Small-craft pilots and others who rely on small planes are upset over a proposal to increase the federal fuel tax by as much as 263 percent. The proposal, directed toward corporate jets and small planes, would pay for improvements to the nation’s air-traffic control system. The current tax on general aviation jet fuel and aviation gasoline runs about 20 cents per gallon. Under the proposed changes, that would rise to 70 cents. Flight schools and charter services fear the increase will slice into their business. The plan also calls for a fee charged each time a general aviation plan takes off from or lands at one of the 30 largest U.S. airports. Those who fly the planes are afraid it eventually would be expanded to cover smaller airports. Backers of the plan, though, which includes commercial airlines, say general aviation accounts for 16 percent of the costs of operating the air-traffic control system but only pays 3 percent of the tab.