Hi guys,yesterday my 767 was in VNAV descending to a waypoint with a speed restriction (no altitude restriction). A few miles before reaching this waypoint I noticed the autoflight system closing the throttles in order to adhere to the speed restriction. :-eekSo far I have been under the impression that a speed restriction without an altitude restriction is "meaningless" in terms of the autoflight system, and would just serve as a reminder to the pilot.Seems I was wrong here.Nevertheless, the question occurs to me, how this situation would be handled by the real aircraft. Since in the real thing VNAV descends always happen with engines in idle (obviously not modelled in PIC due to wrong drag/speed models in FS2002), retarding the throttles in order to comply with a speed restriction does not seem to be an option.Will the aircraft instead pitch up a little in order to allow the speed to bleed off in time? :-hmmmIf so, does the FMC compute a new TOD when a speed restriction at one of the descend waypoints is entered, since it has to take the deceleration phase (invariably combined with lower descend rates) into account when computing the flight path?In PIC this recalculation would not be necessary, I presume, as the aircraft does not pitch up for slowing down, but just closes the throttles completely.And another questions for the FMC buffs: is there any way to cancel a speed restriction of the active waypoint when in VNAV descend without dropping out of VNAV?I ask because I noticed that deleting the speed restriction would not cause the aircraft to speed up again, nor would it change the predicted speeds for the waypoints further down the road to their original values (ECON DES speed or 250 kts when 250/10000 restriction is active).Many thanks for your thoughts.Kind regards,Stefan

"Will the aircraft instead pitch up a little in order to allow the speed to bleed off in time? "Exactly, Stefan.... Just the same as it does for slowing to, say, 240kts at 10,000'."If so, does the FMC compute a new TOD when a speed restriction at one of the descend waypoints is entered, since it has to take the deceleration phase (invariably combined with lower descend rates) into account when computing the flight path?"The FMC looks at the lowest waypoint restriction and works backwards up to TOD, taking into consideration all restrictions, Cost Index, winds, etc."And another questions for the FMC buffs: is there any way to cancel a speed restriction of the active waypoint when in VNAV descend without dropping out of VNAV?"Definitely not a buff, but...The A/P on a 767 doesn't drop out like it does on, say, a 737NG. The mode may change to VNAV SPD instead of VNAV PATH, but it still remains in VNAV. If this change results in you being above the path, you may get a message telling you to extend the boards (speedbrakes), however, but VNAV will still be active.Hope this helps.Cheers.Ian.

In order to decellerate the FMC will apply a 500fpm rate of descent, which will lead to roughly a 10kt per nautical mile decelleration. For example if you put in a speed restriction of 250/130A in the LEGS page whilst flying at 300kts at 5nm from this point the bug will move from 300 to 250, the nose will pitch up and the rate of descent will slow to around 500fpm, all in VNAV path. Unlike the VNAV DES page speed limits this decelleration will not be marked with green donuts.If you deleted the 250/130A from the LEGS page the pitch annunciator will change to VNAV SPD, the bug will move to your present speed while the FMC does the new path calculation, then N1/EPR will annunciate in the autothrottle FMA, which will change to SPD, and the thrust will advance to reduce the rate of descent at the VNAV DES page commanded speed. When the aircraft flies into the new path IDLE and VNAV PTH will annunciate and you're back in business!If you did the opposite and added the 250/130A into the legs page whilst on descent the opposite would happen. As you press execute the FMA will change to VNAV SPD and the new path will be calculated below the present path. To reintercept the path you must either increase speed using speed intervene (not always possible due ATC etc) or use the speedbrake. In order to take advantage of the VNAV calculations you must be back in VNAV PTH at least 5nm from the constraint, otherwise VNAV SPD will remain and the decelleration will not occur unless you speed intervene to 250kts yourself.That's just an example for you, I hope it's understandable!

"Since in the real thing VNAV descends always happen with engines in idle (obviously not modelled in PIC due to wrong drag/speed models in FS2002)" Let me set this misconception straight, the ONLY limiting factor in this regard is between 36'000-30'000 BUT to imply that a *true* idle descent cannot be achieved is simply not true. It might take an awful lot of coding but it can be done right. In fact I am flying a updated NG model with this exact thing...[h4]Best Wishes,Randy J. Smithhttp://img.villagephotos.com/p/2003-5/196432/winglets_lg.jpg

First of all, thanks for your comments.Ian, I think I may have used a somewhat flawed terminology. When I wrote about the autopilot dropping out of VNAV I actually meant the pilot actively deactivating VNAV and instead engaging FL CH or VS mode.I did not intend to imply the autopilot would drop out by itself in the discussed situation.Anthony:"If you deleted the 250/130A from the LEGS page the pitch annunciator will change to VNAV SPD, the bug will move to your present speed while the FMC does the new path calculation, then N1/EPR will annunciate in the autothrottle FMA, which will change to SPD, and the thrust will advance to reduce the rate of descent at the VNAV DES page commanded speed. When the aircraft flies into the new path IDLE and VNAV PTH will annunciate and you're back in business!"That sounds reasonable enough and I would have expected something like this, but unfortunately it is not what happened when I deleted a 220 kts restriction (no altitude restriction there) of the active waypoint.Neither did the airspeed settle at the current speed (aircraft was in the process of slowing from 295 kts to 220 kts at around 260 kts), nor did it go back up to 295 kts, it just kept going down until it had reached 220 kts.Also, the predicted speeds of all following waypoints stayed at 220 kts although no further speed restriction was active anywhere (and the first one was already deleted!).Randy:I didn't say that a realistic idle descent could not be modelled in FS2002, but I haven't come across any airliner that achieves reasonable descent rates with engines idle at higher altitudes. PIC certainly goes down like dive bomber in order to keep speed up (in FL CH, that is).The closest thing I saw so far is the freeware 727-200 by Charles Fox and Richard Probst, 2500 to 3000 fps (max) at high altitudes seem okay more or less.Unfortunately I don't own the PMDG 737 NG, but from what I hear the flight model seems to be superb, so that one may indeed play in another league.All the best,Stefan