When does FMC command aircraft to slow down?
 

Hi,

Can someone point in the direction of the manuals where I can find an explanation on this?

Say on a STAR with a speed constraint, when will the FMC command a reduction of airspeed?

Thanks

Which airplane?

On the B777

The FMS flight plan should show the speed constraints. The A/T will follow those.

Yes, is there a time before the waypoint is reached where the aircraft will start slowing? Understand if its the 250/10000, there will be 2 green rings but other than that, there's no reference when the aircraft will slow down.

What 2 green rings are you talking about? I’m guessing one is the altitude arc? What’s the other?

Deceleration points are indicated on the ND by a small circle (but unlike things such as T/D are not labelled)...it’s in the FCOM under ND symbology.

As for the OPs initial question..”Say on a STAR with a speed constraint, when will the FMC command a reduction of airspeed? ”... I’m not sure what info the OP wants,if you have a constraint to cross a way point at e.g. 250 knots the FMC will command the decel at a suitable point to achieve the 250 knots at the waypoint. How it calculates the point, what the algorithm is, I have no idea. It is not in any document I have seen and of course on the line you just keep an eye on it to make sure it does what it should....no point in worrying about how it works it out.

Your question is too general - there being various states of flight the 777 can be in during descent. Altitude can be commanded and speeds flown to speed and alt constraints in the FMC, FMC speed can be flown by air braking to fly a steeper descent, etc.
Other than speed and alt constraints the FMC numbers are a guide. ATC will, most of the time be issuing speed and altitude commands at you anyway.
Were one to leave the aircraft completely alone to fly the FMC invariably it would try and reach the FMC speed on arrival at the constraint and then be unable to descend at the necessary FPM in order to reach the next desired altitude at the required speed, etc.
During descent the pilot becomes once more the most efficient computer on the aircraft and disposes gradually of the automation systems as control is taken of the flying.

I get caught sometimes when there's a speed constraint at a waypoint and FMC doesn't slow down until we are very close towards it. So i'm wondering if there's a timeframe FMC will start slowing down so at least I know what the FMC is doing and not bust the speed constraint passing the waypoint.

Depending on the amount of the speed to be reduced. usually with the 737NG if the restriction is about 25 kts less, the vnav will start to break the speed and the vertical speed is drastically reduced at 700 ft or 800 ft before the waypoint altitude untill reaching the new speed. For me, if the vnav does not behave as expected at that moment I do it by myself from MCP (lvl change or V/S first then lvl change)

On 737, it commands a 1.3 degree decel segment, IIRC, to achieve the speed restriction. Length of the segment would be based on the difference in speeds and the wind.

Not sure if it's the same for the 777, but it might be similar.

It seems to do it at the last possible moment possible to just make the restriction by selecting idle. Usually just around the time that you are about to take overriding action seeing as it is at the latest possible moment that it starts to slow down.

Sometime though, the waypoint takes longer to change from magenta to white than one expects it will take.

Exactly, that was what I faced, it seems to me that FMS starts decelerating at the very last moment when i'm about to intervene.. I've asked around, it seems to be that FMS starts decelerating about 1.5nm per 10kts prior to the waypoint?

Any thoughts?

777, average weight, nil wind, level descent at the start of the pattern..A ball park 1 to 1 1/2 nm per 10 knots has always worked pretty much ok for me..if the FMC "wants" a bit more I wouldn't be upset.

In the 737, it's an angle - 1.3 down - commanded for the decel.

An ATC friend once told me that can see see altitudes very well, but airspeeds are very hard to tell. If you're within 10 knots and correcting, are you very concerned?

It will be buried in the FMC performance, and will be different all the time due to weight, wind, etc. As others have suggested the best you can use is a rule of thumb. Another way is simply to work out if the FMC is low on profile during the descent to the respective WPT then obviously it has 'fat' built in to slow down, if there's not much fat expect a "DRAG REQUIRED" FMC message.

extricate,

Did you fly the bus previously?

I went from the bus to the 73, and I sure miss to "see" the commanded speed....