Jan's VNAV demo part 2, FMC speed

[Rodeo]

I don't have the plane yet (no point in buying now as X-Plane Desktop will not run on my iOS device), but enjoying the various streams and videos so far.

 

One thing that surprised me was how the FMC seemed to manage speed during approach, in an almost Airbus-like manner. I was under the impression that the FMC would take care of the 10,000 feet speed restriction and also protect from stalls snd flap/gear overspeed, but that any slowdown beyond 240/10000 would have to be initiated 100% manually.

 

I will watch the video again when I have time, but IIRC when getting closer to the runway, the FMC automatically started to slow the plane down to under 200 knots (but waiting for flap extension to avoid stalling etc.).

 

Was there a specific speed constraint in the approach that triggered this, or does the FMC do this on its own when a certain distance from landing is reached (kind of like the approach phase on an Airbus), and if so, where can I read more about it (I assume an FCOM, but which version and chapter, if anyone remembers from memory)?

 

Regards,

Tim

[Rodeo]

More specifically, I referring to 11:45 onwards (and 15:00 onwards), where the FMS gradually commands a target speed of 150 knots, I wonder whether this is approach-specific (restrictions in navdata) or an FMS feature triggered by one or more conditions

[Litjan]

Hi Rodeo,

the FMS (and we don´t have that quite down yet) - will eventually command the approach speed as entered in the FMS (Vref+5). Usually it will try to target the speeds associated with waypoints - and in the case of the intermediate approach waypoint (where the plane starts going down) this is mostly 180kts or so. After that the plane will try to slow down to Vref+5 (heeding minium speeds as dictated by lack of flaps).

Jan

 

[Rodeo]

Cool, thanks. Where would you recommend I look regarding documentation about this feature (as implemented in the 737-300) (like, in addition to the FCOM), and/or do you by any chance know which hardware/firmware combination is simulated for the control display unit and underlying FMS system?

Edited April 28, 2016 by Rodeo

[Rodeo]

I guess this is described simply enough in the FCOM (e.g. section 11.31.x).

I always learned that Boeings should be flown with manual speed control below 250/240 knots, now I wonder whether other addons (like FlightFactor) implement this

It's a very convenient feature and one of the reasons I like Airbusses, but it turns out Boeings are seemingly just as good in that regard.