bookworm, let us take an example:
The calculations below show actual feet/mb for various altitudes assuming ISA:
Zero ft 30 ft per mb
5000 ft 30 ft per mb
10000 ft 31.45 ft per mb
20000 ft 36.25 ft per mb
30000 ft 40.84 ft per mb
40000 ft 47.84 ft per mb
50000 ft 52.75 ft per mb
60000 ft 62.96 ft per mb
If an aircraft is flying at 20000 ft on an RPS of 1033 (20
mbs x 36.25) the aircraft is 725ft lower than an aircraft at FL200. This differs from our traditional assumtion that, using 30ft/mb, the first aircraft would only be 600ft lower.
If we then use this example in an ATC coordination scenario, we may find that ATC would agree for an aircraft not to be below 21600ft RPS to ensure standard separation against traffic at FL200. Under the 30ft/mb assumption this would be OK as 21600ft would equate to 21000ft 1013.2mbs. However, taking into account the 36.25ft/mb accurate equation, in this particular circumstance the aircraft on RPS would actually be at the equivalent of 20 875 ft 1013.2 mbs thus creating actual separation between the 2 aircraft of 875ft. This, in ATC separation parlance, is not enough.
If we take the argument up to the 35000ft territory and are in airspace where the temperature bit of ISA is significantly different from the assumed, then the problem gets significantly more acute.