I have had some questions from some users of the research on this site, particularly from the Major Airline Pilot forecasts comparison page. On this page users will notice a definite difference between Spirit Airlines Seniority progression verse some of the others. The following is an explanation.
Spirit is projected to experience more seniority movement than any of the carriers. This is a result of Spirits size and growth plans. When an airline is small, one additional airplane represents a greater percent increase in fleet size. So in other words an airline that only has 50 airplanes could feasibly double in size to 100 without much problem. If an airline doubles in size in say three years then the pilots who are on the front of that wave can expect a three year upgrade.
A major which has over 700-800 airplanes may experience a growth of an additional 50 airplanes but this represents a much smaller movement in Seniority for those pilots because of the total size. Also, because of that starting size of 700-800 airplanes the chances of the Major doubling in size is pretty small.
From a seniority progression standpoint growth is almost always better than attrition. For example even though Delta may need 600 pilots/year in the future due to attrition it represents only a small fraction of the total pilots thus resulting in a smaller increase in Seniority. Airlines that are in the growth phase likely will see a doubling and tripling in size over a timeline resulting in huge Seniority gains. For example when the regional's doubled in size in the early 2000's they experienced 2 year upgrades because they were doubling in size every 2-3 years!