If one switch turns on multiple lights pointing in roughly the same place, then seeing that area light up means that at least 1 light is working, but not necessarily all of them. $\endgroup$– Ralph J ♦Commented Jul 29, 2020 at 14:46
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$\begingroup$ Each bulb has it's own switch and the coverage area of each bulb may overlap enough not to be able to tell them apart. Got it. See, I'm not a pilot, so I don't know these things... $\endgroup$– FreeManCommented Jul 29, 2020 at 16:32
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Some aircraft have multiple landing lights fitted to them. Often larger aircraft have some of the landing and taxi lights attached to the landing gear and some landing lights are located in the aircraft structure behind transparent aerodynamic fairings. This allows the crew to still make use of landing lights once the gear is retracted (the gear is usually retracted once airborne and in a continuous climb and extended at the FAF on an instrument approach).
Which reminds me of a “there I was...” story of mine. I was flying around San Diego one overcast and cloudy night on a local IFR practicing instrument approaches into the smaller airports around KSAN (not a great idea BTW) in a Cessna 182T. It was a busy night with multiple heavy arrivals into KSAN that evening and Miramar was busy with departures and launches as well as an emergency. I was returning from Brown (KSDM) back to plane’s base at Montgomery (KMYF) when I got the following radio call:
“Cessna xxx, traffic 2 o’clock, three miles at 5000. Boeing 737. Report them in sight.”
“Cessna xxx, no joy. I’m in IMC.”
Now you hope the controllers are professional here - it is a major airport in Class B airspace - and can keep us separated.
And it was about that time that I saw two brilliant white beams of light piercing the clouds and passed right overhead.
Well, there’s the 737. He was not at a safe separation and pretty damned close. If he had the gear down he might have left tire tracks over the wing of my airplane.
answered Jul 26, 2020 at 22:13
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Safety and redundancy (is that redundant?). As a couple examples, if one of the switches breaks, you only lose that one light instead of all of them. These switches do break too, they get flipped multiple times every day and are in pretty harsh environments. Also, you could get a short circuit in one of the lights, or a light circuit. Again, having separate switches allows you to turn off only the problem light, while keeping the rest on.
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$\begingroup$ And yet Airbus replaced the switches with just a single one on all newer models (A330 upwards). I guess they don't break that often... $\endgroup$ Commented Jul 26, 2020 at 18:13
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$\begingroup$ They may have internal logic to bypass malfunctioning circuits, guess they're banking on their switches being invulnerable. I'd venture to guess that I've seen switches break in every type of airplane I've flown. $\endgroup$– KhantahrCommented Jul 26, 2020 at 18:21
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Most of the jets that I fly have their landing lights wired through the down locked switches on the main gear. This means the landing light automatically turns off as the gear is raised.
It also means this is one of the ways the crew can determine if a main gear is down and locked if they have conflicting information in the cockpit. It is easier to turn off all lights except the one landing light in question and ask tower if they see any lights than to have both landing lights on and ask the question if they see one landing light. There is no ambiguity in the first question. There could be in the second question.
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1$\begingroup$ But in a small plane (where the gear is always down), I often turn the landing light on when near airports, just for visibility. I see commercial jets doing the same thing, turning landing lights on maybe 15-20 miles from the airport, long before the gear is lowered. $\endgroup$– jamesqfCommented Jul 26, 2020 at 23:52
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$\begingroup$ Those typically aren't landing lights but recognition lights. They are designed to make the airplane more visible. $\endgroup$– wbeard52Commented Jul 26, 2020 at 23:58
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1$\begingroup$ True, but recognition is the primary purpose of the landing light in a small plane, too. I can certainly land at a lighted airport without the landing light - and have, a couple of times, 'cause the darned things fail quite often - but I'd be very reluctant to try a night landing without runway lights. $\endgroup$– jamesqfCommented Jul 28, 2020 at 0:56
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$\begingroup$ No, they're landing lights. At my carrier, we have the fixed landing lights on below 18,000', day + night. The retractable landing lights are extended + on when the flaps are out. But they're all landing lights. $\endgroup$– Ralph J ♦Commented Jul 29, 2020 at 15:19
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$\begingroup$ @RalphJ Which aircraft are you referring to? $\endgroup$– wbeard52Commented Jul 29, 2020 at 15:22