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sbieg

Waypoints speed restriction

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My 777 does not follow waypoint speed restriction (to slow down before descent). Have to manually reduce speed using speed intervention knob. Either, after selecting lower descent speed on FMC Desc. page, the plane supposed to reach that speed, before T/D waypoint ( as 737ngx does), but it doesn't.  I don't think it is normal for this plane, isn't that ?

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Of course, I can, but this is VNAV job.

It should be a VNAV job.

 

as in you should let VNAV do it's thing and ignore it, or not use VNAV if you want to do something else.

 

VNAV will not slow down before descent***. It will begin descent by pulling throttle to idle thrust (at the same time as it pitches down to descend) and maintain the same airspeed or mach number.

 

At around 11,000ft it will pitch up slightly to slow down to 240 knots.

 

about 15 nm from the runway it will slow to about 210kts or the slowest possible speed with flaps up if higher.

 

It will do this unless there is a speed restriction built into the STAR or IAP.

 

The most efficient profile is to begin your descent at cruise mach speed, and slowly reduce mach speed to the IAS/Mach transition then maintain a constant IAS to 11,000ft. This will look like an Airspeed that slowly increases between the high Flight Levels (FL330 - FL430) then remains constant once it gets to around 270 up to 320 knots till FL110, when it will slow to 240kts (by reducing rate of descent to about 500ft/min or less for a minute or two).

 

If you slow down to 250kts at FL410, 2 things will happen:

1: the FMC VNAV calculation will be well off.

2: All the other traffic behind you will catch up fast

3: You will take longer to arrive at destination

4: The ATC sequencing will be all out of whack.

 

If you want to descend at a slower speed you should enter this speed in the FMC's "VNAV > Descent" page.

 

***it will slow down before descent if the MCP set altitude restricts it from descending on profile and you end up high.

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If you slow down to 250kts at FL410, 2 things will happen:

1: the FMC VNAV calculation will be well off.

2: All the other traffic behind you will catch up fast

 

250KIAS at FL410 is around 455KTAS - not as slow as you think. Still equates to M.079-0.80.

 

Don't get confused between the two.

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250KIAS at FL410 is around 455KTAS - not as slow as you think. Still equates to M.079-0.80.

 

Don't get confused between the two.

 

Which would be great if we were in the 737NGX forum, but randomly dumping 0.04 of a mach number just before your descent? (not During your descent, but before it)

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Trent is correct, absouletly no reason to start slowing down before descent, VNAV if programmed correctly with a wind forecast will do the job starting descent at cruise speed.

 

In a perfect world without other traffic or ATC sequencing the best descent would be idle thrust in FLCH

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It's not better, then brake hard just past T/D ? Which airlines would like that ?

Vnav will give you an (economically) optimzed descend path based n cost index.

Low cost index and you will descen with M.82 which then switches over t 250-260kt.

High cost index will give you something like M.84 and then switch over to 310Kt or so.

 

If you have entered a speed restriction down the road then Vnav will meet this.

But it will not start to reduce to this slower (non economical speed) untill right before getting to that point. Just wait you will see it will meet the speed requirement!

 

If you want the whole descend to be slow, like 260kt from 30000ft then you have to enter that in the Vnav descend page as a fixed speed. Or you can add an additonal line to the 250/below 10000ft (like 260/30000).

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Vnav will give you an (economically) optimzed descend path based n cost index.

Low cost index and you will descen with M.82 which then switches over t 250-260kt.

High cost index will give you something like M.84 and then switch over to 310Kt or so.

 

If you have entered a speed restriction down the road then Vnav will meet this.

But it will not start to reduce to this slower (non economical speed) untill right before getting to that point. Just wait you will see it will meet the speed requirement!

 

If you want the whole descend to be slow, like 260kt from 30000ft then you have to enter that in the Vnav descend page as a fixed speed. Or you can add an additonal line to the 250/below 10000ft (like 260/30000).

It was short flight below 10000ft (I missed that info. in my initial question, sorry). Plane entered into descent segment with 250 knots, which was way to fast, despite modified lower speed in FMC. Thanks for good response, anyway.

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It was short flight below 10000ft (I missed that info. in my initial question, sorry). Plane entered into descent segment with 250 knots, which was way to fast, despite modified lower speed in FMC. Thanks for good response, anyway.

It might be too fast if flying in busy airspace.  In that case, at least in the real world, you'd almost certainly be given a speed restriction.  But as far as the aircraft and airspace rules go, 250 KIAS  below 10000 is perfectly acceptable.  Beyond that, inbound below 10000, you would normally be flying vectors anyway, so you would want to be in FLCH, HDG, and SPD modes, not VNAV.  Again. RW stuff.

Cheers. 

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It might be too fast if flying in busy airspace.  In that case, at least in the real world, you'd almost certainly be given a speed restriction.  But as far as the aircraft and airspace rules go, 250 KIAS  below 10000 is perfectly acceptable.  Beyond that, inbound below 10000, you would normally be flying vectors anyway, so you would want to be in FLCH, HDG, and SPD modes, not VNAV.  Again. RW stuff.

Cheers. 

Yes, I think 737 FMC is a different design as for midsize mostly short flight jet. Thanks.

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It might be too fast if flying in busy airspace.  In that case, at least in the real world, you'd almost certainly be given a speed restriction.  But as far as the aircraft and airspace rules go, 250 KIAS  below 10000 is perfectly acceptable.  Beyond that, inbound below 10000, you would normally be flying vectors anyway, so you would want to be in FLCH, HDG, and SPD modes, not VNAV.  Again. RW stuff.

Cheers. 

It may be glitch in my 777 FMC. Here is, how it's computing descent profile : KDFW- RNAV-RWY17C(last segment): BOSSI 240/6000A-3NM-PENNY 240/5000A-6NM-ZING 240/3000A-2NM-JIFFY 170/2300A-5NM-RW17C170/612. Obviously, no way to reduce speed by  70 knots within 2 nm when descending. After I modified(lowered) speed of all these waypoints, on next attempt, the plane still flew too fast passing waypoints with higher than restricted(modified) speed(drag required prompt). My 737ngx FMC computing this approach right.

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