I Blame the Minimum Fuel Theory Guys

March 18th, 2008

Fuel planning is a complex item when it is comes to balance of commercial vs safety and relative probabilities.

Relative probabilities are laid out by the minimum quantities required by the air law.

This may sound simple – great here we have a document telling us exactly how much the minimum fuel is we legally need to take. But, there is a small trick with that statement.

The air law normally says we must take fuel for a flight from A to B, + 1 an alternated if the weather is relatively okay or two alternates when it’s not, and thereafter for approximately 45 minutes.

The air law, then attempts to ensure the blame is completely shifted from authorities, by including a small statement that says something like “the pilot shall take into account expected delays, weather, and a whole host of other things…”.

The problem the captain then has is how to determine the relative probabilities of “expected” delays. Balanced by the pressures often put on him by a company with respect to what I call the “minimum fuel theory”.

Minimum fuel theory says that every pound of extra fuel we take costs dollars, i.e make sure you have a good reason should you decide take more than required.

This is more complicated by what I call the “commercial-safety battle”. This says you may take extra fuel providing you have a good reason, but if you have to off load passengers to do it, you better have a damn good reason.

These two theories leave a captain in a situation where he now returns to weighing the balance of commercial vs safety and relative probabilities with the scale offset towards the minimum fuel.

Despite constant criticism from the more “commercial” pilots, I am a follower of the “what-if theory”. WShat if the fog does begin to move in even if it is not forecast, what if someone crashes on the intersection, what if we are approaching our alternate fuel when we arrive– any small delay means no alternate, however diversion means arrival at our alternate with absolute minimum fuel – sufficient only for an approach, arriving at the missed approach point with legal minimum reserve (approximately 30 minutes for a turbine aircraft in most countries).

What happens if, when you take off, your destination has a visual approach only but is VMC with CB’s in the vicinity. Neither weather report for your destination or your alternate requires a second alternate, and your flying time means your 10% contingency is equal to about 4 minutes. You decide to take an extra 30 minutes fuel.

As you approach you are advised your destination is closed, your alternate is still open but IMC. You divert immediately to your alternate, which is now your only optyion as you were only required to carry one. Aircraft are stacked in the hold, and ATC advises you to expect a delay. On calculation you now have about 40 minutes + reserves. Time moves on and your 40 minutes is soon past tense, you are approach the 30 minutes final reserve fuel.

Reserve of 30 minutes – still time for two approaches? You inform ATC you are declaring a fuel emergency, get moved to the front of the stack, and begin the approach, but the airfield is now fluctuating around minima. You approach the Decision Altitude, but loose sight of the airfield.

Still time for another approach… ?

b190 on road

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