Inside the FMS

The NGFMS Gold package for the Embraer E-Jets, E170/175/190 and 195 adds several airline-specific capabilities to the Basic NGFMS that deliver quantifiable and measurable value to airlines operating these aircraft. NGFMS Gold offers three main benefits to airline operators:

1. Cost Index (CI) and ECON speeds
2. Wind Trade Steps evaluation
3. Optimum (Off-Idle) descent path creation and management capability

This article will focus on Optimum (Off-Idle) Descent Path Creation and Management Capability and what crews need to know to take advantage of this feature.

Traditionally, the method used to minimize fuel burn during descent was to remain at cruise for as long as possible then perform a descent at idle thrust. However, there is a point at which more fuel is burned in a longer cruise than is saved in an idle descent. In addition, practice has shown that true idle thrust descents can be problematic for several reasons. For example, when unplanned tailwinds are encountered (necessitating the use of speed brakes) or if ATC issues a crossing clearance in the descent that requires the airplane to “slow down and go down” simultaneously; both of which can result in a high workload situation.

The Optimum (Off-Idle) Descent Path provides better fuel savings and reduces the chance of the crew getting into a high workload situation during a VNAV descent. The NGFMS constructs a shallower descent path than a traditional idle descent with an earlier Top of Descent (TOD) point. This descent path allows the autothrottle to manage speed through minor power adjustments during the descent and provide a margin to buffer unplanned winds or revised ATC clearances. The Optimum Descent Path also determines when to slow down for speed restrictions and maintains the optimal constant descent.

Crews can easily determine whether the FMS in their aircraft has Optimum (Off-Idle) Descent Path enabled by viewing the DESCENT 1/1 page on the MCDU. The figure below illustrates how not having the ANGLE present on 2R tells the crew that an Off-Idle descent is enabled in the aircraft personality module (APM) and cannot be disabled by the crew. The Off-Idle descent is flown in VPATH once TOD is reached; the planned descent angles can be viewed on the ACTIVE FLT PLAN page. There is no procedural change required to fly the Off-Idle descent.

Through the use of the Optimum (Off-Idle) Descent Path calculation, you can now sit back, relax, and enjoy the cost savings.

Program Pilot Ryan Milmoe supports Embraer and Dassault EASy for Honeywell Flight Technical Services. He can be reached via email at Ryan.Milmoe@Honeywell.com