Authorities investigating the missing Malaysian airliner are now focusing their attention on a search area north of where they thought the plane may have crashed. The logic, according to Australian officials, is that the plane flew faster than first suspected, causing it to run out of fuel and plunge into the ocean miles before we previously thought.

Wait, you say. If a plane is flying at a faster speed, shouldn't it cover more ground? How does this make any sense? We're going to unpack some of the science behind flight to try to understand what may have happened.

There are two operative factors here: One is fuel consumption, and the other is altitude. (There's also a tricky third matter called weight, but we won't get too deep into that now.)

How far a plane can go is determined by what the Federal Aviation Administration calls endurance. Endurance is a way of describing how planes convert fuel into energy. It's a bit like how we calculate gas mileage for cars — only in the air, environmental conditions play a much bigger role.

Maximizing an aircraft's range means having it sip fuel, not gulp it. "If maximum endurance is desired, the flight condition must provide a minimum fuel flow," according to a pilot's manual published by the FAA.

Those conditions are most easily achieved when you're in stable, level flight. Climbing or descending are energy-inefficient activities that increase the fuel flow, which is bad if you're trying to fly as far as you can. That said, just like a car, going faster requires more power — and more fuel.

But fuel consumption is also affected by altitude. The higher up you are, the less drag produced by the air -- making your engines more efficient. Simon Weselby, an engineer for Airbus, explained in a 2012 presentation that flying just below the right cruise altitude can waste an enormous amount of fuel.

This also makes sense: It would take you longer to cross a given stretch of ground if you have to traverse it rather than fly over it.

We don't know what MH370's altitude was at the time that Malaysian authorities believe it crashed. But given that investigators now think the plane was both moving faster and crashed sooner, it's reasonable to think that the aircraft was flying inefficiently.