To reduce the fleet by 20 percent in 5 years means buying 20 fewer aircraft during that time, or 4 fewer each year. Instead of buying 10 per year, we buy 6 during the transition period. When the fleet reaches 80 aircraft after 5 years, we increase the annual buy to 8 to remain at the new steady state of 80. Aircraft stock assets dropped by 4 percent per year for 5 years, leading to a 20 percent reduction over the 5-year transition period, while the procurement of new aircraft abruptly fell 40 percent, from 10 to 6. Stocks are accumulations.
Flows change the stocks.
To alter a stock, the flow must change rather dramatically. Stocks and flows are essential to understanding the dynamics of system change. This demonstrates the accelerator, a crucial system principle . Changes to stocks lead to accelerated changes in flows. Assets change gradually while production swings widely.