Monday, August 23, 2010

GPS Transition, Part I

If you are a pilot who has avoided flying GPS-equipped aircraft or glass cockpits, you’re not alone. Many pilots, some who are VFR-only pilots, shy away from getting checked out in glass panel aircraft. It’s as if they take one look at a glass panel or read the first pages of a GPS pilot’s guide, get a sinking feeling, and think “This looks way too complicated.” Or they may fly a GPS-equipped aircraft and simply avoid using the GPS for anything but direct-to navigation. Take heart because learning to fly a glass panel or GPS-equipped aircraft is not only doable, these systems offer some important advantages.

A big part of the intimidation factor is that instructors, training courses, and pilot guides often focus on mastering the knob-twisting and button-pushing for a particular brand and model of GPS or worse, they start with nitty-gritty details of how all the components function. The key to learning and then mastering GPS and glass is to start with what you learned about pilotage and VOR navigation, use that as a bridge to learning RNAV concepts, and then you’ll be ready get into the details of the particular model of GPS you plan to use.

Start with the Known

As a student pilot, you undoubtably recall learning to get a weather briefing and then creating a navigation log for your cross-country flights. Most instructors still require student pilots to complete this process using pencil, paper, plotter, a navigation log form and an E6B calculator. The drill goes something like this:

  • Draw one or more course lines on your chart between your departure and destination

  • Determine the altitude(s) for direction of flight based on terrain, airspace and other considerations

  • Using the chart and a plotter, measure the True Course (TC) for each leg

  • Apply the local magnetic variation and determine the Magnetic Course (MC)

  • Determine a planned True Airspeed (TAS)

  • Using the winds aloft forecast, calculate ground speed & a wind correction angle

  • Use the wind correction angle to determine a Magnetic Heading (MH) to fly

  • Apply the compass deviations to determine a Compass Heading (I find this a bit precious)

  • Determine Top-Of-Climb and Time-Of-Descent based on aircraft performance

  • Identify checkpoints along the route and measure the distance for each leg

  • Calculate the leg time and fuel burn for each leg

  • Determine total time and fuel required for the entire trip

If this seems like a lot of work, it is. But it’s a good learning process, too. As soon as the ink is dry on their temporary airman’s certificate, most pilots quickly graduate to using an computer-based flight planner such as the ones offered by DUAT, DUATS, or AOPA. This is smart because it saves time and reduces errors. I like the flight planner that DUAT provides because it has a nice format and it calculates Top-of-Climb and Time-of-Descent, but there are plenty of choices out there.

Whether you create your navigation log by hand or by computer, you’ll refer in flight, make notes on the estimated and actual times of arrival, and update subsequent legs as you gather real-time flight information. This process of dead reckoning is not foolproof: You’re bound to discover some discrepancies in what you planned. After all, the winds aloft forecast is just a forecast.

Old School, meet New School

When navigating via VOR, you either fly to the VOR station or away from the station. Whether you are tracking a particular radial or you simply headed direct to the VOR, this sort of navigation involves basic steps:

  • Tune the frequency of the VOR station you want to use

  • Listen to the Morse code identifier and verify it is the correct station

  • Twist the OBS or HSI course pointer to set the desired radial

  • Choose an intercept heading or determine a direct-to heading

  • Fly that heading and adjust as necessary for winds aloft

When flying on Victor Airways or Jet Routes (course lines connecting VOR stations on the ground), you generally fly away from one VOR and then switch to flying to the next VOR. The point on an airway where you switch from using one VOR to another VOR is called a changeover point.

GPS navigation is sometimes called To-To navigation, not after the little Cairn terrier in the Wizard of Oz, but because you’re always navigation to something called the current waypoint. A waypoint can be anything contained in the GPS database (an airport, VOR, NDB, intersection, computer navigation fix, or user waypoint). At its most basic, GPS navigation involves these basic steps.

  • Press the Direct-to button

  • Enter the name of the waypoint you want to navigate to

  • Confirm you’ve entered the correct waypoint

  • The GPS will display a desired track (DTK) to the waypoint

  • Twist the OBS or HSI course pointer to the desired track (DTK)

  • Fly a heading that corresponds to the DTK and adjust as necessary for the winds aloft

Some of the differences are that with GPS don’t have to listen to Morse code and you don’t have to figure out the correct course since the GPS tells you the desired track. With GPS, pilots can also fly on a desired track that is a course line between waypoints entered in a GPS flight plan. You can even enter waypoints that define a Victor or Jet airway, but more on that later.

Move toward the Unknown

As you apply old school flight planning to GPS navigation, you’ll need to correlate the old school terms to some high-tech terms. Below is an elegant figure from an old Garmin manual that concisely illustrates some core GPS navigation concepts.

Current track (TRK) is essentially the aircraft’s current, real-time magnetic course across the ground. It’s easy to appreciate that calculating TRK by hand in real-time would require significant time and effort. That’s why airliners of yore had dedicated navigators as part of their crew. With GPS, the TRK is automatically calculated, displayed on your GPS screen, and updated in real-time for free. That’s pretty cool.

Desired track (DTK) is either the magnetic course between the current waypoint and the previous waypoint in your GPS flight plan or, if you pressed the direct-to key, it is the magnetic course from your current position to the current waypoint. GPS receivers with a moving map usually show the desired track as a magenta line. Glass panel systems with an electronic horizontal situation indicator (HSI) will automatically set the course pointer to the the DTK. Without an electronic HSI, you will need set the course pointer or OBS to the DTK, just like you would do when tracking a VOR radial.

Track angle error (TKE) is not used by most pilots, but it is the difference between the desired track (DTK) and the current track (TRK). If the DTK and the TRK both read 060˚, you are either on the magenta line or you are paralleling that line. If DTK and TRK differ, TKE can tell you if you are diverging from or converging to intercept the DTK.

Cross-track distance (XTK) tells you the lateral offset (usually in nautical miles) between your current position and the DTK. When XTK is zero or near zero, you’re on the DTK.

A full-scale deflection of the CDI or HSI needle can mean different things, depending on the current GPS course sensitivity. The three sensitivities are ENR (enroute), TERM (terminal), and APR (approach). A full scale needle deflection means the XTK is 5 nautical miles when in ENR mode, 1 mile in TERM, and 0.3 miles in APR (again, there are other possible approach sensitivities we're leaving out for now).

So what about updating the old school navigation log and dead reckoning? The GPS will continuously calculate the time and distance to the current waypoint, leaving you time to focus on other tasks like looking for traffic, evaluating the weather, and talking with ATC. If you still want to create a navigation log and update it in flight, by all means do so. Dead reckoning is a skill and if you don’t use it, you’ll lose it.

What about the magnetic heading and the wind correction angle? You don't really need them to stay on course because the GPS calculates the DTK and TRK in real-time. As for winds aloft corrections, you simply fly a magnetic heading that will keep you on the DTK. Many glass panel aircraft will even calculate and display the winds aloft for you in real-time.

What about the leg time and fuel consumption? GPS and glass panel systems that are aware of your fuel consumption can display the estimated time en route for each leg as well as the estimated fuel burn for each leg, too.

While all of this information is waay cool, it’s important to remember that having GPS on board won't keep you from running low on fuel, flying into restricted airspace, crashing into a mountainside, or flying into a thunderstorm. You still have to use your little gray cells, otherwise flying wouldn't be much of a challenge, would it?

How you go about accessing the above-mentioned GPS information on a particular brand of GPS will vary, but rest assure that this information is available in virtually all aviation GPS receivers. There really needs to be more discussion of these basic concepts in the FAA's Instrument Flying Handbook and in all GPS pilot guides.

Some other important and often-overlooked GPS concepts that are common across different models of GPS receivers are Waypoint Sequencing, Turn Anticipation and GPS flight plans.  We’ll examine at these in GPS Transition, Part II, so stay tuned.


Dave said...

John, how does the G1000 (for example) calculate winds aloft?

John Ewing said...


To calculate winds aloft, one needs the following information:

Ground Speed
True Airspeed
Magnetic Heading
Magnetic Course

The G1000 knows Ground Speed from the GPS, True Airspeed from the Air Data Computer, Magnetic Heading from the magnetometer, and Magnetic Course (or TRK) from the GPS. Of course for the calculated winds aloft to be accurate, one needs to be in stabilized flight for several seconds (at least).