Issue

There are occasions when a flight plan has two or more altitude constraints for descent. These constraints can be manually entered or part of a Standard Terminal Arrival Route (STAR). The Flight Management System (FMS) defines a geometric segment between altitude constraints. Depending on the segment length and altitude constraints, it may consider the geometric segment to be too steep to fly without speedbrakes. Although the speed brake requirement may be for only a few miles, the FMS assumes that during the entire segment, half of the speedbrakes will be extended and predicts an increased thrust requirement for the entire segment. This may lead to erroneous fuel predictions. A geometric segment of approximately 100 NM may lead to an error of 1.5 tons in the fuel prediction at the destination.

Predictions turn back to normal when last altitude constraint is sequenced.

Solution

• Check all constraints and also look for additional “Too Steep Path” message in between flight plan legs in MCDU
• Delete/Modify altitude constraint and compare the fuel predictions (Use secondary FPLN option for this purpose so that primary navigation in managed mode is not affected)