Landing Field Length in FMS

[FlightPathOBN]

Not sure of your FMS version, but this is the criteria:

From the end of runway, the obstacle clearance surface begins at 35' above, with a 40:1 slope origin, or 152'/nm.

The aircraft climb gradient minimum is 200'/nm, the difference is the required minimum clearance. These surfaces go out to 10nm from runway end (1500' ROC).

As an example, right at the end of runway, you have 35' of clearance.

At 1nm, you have 48' ROC, at 2nm you have 96' ROC.

Depending on your FMS, there are certain 'nuances' built in with the ROC for ARR and DEP....(and perhaps in your situation, the Collins box does have certain unexplained differences in the programming)

Edit: this is aircraft variant specific as well...