Thursday, April 17, 2008

T for Two

Continuing on my last post about seldom used G1000 features, here are some uses for the parallel track feature. To keep things interesting, I'll show two examples with one that incorporates T-routes.

T-routes are RNAV routings that have been (or are being) developed for GPS-equipped light aircraft. Many T-routes are designed to help smaller aircraft avoid convoluted ATC routings around arrival and departure paths for large aircraft in Class B airspace. The T-routes I'll use in my example are for the navigating through the Caribbean. T-routes are meant to be used at least 1,200 feet AGL and below 18,000 MSL. Designs for T-routes for the Sacarmento and the San Francisco Bay Area regions are currently making their way through the federal rule-making process. You can read more about RNAV T-routes and Q-routes in the section 5-3-4 of the Aeronautical Information Manual.

A parallel track is simply a GPS computed track that parallels the current leg in the flight plan, either on the right side or the left side, by some number of miles. Once you have created a parallel track, it becomes the active leg and your autopilot can fly it in NAV mode just like it would the original leg. When I first saw this feature described in Garmin G1000 documentation, I thought to myself "Okay, I understand the concept, but why would I ever need it?"

On the aforementioned flight from Petaluma to Oakland, there was another aircraft that appeared near us and it was also headed to Oakland. Since the other plane was about 20 knots faster, Norcal told us to follow them to the airport. We were pretty close to the traffic and though they were 500 feet below us, I thought it a good idea to offset to the left. That's when it hit me that this was the perfect application of parallel track!

You can create a parallel track using either the PFD or MFD flight plan interface, but I'll show the MFD interface. Press the FPL button and note the active leg is shown within the magenta bracket. Press MENU, scroll with the big FMS knob, highlight the Parallel Track option, then press ENT to begin specifying a parallel track for the current leg in the flight plan.

The next dialog that appears lets you select a track that is left or right of the current leg as well as the distance to offset from the current leg. Use the large and small FMS knobs in the usual, Garminesque fashion and then press ENT to activate the track.

Once your parallel track has been activated, you'll see it displayed on the moving map. If your autopilot was engaged in NAV mode, it will intercept and track the parallel track just like it would a normal flight plan leg.

One limitation of parallel tracks is that VNAV (vertical navigation) will be disabled and you won't hear the aural alert "Vertical Track."

Another application of parallel track might be to avoid weather, terrain, or obstacles. Assume you're headed from Ft. Lauderdale Executive to Exuma, Bahamas and you've decided to file IFR, specifying one of the T-routes available in the area. From the airport, you're cleared to SKIPS intersection to join T137, so the first order of business is to put SKIPS into your flight plan.

Make sure you have highlighted the line just after SKIPS in your flight plan, then press the MENU button, select Load Airway, and press ENT. Scroll with the big FMS knob until you see T137 and press ENT.

A convenient intersection to exit T137 for Exuma is FORKK, so select that as the termination.

Now for the application of parallel track. Let's say you're cruising along and you see some nasty cumulus build-ups a few miles to the right of your current track.

You tell ATC you like to offset to the left a few miles and they approve. You create a parallel track 4 miles to the left of your current track and voila!

To delete a parallel track, just enter the flight plan, press the MENU key, then select the Cancel Parallel Track item, press ENT, and you'll be back to the flight plan leg you originally entered.

1 comment:

John said...

I have used the offset track for deviations around weather and for deviation around a TFR where an airshow was being conducted.


John Collins