Originally Posted by
BOAC
... In any case even an instant speed reduction from 160k to 150k at 5d instead of 4D makes, I think, a difference of about 200ft in separation ...
I beg to differ, BOAC, by my calculations:
"Slow" Aircraft A is 150kts at 5DME and, at that speed in still air (2.5nm per minute) will take 24 secs to reach 4DME.
Aircraft B behind, travelling at 160kts will travel 1.067nm in the same 24 secs.
The reduced separation of 0.067nm is approximately 400 feet not 200 feet?
Your 200 feet would I think be the correct answer assuming a linear speed reduction by Aircraft A from 160 to 150kts between 5DME and 4DME not an instantaneous one at 5DME.
I accept your 'instant speed reduction' is not real, but I suggest a single nautical mile of the final approach i.e. 24 seconds is not a very long time period to be analysing as fully "containing" or bounding the problem scenario. I am NOT a PP so I could be very wrong when I say that I guess the problem begins earlier. If that is the case, multiples of 400 feet every 24 secs soon eats into safe separation even if everyone else in the sky has nailed it as instructed.
I have treble-checked my maths and have hit Submit and now stand meekly to be corrected
