moran? >> chairwoman, thank you very much. ms. gilligan, the faa's 2004 sport pilot rule allows private pilots to fly small aircraft without a third class medical certificate under certain safety restrictions. the issue i want to explore is altitude. is there a clearly defined safety benefit for one altitude over another? i think theory is that pilots maybe can fly lower than the 18,000 feet, the floor for class a airspace. what should we know about safety based upon the ability to fly at various altitude levels? >> well, senator, i think there are a couple elements to that. i'm sure mr. baker can expand upon it as well. the one you point out is that the structure within the airspace tends to separate aircraft based on the sort of technical abilities of the aircraft. lower airspace is not as conducive to jet operation, for example, or to commercial operations. higher altitudes allow them to be more efficient. so there are some elements of the airspace design that suggest separation of operations by altitude. >> my question is, can