View Single Post
Old 19th May 2008, 18:26
  #34 (permalink)  
Join Date: Oct 2005
Location: USA
Posts: 3,218
Guppy, you might be right in principle, however the regulations governing GA and airlines are different, at least if mentioned GA aviation is not working under an AOC and of course everything just applicable in europe (JAR), dunno enough about the rules in other parts to comment about that.
This is because you're considering the wrong regulations. Operational regulations are irrelevant. What you need to consider are certification regulations. A two engine transport category airplane, be it a Lear 60 or a B737, must still meet the same performance standards and design criteria with respect to minimum gauranteed performance, and they're still flown the same...especially with respect to takeoff and landing. Transport category aircraft are transport category aircraft, big or small.

Now there are certain things you can do, and get away with in small, light transpor category aircraft that you can't in big airplanes; I've taken off in a Lear 25 and turned downwind at 18,000'. Not something you'll do in a heavily loaded B747. However, each must still meet the same minimum performance criteria. And the training for each is very similiar. Moreover, approved training programs for either one don't teach to stop after V1, and do provide for reduced thrust operations.

But surprised me with this:

"It Increases fuel burn.
Strange, but true. This is because:

1. Assuming an uninterrupted climb, it will take longer to reach the more economical cruise altitude than a full thrust climb.
2. Engines are less efficient when not at full thrust. "
During a reduced thrust takeoff, very often when climb thrust is set after takeoff, it may be to increase thrust, rather than reduce it. During a standard noise abatement departure, climb trust is a thrust reduction. However, during a reduced thrust takeoff, setting climb thrust may mean increasing it to the climb thrust setting.

Turbine engines are most efficient at a high power setting. You can imagine the loss of efficiency at lower power settings somewhat like driving a car up a hill in the wrong gear; inefficient. Part of the reason that a jet engine is most efficient at high altitude is that the powerplant is required to run at a more efficient RPM. At lower altitudes, excepting climb, the power is pulled back into a less efficient power range.

The problem as you can guess is that at low altitudes, airspeed limitations prevent pushing the power up too far. At altitude, the power can be pushed up into an efficient range without exceeding airspeed limitations. At altitude in most turbojet airplanes, you run out of available power before you exceed airspeed limitations...meaning you can cruise in an efficient range. At low altitudes, reducing power even further decreases efficiency...which is part of the reason (performance being the other) that power is often increased after a reduced power takeoff.

Our typical profile includes a departure at reduced thrust, with climb thrust set at 1,000' above the departure elevation. At 10,000' we'll reset the climb thrust, often to a reduced climb by a given percentage; we use .04 EPR if our weight is more than 600,000 lbs, and .06 epr if weight is below that value. This isn't a great reduction, but it's also another place we reduce power slightly below maximum climb values for engine longevity. Around 24,000' we find that we need to restore the climb power to the maximum value for performance reasons, and continue up to our cruise altitude at that power setting.
SNS3Guppy is offline