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Navigator

Take Care When Using ‘RNAV Off the Ground' Departures

Instrument departures known as "RNAV off the ground" (ROTG) now have been in place for a few years at airports including Dallas-Fort Worth International (DFW), Las Vegas McCarran International (LAS) and Hartsfield-Jackson Atlanta International (ATL).

When using an ROTG, pilots are expected to engage the lateral navigation immediately after departure because headings are not assigned by the tower. It is critical that pilots ensure that the correct departure runway is loaded into their flight management system (FMS), since many of the locations where ROTG is in use have parallel runways, each with its own distinct ground track off the departure end of that runway.

The consequences of loading an incorrect runway in the FMS likely will result in a deviation from the expected flight path and the departure controller immediately issuing control instructions to you and/or nearby aircraft to avoid a near miss or midair. There is rarely enough time for the pilot who has loaded an incorrect procedure/runway in the FMS to notice the deviation and take corrective action to avoid it.

Unfortunately, repeated instances of pilots loading the wrong procedure/runway in their FMS have been reported, in spite of reading back the correct information to air traffic control, resulting in the filing of pilot deviations against the air crew. In fact, at DFW, the most recent data indicates that general aviation (GA) aircraft account for over 20 percent of DFW ROTG pilot deviations. Since GA traffic at DFW averages about 1 to 2 percent of the total operations, this is a significant concern.

ROTG procedures are designed to result in consistent, precise and repeatable ground tracks in order to enable simultaneous departure operations off closely spaced parallel runways. The most important concepts for "RNAV off the Ground" operations include the following:

  • Standard operating procedures (SOPs) should ensure that the crew has loaded the correct runway, standard instrument departure (SID) and SID transition. SOPs also should verify with the RNAV SID chart that all waypoints are correctly loaded into the active FMS flight-plan route.
  • During takeoff and departure, crews should arm/engage lateral navigation (LNAV) as soon as Aircraft Flight Manual (AFM) limitations permit and then follow the flight director (FD) guidance.
  • Never hand-fly the procedures.
  • During LNAV operations, manual intervention should never be used to correct for drift. Follow the LNAV guidance. Even in strong crosswinds, the differences in ground tracks between airplanes whose FMS employ VA (heading to an altitude termination) versus CA (course to an altitude) legs will not be sufficiently different to require manual intervention – i.e., loss of separation will not be an issue, provided the correct runway, SID and transition have been loaded into the active route. Generally speaking, the VA legs are relatively short. Crews should continue to follow the LNAV FD guidance throughout the procedure.

A typical deviation occurs when a runway change is issued after the aircraft starts taxiing. Some crews assume their FMS route is okay after making a runway change. However, the FMS actually deletes the transition route that was previously loaded. Crews should be aware that all three elements of the RNAV procedure (the runway, the SID and the transition) must be reentered into the FMS following a runway change. Pilots should then crosscheck the waypoints loaded into their aircraft's FMS with the waypoints listed on the RNAV SID chart.

“The only time manual intervention should be required is in the case of an inadvertent, gross deviation toward a parallel runway.”

New Spring-Flying Tool Available

As convective weather season approaches, it is time to look at a new product that will help mitigate Mother Nature's impact on the National Airspace System (NAS). The resource, available through the Storm Prediction Center, is called an "Aviation Impact" tool. This tool is based on the short-range ensemble forecasts (SREF), which is a set of forecasts that use several versions of the same model, but each model version is initialized with slightly different initial conditions.

The next step is to add historical airspace data, including routes through the NAS, as well as arrival and departure corridors for airports, to determine where the storms would have an impact on air traffic within the NAS.

The final product shows where storms might have an impact on traffic. So, while there may be storms in Montana, operators won't see any areas of concern shaded in, because no major airports or traffic flows would be impacted in that area.

The other extreme, of course, would be that one cell in the New York metro area that could generate a huge red area over the airports, and some heavy red spokes along the airways leading to and from the New York area.

To access the Aviation Impact tool, go to www.spc.nssl.noaa.gov/exper/sref1hr/ and click on "Most Recent Model Run" or "Previous Run." Then click on "Aviation Convective Guidance." The impact maps for all flight levels are listed in that menu. To view the different maps, move your cursor along the top numbers. Remember, because this is an experimental product, it may not have the same degree of reliability as operational National Weather Service products.

Navigator is a biannual department contributed by NBAA's General Aviation (GA) Desk. Based on the control room floor of the FAA Air Traffic Control System Command Center, GA Desk specialists act as business aviation representatives as they participate in real-time national airspace flow control and decision making. Learn more at www.nbaa.org/gadesk.

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