If you're talking SWA, I've jump seated in the cockpit enough to know the inability to maintain VNAV path is self induced. When the FMS says "drag required" you have to increase drag. The SWA rule against speedbreaks with any flaps makes that hard to do unless you want to start throwing your landing gear out.
Not exactly. (And I read your later post on the subject as well)
Sorry, Jim, but the inability to maintain VNAV path almost always has nothing to do with drag. It's more to do with the VNAV software we use, and not keeping up with it, or failing to program it properly (incl. descent forecast). However, it happens fairly often even with proper programming. And 98% of the time, inability to maintain VNAV path occurs during 'idle descent' from altitude, and
not while in 'geometric descent' in the terminal area, so flaps are
not a player here. In 'idle descent,' it will revert to VNAV speed, and you will generally get high on path when it attempts to capture the desired speed first.
If, however, you capture VNAV path in 'geometric descent,' the speed range opens, and vertical path is maintained automatically
without regard to airspeed (short of overspeeding the aircraft). The "drag required" message may appear if you're above desired speed, but you
will stay on the path; you'll just be fast.
Some observations on your stated 737 theory/techniques, Jim:
-- "Drag required" messages leading to losing VNAV path occur during 'idle descent' from altitude, so using flaps is inappropriate in this regime in the first place. Speedbrakes
may be used, or better yet, simply increase the descent speed in FMS or with 'Speed intervention' to regain the path without leaving VNAV or shaking the plane.
-- "Drag required" messages during 'geometric descent' with a captured speed range indicate excessive
speed, not an impending departure from the vertical path. Speed brakes
may be used if you
have to slow down (assigned speed, etc.). However, if somewhat higher speed is not a problem, don't worry about it; you've already paid for that energy, you might as well use it rather than bleeding it off with speedbrakes.
-- using flaps and speedbrakes together puts more stress on the airframe, and causes a pretty good buffeting for your passengers. If you understand the relationship between speed and vertical path during the two types of VNAV descents, than you should be able to avoid using speedbrakes the overwhelming majority of the time, and give your passengers a smoother ride.
-- blindly following "instructions" from your FMS scratch pad (i.e. pulling the speedbrake handle just because the "drag required" message appears) is probably not a good idea in the first place. Understanding the system and how it works goes a long way to making your life easier, and your flight smoother.
At any rate, Jim, I'll grant you that a fair number of our guys don't necessarily use the best techniques with VNAV. I suspect that this stems from being taught from day one to fly the plane one way, and then when SWA finally got around to embracing VNAV, the training these older guys got consisted mainly of assigned self-study. When they're out there, they often revert to the way they "used to do it." But having said that, your response of merely throwing out drag in response to a FMS message isn't always the best idea either.
Bubba
