The FAR/JARs part 33.77 do specify a hailstone impact test which nobody bothers with nowadays. It orriginally came in with the SST and assumed cruise speed and a mixture of 1 and 2" size stones. It turned out that only a few minor dents were experienced on the engine blading in the test and yet no single confirmed incident of such in service.
However some new thinking was coming about on the hail mixed with rain where the stones were very small but the mush effect was causing the compressors to work so hard to process it at idle power, that the engines were being damaged internally. Thus the push for testing moved on to more complex rain/hail/mush and and the bigger birds to sort out the new engines today.
[ 03 November 2001: Message edited by: lomapaseo ]