Airlines have responded by fuel hedging - locking in the rate of fuel sometimes years in advances. However, in times of prices increases that exceeded most expectations, many carriers have found themselves very vulnerable to cost increases. As their ability to raise rates has been limited, airlines have introduced a different method to generate revenue to offset these escalating costs-namely, surcharges. Though each carrier monitors the development of fuel costs and imposes a specific surcharge amount per kilogram of cargo, which is tied to the fuel price level. Whenever the fuel price increase above or decreases below a certain threshold, the surcharge is adjusted upward or downward.