CHICAGO -- Amendments tacked onto an Airport Improvement Program authorization bill by the U.S. Senate could harm the credit quality of airports and may trigger delays in airport bond sales, Duff & Phelps Credit Rating Co. said this week. The amendments, which would allow airlines to appeal airport fee increases to the U.S. secretary of transportation and would place disputed revenues in an escrow account until the dispute is resolved, could reduce airports' management ability, the rating agency said on Wednesday. "At Duff & Phelps, our concern is that any change in that ability is a risk factor," said Michael Ross, a group vice president at the rating agency. Duff & Phelps is the second rating agency to issue a warning this week about the potential impact of the amendments on airports. On Tuesday, Standard & Poor's Corp. said the proposed changes could impair the ability of airports to raise fees on a timely basis -- an ability the rating agency called a key rating factor for airports.