A software flaw in Patriots fire control system is thought to be causing at least two cases of the system ignoring the IFF (Identify, Friend of Foe) transmitters on friendly warplanes. In one case, a missile was fired and downed a British Tornado aircraft, killing the two man crew. In the second incident, on March 25th, an American F-16 was also tagged as "hostile" by the Patriot fire control system. But this time, a missile was not fired. Patriot crews are on the alert for the for further occurrences of the flaw. In the past, there have been some problems with IFF equipment on some NATO aircraft, but this does not appear to be a factor here. During the 1991 Gulf War, there were instances where friendly electronic warfare aircraft, testing some of their equipment as they headed north, triggered a hostile response from nearby Patriot systems. There were no shoot downs then.