CRZ speed slowly dropping

[sagitta424 0]

Hi Captains,When flying the NGX I noticed that de CRZ speed on the FMC is slow decreasing all the time. Does anybody know why? The only thing I can come up with (from my experience as a competitor in gliding contests) that the speeds for optimal performance are higher when a plane is more heavy. (That's why a glider takes water ballast on board as long as the thermals are strong enough to compensate for the extra weight). So I could imagine that, while the plane is loosing weight while burning fuel, the optimal CRZ speed (according to the Cost Index that is flown with) would accordingly drop.Any ideas?Best regards,Herman.

[ChopperSteve 6]

Hi sorry for this long windwd reply! Hope it doesnt bore too many:)Mach schedules are the most common style of speed control in cruise. Constant Mach simply means maintaining the same Mach number for the entire Crz section.At constant flight level a constant Mach number is also a constant IAS, As fuel is burnt and weight reduces the lift required to maintain level flight at that speed also reduces.To reduce lift at constant IAS the angle of attack must also reduce. This results in less drag and so the engine RPM will need to e gradually reduced as the flight progresses.Constant Mach CRZ involves reducing both angle of attack and RPM as weight reduces, the wing Achieves best range only at one able of attack. It is not possible I maintain that angle of attack and constant speed and constant flight level as weight reduces, so constant Mach cruise involves a range of angles of attack and is not as fuel efficient as cruising at a constant angle of attack IE(long range cruise LRC)At LRC schedules this involves a constant angle of attack lose to that for best range. As weight reduces the Mach number and RPMalso reduces.

[sagitta424 0]

Thank you Stephen. So if I understand you well, it depends on the Cost Index setting whether the NG will operate in Constant Mach or Long Range Cruise mode? The first being faster, the second more economic?Herman.

[ChopperSteve 6]

Hi Herman,In the CDU you have an LRC select key that will apply got those long legs

[sagitta424 0]

Thanks Stephen, all clear!Herman.