Recent discussions on another forum have raised a few questions (as well as highlighting my poor knowledge on the above subject
)
On the 747-400, after every monthly FMC Nav Database update, I check the Manoeuvre/Maneuver Margin value on the PERF FACTORS page (ours is set to 1.30). As I understand it, the FMC uses this data for "flight envelope and bank limits calculations" and allows the FMC to draw little yellow "hockey sticks" on the Primary Flight Display above/below the Vmin/Vmax red bands on the airspeed tape.
http://members.ozemail.com.au/~b744e...vreMargins.gif
I had, until now, assumed that the yellow hockey sticks were related to Vmin and Vmax, but after discussions on the subject, now I'm not so sure. Speeds such "Va" and "
Vb" have been mentioned, but what these are and how these relate Vmax and Vmin I don't exactly know (I haven't studied aerodynamics).
Q1. On the 747-400, the bottom segment/brick of the
upper red band reflects "Vmax". This reflects either gear placard speed, flap placard speed or Vmo. The upper yellow hockey stick indicates "margin to buffet" (according to my text). Would Vmo and buffet be the same in straight and level cruise flight or do the two values have no direct relationship? What kind of buffet are they talking about here?
Q2. Regarding the lower hockey stick and the
lower red band... My notes tell me that the top red brick of the lower red band is "Vss" (Stall Warning Management Card-calculated stick shaker speed) when the flaps are down... or, when the flaps are up, the greater of Vss or "Bbflo" ("FMC-calculated initial buffet speed"). What, exactly, is Vbflo. Is it Va (the speed at which, I am told, the aircraft is designed to stall to prevent damage to the wings when the aircraft is subjected to high g's during strong updrafts) or is it another buffet value,
Vb, or something completely different?
Q3. When the aircraft banks, do the hockey sticks or the red bands change length or do they remain static. There are a lot of pitch-type inputs to the Stall Warning Management Cards, but there doesn't seem to be any roll inputs (Having said that, I realize that pitch does increase during turns, especially steep ones).
Thanks for any input.
Cheers.
Q.