B744 Descent ECON SPD v. SEL SPD - Effect on Descent Logic

[cf6-80c2b5f]

The B747-400 manual splits up the descent logic regarding how fast (or slow) the VNAV will allow the aircraft to go in order to stay on path based on whether you have passed (below) your first speed constraint (presuming a VNAV PTH descent with the speed window closed). If you haven't passed the first speed constraint, it will allow up to VMO/MMO-11; but if you have passed your first speed constraint, it will only allow up to FMC command speed plus 15 knots.

That's easy to understand if you have a speed constraint on an arrival down low and you see it on the LEGS page next to an arrival waypoint. You know that after that point, the logic changes to limit how fast you can go to stay on the path.

But what if you decide not to descend in ECON SPD and instead enter your own speed in the descent page prior to T/D? ECON SPD is immediately replaced by SEL SPD. So, if you entered .840/310, that will show up in 2L on the descent page.

Question: After replacing ECON SPD with SEL SPD (say .840/310), would that immediately put the aircraft under the second set of rules that apply after you have already passed the first speed constraint? So the fastest it would go right out of T/D would be 310 + 15 = 325 knots?

In other words, does the very act of inserting a speed other than ECON in the DES page cause the aircraft to behave as if it has already crossed its first speed constraint?

Thanks in advance.

[Long Haul]

Yes, entering a speed on the VNAV descent page effectively creates the first speed constraint.

Although the FCOM splits the speed logic into situations "above and below the first speed constraint," it may be more useful to think of it this way: if the FMC is allowed to choose it's own target speed, it will choose a new target speed upto Vmo/Mmo -11 kts in order to stay on path. If, on the other hand, it has been programmed to maintain a speed chosen by the pilot, or dictated by a terminal procedure, it will only deviate up to 15 kts from that speed in order to maintain the descent path.

[cf6-80c2b5f]

Thanks for the reply, Long Haul.

This would explain the confusion and conflict among eveyone I've asked about this. Some guys insert a descent speed on VNAV 3/3 and some will let it descend in ECON descent speed. So, without even realizing it, by doing that, it changes the logic as to how fast it will go to stay on path (VMO-11 versus FMC command + 15).

[cf6-80c2b5f]

One more clarifying question about this:

Does it matter whether this is done prior to T/D or after it starts a descent. One person I talked to believes (and maybe correctly) that the first speed constraint is only created if it is done during the descent and that it will still go up to VMO-11 if you insert a descent speed while prior to T/D.

In other words, if during cruise I replaced the ECON DES speed in 2L of the descent page with something like M.840/310, would it have any different effect if I did this during cruise as compared to the effect of doing this during the descent (while the descent page is active)?

Would it still be limited to FMC command speed (310) + 15 knots in either case?

Thanks!

[cf6-80c2b5f]

Any B744 FMS experts care to take a crack at this question?