Question about Airline ops

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Question about Airline ops

Postby redwoodcyclist » Tue Jul 06, 2010 7:16 pm

Hello, I've been reading a bunch of commercial pilot blogs, of which Rand Peck's is clearly the best. In commercial ops, I notice that pilots often refer to climbing to a certain altitude, such as FL28, and they "burn off some fuel" or wait to reach a weight where they can climb higher. I was wondering if either the forum members or the podcast could address how this works and the reasons why this is an efficient way to fly.


Greg Bockelman
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Re: Question about Airline ops

Postby Greg Bockelman » Wed Jul 07, 2010 10:54 pm

Wow. No one wanted to tackle that? I will give it a shot.

For any given weight, there is a "Sweet Spot" altitude, for lack of a better description, at which an airplane is most efficient. Too low, and the engines are not as efficient as they could be, and too high, the engines have to overcome the drag of the increased angle of attack. That takes more power. Either condition equates to an increase in fuel flow.

As weight increases, either the speed of the airplane has to increase at a given angle of attack, or the angle of attack has to increase for a given speed to support that weight. Both of those scenarios increase drag on the airframe, and subsequently it takes more power to move the airplane.

Now, another factor that goes into it is a phenomenon called Coffin Corner. What happens is as weight increases, the angle of attack has to increase. There is then less margin between that angle of attack and the critical angle of attack where the airplane stalls. That is on the low speed side. On the high speed side, if an airplane is going too fast, airflow over parts of the airframe could exceed the speed of sound. When that happens, the airplane goes into what is called Mach Tuck. The nose pitches down.

Where stall speed meets Vne, that is called coffin corner and it is difficult to recover from that.

Bottom line is that an altitude is selected that gives reasonable fuel consumption AND an airspeed envelope that is wide enough to provide protection from both stall and over speed.

If that does not make sense, ask again and I will try to explain better.

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Re: Question about Airline ops

Postby champguy » Thu Jul 08, 2010 10:26 pm

Until they burn off the fuel, they are too heavy to get high enough. The flight envelope gets very narrow, only a few knots, and flight management is strictly for professionals with computers.
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