The ARM methodology(?) now in use by UK CAA gives the F15E/J41 airprox an ERC of 502 and the 747/747 an ERC of only 102. I assume there is supposed to be some steady linear progression of measured risk between these two scores and not some logarithmic scale as with decibel measurement
Not really a steady scale. The method uses a matrix which has effectiveness of remaining barriers along one side and most credible accident outcome (if it progressed to an accident) along the other. The outcome maxes out with Catastrophic at 3 or more deaths so the only difference between the two scores would be the effectiveness of remaining barriers. It may have been developed since I last looked at it but my notes say 13 possible scores ranging from 1 to 2500.