After the loss of the Avianca(?) B707 a declaration of "minimum fuel" was introduced in the USA the idea being to make this declaration BEFORE an Emergency situation arose. On declaring "Minimum Fuel" the controller is obliged to tell you whether there is any unusual delay ahead ..... if you can't accept the delay then you better shout "MAYDAY" I have used "Minimum Fuel" once when a runway was changed at SFO and I was taken out of the approach flow for re-sequencing which involved an exceptional delay / holding at low altitude. The response couldn't have been better and we were re-inserted into the approach sequence to an uneventful landing.