SMAC - Sabang Merauke Raya Air Charter, Medan, Sumatra, Indonesia, 1969 - 2011.  -  #1:

Small Planet Airlines, Vilnius, Lithuania, 2010-2018.  Was also registered in Germany and Poland.
-            #1,         #3(front small planet, rear Neckermann)/#4(front and rear same small planet):

Smartwings, Prague, Czech, founded 2018 by renaming from Travel Service completed.  - #1:

SN Brussels, Belgium, 2002- 2006,  successor of Sabena (q.v.  Brussels Airlines):

Sobelair, Brussels, Belgium, 1946-2004.      -   #1    and   #2:

SOL Lineas Aereas, Rosario, Argentina, 2006-2016.   - #2:

SOL Linhas Aereas, Cascavel, Brazil, 2008-2016.   -#1:

Solaseed Air, Miyazaki, Japan, founded 2011 by rebranding from SNA Skynet Asia (q.v.).  - #1:

Solid Air, Eindhoven, Netherlands, 1996-2011.    #1:

Solomon Airlines, Honiara, Solomon Islands, founded 1968 by renaming from Megapode Airlines. - #2, #3:

Somali Airlines,  Mogadishu, 1964-1991.    -   #2a:

Somon Air, Dushanbe, Tajikistan, founded 2008.    -  #3:

SonAir, Luanda, Angola, founded 1998.     -   #2:

SONG LLC, Atlanta, Georgia, USA, 2003-2006. Song was a low fare brand of Delta(q.v.).  - #1:

South African Airways, Kempton Park, founded 1934. 
-           #6,                        #4/#9(diff height stripes area),             # 2,                         #3:

-   #12/#19/#20(diff base n maker),   #5normal/#5Aglossy/#7diff maker,       #1,          #10:

Southern Winds Airlines, Córdoba, Argentina, 1996 - 2005.    -  #1,       #5:

Southwest Airlines, Dallas, Texas, USA, founded 1971:
 
SPANAIR, Barcelona, Spain,  1986 - 2012.
-             #4/#9(diff paper),                     #6,                          #1,                              #2:

-                            #7,                                 #8,                                   #12:

SPANTAX, Madrid, Spain, 1959 - 1988.   -          #1,            #3:

SPICEJET, New Delhi, India, founded 2005.
-      #2,                                           #6,                            #7/#8,                        #9:

-                  #12,                                             #14:

Spirit Airlines, Miramar, Florida, founded 1980.    - #2:

SPRING Airlines, Shanghai, China, founded 2004.
        -    #2,                                       #3,                                 #4,                               #5:

         -  #7,                     #8...may be UESD.../#10...may be USED,

Sri Lankan, found 1979 as Air Lanka (q.v.) and renamed 1999.  
         - #3,               #1/#2(with resp w/o Tear off strip),               #4:

Sriwijaya Air, Jakarta, Indonesia, founded 2003.   -   #3,        #1,       #6,           #7:

  -               #8:

Star Air of Indonesia, founded ?, ceased 2005:

Star Airlines, Paris, France, 1997 - 2006. Renamed to XL Airways France(q.v.).
-              #2,                                             #3:

StarFlyer, Kitakyushu, Fukuoka, Japan, founded 2002.
  -            #2(flat base),                  #3(sealed base),                             #4,                            #5:

Star Peru, Lima, founded 1995.        -   #3,              #4:

Sterling Airways, Copenhagen, Denmark, 1962-1993.  - #5(matt)/#5A(glossy),   #3,    #7:     
 
Sterling Airlines, Copenhagen, Denmark, 2005-2008.
-                        #1,                                  #2,                                        #4:             

-            #6,                                                #8:

SEA Sterling European Airlines,  Copenhagen, Denmark, 1993-2005.    -  #1:

STP Airways, Sao Tome and Principe, founded 2008.     -   #1,           #2/#3(diff cut top rim):

Stuttgarter Flugdienst (SFD Aviation), Stuttgart, Germany, 1956-2019.   -   #1:

Sudan Airways, Khartoum, founded 1947.   -   #2,       #11,                      #8:

Summit Air, Kathmandu, Nepal, frounded 2017 by renaming from 2011-founded Goma Air.  -#1(front and rear):

Sun Country Airlines, Minneapolis, Minnesota, USA, founded 1982,    -   #1,    #4:        

Sund Air, Stralsund, Germany, founded 2016.     - #1:

SUN D'OR,  Tel Aviv, Israel, founded 1977 as EL Al Charter, renamed 1981.    -  #1:

Sunday Airlines, Symkent, Kazakhstan, founded 2013.     -  #1:

Sun Express, Antalya, Turkey, founded 1989.        -       #1,        #2:    

- #3/#3a(glossy/normal paper),   #4/#5(normal/glossy),  /#6/#7(glossy/normal),
diff. makers in gussets:

Sunu Air, Dakar, Senegal, founded 2003.   - #1:

Sunwing Airlines, Toronto, Canada, founded 2005.   -     #1,            #3:

Suparna Airlines, Shanghai, P.R.China, founded 2017
by rebranding from 2003 founded Yangtze River Express.  -    #3:

Super Constellation Flyers, Basel, Switzerland.   -   #1:

Surinam Airways, Paramaribo, Suriname, founded 1955.  -  #2,       # 4,           #5,     #6:

Swedair, Stockholm, Sweden, 1975-1994.   -  #3:

Swissair, Zurich-Kloten, 1931-2002.   -  #17(old, 30cm tall bag)front and rear shown:
    
#4/#16(diff paper),              #6      and       #12/#14/#15(diff cut, base, tear off strip):

#3/#5(diff paper),                       #10/#9(Name at base upright low),           #11:

Swiss International, Basel, Switzerland, founded 2002.
#3,                         #4,                          #8                      and                 #5 / #6:

#13(Name at rear),      #9(normal paper)/9a(glossy paper),         #10/#11,               #1:

-    #14(Front: Waste, rear:Swiss) / #15(Front: plain),     #2:

Syrianair, Damaskus, Syria,  founded 1961.  
    #2/#5(diff paper),          #3,                     #8/ #10(diff maker),                        #7: