Why Don't Use Ias Above Fl300?

[wingtip777]

Hi Guys! Normally We Use Mach Above Fl300 And Use Ias Below. But I Find There Are Two Benifit With Using Ias At High Level.
First When Atc Give A Descend Clearance With A Good Rate Requirment, The Thrust Will Retard Faster On Flch Mode When We Set Ias On The Window.
Second If I Need Hand Over To My F/o In Case Of An Earlly Descent When I Haven't Finished My App Brief, Then I Wondn't Worry Too Much The Airplane Will Overspeed(i Don't Mean The F\o Is Not Good .).
Does Anyone Agree With Me?

THANKS TO ANY REPLIES!

[ricky-godf]

First: not true. The airplane will accelerate during the descent if you descent in MACH, because the same mach number provides a higher IAS at lower level, until the point you change-over to indicated and then the flight path angle will decrese and stabilize.
I have no idea what aeroplane you fly, but at higher level your airspeed margin is decresed (between min and max speed, I mean) so maintaining a constant mach provides a good safety margin. Otherwise you would have to wind up the speed (IAS) constantly during the descent to keep a good rate.
Second: the aeroplane should not overspeed until you get to the point where your cruise/descent Mach equals the max IAS of the aeroplane, and this should happen at a low level (and a good autopilot should have corrected your error already).. if it takes this long to complete a briefing there is something badly managed there...

No, I don't agree with you....sorry!!

[ppppilot]

Neither do I
First.- Look the emergency descent on your SOP, how to get a good rate of descent.
Second.- Your FO will not overspeed but could fall below minimum clean speed on the descent.

[wingtip777]

Thanks To You Both!
I Agree With You Two Theoritically. But Overspeed Did Happen Few Times Around . The 777's A\p Doesn't Work Perfectlly In This Situation. But I Think Maybe Is Better To Follow The Sop And Keep Eyes On It .