Thursday, June 18, 2009

RNAV departures

GPS (Global Positioning System). Those three letters have changed the way we live our lives. Twenty years ago most planes got from point A to point B by flying along airways created by ground based radios (VORs or NDBs). Ten years ago most people in the United States got from point A to point B with paper maps and coffee. Today both planes and cars use GPS to get from point A to point B as efficiently as possible.

With GPS on board we can go direct to a VOR that is 500 miles away (too far to be picked up the old fashioned way). Additionally we can go directly to a fix that is either created by the intersection of two airways, a distance from a VOR or just exist in open space. In the old days a plane would have to fly directly to one VOR then follow the airway until they arrived at a set distance or intersection of two airways to get to a fix.

With ground based navigation, the airways are 4 miles wide on each side of the centerline. The margin for error is huge! This margin is due to slight variances in the radio equipment onboard airplanes.

With GPS, airplanes are able to fly an EXACT route. This really helps move traffic in and out of busy airports.

DPs or Departure Procedures, are used to route airplanes from an airport to an airway and onto their destination. DPs have been around for years. Most are still designed around ground based navigational aids and RADAR vectors. Due to the higher workload on the controller and pilots, fewer aircraft are able to be funneled through a given space.  Increasingly many are being designed solely using GPS.

These new DPs are called a RNAV Departure Procedure (aRea NAVigation...hey I didn't make it up!). With RNAV DPs, planes can be routed to very specific routes and to a very specific space and then onto their destination.

Many RNAV DPs are designed down to the departing runway. One RNAV DP might have 3 different routes depending on which runway a plane is departing from.

When loading a RNAV DP I have to be very careful to load the correct runway. Before takeoff the runway assignment is checked (via checklist) at least three times. Why? Well look at the RNAV DP from LAX called the Oshnn Three Departure.

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Assume the airport is in a west flow. Planes taking off from the 24R will proceed to FABRA as the first fix. Planes taking off from 24L will have DLREY as the first fix. Planes taking off from 25R will have DOCKR as the first fix and planes from 25L will proceed to HIPR as the first fix. Got it?

LAX is a busy airport. They typically launch one plane from each side of the airport at the same time. Let's assume they use 24L and 25R as the departing runways. If the plane taking off from 24L accidentally loaded 25L as the departing runway, upon takeoff the plane would veer to the left toward HIPR instead of heading toward DLREY. The plane taking off from 25R would be proceeding to DOCKR. The two would likely mate if no one intervened.

This sounds far fetched...but it happens often. I have had runway assignments changed on me numerous times. Each time the Captain and I take the time to verify both the new runway and the RNAV fixes.

To combat planes having the wrong runway loaded, many airports have another hurdle for pilots to cross. Here is a new exchange that happens when a plane is cleared to takeoff.

LAX tower -"Geek flight 398 cleared for takeoff, runway 24L RNAV to DLREY"

Geek 398 - "Cleared for takeoff runway 24L, RNAV to DLREY Geek flight 298"

By confirming the first fix in the RNAV DP the tower helps the pilots make absolutely sure they correct RNAV DP is loaded and they are lined up on the correct runway.

While flying in the terminal area, the GPS routes are just .3 miles wide....1/3 of a mile. Pilots hand flying while at 250 knots can easily be off course while turning if they don't pay 100% attention.

Some pilots have been reprimanded for making turns too shallow and overshooting the course. They turn shallow for passenger comfort. Using the same DP, look at the sharp turn made when taking off from 25R between WEILR and SHAEF and SHAEF and PEVEE. RNAV DPs are designed to be flown by an autopilot. Pilots, for the most part, like to actually fly. When I fly a RNAV DP my eyes are inside focused on the flight director and the CDI needle the entire time. Even a momentary glance outside can cause me to fly off course. When I want to look outside I click on the autopilot at 600 feet and let "George" do the flying.

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