question for pilots

[van der vart]

when controllers are doing sequencing and gave you a descent speed of 270kts, and then tells you to increase to 290kts (or even 300kts), is this a nuisance and will it mess up your plan? or is this increase of speed welcome?

And vice versa, a decrase of descent speed - welcome or nuisance?

[Intruder]

A speed increase is usually no problem; burns a bit more fuel, but saves a bit of time.

A speed decrease, especially if the airplane is already on an optimum VNAV descent profile, could cause problems. Many airplanes do not descend and decelerate well, simultaneously. Excess use of speedbrakes and early deployment of flaps and landing gear may be required, causing even more fuel burn, and possible discomfort to those in the back.

[van der vart]

what's VNAV descent profile?

[Intruder]

VNAV (Vertical NAVigation) is an automated feature of the FMS (Flight Management System) that dynamically calculates the optimum climb, cruise, and descent speeds and altitudes.

In this context, VNAV calculates a TOD (Top of Descent), taking into account all preprogrammed altitude and speed restrictions, so as to remain at cruise altitude until an idle descent at optimum speed can be made to the most restrictive point (soonest adverse altitude or speed restriction). Any unplanned speed restriction will require more fuel.