VNAV and autothrottle during flap deployment

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navymustang
Posts: 289
Joined: 12 Aug 2014, 21:30
Location: Orlando, FL USA

VNAV and autothrottle during flap deployment

Post by navymustang » 04 Apr 2019, 14:06

Had a real-world pilot fly my sim last night. He brought up a point that was briefly mentioned in another post without an answer.

In a VNAV descent below 10,000 and 250kts, when selecting flap positions, he indicated that the VNAV mode will compute a new speed and automatically set that speed. I think he is wrong but I can not find any hard evidence that says what happens. (he does not currently fly the 737)
The question is: will VNAV compute new speeds and set that speed with changes in flap position? Currently Prosim does not do that.

Thanks

bravolima
Posts: 31
Joined: 15 Nov 2015, 21:13
Location: Zurich Area

Re: VNAV and autothrottle during flap deployment

Post by bravolima » 04 Apr 2019, 20:26

It should do so. My manual (FMC Version 10.8) says:

If descending in VNAV PTH before final approach and the situation permits a continuous descent through final approach, remain in VNAV PTH while
configuring the airplane for approach and landing. The airplane slows automatically to the maneuver speed for the current flap setting.
I add: be sure to be in VNAV PATH and SPD INTV is off.

Bernhard (LSZH)
P3D v4.3 FDS: MIP, CDUs, Clocks, Shell, Liners,Tiller; CPFlight: Overheads, Pedestal; MCP, EFIS; OPENCOCKPIT Pedals; FI: Brake-pressure, jaw-damper, compass; FCS: linked Yokes (modified); Weber-Seats, PROSIM: B737&Utility; AIVLASOFT: EFB 2,

navymustang
Posts: 289
Joined: 12 Aug 2014, 21:30
Location: Orlando, FL USA

Re: VNAV and autothrottle during flap deployment

Post by navymustang » 04 Apr 2019, 22:53

Bernard, thank you for the flying lesson. Shame on me for not knowing this. I tend to come out of VNAV PATH on my descents and never carried them all the way to the point where I saw this function operate.

Great learning experience.

rs3tier
Posts: 975
Joined: 24 May 2014, 04:38
Location: Hobart Tasmania

Re: VNAV and autothrottle during flap deployment

Post by rs3tier » 05 Apr 2019, 08:58

I think the simplest answer (rule) - is this.. if the Speed Window on the MCP is open/available - then you are in charge of setting the speed - if it is closed, then the FMC is in charge of setting the speed. When the FMC is in charge it will include bugging the correct speed for flap settings etc.

Cheers,
Rob

bravolima
Posts: 31
Joined: 15 Nov 2015, 21:13
Location: Zurich Area

Re: VNAV and autothrottle during flap deployment

Post by bravolima » 05 Apr 2019, 09:58

For training purpose try to perform complete VNAV-Approches down to the MDH. You can take every RNAV-Approach stored in the FMC. Don`t forget to activate APP prior your FAF, after passing the last waypoint before (not earlier!!). Do not put an ILS-Frequence in the box. You only have to set flaps and lower gear. These VNAV approaches are a substitute for ILS CAT II ( good for airports with no ILS-equipment). The speed in VNAV-approach can be modified with SPD INTV if necessary. There are some tricky VNAV approaches around. Try LOWI RWY08 if you are more familiar....

Bernhard
P3D v4.3 FDS: MIP, CDUs, Clocks, Shell, Liners,Tiller; CPFlight: Overheads, Pedestal; MCP, EFIS; OPENCOCKPIT Pedals; FI: Brake-pressure, jaw-damper, compass; FCS: linked Yokes (modified); Weber-Seats, PROSIM: B737&Utility; AIVLASOFT: EFB 2,

rs3tier
Posts: 975
Joined: 24 May 2014, 04:38
Location: Hobart Tasmania

Re: VNAV and autothrottle during flap deployment

Post by rs3tier » 06 Apr 2019, 00:21

Just a note, in order to use APP mode for VNAV approaches, you will need to have IAN (Integrated Approach Navigation) function enabled in the instructor station.

Rob

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