Can I change waypoint speed without also changing the predicted altitude?

[tfm 169]

Here's a little quirk that's niggled at me for a while:Suppose a waypoint on the LEGS page shows that it will be crossed at "240/3400", meaning 240 knots at 3400 feet. If I want to cross the waypoint at 220 knots instead, I should be able to enter "220/" into the scratchpad and then line select the "240/3400" entry in the LEGS page of the FMC. This ought to substitute 220 knots for the existing 240 knots. It is a helpful little shortcut.But with the PMDG 747 (FS9 and FSX versions), this only works if the existing entry of "240/3400" is a "hard" entry (in large type on the FMC), as distinct from a predicted entry. If the waypoint speed I want to change is predicted rather than hard, I must enter BOTH the speed and a hard alititude: ie, in my example, I must make the full entry of "220/3400" instead of just "220/".The same applies (mutatis mutandis) to entering hard altitudes where the existing speed is merely predicted.This is different from other implementations of the Honeywell FMC, including the Level-D 767X. Is it a known limitation of the PMDG 747? Or does the FMC on a real 747 actually work in this way?Thanks,Tim

[Pod562 0]

The same issue has occasionally arisen for me so I would also be interested to read any feedback on your question.Renzo

[DLP084 5]

Hi, RTFM: Page 350: "Speed constraints must always be entered in association with in altitude constraint." Jan-Paul

[tfm 169]

RTFQ. Is this a known limitation of the PMDG implementation? Or is it true to life?Tim

[Tabs 2,790]

It's that way on the real aircraft.

[zubart 5]

You should be able to just change or add a Speed Restriction just like you can in a real NG's FMC. The 744 I'm not sure but probley is the same as the NG's.BTW, All speed retrictions are considered by the FMC as at or below restriction.Good luck,John Floyd

[tfm 169]

Many thanks: that's all I wanted to know. It seems strangely backward compared with the FMCs on other aeroplanes, but perhaps there is a safety advantage. Tim

[jordanal 70]

The first hard-entry must contain both the speed and altitude (220/3400) according the manuals. After that, you can change just the speed or the altitude of that hard-entry 220/ or /3400 seperately. At least that's the way I do it.