Dynamite D:
Most of our fellow ppruners have answered your questions. If I may I would like to add...
How is range increased in a headwind ?
Take the worst case, one in which the headwind is equal to cruise speed. The only way to increase range would be to increase speed. Therefore, flying at an increased speed in a headwind improves range.
Which is higher Vmcg or Vmca on a B747 ?
Vmca is always higher that Vmcg and this fact is not airplane related. Actually the only factor which affects Vmca and Vmcg is Air Density and no other variable. Example: In High Air Density the engine produces higher thrust therefore the loss of a critical engine causes more asymmetry requiring higher speeds to maintain control. Therefore both speeds are lower in Low Density Air and vice versa.
What happens to stall speed at high altitudes?
If you have had the chance to see any jet QRH performance page indicating Vref speeds for different flap settings, you will notice that no correction is provided for altitude. Vref is determined from 1.3 Vso and the absence of any altitude correction implies that INDICATED stall speed is not affected by altitude. For every AOA there is a corresponding IAS. In addition, the coefficient of Lift does not vary with altitude for the same IAS. Since we as pilots have only the IAS Indicator, Stick Shaker and maybe the AOA sensor, the calculation of TAS Stall speed is not critical. However, you should know that TAS Stall speed will vary with altitude.
If these questions are in prep for an interview good luck.
Chryse