  We report on the performance of lumped--elements Kinetic Inductance Detector
(KID) arrays for mm and sub--mm wavelengths, operated at 0.3K during the
stratospheric flight of the OLIMPO payload, at an altitude of 37.8 km. We find
that the detectors can be tuned in-flight, and their performance is robust
against radiative background changes due to varying telescope elevation. We
also find that the noise equivalent power of the detectors in flight is
significantly reduced with respect to the one measured in the laboratory, and
close to photon-noise limited performance. The effect of primary cosmic rays
crossing the detector is found to be consistent with the expected ionization
energy loss with phonon-mediated energy transfer from the ionization sites to
the resonators. In the OLIMPO detector arrays, at float, cosmic ray events
affect less than 4% of the detector samplings for all the pixels of all the
arrays, and less than 1% of the samplings for most of the pixels. These results
are also representative of what one can expect from primary cosmic rays in a
satellite mission with similar KIDs and instrument environment.
