As a means of increasing efficiency by not having to carry unnecessarily extra fuel, plus the fuel required to carry the weight of that extra fuel, airlines have developed a scheme to reduce the required fuel without jeopardizing safety.
This is a bit simplified, but the regulations require an airliner to be dispatched with enough fuel to fly from the departure airport to the destination + 10% of that + continue to the alternate airport + 30 minutes.
One way to reduce the amount of fuel required is to dispatch the flight to an airport that is different and closer than the actual, intended airport, then put 10% of that fuel, or about 5–9% of what was required in the original formula, not 10%. The focus here is reducing the 10%. All other extras (alternate, 30 minutes, contingency) remain unadjusted.
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So, for example, let’s say we’re flying a 747 from Bahrain to Hong Kong. The flight time is 6:40 and the 10% part of the fuel required is 400 minutes, or 5300 kg. Instead, we are dispatched to Bangkok (with no intention of actually going to Bangkok) with an enroute, planned redispatch to Hong Kong at what’s called a Point of Redispatch (POR). In our example, the POR is RAJ- Rajshahi VOR at 3:40 enroute. We only have to carry an extra 10% of the fuel required to go to from the POR (RAJ) to Hong Kong (plus the other extras), not 10% of what’s required all the way from Bahrain to Hong Kong. In this example, we’re carrying an extra 2300 kg extra, not 5300 kg extra, but we’re still landing with over 14000 kg onboard. If we hadn’t Redispatched then we would have burned about 1000 kg more fuel just to carry the unnecessarily extra 3000 kg of fuel.
Within 2 hours of the designated POR (RAJ), assuming the flight has the fuel required (110% of the fuel required from the POR (RAJ) to Hong Hong + alternate + 30 minutes), the airline’s Dispatcher will send a Redispatch Message to the flight. The Flightcrew confirms that they will have at least the minimum fuel required when they pass the POR (RAJ) and the Captain accepts the Redispatch. If they won’t have enough fuel remaining then the Captain rejects it and, if no other options are available, they divert to the airport that was in the original Dispatch and the Flightcrew has a really amazing layover in Bangkok instead of Hong Kong. I have never had to divert for this but I’ve come close a couple of times.
At first, this might appear unsafe. The regulations required 10% extra, right? Why should they be allowed to have less? Think of it this way: It’s identical to how much fuel they’d have loaded if they’d originally taken off at the POR (RAJ) to Hong Kong and the whole flight from Bahrain to the POR (RAJ) was just another, earlier flight.
Keep in mind that this is all invisible to everyone except the airline and the Flightcrew. As far as ATC and everyone else is concerned, the flight is and always was going to Hong Kong, except the Earth and the airline’s shareholders are a little happier, Exxon not so much.