A method of assisting operation of an aircraft en route to an airport, the method comprising: identifying a reference point in advance of a runway at the airport using a procedure database, the reference point comprising a final approach fix associated with the runway; dynamically determining a gliding vertical trajectory for the aircraft en route to the reference point based at least in part on a current altitude of the aircraft at a current aircraft location and gliding characteristics of the aircraft, the gliding vertical trajectory resulting in a predicted altitude of the aircraft at a location corresponding to the reference point; constructing a reference vertical trajectory having a constant flight path angle backwards from the runway to the final approach fix; and providing, on a primary flight display, a graphical indication of a vertical height margin comprising a difference between the predicted altitude at the location corresponding to the reference point resulting from the gliding vertical trajectory and an altitude criterion associated with the reference point, the altitude criterion comprising a target altitude at the final approach fix identified as an altitude of the reference vertical trajectory at a distance in advance of the runway corresponding to the location of the final approach fix, wherein the graphical indication of the difference dynamically updates as the aircraft travels, and wherein the graphical indication comprises a graphical representation of the difference between the predicted altitude at the location corresponding to the final approach fix resulting from the gliding vertical trajectory and the target altitude at the final approach fix, wherein the graphical representation of the difference is rendered in a visually distinguishable characteristic influenced by a sign of the difference. identifying a reference point in advance of a runway at the airport using a procedure database, the reference point comprising a final approach fix associated with the runway; dynamically determining a gliding vertical trajectory for the aircraft en route to the reference point based at least in part on a current altitude of the aircraft at a current aircraft location and gliding characteristics of the aircraft, the gliding vertical trajectory resulting in a predicted altitude of the aircraft at a location corresponding to the reference point; constructing a reference vertical trajectory having a constant flight path angle backwards from the runway to the final approach fix; and providing, on a primary flight display, a graphical indication of a vertical height margin comprising a difference between the predicted altitude at the location corresponding to the reference point resulting from the gliding vertical trajectory and an altitude criterion associated with the reference point, the altitude criterion comprising a target altitude at the final approach fix identified as an altitude of the reference vertical trajectory at a distance in advance of the runway corresponding to the location of the final approach fix, wherein the graphical indication of the difference dynamically updates as the aircraft travels, and wherein the graphical indication comprises a graphical representation of the difference between the predicted altitude at the location corresponding to the final approach fix resulting from the gliding vertical trajectory and the target altitude at the final approach fix, wherein the graphical representation of the difference is rendered in a visually distinguishable characteristic influenced by a sign of the difference. <EOS>