Before the jet age, most cross-country routes had to make multiple stops[a], so flight planning was done in chunks and was more manageable. And as @mins commented, navigators were part of the crew.
Dedicated meteorological departments were also required; the departments shrank as networked computers took over and improvements in forecasts lessened the requirements, and eventually eliminated at most European and at several US carriers. More so in Europe when fuel prices rose (I suppose their salaries exceeded the cost-saving benefits of thoroughly and manually calculated efficient routes); in the US the situation was "ambivalent":
Airline communications were improving, longer range aircraft were becoming available and a number of countries now began providing terminal and in come cases even upper air forecasts. The requirements for dispatchers/meteorologists in the field began to lessen and as a consequence most European, as well as several U.S. carriers eliminated their meteorological staffs. As fuel costs also began to rise, all personnel including meteorologists was reduced. In the United States, the attitude was somewhat ambivalent. The approach carriers took ran from total dependence on the digital forecast with no human intervention, through little more than providing an amendment service, to manually developing a separate upper air forecast. [Steinberg. p. 2; emphasis added]
Beginning in 1962[b] (the scope of the question), computers began to take over (inc. optimum route selection) at the major operators:
American Airlines contracted with IBM to develop jointly a flight planning program for a 60,000 digit IBM 1620 computer [...] Operating cost savings are on the order of several million dollars annually. The program continuously surrounds an aircraft with a set of prognostic temperatures and winds valid by the craft's clocks at the plane's altitude. The program also selects the optimum route and the optimum altitude profile on the selected route. Flight plans on the optimum route at the optimum altitude are automatically delivered to the flight crew and the controlling dispatchers. [Kraght. p. 1;355; emphasis added]
And, by 1972 the predictive weather models in the Northern Hemisphere allowed for fully automated flight planning. [Steinberg. p. 1.]
In essence, the range and altitude leaps of the jet age coincided[c] with the beginnings of the information age, and prior to that more staffs were needed.
a: country-size dependent (:
b: 1962 also saw TWA's introduction of Doppler navigation and subsequent FAA authorization to not have a navigator on board for over-water flights
c: James Burke would find a connection though
References and further reading:
- Kraght, Peter E. "Flight Planning with a Digital Computer." Bulletin of the American Meteorological Society 44.6 (1963): 355-363. (PDF)
- Steinberg, Robert. "Airline flight planning-The weather connection." (1981). https://ntrs.nasa.gov/citations/19820040851