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  • RetroTech: Lighted Airways and the Radio Range

    Posted by David Foster on April 18th, 2014 (All posts by )

    When airplanes first started to be used for serious transportation purposes, sometime after World War I, the problems involved with flight at night and in periods of low visibility became critical. Transcontinental airmail, for example, lost much of its theoretical speed advantage if the plane carrying the mail had to stop for the night. Gyroscopic flight instruments addressed the problem of controlling the airplane without outside visual references, but there remained the problem of navigation.

    An experiment in 1921 demonstrated that airmail could be successfully flown coast-to-coast, including the overnight interval, with the aid of bonfires located along the route.  The bonfires were soon displaced by a more permanent installation based on rotating beacons. The first lighted airway extended from Chicago to Cheyenne…the idea was that pilots of coast-to-coast flights could depart from either coast in early morning and reach the lighted segment before dusk.  The airway system rapidly expanded to cover much of the country–by 1933, the Federal Airway System extended to 18,000 miles of lighted airways, encompassing 1,550 rotating beacons. The million-candlepower beacons were positioned every ten miles along the airway, and in clear weather were visible for 40 miles. Red or green course lights at each beacon flashed a Morse identifier so that the pilot could definitely identify his linear position on the airway.

    Lighted airways solved the navigation problem very well on a clear night, but were of limited value in overcast weather or heavy participation. You might be able to see the beacons through thin cloud or light rain, but a thicker cloud layer, or heavy rain/snow, might leave you without navigational guidance.

    The answer was found in radio technology. The four-course radio range transmitted signals at low frequency (below the AM broadcast band) in four quadrants. In two of the quadrants, the Morse letter N (dash dot) was transmitted continuously; in the other two quadrants, there was continuous transmission of  the Morse A (dot dash.) The line where two quadrants met formed a course that a pilot could follow by listening to the signal in his headphones: if he was exactly “on the beam,” the A and the N would interlock to form a continuous tone; if he was to one side or the other, he would begin to hear the A or N code emerging.

    The radio range stations were located every 200 miles, and were overlaid on the lighted airways, the visual beacons of which continued to be maintained. The eventual extent of the radio-range airway system is shown in this map. All that was required in the airplane was a simple AM radio with the proper frequency coverage.

    The system made reliable scheduled flying a reality, but it did have some limitations. Old-time pilot Ernest Gann described one flight:

    Beyond the cockpit windows, a few inches beyond your own nose and that of your DC-2’s, lies the night. Range signals are crisp, the air smooth enough to drink the stewardess’s lukewarm coffee without fear of spilling it…Matters are so nicely in hand you might even flip through a magazine while the copilot improves his instrument proficiency…

    Suddenly you are aware the copilot is shifting unhappily in his seat. “I’ve lost the range. Nothing.”

    You deposit the Saturday Evening Post in the aluminum bin which already holds the metal logbook and skid your headphones back in place…There are no signals of any kind or the rap of distance voices from anywhere in the night below. There is only a gentle hissing in your headphones as if some wag were playing a recording of ocean waves singing on a beach.

    You reach for a switch above your head and flip on the landing lights. Suspicion confirmed. Out of the night trillions of white lines are landing toward your eyes. Snow. Apparently the finer the flakes the more effective. It has isolated you and all aboard from the nether world. The total effect suggests you might have become a passenger in Captain Nemo’s fancy submarine.

     

    Gann continued the flight via dead reckoning, aided by rough cross-bearings on commercial broadcast stations, until the range became audible again.

    The range could also be used for instrument approaches, as shown on this approach plate.  Gann describes a range approach to Newark Airport in light blowing snow.:

    The on-course signal is building very sharply. You turn the volume down to save your ears and estimate fifteen seconds to the cone…In the tight little world of professional flying there are in the whole nation less than a thousand pilots who fly instruments regularly, and fewer still who shoot 300-foot approaches in blowing snow at night. Therefore, when the peanut light glows to indicate the cone and the signal crescendos and almost immediately falls away to complete silence, it behooves you to fake a yawn…

    The range signals are now reversed with the Ns and As changed sides. As you leave the station the signal volume diminishes as rapidly as it mounted. The marker light goes out. You have allowed your speed to drop off the 105…

    Twenty seconds to go and 105 miles per hour, which is about as slow as you care to proceed until you break out underneath–if you do. Altitude 400 feet and still descending…

    You halt the altimeter at 250 feet…and start counting.

    “I have the perimeter lights.”

    You glance out ahead and immediately flip on the landing lights. Two bright lozenges appear in the snow. They seem to be racing each other. It is all over and you have only cheated by fifty feet.

    The perimeter lights slip past as you flare very slightly to make a wheel landing which is most pleasant and satisfying upon a cushion of snow. “Are we down?”

    In the 1950s, a new electronic navigation technology was introduced: the Very-High Frequency Omnidirectional Range.  Instead of the four courses of a classical radio range station, there are a theoretically-infinite number of courses to or from a VOR station. Each VOR ground station (there are about 1000 in the US) transmits an omnidirectional master signal and a highly directional signal that rotates 30 times a second: equipment in the aircraft compares the time relationship of the two signals. The pilots selects the desired course TO or FROM the station, and a pointer shows his position to the LEFT or RIGHT of that course line. The VOR system is much less sensitive to static and weather than was the radio range system, but it does suffer from line-of-sight range limitations. VOR has been the primary aeronavigation system in the US for decades, but it is expensive to operate all those ground stations, and the FAA is now selectively reducing their number, intending eventually to only maintain a limited system as backup for GPS.

    The last visual airways beacons operated by the FAA were shut down by the early 1970s; however,  there are still some beacons in western Montana which are operated by the state government.  The radio range system was largely phased out in the 1960s, but a range segment in Alaska continued to operate until 1974. See the FAA’s photo album: building the airways.

    The Ernest Gann excerpts are from his book Ernest K Gann’s Flying Circus.

     

    18 Responses to “RetroTech: Lighted Airways and the Radio Range”

    1. MikeK Says:

      Gann’s novels and his memoir, Fate is he Hunter are a wonderful history of flight in the early days.

      When I was a child, we could see the Lindbergh Beacon sweep overhead in its rotation as we lay on the beach at night in Michigan across the lake.

      A beacon named for the aviator Charles Lindbergh was added to the building in 1930. The Beacon was renamed the Palmolive after Lindbergh’s support of Nazi Germany.[4] It rotated a full 360 degrees and was intended to help guide airplanes safely to Midway Airport.[5] The beacon beamed for several decades, and ceased operation in 1981 following complaints from residents of nearby buildings.

      When I was a child there were no taller buildings to complain. We never knew it as anything but the Lindbergh Beacon.

    2. Roy Says:

      Probably fascinating only to those who fly, but, my oh my. I had no idea that the Omni ranges dated only to the 1960’s I first used them.

      I chose “highways in the sky” as the subject for one of the speeches I had to give in a college speech class. 8 minutes to explain Omni and make plain how pilots could know where they were when flying in or above the clouds. Wish I’d known some of the history the blog post above provides.

      In later years rather than fly the “victor skyways” (vectors, or paths, on the Omni ranges), I took to flying Visual Flight Rules (not in clouds) dead reckoned (use of landmarks) great circle routes with Omni as backup. Still later, as I shared the experience of nearly everyone by driving aided by GPS, I wondered how GPS would change the Federal Aviation Agency flying regulations for private pilots. Technology makes a flat screen GPS display of one’s location on a map more accurate and easier to visually interpret than Omni. Further, this system requires less weight than did the old Omni receivers and displays. I wonder how soon the blog post’s history will get supplemented with the story of the Omni stations being a matter of history?

    3. MikeK Says:

      Having navigated my way to Hawaii in 1981 with only a sextant and DR, I am envious of boats that have GPS now. But, this changes the race. In 1981, I was only certain of my position as we approached the islands within 50 miles, or so. Latitude was a bitch because in the Pacific, the night sky is always overcast with “trade wind clouds.” I could only shoot Polaris once when the cloud cover broke just before dawn. Had I been more certain of my latitude, I could have “chased” rain squalls I could see to the northwest. We didn’t and lost first overall to another faster boat by 9 minutes. He got in at 3 AM and gave us three hours of handicap time. We finished at 6:09 AM. Another squall or two might have made up the difference for us. The Transpac Race is a latitude problem as it is run about July 1 when the sun is 20 degrees north, the latitude of Hawaii.

      The Mexican races I used to compete in were longitude races where latitude is easy in the winter and we are headed south. In the Transpac, the race goes southwest, especially west. Longitude gives you your miles made good while in Mexican races latitude does the same thing. Latitude with the sun overhead is inaccurate. In Mexican races, longitude can be corrected by looking to port. The coast there is hilly and visible 30 miles out. We rarely went out of sight of land.

      I am not a pilot but have always been interested in the similarities between sailing and flying. My undergrad major was aeronautical engineering. The only good sailing movie is Wind , a movie about the America’s Cup. By the way Amazon’s search function has been modified and is now terrible. I tried to find that movie on Amazon and was unsuccessful

      Sailing also gave me an abiding interest in meteorology.

    4. MikeK Says:

      Sorry about the reversed symbol in the link. I missed it on preview.

      [fixed – J]

    5. cris Says:

      What’s that old joke? IFR stands for,” I Fly Railroads.”

    6. Bill Brandt Says:

      Cris – that was probably standard navigation to the 1920s mail plane pilots – if you can find it a great movie on that era is with Christopher Reeve and Rosanna Arquette called ‘The Aviator” – those pilots had major cojones. SOP to descend quickly though cloud was to spin the airplane.

      Another interesting old remnant of 1920s navigation were these giant concrete arrows visible from the air

      http://wchsutah.org/aviation/navigation-arrows.php

      I haven’t been flying since the 1980s – but I am shocked at the revolution in general aviation cockpits – led by Garmin. You don’t really need VORs anymore – now that GPS is affordable.

    7. David Foster Says:

      Another interesting aspect of radio range history: at the time this technology was first being rolled out, there was a competing approach in which the airplane’s position relative to the course leg would be seen by the pilot via a pointer system, rather than heard over headphones. There was disagreement between air mail pilots, who preferred the pointer system, and army pilots, who liked the audio approach. More here:

      http://ntl.bts.gov/lib/000/700/744/JAT_8-2-7.pdf

    8. Kirk Parker Says:

      Minor nit: VOR receivers work by comparing the phase of the two signals.

    9. David Foster Says:

      Kirk…true that VOR receivers measure phase. But phase and timing are two sides of the same coin. If the reference signal is 90 degrees out of phase from the course phase…as it would be if you were on the 90 degree FROM radial the 270 TO radial…then each point in the sine curve of the course signal would be delayed 1/4 of 1/30 second from the equivalent point of the sine curve of the reference signal.

    10. Mike K Says:

      I think this is how LORAN works, too. Isn’t it ? LORAN for small boats was useful in coastal sailing but useless once we got out into the ocean beyond 400 miles. It was of no use approaching Hawaii and I was told the reason was that the transmitter faced west. I know it was inaccurate and it had been quite good in coastal water.

    11. Jimmy J. Says:

      A tip toe through the tulips. In flight training (1955-56) we were required to learn how to navigate on the old radio range course using the A-N signals to determine where we were. We also had the auto direction finder or ADF for use with low frequency beacons. The Omni ranges were just coming into use, but most Navy single engine aircraft at that time had only Tactical Air Navigation (TACAN) systems. When we went cross country we had to use low frequency beacons and ADF to navigate point to point.

      In those days single engine airplanes didn’t do that much instrument or night flying because most missions required us to see the target, thus VFR was the order of the day. If the weather went bad while out on a training mission we had the TACAN and Ground Controlled Approaches (GCAs) to get us back in minimums down to ceiling 500 feet and 1/2 mile visibilities.

      As time passed, the VORs and TACANs were co-located as VORTACs, which made it much easier for us to fly cross country on instruments. We could also descend to lower minimums on approaches at civilian fields, but we didn’t have ILS.

      We became more adept at flying instruments and our equipment improved with time. So, our night and instrument flying increased. When I was instructing at NAAS Whiting Field in 1961, the only instrument approach to the field was a low frequency radio range, which had a minimum ceiling of 500 feet and 3/4 mile visibility. Practicing that approach (I made a practice approach about once a month.) paid off one day when I had a radio failure during IFR conditions and had to use the radio failure procedure to make the approach to minimums.

      By 1965 when I was flying in Vietnam we regularly went on night road reconnaissance missions. We used parachute flares to illuminate the targets and air to air TACAN to keep track of one another. Back at the ship the Carrier Controlled Approach (CCA) was the standard at night. They controlled you down to the acquisition of the meatball of the mirror landing system, which lead you down the glide slope to the deck. By the time I started flying for an airline in 1968, I was pretty experienced flying a single engine Navy aircraft (Mostly A-1s) in all kinds of instrument situations.

    12. rcocean Says:

      Charles Lindberg never “Supported Nazi Germany”. The Wikipedia entry references “Bill Bryson” as the basis for that statement- who ever the hell he is.

    13. Kirk Parker Says:

      David,

      My point is: there’s no high-precision timebase in the receiver that would be required to directly measure the time difference. Instead, the measurement of phase is being done by phase-shift networks and combining or multiplying the resultant signals, from which the equivalent time difference could be derived (though I don’t think it ever is in these instruments: the phase difference itself is used to drive the indicators.)

    14. Jonathan Says:

      SOP to descend quickly though cloud was to spin the airplane.

      Another way to do it, not as fast, was to descend while maintaining a constant heading on the magnetic compass.

    15. Gary Snodgrass Says:

      Great Article. You might remember the U.S. Coast Guard Ocean Station vessels. These patroled in tight circles in the Pacific and Atlantic providing Radio Beacons for trans-continental flights. I believe they operated up until the late seventies.

    16. David Foster Says:

      Gary…I actually did not know this…I thought once you were out of shore radiobeacon and LORAN range, all you had was celestial navigation and dead reckoning.

      I wonder how these CG vessels maintained *their* station accurately?….far too deep to anchor in most areas of the ocean, their celestial navigation views would have been inferior to those of an aircraft (although more accurate)…I’m not sure if LORAN reception was better at high or at low altitudes, but doubt that there was much difference.

    17. Kirk Parker Says:

      Gary: fascinating! I too, did not know about these.

      David: here’s a brief USCG note on the stations, which is interesting but doesn’t answer your exact question: http://www.uscg.mil/history/webcutters/rpdinsmore_oceanstations.asp

      My uneducated guess (and MikeK is the resident ocean navigator around here, I restrict my sailing to inside Puget Sound) is that it’s mostly a matter of relative speed: keeping yourself inside a 10-mile square is a lot easier at a few knots than it is at even piston-engine aircraft speeds. Combine that with a known approximate current speed and direction… even so in bad weather one wonders.

    18. Kirk Parker Says:

      Ok, y’all go read this:

      http://www.jacksjoint.com/ocean_stations-forum.htm

      Truly the bored male is a very dangerous (albeit hilarious) species.