Climb Schedules

General

In the "ENROUTE" climb jet aircraft generally conform to a schedule which specifies an initial climb speed as an indicated airspeed (IAS), and a Mach number for the upper part of the climb until cruise altitude is reached. Mach numbers are then used as a cruise schedule.

In the ATPL performance, or Navigation exam you may be asked to calculate the pressure altitude (FL) at which the IAS climb schedule ends, and the climb Mach number schedule starts. This point is called the "Changeover Level", and we will concentrate on calculating it in this editorial.

In the early stages of the climb at a fixed IAS, the Mach numbers rise until changeover level is reached. Changeover level is that flight level where the climb IAS value and climb Mach values occur simultaneously.
For any given IAS/Mach climb schedule there can be only one changeover level. Different aircraft types may have different climb schedules specified in their Aircraft Flight Manual. (AFM).

Let's use the B727-200 aircraft for example. The AFM specifies a climb schedule of 310 IAS/Mach 0.78.
What is the FL at which 310 IAS equals Mach 0.78 ? (ie: Changeover Level for this combination).

Working:

Step 1. Set Mach 0.78 in the Mach window.	Step 2. Opposite 310 IAS, read FL280 (nearest)

So the flight crew of the B727 will hold a constant enroute climb IAS of 310 IAS until the Machmeter value has risen to Mach 0.78 (at FL280), after which FL they will hold Mach 0.78 until Top of Climb (TopC).