What's the reason a military/reconaissance plane would have a light on top of its roof? The SR-71 was the highest-flying airbreathing plane, after all. There was little danger from any collision from above.

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    $\begingroup$ It's hard to see a black plane on black tarmac... Planes operate on the ground too. There's also a lot of airspace up to cruise altitude. $\endgroup$
    – Ron Beyer
    Oct 12 at 14:54
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    $\begingroup$ @RonBeyer But lights could have been put on its sides or elsewhere. $\endgroup$
    – Giovanni
    Oct 12 at 14:56
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    $\begingroup$ @Giovanni On its way to altitude, any plane can potentially bump into a higher flying plane from below, no matter the service altitude. Hence, the lights on the top. $\endgroup$
    – Dohn Joe
    Oct 13 at 7:44
  • $\begingroup$ @DohnJoe That doesn't explain why it should have a light on its roof, not on the sides where they can be seen from all directions. $\endgroup$
    – Giovanni
    Oct 13 at 8:39
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    $\begingroup$ @Giovanni It probably has those too. Most planes have lights at the wingtips, top, bottom, and back at least (in some cases the same light(s) may fill multiple roles). I assume they can be turned off when running stealth operations. $\endgroup$ Oct 13 at 17:31

That is an anti-collision light as described in the flight manual:

Anti-Collision/Fuselage Lights and Switch

Two combination retractable anti-collision and fuselage lights are located at the top and bottom of the fuselage, near the middle. The lights are controlled by a three-position toggle switch on the pilot's lighting panel. The switch positions are: ANTI COLLISION (forward), FUS (aft), and OFF (center). In ANTI COLLISION, the lights extend, illuminate red, and rotate at 45 rpm (which produces 90 flashes per minute).

(SR-71 Blackbird Flight Manual)

The US AIM 4-3-23 recommends the following (see this answer for more details):

In addition, aircraft equipped with an anti-collision light system are required to operate that light system during all types of operations (day and night).

The idea is that the ground crew and other aircraft on the ground are alerted that the engines are running (or about to be started) or that the aircraft is moving (or about to move). The ground crew can see the lower light (bottom of fuselage), while other aircraft can see the upper one (top of fuselage).

In flight, the SR-71 would switch to FUS or OFF mode:

In FUS (fuselage), the lights are retracted and illuminate white. The lights are rectracted and off when the switch is in OFF.

(SR-71 Blackbird Flight Manual)

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    $\begingroup$ So was the SR-71 violating any FARs by not operating the anti-collision light with which it was equipped, in US airspace? Hmmm-- another ASE question there? Someone should ask it-- $\endgroup$ Oct 12 at 15:41
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    $\begingroup$ @quietflyer The AIM is not regulatory (see this answer). Not sure if the FARs require anti-collision lights, and even if they do, military might be exempt. $\endgroup$
    – Bianfable
    Oct 12 at 15:45
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    $\begingroup$ @quietflyer: How much jurisdiction does the FAA have over military aircraft? $\endgroup$
    – mins
    Oct 12 at 15:58
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    $\begingroup$ IIRC when flying above FL600, one is outside the scope of regulation so... I may be very wrong here though... however, general rule in military aviation (the jurisdiction I'm familiar with) is that there's always the "or as commanded" statement. So minimum alt is as per regulations or as commanded, speed limitation 250kts below 10000ft or as commanded. Military pilots always fly according to a mission plan commanded by their superiors, and the mission plan may override pretty much any civilian rule there is, especially in cases where national security is directly involved. $\endgroup$
    – Jpe61
    Oct 12 at 17:17
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    $\begingroup$ @Jpe61 Above FL600 is not outside the scope of regulation. It just isn't controlled airspace. It's Class E. You don't have to be IFR or have a clearance from ATC, but you do have to follow the FARs... if you're a civilian aircraft, which the SR-71 is not. Of course, if you're a civilian, you will need to be IFR and have a clearance to get to FL600 in the first place, unless you entered the NAS above that level or were dropped off of some air/space craft flying above it. $\endgroup$
    – reirab
    Oct 13 at 5:30

During missions the SR-71 had to be refuelled air to air many times. These missions took place also at night, so visual cues were absolutely necessary.

Also the mission obviously were often flown lower and slower in "civillian" airspace for departure and approach legs, so lights were there for general safety also.

At high altitudes they were of course not necessary.

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    $\begingroup$ This could, I suppose answer the "In FUS (fuselage), the lights are retracted and illuminate white. The lights are retracted and off when the switch is in OFF." question. Retracted for aerodynamics and white for visibility. After refueling, a quick flick to OFF and the plane goes dark again and returns to spying. $\endgroup$
    – FreeMan
    Oct 12 at 17:16
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    $\begingroup$ Refuellings were never done in hostile airspace, but lower visibility may still have been desired. Retracted lights probably did not show as widely around the acft as when they were "out". Speed/drag was not of concern during refuel, as the Blackbird was flying very slowly considering its performance envelope. Fun fact: for the latter part of refuelling, one of the engines was set on afterburner to keep the increasingly heavy plane's speed up. Imagine flying that thing with assymmetric thrust at night, thethered to a tanker... I have the utmost respect for those guys! $\endgroup$
    – Jpe61
    Oct 12 at 17:31
  • $\begingroup$ "Returns to spying" also indicates (in my head, at least), "returns to airspace where spying would take place". ;) Also, yeah, only one afterburner in the dark tethered to a flying bomb? Yikes! $\endgroup$
    – FreeMan
    Oct 12 at 17:45
  • $\begingroup$ @FreeMan totally guessing, but the retracted light might be for while-tethered or during the hookup process? So to minimise loss of night-sight on the boom operator but still being visible in other directions. $\endgroup$
    – Criggie
    Oct 13 at 2:36
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    $\begingroup$ @Criggie: ANTI COLLISION mode is probably even better for night sight: it flashes red (while extended) in that case, vs. solid white in FUS setting (while retracted). IDK whether flashing at 90 BPM or solid would be more useful for boom operators, but red lights are more friendly to night vision; red light doesn't trigger the human eye's reaction to contract the iris. $\endgroup$ Oct 13 at 22:44

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