If you have taken flight plan fuel and it all worked out as per the theory (in practice you often make a bit during the flight) YOU DON'T have 30 mins holding fuel, you have none. You must land with a 30 minutes reserve (which is calculated on how much fuel you would require to hold for 30 mins) or declare a mayday if it looks like you won't. If you elect to use your alternate flight plan fuel to hold at destination and the runway becomes blocked it just isn't your day, the basis of the regulations is that it just as likely that your alternate may become blocked as your destination. This is why one of the requirements to use alternate fuel says that the commander must take into account the traffic and operational conditions prevailing at the destination, the route to the alternate and the alternate. An example of the thinking might be:
Holding at single runway destination, short, wet runway, strong crosswind and busy. Alternate is into wind and quiet. Would I burn my alternate fuel?. No, much more chance of getting a blocked runway at destination so I would keep our alternate fuel and divert a little while before the fuel state reaches the critical point so as to be sure to land at the alternate with final reserve. On the other hand it clearly makes no sense to divert if the situation was the other way around.
If as you said you had committed to land at destination and it then became blocked you may have to set off to your alternate (assuming you still have enough fuel to get there, another thing to consider) and land with less than final reserve but you are then in a mayday situation.
Basically it is quite safe to use your alternate fuel to extend your holding at destination but you MUST consider where you are and what options you are losing by doing so, don't do it too lightly just because the rules say you can.
PS. As I am sure has been said before the number of runways makes no difference to the ability to dispense with your alternate but may well influence you decision making, I know it would mine.
Last edited by Max Angle; 12th November 2007 at 17:30.