A few years ago I was in the hold at Heathrow with about ten minutes to run before committing to final reserve or diverting. A BA B747 had an engine fire on finals and blocked RW 27L. Shortly afterwards a BA B757 called up stating he was '5 minutes to a fuel emergency'. The ILS was not radiating for RW27R and the B757 was offered an SRA. However . . . the cloudbase was below his minima.
For a couple of minutes there was an aircraft with insufficent fuel to divert flying round an airfield at which he could not land.
The pitch of the RT conversation rose noticeably whilst they switched on the ILS for RW27R. The holding traffic was given 'Delay not determined' which means no landing with final reserve.
JAR allows you to commit to landing at your destination. Whilst Heathrow has 2 runways it also has more traffic than any other airfield in the UK.
If operators plan on anything less than 20 minutes holding at Heathrow one day we will have a big problem. Sh*t happens despite the most cleverly crafted rules.
[This message has been edited by Caractacus (edited 07 May 2001).]