Source: Airlines for America & BTS. Both domestic and international flights. Load factor is the number of passenger miles as a proportion of available seat miles.
The elaboration and consolidation of hub-and-spoke systems combined with fiercer competition have been among the factors propelling load factors (the percentage of seats filled) higher, both in the United States and around the world. While the average load factor was around 65% in the 1990s, better scheduling and yield management have enabled American airlines to substantially increase the load factor of their fleets above the 80% landmark by 2010 (this threshold was reached at the global level in 2013). In light of increasing competition, airlines have been able to improve the utilization level of their assets. This has also involved reducing capacity on some segments of the network with the dual intent of increasing fares as well as asset utilization.
However, while high load factors imply higher profit levels for airlines, they also involve lower levels of flexibility to accommodate demand fluctuations and disruptions. For instance, if a flight is canceled, an airline may have difficulties rebooking passengers on alternative flights within a reasonable timeframe since there are few extra seats available. It may therefore be difficult to have load factors above 85% since it could create a context where small disruptions (a few canceled flights) could have substantial system-wide impacts. This appears to follow the rule in transportation that once an asset (vehicle, terminal) has reached a utilization level of 80%, this level is close to optimal use of this asset.