Thanks for your replies
ПАК - ? (context is needed)
ПНС - ? (context is needed)
УПБ - ? (context needed)
As for the context, these abbrevations are those used of the allaces.ru website.
The webpage below has a list of WWII Soviet units:
http://allaces.ru/cgi-bin/s2.cgi/sssr/struct/p_all.dat
A good part of the units then have a webpage showing personnel list.
Example:
http://allaces.ru/p/ls.php?div=bap0001001
The table has columns with date of leaving the unit (that will also be the date of death/disparition) and fate. The above abbrevations will be in the late column.
By clicking on the name, more details are available
So by clicking on some names, my guess about the above codes:
ПАК : killed in crash (K for "katastrophy")
http://allaces.ru/p/people.php?id=00000009921
MHC : killed in a ground accident ??
http://allaces.ru/p/people.php?id=00000009952
http://allaces.ru/p/people.php?id=00000010969
These two incidents if I understant well the automatic traduction by Google and some wild guess of my part are for one airman who was killed by walking into a running engine, and another killed by the accidental discharge of a gun.
УПБ: killed on ground by an enemy air raid
http://allaces.ru/p/people.php?id=00000010969