This will actually be a somewhat comprehensive lesson on aerial navigation. In other words, how do we find our way from Point A to Point B? After all, that's why we're on an airplane.
This is a big subject and will probably take at least 3 chapters to do it justice. In this first chapter we'll look at how we did it in back in the prehistoric days before GPS.
First, the basics. The purest form of navigation is using a map (we call them charts) and looking at landmarks. We don't do a lot of this in the airline world. On days with good weather we may be cleared to fly the last 20 miles or so to the airport visually but that's about it. Most of the time we're navigating using our instruments.
He's on instruments
Navigating via instruments requires understanding a few concepts:
Heading - This is just the compass direction the nose of the plane happens to be pointing. Measured in degrees. 90 degrees is East, 270 is West, 180 is South, 360 (or 0) is North. On a perfectly calm day, our actual track across the ground is the same as our heading. Since perfectly calm days are pretty rare, we have to take wind into account.
Since the air we're flying through is probably also moving, our Course or Ground Track probably won't be the same as our heading.
In this diagram we have an aircraft heading 360 (North) but the wind is blowing from the West. Our actual track across the ground is more like Northeast. The difference between our actual track and where we want to go is called "drift". The stronger the wind, the greater its effect.
Effect of wind on ground track
So in order to fly a straight line to where we want to go, we have to point the nose of the plane somewhat into the wind. How much? Depends on how much wind there is and how fast we're going. I used to know a handy little rule of thumb for this but I've forgotten over the years.
This picture shows an aircraft "crabbing" into the wind to correct drift. In order to fly North we have to take a heading towards the Northwest to counter the westerly winds.
Heading corrected for wind drift.
This is simple enough if you can see the ground. Way back in the day some airliners had a "drift meter" which was basically a downwards pointing periscope. Back in the early days of air mail, aircraft were sometimes guided at night by following a string of fires spaced out along the route. Later they used rotating beacons, similar to a lighthouse. A legacy of this still exists to this day. Airports have a rotating beacon that alternates green-white, usually located on top of the control tower.
Obviously this only worked if you could see the ground. Navigating when the ground was obscured by clouds or fog required the use of radio signals.
The two earliest forms of radio navigation were Low Frequency Radio Range (LFR) and the Non Directional Beacon (NDB). NDBs are still around, mostly in less developed parts of the world. The Low Frequency Ranges were mercifully long gone by the time I started flying.
The LFR transmitted four beams, similar to the spokes of a wheel. If you had the station tuned and were near one side of a beam, you received a Morse Code "N" (dash-dot) in your headset. On the other side of the beam you received an "A" (dot-dash). If you were centered on the beam you heard a steady tone. You knew you crossed the station when you entered the "cone of silence" directly over the transmitter.
Low Frequency Radio Range
This allowed you to intercept the beam, fly to the station, and then fly outbound on the opposite beam. By spacing these across the country they were able to construct the first "airways". We still have airways, but we'll talk about those some more later.
Obviously there were a lot of limitations with the low frequency ranges. You could only approach or depart the station on one of four courses. Listening to a steady tone in your headset probably wasn't much fun either.
A non direction beacon is basically just an AM radio station. We have an instrument in the aircraft called an Automatic Direction Finder (ADF) that points to the station. They are still pretty common in Eastern Europe and South America. You don't see them so much in the United States but there are still a few around.
Here's what you might see in the cockpit. The green needle on the compass is the ADF and it is currently pointing to a station bearing 60 degrees. We also happen to be on a heading of 60 degrees, so we should be headed direction towards the station assuming no wind drift.
ADF Needle showing a bearing of 060 degrees to the station.
Here's a poorly drawn picture to help you visual it. If the station is 60 degrees bearing then we're somewhere on the reciprocal of that or 240 degrees from the station.
Bearing 060 heading towards the NDB.
This is all somewhat of an approximation. Why? Because the beacon is
non directional. The needle points to the station but our bearing is really only as accurate as our compass. Even if the needle isn't bouncing around (and they usually are) if our compass is accurate to two degrees, the actual bearing might be anywhere from 058 to 062 degrees. That could mean several miles left or right of our desired track, depending on how far from the station we are.
That brings up the other problem. We don't really know how far from the station we are. An NDB doesn't provide any distance information. The only way to truly "fix" your position is to dial up two different stations and plot where the bearings intersect.
Fortunately these are on their way out, at least in the US. I find the ADF receiver is more useful for dialing up AM radio stations on long flights. Not that there's much worth listening to on AM radio anymore.
Fortunately in the 1950s they came up with something better. It's called a VOR or VHF Omnidirectional Range. The VOR receiver uses the phase of the signal to tell you exactly what direction you are from the station. Better yet, a lot of them also have Distance Measuring Equipment or DME, so we also know distance from the station. Now we're able to accurately fix our position using a single instrument.
Let's take a look at a Horizontal Situation Indicator or HSI. This handy little instrument shows you where you are and where you're headed. These are so useful that even in a modern glass cockpit aircraft I can still call up a virtual HSI on my nav display.
Horizontal Situation Indicator (HSI)
What this picture is showing us is:
1. We're 107 miles from the VOR/DME station we have tuned in the receiver.
2. We're roughly 122 degrees bearing from the VOR (tail of the red arrow)
3. Our bearing to the station is 302 degrees (red arrow)
4. We'd like to fly a course of 300 degrees to the station.
5. We're just to the south of our desired course (Course Deviation Indicator)
6. We currently have a heading of about 348 degrees
7. The To/From indicator is in the top of the case so we're heading towards the station
Here's another of my crappy clip-art drawings that shows our real-world situation based on the HSI.
Here's the big picture
If we stay on this heading we will intercept the course when the Course Deviation Indicator needle centers up. At that point we'll need to turn our heading to 300 degrees and (correcting for wind drift of course) fly that course towards the station.
Actually, since we can't turn on a dime, we'll lead the turn so that we roll out just as the needle centers. How much of a lead? That takes a little math. There's a thing called the "60-1 Rule". At 60 NM from the station the degrees are 1 NM apart. At 120 NM they would be 2 NM apart. We're closer to 120 NM so I'll use the 2 NM since this is just a WAG anyways.
There's another rule of thumb that says our turn radius is our mach number - 2. So if we're flying along at .80 mach it should take us 6 miles to make a 90 degree turn.
If the radials are roughly 2 NM apart and we need to lead our turn by roughly 6 miles so we'd want to start our turn when we're 3 degrees from the desired course. Since we're already 2 degrees out so we should have started our turn already. Doh!
We can do all kinds of things with a VOR and DME. We can follow a course to or from a station. We can fly to a specific radial/DME fix. We can fly an arc around a station. We can hold at over a fix. If we put one of these at an airport we can find the airport and even fly an approach to a runway (but that's another chapter).
By spacing them across the country, we built our current system of airways. So let's take a look at one.
Jet Airway
This chart shows a segment of the Airway J78. The "J" stands for "Jet Airway" because these are at altitudes normally flown by jets (FL 180 and above).
Let's suppose we're heading east on the segment from Pocket City (PXV) to Louisville (IIU). We would fly a course of 081 degrees outbound from Pocket City until we reached a distance of 52 nautical miles from Pocket City. Since we're now halfway to Louisville we dial in the frequency for IIU. Now the chart shows a course of 264 degrees outbound from Louisville. Since we're going inbound, or towards IIU we need to fly the reciprocal course of 084.
We know we've crossed the station when the DME stops decreasing and the To/From needle flips from the top of the case to the bottom. That's it. Pretty simple really.
The entire country is crisscrossed by these airways. It gets pretty confusing up in the Northeast. Here's a section of the chart showing the Jet Airways around the New York area.
Jet Airways in the Northeastern US
Most other countries have an equivalent system. The airways in Europe are every bit as congested as the Northeastern US.
This works pretty well, although with GPS being so widespread the VOR stations will likely be taken out of service one of these days. Some of the "classic" airliners don't have GPS, so people are still out there flying from VOR to VOR.
Unfortunately there aren't any VORs floating around on bouys in the middle of the Atlantic Ocean. Crossing an ocean presents a whole different set of navigational problems which we'll talk about in another chapter.