When does the FMS in the 737NG switch to approach mode?

Question

I fly a B737-800, and we talk about when the FMS enters "Approach mode". At this point, the FMS will command the Vref (plus additive) at the appropriate flap setting. This is helpful to know especially when there is a flap malfunction.

When does the FMS switch to "approach mode" for calculating speeds, and where do I find this information? I haven't seen it in the FCOM.

Answer 1

If the 737 is like most other FMS installations you will find the approach will activate at the capture fix. It is typically the waypoint just before the FAF. If you were flying into KMEM RNAV RWY 36C the capture fix would be NESBT and the FAF would be GINIE.

In the FMS that I use, the capture fix is typically loaded twice into the flightplan. Once from the enroute structure and once again for the approach after the approach name.

• NESBT
• ** RNAV 36C **
• NESBT
• GINIE

Answer 2

The FMC transitions to “on approach” when the airplane is within:

• 2 NM of the first approach waypoint (including approach transitions such as arcs and procedure turns), or
• 2000 feet of airport elevation, whichever occurs first.

^{(Boeing 737 NG FCOMv2 11.31.37 - Flight Management, Navigation - Flight Management System Operation)}