jonny dangerous
20th Jan 2006, 21:24
I've recently observed an FMC charcteristic of the 737NG and was wondering if the 320 might do the same. I was typed on the 310/320/330, but that ended 4 years ago and can't remember such a minor thing...
SITUATION
While cruising at FL350, at mid range weights, with a TW component of let's say 40 knots, the Cruise Mach is 0.75 (ish).
Plug in a new cruise altitude of FL370 and execute that, and the airplane wants to accelerate to Mach 0.78 (ish) and then begin climbing. That's what it would do in VNAV(CLB for Airbus guys). Once levelled off at 370, the airplane then decelerates to 0.76 (it's new cruise mach at 370, with the winds etc).
NOTE: The original climb mach to 350, was at the planned cruise mach of 0.75. If the cruise altitude had been 370, I'm sure the planned cruise mach would have been 0.76, and the climb mach would have been the same. Now that I'm already at 350 and want to climb to 370, the "box" thinks I should accelerate first to .78 and then go up.
Question for the Airbus guys: Does the FMS do the same thing in that situation. I won't expect answers anytime soon, as it's the type of thing that you have to be there to see.
I don't really mind the aircraft doing this, but once or twice I've decided to use LVL CHG (Open CLB for Airbus) and incrementally increase the climb mach as required to end up at the new cruise mach. i.e just add power and go...the results (time to altitude, distance in climb, etc) seemed favourable.
I've looked at it from the buffet margin angle, from the Best Rate angle (no pun!) and can't seem to rationalize it.
Anyway, it's winter, it's cold where I am (a non descript Holiday Inn) and I have a few hours on my hand.
Hope that was clear...
Charlie Charlie
SITUATION
While cruising at FL350, at mid range weights, with a TW component of let's say 40 knots, the Cruise Mach is 0.75 (ish).
Plug in a new cruise altitude of FL370 and execute that, and the airplane wants to accelerate to Mach 0.78 (ish) and then begin climbing. That's what it would do in VNAV(CLB for Airbus guys). Once levelled off at 370, the airplane then decelerates to 0.76 (it's new cruise mach at 370, with the winds etc).
NOTE: The original climb mach to 350, was at the planned cruise mach of 0.75. If the cruise altitude had been 370, I'm sure the planned cruise mach would have been 0.76, and the climb mach would have been the same. Now that I'm already at 350 and want to climb to 370, the "box" thinks I should accelerate first to .78 and then go up.
Question for the Airbus guys: Does the FMS do the same thing in that situation. I won't expect answers anytime soon, as it's the type of thing that you have to be there to see.
I don't really mind the aircraft doing this, but once or twice I've decided to use LVL CHG (Open CLB for Airbus) and incrementally increase the climb mach as required to end up at the new cruise mach. i.e just add power and go...the results (time to altitude, distance in climb, etc) seemed favourable.
I've looked at it from the buffet margin angle, from the Best Rate angle (no pun!) and can't seem to rationalize it.
Anyway, it's winter, it's cold where I am (a non descript Holiday Inn) and I have a few hours on my hand.
Hope that was clear...
Charlie Charlie