Flying “Blind”—How Do They Do It?
We were flying from Paris to Rotterdam. The weather was very bad. I prepared to make an instrument landing. When my nervous passenger-friend in the copilot’s seat heard the landing gear go down, he asked with concern, “Can you see the runway?”
“It’s down there on the left,” I calmly replied. He had no reason to be nervous.
SOME TIME later, I was spending an evening with this friend back in France when suddenly he said: “You’ve got absolutely incredible eyesight!” Noting my surprised silence, he continued: “When we landed in Rotterdam, you spotted the runway ages before I did. I couldn’t see it until just before we landed.”
“Really, my eyesight is not so exceptional,” I said. “Radio instruments, in effect, replace my eyes. Thanks to them I am able to ‘see’ the airport even when visibility is zero.”
“But how is that possible?” he asked.
It dawned on me just how mysterious an aircraft’s array of buttons and dials must seem to the average person. And no doubt many commercial flight passengers feel a twinge of panic as they see their plane descend toward a seemingly invisible runway!
“Seeing” by Radio
“First of all, remember that flying is not all done by sight. True, a pilot can and does use landmarks and maps to guide him, but he also depends on his radio. Often it’s not practical to follow a route visually. So along the way there are radio stations that beam out a special signal; each has its own particular frequency and identification code. On board, the pilot receives these signals on his ADF (Automatic Direction Finder) or radio compass.”
“How does that work?”
“The old radio compass was simply a radio receiver with a rotating loop-type antenna. This type of antenna gets the best signal when the edge of the loop points toward the transmitter. But the signal weakens when the loop faces the station. Thus a pilot could determine the direction of the transmitter.”
“Was this like swiveling a transistor radio and having the music get louder or softer?”
“Yes, it basically followed the same principle.”
“But how could you tell if the transmitter was behind the aircraft or in front of it?” my friend asked. “When you turn a radio 180 degrees, you get exactly the same reception.”
“That used to be a problem,” I admitted. “In times past we therefore had to take two consecutive measurements to get our bearings. Nowadays the ADF has an electronic circuit that solves this problem. And once you’ve tuned to a station an indicator dial on the pilot’s control panel constantly points toward the transmitter.”
“Do you mean that if your plane was heading directly toward a transmitting station, the needle would point straight ahead?”
“Exactly!” My friend was beginning to catch on.
“Well, could the pilot just tune into any land-based transmitter like an AM music station?”
“Yes. But radio stations are not always located where they are useful for navigation. So special transmitters are set up in strategic locations and at airport approaches. It’s really very simple to follow any flight path because of these beacons. Once a pilot identifies the station he’s tuned to (remember, each station has its identification code) he can, with the aid of a chart, easily ‘see’ where he is and plot his course.
“Of course,” I continued, “no system is perfect. While these beacons have a long range, even for low-flying aircraft, they do pick up interference—like lightning.” (Indeed, it could even lure an inexperienced pilot right into a thunderstorm! But this is unlikely.)
“Why don’t they use FM broadcast bands, then? They are not affected by storms, are they?”
“No, they aren’t. But the high frequencies generally used for FM transmissions have very limited range and do not work when there is an obstacle between transmitter and receiver.”
Fortunately, there are other systems. I next told my friend about the VDF (Very-High Frequency Direction Finder).
“The difference with this system,” I explained, “is that the pilot does the transmitting and the air-traffic controller at the airport receives the signal.”
“Does he have a receiver with the same sort of dial as the one in your plane cockpit?”
“He used to, but nowadays information is given by digital readout. Also, the air controller only guides the pilot when he is requested to do so. The controller does, nevertheless, regularly check the aircraft’s direction as a security measure.”
“Does this system have any drawbacks?”
“The primary disadvantage is that only a few planes can be handled at a time—and only one for landing. Also, the range is limited (about 160 miles, or 260 kilometers) and the plane must be high enough for there to be no obstacle between receiver and transmitter. So this system is used only for the airport approaches and landings.”
“Has anything been found to overcome this weakness?”
“Yes. There is the VOR (Very-High-Frequency Omnidirectional Radio Range) system. It is immune to meteorological disturbances, it is accurate and it can be used by several aircraft at the same time. In this case, the transmitters are land based. The pilot tunes into the correct station and watches the needle of his 360-degree VOR indicator. When he’s on course, the needle moves to the center. This instrument also shows the pilot if the plane is flying toward the beacon or away from it. And an OFF indication appears on the panel if the airplane is too far away or flying too low for satisfactory reception.”
“But what can the pilot do if that happens?”
“Tune in to another beacon. That’s really not difficult because each beacon has its own frequency and identity code.”
When the Landing Strip Is Invisible
“Well, I think I’ve followed you up until now,” my friend said. “But I still can’t see how these instruments could be of any use during the approach and landing if you can’t even see the runway!”
“This is where the ILS (Instrument Landing System) comes into play. The pilot tunes his receiver to the proper frequencies. He wants to receive two radio beams emanating from the airstrip. They provide an ‘electronic ramp’ leading to the runway. The ‘vertical localizer’ beam lets him line his aircraft up with the runway right on center. At the same time, a second radio beam provides the proper angle of approach, at which the pilot should descend. To utilize these beams, he watches the vertical and horizontal indicator needles and flies so as to keep them centered. (See illustration.) Thus he flies on the correct course to the runway, even if he cannot see it. And just before he approaches the runway, three small ground transmitters send out a special signal. This tells the pilot exactly how far he is from the runway, just before touchdown. Of course, while it sounds simple, strong winds can send an aircraft off course, so it takes much skill and experience to fly a plane blind.
“Well, all of this defies imagination!” said my astounded friend.
“It is amazing,” I replied. “And we have only discussed a few navigational instruments.”
My friend had much to think about, and I gave him a little more: “But I’ll tell you what really ‘defies the imagination.’ The fact that Jehovah God has already created communications systems that make man’s equipment seem crude by comparison. Bees, fish, dolphins and many birds navigate with extraordinary precision—and without the aid of any of these instruments!” (See Job 12:7-9.)—Contributed.
[Diagram/Picture on page 26]
(For fully formatted text, see publication
Localizer beam
Localizer transmitter
Runway Glide-path transmitter
Glide-path beam
Too far left
On course
Too far right