It may well have been a status thing, you are not a real airline in the 70s unless you operated 747s. Some carriers such as Delta & National replaced their 747s with trijets when they became available but often phasing out a very few 747s (two in the case of National) and acquiring a much larger fleet of trijets. Also bear in mind that the 747 had claimed direct operating costs 30% below that of the 707 (the actual saving was less once you took passenger service and other overhead costs which didn't reduce as much) so a business case could have been made. In summer 1969 they operated about 6 round trips per day and growth rates were around 15% - and expected to continue at that rate.
I don't know if Aer Lingus was part of ATLAS or KSSU, they certainly did a lot of third party maintenance work but it may have been that the the small fleet size was not such a problem. I think that in the off season in the 80s they would typically operate a daily 747 to JFK & weekly to BOS with maybe the odd charter flight to places such as the Canary islands. I doubt if the load factor was high but that was an accepted part of operating an airline back then. (Deregulation and the twin jets changed that.) They would doubtless had made money leasing out the third 747 at quiet times for Haj traffic and other purposes. Its interesting how much off season traffic trans Atlantic traffic from Ireland has grown in recent years, doubtless a combination of the economic growth of Ireland and establishment of Dublin as a hub with the ending of the rule that all aircraft had to stop at Shannon.
Last edited by Peter47; 23rd Oct 2020 at 11:33.