History of Trans-Oceanic Navigation and Communication at Seaboard

aira62_starchart-c.jpg 1962 Air Almanac, U.S. Naval Observatory 

by Capt. Ken Kahn

When Seaboard began operations with DC-4s in 1947, it was necessary to carry navigators and radio operators on trans-oceanic flights. The radio operators communicated with distant ground stations via Morse code. In order to increase radio range, the DC-4s were was equipped with a trailing wire antenna about 50 ft long which had a weight on the end. It could be let out by the operator and reeled in before landing. In the early 1950s, high-frequency radio sets that enabled the pilots to conduct voice communications with distant ground stations became available and the radio operators were eliminated from Seaboard flight crews. The early high-frequency radios required the crew to install crystals for the specific frequencies to be used. When the first CL-44 was delivered in 1960, it came equipped with Collins Radio HF 618-T2 Transceivers. Seaboard was the first airline to operate these units which permitted selecting any frequency from the control head without the need to change crystals.

In the early days of Seaboard, a navigator reporting for duty would study the weather charts for the scheduled flight, plot the courses to be flown, and calculate the estimated time for each segment of the flight. This information was written down as the flight plan. Over the ocean, the sextant was the primary tool available to navigators. It was used with a accurate timepiece, data from three volumes of sight-reduction tables and an Air Almanac. The sextant was used to measure the altitude of celestial objects above the horizon. DC-4s had a clear astrodome over the cockpit in which the navigator stood on a small stool and used a bubble sextant to take his measurements. On pressurized aircraft, starting with the Lockheed Constellations, navigators used Kollsman D-1 periscopic sextants that were inserted into an opening in the ceiling of the cockpit. It was tricky business in any sort of turbulence and impossible in or under cloud cover, a common occurrence in unpressurized DC-4s which usually flew at 8,000 to 9,000 feet. The measurements taken by the navigator, data from the Air Almanac and from three volumes of sight-reduction tables about the celestial objects observed, together with accurate time readings, were used to determine position (latitude and longitude). The navigator could thereby monitor the progress of the flight, issue course corrections to the pilots as necessary, and plot new courses if necessary or desired. Click on the image above to see the full chart.

During its participation in the Pacific Airlift for the Korean Conflict, Seaboard developed an advanced form of pressure-pattern flying. A ground navigator studied the weather charts and selected a route that would give the fastest flight time and thereby minimum fuel consumption. During the flight, he would send updated wind information to the flight navigator who used a transparent overlay developed by Seaboard. A separate overlay for each leg showed several possible routes for each leg on either side of the great-circle route that gave the shortest distance. The overlay also listed the distance for each such route and the wind component necessary to compensate for the additional distance. The flight navigator would place the overlay on top of a weather map with the latest wind information and adjust the route to minimize time. In the 1960s, the flight plans were generated by a computer but the navigators' in-flight duties remained otherwise unchanged.

During World War II, LORAN (long-range navigation) was developed by the U.S. Government. It used ground-based radio systems to transmit signals processed by receivers carried by ships and aircraft. In the 1950s, military-surplus LORAN systems became available and Seaboard installed surplus RCA AN/APN-9 LORAN units at the navigator's station in DC-4s. EDO 345 LORAN systems were installed at the navigators stations in CL-44 and DC-8-55F aircraft. The last LORAN model installed in Seaboard aircraft was the EDO 600T. An EDO LORAN indicator can be seen at the bottom of the navigator's instrument panel in this photo showing the navigator's station at the left rear of the cockpit in N8632, a DC-8-63CF. The EDO receiver can be seen behind the transparent window of the radio rack. The system of LORAN ground stations was taken over by the U.S. Coast Guard in 1958.

During the 1960s, international airlines, including Seaboard, starting testing various electronic systems to see if they could replace navigators. Seaboard tested Doppler Radar navigation systems made by both Collins Radio and by Bendix Avionics. One such system tested by Seaboard was the Bendix Avionics DRA-12 / CPA-24 Doppler Radar navigation system. It used a radar altimeter to measure ground speed and drift angle, and it got heading information from the aircraft's fluxgate compass system. The system's computer used that information to calculate distance and direction traveled. The system lacked flexibility in that the crew had to know the distance and course to the next waypoint. One problem was that the Doppler radar could be stymied by smooth water and fail to provide current groundspeed and drift information. In addition, the cooling in the DC-8 radio racks was insufficient to keep the systems' electronic units from overheating and the Bendix system was rejected. The groundspeed/drift indicator was retained, providing useful information to navigators and pilots. The Bendix system was used successfully by some airlines, including TWA from 1962 in their Boeing 707s.

After rejecting the Doppler systems, Seaboard tested the Decca Omnitrac system, as seen in this 1967 magazine ad. Decca, a British company, used its own ground-based radio transmitters, similar to the LORAN system. The cockpit units included moving map displays that used paper maps printed on long rolls. The maps scrolled back and forth and a cursor moved across the roll to indicate the aircraft's position. It was extremely unreliable and ultimately rejected by Seaboard.

In the late 1960s airlines started testing pilot-operated Inertial Navigation Systems (INS), a technology developed for rockets. Seaboard tested INS systems manufactured by Litton Aero Products. They proved to be reliable and accurate and were the first INS units certified by the FAA. They also provided more flexibility then the other systems tried. The route could be changed at any time with no calculation required on the part of the pilots. By the end of 1970, Seaboard had installed two Litton LTN-51 systems in each of its aircraft and Seaboard's navigators were gone. Seaboard was the first airline in the world to equip its entire fleet with INS equipment.

Note: Celestial navigation dates back at least 2,300 years to Pytheas of Massalia, a Greek geographer and explorer. During his sailing voyages to northwestern Europe, he estimated the latitude by measuring the altitude of the noonday sun and of the North Star. A current version of the Air Almanac is still available in electronic form only from the U.S. Naval Observatory. It is published once per year, as opposed to three times per year for the original printed versions. The cover and first of the 908 pages of the 2015 edition can be seen here. Whereas the paper versions came with a cloth-backed white-on-black star chart of the night sky, the digital version has a black-on-white star chart.

Information about DC-4 avionics courtesy of Capt. Bill Eastwood
Information about later avionics courtesy of Andy Flood, Seaboard's Avionics Maintenance Manager
Scans of Decca ad and CL-44 Operating Manual courtesy of Capt. David O. Hill
Scans of Air Almanacs courtesy of Capt. Ken Kahn
Insight into navigation and navigation equipment courtesy of Capt. Tom Reinke
Reproduction of any material on this Web site is prohibited without prior written consent