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When LHR ATC gives the miles to touchdown, enter the data into the app and it will calculate the FPA required to intercept the glide slope at the cleared altitude.

Subtract 0.2 from the calculated FPA and enter in the MCP V/S-FPA window. This is to account for variance in LHR’s miles-to-touchdown number.

Note: This app is NOT a constant descent-rate calculator—it's a flight path angle calculator. Descending at a constant angle is very different from descending at a constant rate, because for a constant angle the descent rate will vary considerably depending on ground speed. For example, for an FPA of 2.4, on downwind at 220 KIAS with a tailwind and a groundspeed of 270 knots the descent rate will be 1100 fpm. On final with a groundspeed of 150 knots the descent rate will be 650 fpm.

The Descent Rate Cross-Check box is for cross-checking that the aircraft is descending at the proper rate for the given FPA, not for making descent rate calculations.