AEROSPACE TECHNOLOGIES
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Honor Thy Speed Constraints
Honor Thy Speed Constraints
In this month’s PIREP article, we examine speed constraints during departures and arrivals to answer a common question: Will the FMS honor this speed constraint? The answer is – it depends.
The chart below shows the TEJAS 5 RNAV Arrival into Houston, Texas. If you were landing on runway 26L, the notes say that after PRAYY, fly a heading of 087° and expect vectors to final approach course.
There are a couple of things to note about the PRAYY waypoint. It is a flyover waypoint, but it also has an altitude and speed constraint of AT 6,000’ and AT 210 knots, respectively.
Figure 1 – TEJAS 5 Arrival
Copyright Jeppesen, Inc. - Used by Permission - Not for Navigation
If an AT constraint is charted and provided in the source data to Honeywell, it will be included in the navigation database, but without a descriptor, as the software interprets all constraints as AT or BELOW. The AT constraint will be met, and followed, in cases where the constraint limits the FMS planned speed target (e.g., the optimum/pilot selected climb or descent speed). This is the typical case. However, if the AT constraint is above the FMS planned speed, it will not be obeyed.
In Figure 2 below, the TEJAS 5 arrival is loaded into the FMS. As expected after PRAYY, there is the “FLY 087° or as assigned”. But notice the speed constraint at PRAYY: It is an AT or BELOW 210 knots, not AT 210 as depicted on the chart. So, there is nothing here stopping the system from meeting the 210 at PRAYY, but…
Figure 2 – Flight Plan Showing TEJAS 5 Arrival
…because it is AT or BELOW 210 the FMS can fly slower. So, why would it?
In the speed schedule shown below in Figure 3, when the aircraft is in the approach volume (within 15 NM of the airport for this aircraft) the speed schedule is 180 knots and decreases with the flap setting. The system will honor the speed schedule because the speed constraint is AT or BELOW.
Figure 3 – Speed Schedule
The flight control system provides several methods of raising the speed target manually to meet the AT constraint. Typically, the pilot would either select a manual speed on the autoflight guidance panel or program a new speed into the FMS.
In the next example, we see if the pilot doesn’t thoroughly brief the SID, it can result in the dreaded query from ATC to “say airspeed”. Dependent upon which fix the flight is filed to cross, the GLDMN 5 RNAV Departure shown has a max speed associated with passing a fix (i.e., MASTT) as well as a max speed associated with reaching an altitude (dependent on which fix the flight is filed over – BIGGY, ELIOT, LANNA, NEWEL, PARKE or ZIMMZ). See Figure 4, below.
Figure 4 – GLDMN 5 Departure
Copyright Jeppesen, Inc. - Used by Permission - Not for Navigation
The speed restriction associated with the fix MASTT is codable because AIRINC-424 standards can accept a speed limit associated with a specific fix. The restriction associated with the fix will be shown on the FLT PLAN page (see Figure 5, below). The AIRINC-424 database standard cannot, however, accept a speed limit associated with an altitude (in this example “Traffic filed over BIGGY, ELIOT, LANNA, NEWEL, PARKE, ZIMMZ do not exceed 250 KT until reaching 11000.”)
Figure 5 – Example Flight Plan Page
One technique for this departure would be to switch from FMS Speeds to Manual Speeds and set 250 KT upon passing MASTT and then switch back to FMS Speeds upon reaching 11,000 ft. Another technique is setting the climb SPD/ALT LIM on the PERFORMANCE INIT page to 250/11000 (see Figure 6, below).
Figure 6 – Example Performance Init Page
Understanding the types of speed restrictions that can and cannot be coded into the navigation database will help you to better set up, brief and fly these types of departure and arrival procedures.
