What characterises people who stay calm in crisis situations?
This is measured with the Big5 Neuroticism#6 facet: Vulnerability.
This trait measures how an individual reacts to stress, including emergency situations:


  High scorers on Vulnerability experience panic, confusion, and
  helplessness when under pressure or stress. Low scorers feel more
  poised, confident, and clear-thinking when stressed. - See more at:
  http://www.psychometric-success.com/personality-tests/personality-tests-big-5-aspects.htm#sthash.PmuKOUm0.dpuf


http://www.psychometric-success.com/personality-tests/personality-tests-big-5-aspects.htm

This facet has shown to be the one more important for being able to make decisions and depending on others (page 153):
\http://www.researchgate.net/profile/David_Watson14/publication/257044687_Procrastination_and_the_five-factor_model_a_facet_level_analysis/links/0a85e52fd0aa495d2f000000.pdf

This suggests that the panic, confusion and helplessness experienced may be due to being unable to make a decision on how to act and react.