The US Federal Aviation Administration (FAA) has issued an Airworthiness Directive (AD 99-15-06) mandating changes to the ALF502 engines installed on the BAe 146. The AD requires that an improved fan core inlet anti-ice system be installed at the next shop visit, but no later than the end of 2002. The modifications are intended to solve the problem of uncommanded reduction of engine thrust (“rollback”) and loss of thrust control in icing conditions. An earlier AD (AD 96-14-09) had imposed restrictions on flights above 26,000 feet in icing conditions as a means of avoiding this problem. Although some operators advocated simply retaining this restriction, which is seldom constraining on the relatively short flights BAe 146s tend to operate, the FAA decided to mandate the more permanent solution. The modifications basically involve fitting parts developed for the later LF507 engine, which is fitted on the Avro RJ and on some late-build BAe 146-300s, and which is not subject to the rollback problem. The FAA estimates the modifications will cost US$75,000 per engine. Although the AD is only binding on US-operated 146s, other aviation authorities usually follow the FAA lead on such issues.