Friday, August 05, 2005
Cross-Country Flight Planning
Tomorrow is my first cross-country flight, so I've spent the last couple of hours doing the planning for the trip. It's a little like doing one's income taxes, except it is more tedious and cheating doesn't help you. There are software applications that automate this task, but we student pilots are expected to do it the old-fashioned way, with pencil and paper.
You start by drawing your flight path on the chart (what most people would call a "map"). The naive thing to do would be to just draw a straight line from my home airport, Atlanta/Dekalb-Peachtree (PDK) to the destination airport, Chattanooga (CHA). The problem with taking that approach is that there is a whole lot of nothing between PDK and CHA. Without any landmarks, I wouldn't know how far along I am or how far off course I might be. I could rely on radio navigation systems, but the point of VFR cross-country training is to learn to use pilotage (comparing what you see with what's on the chart) and dead reckoning (calculating where you should be based upon heading, speed, and time). So I picked a few mountains, lakes, and airports to fly over, all spaced closely enough together that I'd be unlikely to be unable to find the next one or find my way back to the last one at any time.
One annoying thing about a trip from Atlanta to Chattanooga is that you have to fly across the edges of the chart. The northern part of the area is on one side of the paper and the southern part is on the other side, so you have to flip back and forth to see the whole flight path, and go through a few geometric contortions to measure course and distance across the edge. Luckily, I have a second copy of the chart, so I could just lay the two opposite sides next to one another.
Once you have all the "checkpoints" picked out, you draw straight lines between them. Then you fill out a large tabular form called a "navigation log," which has one row for the path between each pair of checkpoints. For each pair of checkpoints, you use the chart and the plotter (a combination ruler-protractor) to fill in the following columns:
- distance between the checkpoints, measured using the chart and the plotter, in nautical miles
- course between the checkpoints, measured in degrees, where 0/360 is due north, 90 is due east, etc.
- magnetic variation, which is the difference between true north and magnetic north, found by looking at the "isogonic lines" on the chart
- altitude to be flown, making sure that one stays above or below any obstacles or controlled airspace, and making sure that one follows the rules about which cruising altitudes are to be used based upon one's course
- determine whether there is any radio navigational aid that would help you keep the course, and if so, write down its identification and frequency
Then, for each airport to be visited, you write down all the associated radio frequencies (ATIS, tower, ground, approach, flight service) so they will be at hand when you need them.
That's all I've done tonight. Tomorrow morning, when I get updated weather forecast information, I'll have to fill in the following columns for each checkpoint:
- forecast wind speed, wind direction, and temperature at the flight altitude
- the true airspeed of the airplane, based upon pressure altitude and temperature
- the wind correction angle (WCA), true heading (TH), magnetic heading (MH), and estimated groundspeed (GS), described in more detail below
- estimated time enroute (calculated by dividing the length of the path by the estimated groundspeed)
- fuel consumption (calculated by dividing the airplane's fuel burn rate at the desired power level by the estimated time enroute)
I'll describe the calculations. Consider an example: say you are flying from your home airport to another airport that is 240 nautical miles due north, and your airplane flies at 120 knots. If there was no wind, then the calculations would be easy: you head due north, the flight will take two hours, and so you'll burn two hours' worth of fuel. However, what if there is a 30 mile wind coming from the west? You wouldn't want to point the plane due north, because the wind would blow you 60 miles to the east during the two hours in the air. Instead, you'll need to point the nose left of true north to offset the drift. If you point the nose to the left, you'll be heading into the wind a bit, so your speed over the ground (groundspeed) will decrease.
Doing a quick calculation with the E6B flight computer (you'll have to trust me that it was quick), we find that we would have to point the nose 14 degrees left of due north to maintain a true-north course. This is called the wind-correction angle (WCA). The true heading will be 346 (360 minus 14). Our groundspeed would be only 117 knots, so the trip would take two hours and three minutes instead of just two hours. That extra three minutes isn't much in this example, but if the wind was stronger or coming from more of a northerly direction, it could have a significant effect on groundspeed and fuel usage.
We've figured out the true heading (TH), but we haven't really figured out which way to point the plane yet. The magnetic north pole is not at the same place as the geographic north pole, so if we want to use a compass we have to take into account the difference between magnetic north and true north. This is called variation, and for the area between PDK and CHA, the variation is about three degrees west. So we add three degrees to our true heading to get the magnetic heading (MH), which would theoretically be the heading we want our compass to show.
But wait, that's still not it. A compass is affected by electromagnetic fields in the plane and other imperfections, so we have to factor in the compass's deviation. There is a small card attached to the compass in the plane that gives the deviation, so we can't really write down the final compass heading (CH) until we get into the plane.
So, we do all those steps for every checkpoint in the navigation log. I've got six checkpoints for my trip up to CHA, and six different points for the return trip, where we'll do a touch-and-go at Rome (RMG) to make it a three-leg cross-country trip. In addition to these checkpoints, I also have to calculate the "top-of-climb" and "top-of-descent" points for each leg, which are the points where I'll reach my desired cruise altitudes and start my descents, respectively. I have a total of nineteen rows in my navigation log.
After you fill in all the rows, then you add things up to get the total distance, total estimated time enroute, and total fuel consumption. If total fuel consumption is greater than total fuel capacity, then you have a problem.
When I've finished all these calculations, we'll get in the plane and know exactly what headings to fly and how fast we'll get to the destinations, right? Wrong. The calculations are all based on forecast winds, which won't match the real winds, and on ideal aircraft performance. So while flying, I'll be keeping track of what time we reach each checkpoint, and calculate the actual groundspeeds. If we get blown off course I'll have to adjust my wind-correction angles accordingly.
After my trip, I'll post the nav log to give everyone a better idea of what it looks like. Tonight, I need to get some sleep.
Some World War II General (MacArthur or Patton?) said something like "Battles are always fought at the corners of maps" (Although that's not the exact quote, according to Google).
So your experience is not unique.
I used to be an Air Force navigator, where I learned the difference between a chart and a map.
A chart is a mathematical projection of the earth's surface. On a chart, distances and headings can be measured accurately (within the rules of the chart's projection algorthium).
A "map" is a schematic representation of the relationship between objects. Heading and distance measurements are only promised to be approximations on a map.