AOPA will be closing at 2:30 p.m. EDT, August 29th, in observance of the Labor Day Holiday. We will reopen on 8:30 a.m. EDT, Tuesday, September 2nd.
May 1, 2010
By Barry Schiff
My last international flight for TWA was in 1998 and involved navigating across large chunks of ocean by following a magenta line on a moving-map display from coast-out to coast-in. It was little different than the way we now use GPS, a matter of flying direct from one waypoint to the next. Navigating, frankly, had become a relatively boring chore, a far cry from the challenges of finding one’s way in the days before inertial and GPS navigation.
I began flying for TWA not long after the last of our professional (celestial) navigators had been retired. Our intercontinental Boeing 707s, however, still had a sextant port (opened by a valve in the cockpit ceiling) through which the navigator inserted his periscopic sextant to measure the altitudes (angles above the horizon) of his twinkling compatriots of the night sky. As a new co-pilot, however, I was unaware of this obsolescent sextant port.
On my first Atlantic crossing, a red-eye from New York to Rome, we were south of Greenland when the flight engineer returned to the cockpit from a visit to the forward lavatory. Without my being aware of it, he reached up to the cockpit ceiling and pulled the valve open. The roaring sound of air shooting out of the aircraft bolted me from my reverie and I grabbed the emergency checklist, waiting with nervous anxiety and an increased pulse rate for the captain to have me begin reading the Cabin Pressure Loss Checklist. But instead, he and the engineer broke into a fit of hilarious laughter. I had been initiated to international operations. (Air loss through the valve is minimal and does not affect cabin pressure, but it sure is loud.)
Guidance following the era of celestial navigation was provided by Doppler navigation, the first totally self-contained system that required neither a view of the sky nor the reception of distant radio signals. As its name implies, it was based on the Doppler Effect, a change in the frequency of a sound or radio wave for an observer moving relative to the source of the wave. You’ve undoubtedly noticed the change in the sound frequency of a train whistle as the train approaches, passes, and moves away. The same happened to radar waves transmitted fore and aft, and right and left of an airplane, by a Doppler navigation system. Groundspeed was computed from the change in frequency of the returning fore-and-aft radar waves, and drift (sideways speed) was determined by the change in frequency of the returning right and left radar waves. The Doppler computer used groundspeed and drift to compute and display to pilots the distance to go to the next waypoint, as well as lateral offset from the selected course. It was quite simple.
As wondrous as Doppler was when introduced, it was not perfect. Its accuracy depended on the state of the sea from which the radar waves were reflected. Airline crews had to have some method of obtaining en route fixes to check on Doppler accuracy. Such fixes often required us to update the Doppler, which meant that we had to make corrections to “distance to go” and “miles of lateral offset.”
The only way to obtain a fix without celestial navigation was with loran A, a system developed during the latter stages of World War II. It was much more difficult to use than the automated loran C system used by general aviation years later.
Using loran A involved tuning a high-frequency receiver to a pair of synchronized loran stations as far away as 800 miles during the day or 1,600 miles at night. The pilot studied an oscilloscope and used the gobs of knobs on a console to isolate and superimpose a pair of dancing waves that appeared on the scope. It seemed more art than science and enabled a pilot to establish a single line of position (LOP). He then quickly tuned in another pair of stations and fiddled with the knobs to obtain a second LOP. Aircraft position was where the LOPs crossed. Fixes generally were accurate to within 10 miles. A third LOP could decrease position error to three miles.
Lines of position were pre-printed on Aircraft Position Charts. They could not be plotted by the pilot because they were hyperbolically curved. Different colors represented LOPs from different station pairs. As a result, a loran chart was a confused, kaleidoscopic spider web of arcing lines. Just imagine a map of the western U.S. with all radials (in 10-degree increments, for example) from all VOR stations printed in different colors.
Navigating in those days was fascinating and challenging.
A flight engineer once told me his favorite story from when international crews flying the Lockheed Constellation included professional navigators. The navigator would plot an occasional fix and then hand the captain a slip of paper with the desired heading correction. On one particular flight, “Magellan” handed the captain a note that read, “Turn right one degree.”
This captain spun in his seat and barked, “I can’t fly this [expletive deleted] airplane within one degree. Don’t bother me with any heading changes unless you have something significant!”
Ten minutes later, the navigator handed the captain a note that read, “Turn right 10 degrees.”
“That’s more like it,” the captain said.
Ten minutes after that, the navigator handed him a note that read, “Turn left nine degrees,” a classic example of one-upmanship.
Barry Schiff was inducted into the Aviation Hall of Fame of New Jersey in 2002. Visit the author’s Web site.
FAA Information and Services,
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Nevada’s governor is being asked to add funding to the budget for the state aviation trust fund.
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