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A ship at sea would fly its flag upside down to signal it's in distress. How does a plane or airliner that has no radio or transponder visually signal that it is in distress?

This question: How do commercial pilots send distress signals? explains how it is done using a radio or transponder, but how could the message "please don't shoot me down" be relayed visually to a fighter pilot?

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    $\begingroup$ To fly a flag upside down the aircraft would probably have to fly inverted :D "Look at me I'm in distress, I'm inverted" LOL $\endgroup$
    – Jan
    Jun 2 '19 at 19:41
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    $\begingroup$ A flag upside down? Wouldn't do much good for Romania, France, Belgium and Japan, would it? $\endgroup$
    – Mast
    Jun 3 '19 at 5:47
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    $\begingroup$ @Mast That's some very impressive flag knowledge. Username checks out. $\endgroup$
    – Cloud
    Jun 3 '19 at 10:35
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    $\begingroup$ Annex IV of the "International Regulations for Preventing Collisions at Sea" (a.k.a., "COLREGS") lists fifteen different things that the crew of a ship can do to signal a dire emergency. Flying an up-side down flag is not one of them. $\endgroup$
    – Solomon Slow
    Jun 3 '19 at 17:18
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    $\begingroup$ I would presume that every aircraft wants to communicate that particular message. $\endgroup$
    – chrylis -cautiouslyoptimistic-
    Jun 3 '19 at 20:52
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For the US, the FAA's Intercept Procedures list several things a pilot can indicate without radios:

  • Acknowledge instructions: rock wings, flash nav lights
  • Unable to land at indicated airport: flash landing lights
  • Cannot comply: switch all lights on and off at regular intervals
  • In distress: switch all lights on and off at irregular intervals
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    $\begingroup$ Being creative: If near enough, sheets of paper with one big letter on each? A torch or mirror? Cellphone if low enough (normally a nogo, but if in distress...). $\endgroup$
    – Peter
    Jun 2 '19 at 19:31
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    $\begingroup$ @Peter I'm guessing you're in the UK? The thought of signaling with a torch while flying an airplane sounds like a very bad idea in American English. - haha $\endgroup$
    – reirab
    Jun 3 '19 at 6:15
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    $\begingroup$ For your convenience „flashlight“. I‘m not a native speaker. $\endgroup$
    – Peter
    Jun 3 '19 at 6:19
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    $\begingroup$ @Peter haha - Yeah, I knew what you meant. It just caused a funny mental picture. I suppose flames in the cockpit would be very effective at signalling distress, though. $\endgroup$
    – reirab
    Jun 3 '19 at 6:26
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    $\begingroup$ @Peter "torch" is actually correct in British English; "flashlight" in American English :) $\endgroup$
    – Muzer
    Jun 3 '19 at 11:03
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ICAO describes the following distress signals in Annex 2 (Rules of the Air):

The following signals, used either together or separately, mean that grave and imminent danger threatens, and immediate assistance is requested:

a) a signal made by radiotelegraphy or by any other signalling method consisting of the group SOS (. . . — — — . . . in the Morse Code);

b) a radiotelephony distress signal consisting of the spoken word MAYDAY;

c) a distress message sent via data link which transmits the intent of the word MAYDAY;

d) rockets or shells throwing red lights, fired one at a time at short intervals;

e) a parachute flare showing a red light.

Note that a) mentions "any other signalling method", which could, for instance, be flashing aircraft lights. d) and e) require no radio or transponder.

In addition, the following urgency signals exist:

The following signals, used either together or separately, mean that an aircraft wishes to give notice of difficulties which compel it to land without requiring immediate assistance:

a) the repeated switching on and off of the landing lights;

b) the repeated switching on and off of the navigation lights in such manner as to be distinct from flashing navigation lights

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    $\begingroup$ Are there any signals which could be sent from an aircraft which lost all electrical systems? (I'm assuming most planes don't carry flares or rockets.) $\endgroup$
    – Skyler
    Jun 3 '19 at 15:22
  • $\begingroup$ @Skyler In that case, rocking the wings could still be useful on short range. $\endgroup$
    – Ferrybig
    Jun 4 '19 at 7:16
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    $\begingroup$ @Skyler: "...a [SOS] signal made by radiotelegraphy or by any other signalling method..." -- Interceptors will attempt to get a look into the cockpit for an idea of what's going on. A torch or smartphone light will do nicely. $\endgroup$
    – DevSolar
    Jun 4 '19 at 15:29
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Commercial flights are very regular throughout the world. One sure fire way to signal "don't shoot me I'm in distress" would be to fly low, and slow. a 737 at 5000 AGL would certainly signal something is wrong (most of the time) and would certainly get someone's attention.

Finding an airport and following the failed comms procedures and making a landing there would also be a sure sign that you needed help.

Remember you don't need a transponder and radio to land, and if you're doing something unusual, generally speaking, everyone else gets out of the way. Also, it would depend on the kind of distress your in. With no radio and no transponder, you probably have a pretty big electrical issue and need to land right away. So land.

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    $\begingroup$ Your advice goes against the procedure linked to in Pondlife's answer, which says: "Do not adjust your altitude, heading, or airspeed until directed to by the intercepting aircraft." $\endgroup$
    – Terran Swett
    Jun 3 '19 at 5:29
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    $\begingroup$ Those rules are for being intercepted. i.e. a fighter escort. I was speaking to letting others know you're in trouble. $\endgroup$
    – coteyr
    Jun 3 '19 at 5:31
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    $\begingroup$ Yeah, I think the question isn't very clear about exactly what it's asking for. This seems to answer the title question, but not the part in the body about, "how could the message "please don't shoot me down" be relayed visually to a fighter pilot?" That part would indeed imply being escorted. Having said that, if your goal is to indicate that you are not a threat and should not be shot down, flying a 737 at 5,000 ft probably isn't the best first move. It will signal distress alright, but not so much the "Please don't shoot me down" part. $\endgroup$
    – reirab
    Jun 3 '19 at 6:10
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    $\begingroup$ I think the downvoted are for suggesting flying a 737 at 5000 ft to get attention. While it would get you attention, I think it’s also a good way of getting shot down. $\endgroup$
    – Notts90 supports Monica
    Jun 3 '19 at 12:03
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    $\begingroup$ @reirab Why would you need to signal to your own escorts to not shoot you down? You'd signal that to interceptors if anything. $\endgroup$
    – Kapten-N
    Jun 3 '19 at 12:29

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