Why FMS Slows Early, How Speed Constraints Conflict, and What Pilots Can Do to Manage RNAV Arrivals

The aim of the current FMS logic is to have the aircraft reach the clean approach speed (i.e., 200 knots for NG FMS and 190 knots for EASy) precisely as it enters this volume. As part of this process, the speed reduction commences from an earlier faster speed, such as 250 knots, and from an earlier point in the flight to allow for a smooth planned deceleration prior to the boundary (see Figure 2). 

This logic is not deterministic though, meaning the timing of the approach speed target change depends on a combination of variable factors such as the aircraft’s weight, altitude, descent angle, any speed constraints, aircraft performance, and other aspects. One nuance with the current FMS design is the variability in how approach speed targeting is implemented. This lack of being deterministic can lead to many pilots expressing frustration about the variable and seemingly premature targeting of the approach speed change, as it occurs before entering the designated approach volume space or “boundary”. Not to mention more importantly how it could affect compliance with any upcoming “At” speed constraints on a published navigation procedure.

The activation of approach speed in the clean configuration is a crucial function that ensures the aircraft approaches the vicinity of an airport at a safe speed to begin configuring for final approach and landing. However, the design and logic of the FMS that governs this process along with modern navigation procedure speed constraints have created several challenges that can complicate flight operations during arrival. Let’s look at this in more detail, next!

FMS Speed Restriction Logic

Many airspeed restrictions in the airspace we operate are typically maximum speeds, such as 250 knots below 10,000’ MSL and 200 knots within certain departure volumes. Because of this, and in general, the FMS has been designed to treat all speed restrictions as “At-or-Below” speed constraints. Think of these as speed limits. Historically, most navigation procedures have utilized upper limit speeds, as mentioned above, but that has been changing with more often seen “At” speed constraints on certain arrival procedures, as an example. 

In practice, the FMS treats all speed restrictions in terms of "at or below," irrespective of whether the constraint might be "at" or "at or below". This has become challenging because pilots can find themselves in situations where the FMS's computation and use of these constraints does not align with the operational requirements seen on a navigation procedure. 

If the FMS mandates a deceleration prior to reaching an “At” speed constraint, it can result in an awkward operational scenario where compliance with speed restrictions becomes challenging. For example, while utilizing the KLAX IRNMN 2 RNAV Arrival procedure (excerpt seen in Figure 3), pilots may encounter navigational speed restrictions that lead to conflicts between the planned FMS approach speed (s) and the published or charted constraints. 

Specifically what may occur during this arrival procedure into KLAX, the destination airport, is that the FMS would target clean approach speed (in this example, 200 knots) and a speed reduction would commence prior to waypoint BAYST, as the 15-NM approach volume boundary (from KLAX) is just northwest of that fix. The aircraft would cross the approach volume boundary at 200 knots. This would cause a conflict because of the “At” speed constraint of 240 knots at BAYST.

In the event the FMS fails to provide and/or hold an appropriate target speed, pilots are advised to switch to Manual Speed (MAN SPD) mode. This allows the pilot to maintain better control and ensure adherence to the relevant speed constraints, especially if the flight plan includes waypoints with specific "At" speed restriction parameters.

Coming Soon, FMS Updates

The situations stemming from premature speed targeting and the treatment of speed restrictions as upper limits highlight the need for a more deterministic approach to speed management. Looking ahead, enhancements are anticipated in future software releases of the FMS, particularly aimed at addressing the challenges associated with earlier than desired clean approach speed targeting.

The envisioned logic redesign proposes that commencement of speed reduction towards the target clean approach speed begins precisely at the boundary of the approach volume and not prior to it (see Figure 4), creating a deterministic behavior where the speed target change is fixed to this same boundary that we have previously defined. In other words, the speed reduction commences at the boundary instead of prior to the boundary.

Figure 5 below of the GTOUT 1 RNAV Arrival into Orlando shows how this method could significantly reduce the chances of encountering larger conflicts or discrepancies with "At" speed constraints, allowing for smoother approaches without unnecessary early deceleration. 

The current FMS logic would change the speed target to clean approach speed and begin a deceleration prior to BANYA so that it reaches the approach volume boundary at the clean approach speed (in this example, 200 knots). The “At” 210 knot speed constraint at BANYA (just prior to the boundary) would be violated.

Taking the example further, but with the updated FMS logic this time, the aircraft FMS would change the speed target to clean approach speed only when it has reached the boundary itself and would only commence a deceleration effectively after passing the approach volume boundary. The “At” 210 knot speed constraint at BANYA would be honored.

Looking at Figure 6 below, and returning to the IRNMN 2 RNAV Arrival example, the updated FMS logic would commence the speed reduction at the approach volume boundary, would help to honor the 250 knot “At” constraint at SYMON, and the deceleration would be located much closer to BAYST (and the aircraft would not reach 200 knots until beyond or after BAYST). But again, this enhancement does not completely solve the “adhering to At-Speed constraints situation”, but should significantly reduce the scenarios in which it may occur.

It is still the pilot’s responsibility to manage aircraft speeds as the FMS treats all speed constraints as At-or-Below speeds. It is recommended to utilize MAN SPD mode at BAYST, CLIFY, and DAHJR because of the 240 knot and 210 knot “At” constraints at those waypoints, respectively. Otherwise, the FMS will target the clean approach speed instead. 

We encourage you to review your avionics Pilot Guide as some avionics platforms also provide the pilot with the capability to modify the approach volume boundary we have been discussing to a different distance and/or altitude dimension from the destination airport. In addition to MAN SPD mode, this would be another tool to be able to manage automatic speed target changes and subsequent timing of speed reductions on and during arrival.