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PIREP: When Good SIDS Go Bad
When Good SIDS Go Bad
Editor’s Note: The title of this series highlights the inspiration for each article: every issue covered was reported to Honeywell by a pilot. PIREP will look at the things a system can’t do, either because of procedure design, database coding, the FMS, or physics. The hope is that pilots will begin to recognize and understand these limitations.
An operator reported a condition on the EPIC platform in which after loading a departure (RUUDY 6) from Teterboro (see Figure 1), the lateral path of the SID showed an erroneous loop that appeared prior to the aircraft taxiing to the active runway. It should be noted that this will occur on all EPIC platforms, regardless of software load.
This procedure has a lot of things happening at once that demand the crew’s attention. A closer look at the SID in the figures below will examine its complexities and why the loop appears. Notice the SID has a climb gradient of 500 ft/nm (8.3%) to 520 feet after departure. That is coded as a temporary (meaning it occurs at an altitude rather than a point over the ground) waypoint in the FMS and a CA (course to an altitude) leg at 520 feet, to represent the climb gradient. Next, it flies a DF (direct to a fix) leg at 520’ to DAVIM. After DAVIM, there is a quick level-off at Wentz at 1500’ (see Figures 2, 3 and 4). Finally, the top altitude for the procedure is 2000’. That’s a lot to keep track of as the crew. The geometry of this departure may result in FMS predictions that stem from the same root cause and will be discussed further.
The first issue is caused by where the SID is loaded. EPIC predicts the climb performance based on the aircraft’s current position (see Figure 5) and assumes a nominal climb gradient. When the SID is loaded from the FBO ramp, especially when the FBO is near the opposite end of the airport (Signature South or Jet Aviation) from intended departure runway, this can cause a problem. Let’s assume the aircraft is located at Signature South.
Since the distance between its GPS position and the first fix is much shorter, it often predicts the altitude constraint past the first named waypoint (DAVIM) on the departure. When the FMS plans the climb, it assumes roughly a 6% climb gradient. The FMS calculates the altitude constraint (*ALT01 in the flight plan) leg to be past the first waypoint (DAVIM) and then plans to go direct to DAVIM after the altitude is met. This causes a loop back to DAVIM and will not look correct during preflight planning (See Figure 6).
Fortunately, when this occurs, the aircraft continuously updates position and calculations so as the aircraft taxis out, the loop gets smaller and smaller and eventually disappears, displaying the procedure correctly. This should require no input from the crew.
A second condition, which is a manifestation of the first scenario, can occur especially when the departure contains a low AT constraint such as WENTZ (at 1500’) on the RUUDY 6. Many crews are leery of departing in a high-performance airplane using max power and then having to do a simultaneous power reduction with a reduction in pitch to avoid violating the altitude at WENTZ.
This can result in a similar manifestation of problem 1. The crew lowers the nose prematurely and reduces power, anticipating the level off at 1500 feet. In doing so, they fail to achieve the gradient required by, and coded in, the departure procedure. The result is similar to before but now with much greater consequences. The FMS continues to move the *ALT01 waypoint out, eventually moving it past the DAVIM. Once the crew does sequence it, it performs a Direct To DAVIM, which is now behind it, resulting in an unexpected turn command of 360 degrees toward the runway and likely catching the crew off guard.
This is a common type of procedure design that can be found in numerous departures around the world. The best mitigation strategy is adequate pre-briefing and planning around the complexities of the departure. This way, the crew can plan an initial climb adequate to satisfy the procedure design with a pitch and power reduction still early enough to minimize passenger discomfort or overshooting the altitude constraint further down the procedure.
As always, if you have questions or comments about this, or any other article in the PIREP series, please contact us at FTS@Honeywell.com.