Inside the FMS: Step Climbs and Capabilities

Step climb functionality can be a bit confusing for pilots, even though step climbs have been a function of flight planning since the early days of FMS. This article sheds some light on the original step climb functionality as well as the new capability to plan step climbs using Honeywell’s NG FMS.

Original Step Climb Function

The original step climb functionality found in legacy flight management systems is referred to as Optimum Step Climb. This is because it attempts to optimize the flight profile by raising the aircraft’s altitude as high as possible, using the step increments specified by the crew. To calculate a step climb, the crew can enter a step climb increment in PERF INIT page 3, as shown in the figure below. The step climb can be entered in thousands of feet or with a single number (i.e. 4,000 or 4).

Once the aircraft has the performance to reach the next altitude specified by the step increment (i.e. FL410 to FL450), it will calculate a climb and compute fuel predictions for that higher altitude. In this case a 4,000 ft. increment was specified, so it would plan to climb in 4,000 ft. increments starting from the initial cruise altitude until it reached either the aircraft ceiling or the top of descent. Once an entry is made, the FMS looks to the aircraft’s ceiling altitude found on PERF DATA page 1 (shown in the figure below) and monitors the aircraft weight. When the weight is reduced such that the aircraft can make a climb by the step increment, it will direct the aircraft to the next step altitude.

The FMS calculates a point at which the climb can be initiated, using the criteria that the climb must be completed at a rate of at least 200 FPM.

Another characteristic of the Optimum Step function is that immediately after takeoff, if the aircraft is capable of climbing to a higher altitude, it will plan that, even if the crew has entered a lower initial cruise altitude during PERF INIT. This does NOT mean the FMS will override the crew’s inputs and violate an altitude assignment. It simply means the FMS will display to the crew that the aircraft may achieve a higher cruise altitude. Remember the FMS will never violate the altitude preselect during any en route phase of flight. Additionally, the FMS will transition from climb speed to a cruise climb once the initial cruise altitude is sequenced.

Take the example of a crew entering FL410 for their initial cruise altitude when the aircraft is capable of attaining FL450. The FMS would plan to climb directly to FL450 and the system would transition from the climb phase of flight to the cruise phase of flight immediately after passing the pilot-entered altitude (FL410 in this case), as shown below.

In another example, shown below, the cruise altitude is set to FL320 and the step increment is set to 2,000 feet. The system will transition into cruise phase of flight and continue in a cruise climb until reaching the CEIL ALT (altitude at which it can no longer meet the 200 FPM climb requirement – 46,000 feet in this example).

The FMS will continue this pattern of climbing until reaching the aircraft’s maximum operating altitude and will base time and fuel estimates on the higher altitude. If the crew doesn’t plan to step climb as presented in the FMS, they must go back to PERF INIT and delete the step increment. This is the only way to accurately reflect time and fuel predictions for an intermediate altitude. The crew will also need to change the cruise altitude to the desired altitude. The Optimum Step feature is most beneficial for operators who want to obtain maximum range by flying as high as possible.

Planned Step Climb Function

A new step feature was implemented into the NG FMS that allows for more versatility: Planned Step Climbs. Planned Step Climbs are available for both step climbs and step descents during the cruise segment of flight. They are entered by the crew and direct the FMS to perform a step climb or descent at a desired waypoint.

The crew enters the desired altitude (to step to) followed by the letter S (e.g. FL450S) on the right side of the flight plan, adjacent to the desired waypoint, as shown in the figure below. When the aircraft reaches the selected waypoint and the altitude preselect is set to a higher or lower altitude, the system will automatically begin a climb or descent.

If the aircraft is too heavy to climb to the desired altitude, the crew will receive the message UNABLE STEP and the aircraft will remain at its current altitude. The VSD will depict the planned steps as part of the vertical profile, as shown below.

Optimum vs. Planned Step Climbs

The primary difference between Planned Step Climbs and Optimum Step Climbs is that the former will coincide with a specific waypoint. Optimum Step Climbs, however, will occur anytime the FMS determines the aircraft has the performance to reach the next step climb increment as input by the crew. Optimum Step Climbs are also only available to climb the aircraft higher, while Planned Steps can also facilitate descents. Also, multiple Planned Step Climbs/Descents can be programmed, which will result in accurate fuel/time predictions.