 In fact, the most efficient altitude for a small general aviation aircraft may be only a some 300-1000 meters (a few thousand feet) above the ground, and increasing altitude may diminish efficiency rather than improve it (propellers tend to lose efficiency in thinner air, and many small general aviation aircraft lack supercharging, effectively decreasing the engine's compression pressure, and therefore efficiency, as the aircraft climbs into more rarified atmosphere).Some pilots use "rules of thumb" for determining when to perform a step climb.These "rules" do not consider the effects of different winds at different levels; computerised flight planning systems may be better at height optimisation, and may even include 'step descents' in certain weather conditions.