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I thought SOS means "save our souls", but apparently it doesn't.

But ships send SOS when in danger and they used Morse code. Why do airplanes use MAYDAY?

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    $\begingroup$ SOS is not used since almost 20 years en.wikipedia.org/wiki/SOS : "SOS remained the maritime radio distress signal until 1999, when it was replaced" $\endgroup$ – Federico Dec 19 '17 at 13:46
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    $\begingroup$ No it doesn’t stand for save our souls. $\endgroup$ – Notts90 Dec 19 '17 at 18:50
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    $\begingroup$ Of interest from that wikipedia article, SOS is actually a mnemonic for a 9 element sequence. It's not actually 3 letters, but rather "The SOS distress signal is a continuous spaceless sequence of three dots, three dashes, and three dots." As such, other letters could be used as a mnemonic, such as VTB. To distinguish between SOS and VTB, one would need to add spaces between the letters which are not there as part of the distress signal. $\endgroup$ – Cort Ammon - Reinstate Monica Dec 19 '17 at 18:59
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    $\begingroup$ Back formation or not, SOS doesn't mean "Save Our Souls", if for no other reason than that (if you follow certain religions, anyway) sailors' souls would be in far more peril while ashore than at sea :-) It means "Save Our SHIP". $\endgroup$ – jamesqf Dec 19 '17 at 19:49
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    $\begingroup$ This question is based on a false premise. Ships also use "Mayday". This reference is from the US Coast Guard, but the terminology is also standard in many other countries (Canada, for example). $\endgroup$ – J... Dec 19 '17 at 20:17
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The difference here isn't between ships and aircraft: it's between Morse code and voice.

The SOS signal is only for Morse code. It's short, easy to send, and easy to recognise. But it's not as convenient to say. It doesn't actually mean "save our souls". The letters were chosen just to form the simple Morse pattern, and "save our souls" is a backformation: it was made up by sailors later, partly as a joke, partly as a mnemonic.

"Mayday" is an English-looking spelling of French m'aidez, "help me". Spoken out loud, it's short, easy to send, and easy to recognise. It doesn't have any sounds that some nationalities can't say (such as r, th, or v). It's a good signal to use as voice, but would be much worse as Morse code, because it's too long.

Back in the days when aircraft were equipped with Morse code transmitters, they would have sent SOS as a distress call, just like a ship. And a ship with voice radio would send "Mayday" instead of SOS. Now that Morse code has fallen out of use, SOS is also disused. You only hear it in movie plots where the plucky hero doesn't have a working radio but can somehow improvise a way of signalling Morse code (usually by holding two wires together on a broken radio).

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Federico Dec 20 '17 at 20:28
  • $\begingroup$ many Chinese can't spell it because there's no /d/ sound in Mandarin, only /t/ and /tʰ/. Actually there are only voiceless consonants in Mandarin $\endgroup$ – phuclv Dec 21 '17 at 8:34
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    $\begingroup$ @LưuVĩnhPhúc True (and I would in general take issue with “any sounds that some nationalities can’t say”), but that’s also true of German; and because in English voiced stops are generally unaspirated and aspiration is not distinctive, native English speakers will tend to interpret unaspirated voiceless stops as their voiced counterparts. Meanwhile, Spanish /d/ is realized as /ð/ in most dialects. In other words, the sound transcribed in Pinyin as “d” is no farther from the original French than many European pronunciations, and just as comprehensible, at least to Westerners. $\endgroup$ – David Moles Dec 21 '17 at 23:03
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    $\begingroup$ '"Mayday" is an English-looking spelling of French m'aidez, "help me"'. I often read this, but as a French I'm very skeptical about it, it's grammatically incomplete and unnatural. "Help me!" translates to "aidez-moi !" imperative tense". If I shout "m'aidez", nobody will understand this is the verb aider (btw "help!" translates to "à l'aide !"). It could be an incomplete sentence for "I need your help!" ("il faut m'aider !" the infinitive, grammatically correct). -ez and -er endings have the same pronunciation. $\endgroup$ – mins Dec 22 '17 at 9:54
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    $\begingroup$ @LưuVĩnhPhúc I speak Mandarin and, while this may be technically correct, in practice it doesn't matter. Mandarin speakers have common words that sound extremely similar to "may" (eg. 没 -- meɪ̯³⁵) and common words that sound extremely similar to "day" (eg. 得 -- teɪ̯²¹⁴ -- which despite what it looks like in IPA is very similar to "d" in practice). I doubt that anyone would have trouble understanding what a Chinese speaker vocalising "没得" meant in the context of aviation. $\endgroup$ – Chris Down Dec 22 '17 at 10:10
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Ships use Mayday.

This is the transcript of M/S Estonia disaster from 1994, a major passenger ferry sunk.

01:23.11
Estonia>    Europa, Estonia.    
01:23.15
Estonia>    Silja Europa, Estonia.  
01:23.19
Europa> Estonia, this is Silja Europa replying on channel 16.   
01:23.27
Estonia>    Silja Europa *  
01:23.34
Europa> Estonia, this is Silja Europa on channel 16.    
01:23.55
Estonia>    Silja Europa, Viking, Estonia.  
01:23.59
Mariella>   Estonia Estonia.    
01:24.02
Estonia>    MAYDAY MAYDAY.  
01:24.07
Estonia>    Silja Europa, Estonia.  
01:24.10
Europa> Estonia, Silja Europa. Are you replying— calling Mayday?    

Source: http://oona.windytan.com/estonia/

You can also listen the same in Youtube:

SOS is for morse code, Mayday for voice.

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    $\begingroup$ This was painful to listen to and read about, but thanks for sharing. $\endgroup$ – Mehrdad Dec 21 '17 at 23:38
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    $\begingroup$ It was a very painful event, national tragedy for Estonia, Sweden and Finland. I remember seeing the head of rescue, Raimo Tiilikainen, cry on television. You don't see major officials cry very often. $\endgroup$ – Tero Lahtinen Dec 22 '17 at 9:08
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Merely being a ham operator for 58 years, this is my understanding. "SOS" means nothing. It's easily sent and easily copied (understood), and it follows "CQD" which was also used on the Titanic. CQD is most aptly "COME QUICKLY DISTRESS" (the D has various meanings / uses).

The senior radioman on the Titanic told the jr. to go ahead and try the new call. As an aside; a young Welsh lad, about 15 being a young ham, heard the Titanic's distress call, sic (ITS A CQD DE MGY CQD DE MGY SOS AM SINKING HAVE STRUCK A BERG). ("DE" means from,- MGY is the TITANIC'S Marconi owned wireless station call letters). He copied the text and went to the local police. When he told them, they laughed at him saying sic. ("the Titanic is unsinkable, go home").

They were surprised the next morning to hear / read the news. MAYDAY is from a French word "French "m'aidez", "mayDAY," [one word], and means in actuality - "help me". It is a short form of venez m'aider - come and help me'.

This first URL is a simulated spark gap transmission.

The second is an exchange between the Titanic and the Carpathia.

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MAYDAY is from the French "m'aidez" meaning Aid Me, or HELP. Try it in a French to (your language) translator and it becomes clear. Hard to believe no one came up with the right response.

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