Celestial marksmen

The Fairchild A-10 aerial sextant weighs only three pounds yet in competent hands can guide — night or day — huge bombers to any given spot on the globe

HAVE you ever gone on a ten-minute flight from an airport and gotten lost? Don't laugh, because it can and is being done all of the time. There are two extremes of aerial navigation, and admittedly getting lost on a short flight is one of them. The other extreme is a prolonged flight at night or over water when there are no radio beams available to guide the plane to its destination.

The problems involved in the first incident above are relatively simple and are easily overcome. On a contact flight around a local area, or on short country hops, a pilot keeps himself oriented by visual cognizance of well known landmarks and by constant reference to his map of the terrain over which he is flying. If his plane has a radio he may fly a radio beam, which makes the navigating problem still easier.

On long night flights when there is no beam to ride, on any extended over water hops, or when the ground or water is obscured by inclement weather, navigation is not so simple a matter to the neophyte, but to the expert navigator the problems incurred on such flights differ little from local flights. Instead of pilotage navigation — ruling a prospective course on a map and marking pilotage points, towns and other landmarks he is to watch along the way — he turns to the mathematical science of instrument navigation.

By the use of a drift meter, chronometer, and airspeed indicator, together with a knowledge of wind direction and velocity, a navigator can fly a course and estimate his ETA. By constant drift meter reading (time lapse between readings is dependent upon the wind velocity), and the resulting corrections every few minutes in his compass heading, a navigator can hit his destination and ETA on the nose, and to the minute.

Called "dead reckoning," this is a very reliable form of navigation, and sometimes in bad weather, the only form possible. However, on long flights the unavoidable accumulation of tiny errors can mean trouble if it were not for means of cross checking dead reckoning navigation so accurately that a position may be determined in the roughest air to within five miles: celestial navigation.

The sun, moon, and stars are always at the disposal of those who wish to use them. In daytime flights a sun line, moon line, and a dead reckoning track can give a navigator a fix that is (within the limits of human error) unfailingly reliable. At night the stars are the navigator's signposts in the sky.

Mariners have used the stars for hundreds of years to guide them in their wanderings. When navigators took to the air, the pioneers of the Army Air Forces realized that the science of ocean navigating would have to be altered and enlarged upon for use in airborne craft. Aerial navigation was developed and with its development there arose the need for a suitable instrument — an aerial sextant.

The aerial sextant (which is not really a sextant at all, but an octant) most widely used today was first used by Howard Hughes on his around-the-world flight in 1938. Called the A-10, this aerial sextant is the result of work on the part of two experts in air navigation: the late Colonel Thomas Thurlow and Dr. Samuel Burka. Manufactured by the Fairchild Camera & Instrument Corporation of New York, this small inexpensive, three pound instrument is now guiding 10-ton bombers to and from targets all over the world.

The various steps involved in the process of computing may sound like the highest branch of mathematics to the layman, but to the experienced navigator they become like second nature and he can go through them and have accurate directions ready to give the pilot in less than five minutes.

The information the navigator derives from the sextant is the angle of elevation of a star above the horizon. From the air, the horizon is visible only very seldom, and at high altitudes not at all. So an artificial horizon has to be provided. In the Fairchild A-10 this consists of a bubble of air floating in a glass chamber full of an alcoholic solution. It shows up in the eyepiece as a shining circle. Rays from the heavenly body enter through a rotating field prism, are reflected down into a fixed prism and from there to the eyepiece for the navigator's observation. With a control knob he moves their image until it is centered within the bubble. Then he takes his readings and starts computing.

The universe he travels in is that of Ptolemy, with the earth as the center. A geography of this universe is the "American Air Almanac," published by the US Naval Observatory, and carried along on each flight. It shows the positions of the sun, moon, planets and 50 navigation stars for every instant of the day throughout the year, relative to Greenwich, England; their SHA (sidereal hour angle, or longitude in the celestial sphere), and declination (latitude). Beneath every star there is a point on earth, the substellar point, at which the star is directly overhead. By measuring the star's angle of elevation above his horizon the navigator can locate its substellar point and a line of position: circle that uses the point as a center. His plane is located somewhere on this circle. By sighting one more star and figuring out another circle that intersects the first, he can get what is known in Army vernacular as an "approximate" position. A third star provides him with the much-desired "fixed" position. For this part of the job he uses a small collapsible desk, a library of almanacs and reference books, a chronometer set on Greenwich time to coincide with the almanacs, a Mercator plotting chart for finding the plane's latitude, and a Weems plotter for measuring angles.

One important phase of the navigator's training is getting the "feel" of his sextant. Several photos accompanying this article, taken at the Army Air Forces Navigation School, Hondo, Texas, show former cadets (now officers wearing decorations for gallantry in combat theaters throughout the world) becoming acquainted with their A-10 sextant. It is the navigator's basic tool, just as the Garand rifle is the basic weapon of the doughboy. He soon gets the habit of keeping it with him and putting in additional off-duty practice, making "shots" at the sun in the daytime and at the stars after nightfall.

Skill in the hands that hold the instrument is not, however, the entire story behind the outstanding feats of celestial navigation that are taking place daily in this war. The navigator must also have a thorough knowledge of the stars — be as familiar with them as he was with the sign-posts on his own main street. The cadet studies and makes friends with heavenly bodies which before had always seemed too remote for his attention. He becomes acquainted with stars that are never seen by people in this hemisphere — a knowledge which is necessary in the event he is sent to combat in some theater of war where these stars will be his only guideposts.

Such are the requirements in the man. The instrument, too, must play its part. In the sextant the most essential requirement is. complete accuracy — a tiny variation in an instrument may put a bomber far off course.

When the bomber arrives back at the home field she is greeted by a gang of men, the mechanics, the "grease monkeys" who will go over her from stem to stern, cleaning, lubricating, checking for and making any necessary repair. Similar attention is given the sextant. After each flight it is carefully wiped, to remove the moisture which forms on any instrument in the intense cold encountered at high altitudes. It is also lubricated and if necessary repaired. And just as the bomber gets 500-hour and 1,000-hour checkups the sextant is periodically disassembled, examined and tested. Any part: lens, prism, eyepiece assembly, bubble chamber screw or pin that shows wear or damage is repaired or replaced. Then the instrument is put back together again and the process of collimation takes place. This is a checkup to make sure that any part that has been replaced is functioning properly along with the other parts. A final test is made by taking twenty-odd observations of the sun and noting the time of each. The observed altitudes are then plotted on a sheet of paper along with computed altitudes for the same period of time. Smooth curves are drawn through the two series of points, the vertical distance between the curves indicating any necessary correction.

In such a way is this precision instrument kept functioning precisely. Instruction in maintenance of the A-10 sextant is given right at the place of its birth. At the Fairchild plant in New York, there is a school to which officers and men come from all over the country to learn about assembly, disassembly, cleaning, lubricating and collimation.

Navigators give names to the heavenly bodies: "Lana Turner," "Betty Grable," "Hedy Lamarr." But this irrepressible Yank humor doesn't detract from their determination to master the sextant and all the intricate aspects of celestial navigation and then to go and join the other navigators who, with their little gadget, are writing Tojo's horoscope on ferrying flights and bombing missions all over the world.

This article was originally published in the October, 1945, issue of Air News with Air Tech magazine, vol 9, no 4, pp 73-74.
The PDF of this article includes diagram and photos showing the instrument, its parts, repair and use.
The original was printed on 9½ by 12¾ inch paper. The photos in the PDF have been reduced to fit on letter-size paper.
Photos credited to Fairchild Camera & Instruments, USAAF.