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Transition from VNAV PTH to VNAV SPD

Posted: Sat Feb 02, 2019 2:07 pm
by GeographyFlyer
Hey guys,

I have another question regarding 777.
Under which conditions VNAV PTH changes to VNAV SPD during descent/approach?

Every answer is highly appreciated. GeographyFlyer

Re: Transition from VNAV PTH to VNAV SPD

Posted: Sat Feb 02, 2019 9:03 pm
by BA777FO
Couple of reasons:

When in cruise, if you hit Alt inhibit (the Altitude selector knob on the MCP) you'll commence a descent (as long as the MCP alt is lower than your current altitude). This would be when ATC clear you down prior to your T/D point for example.

Opening the speed window (pushing the speed knob on the MCP) gives the pilot control over the speed and the aircraft will descend at the speed in the speed window rather than following the FMC-computed path.

Re: Transition from VNAV PTH to VNAV SPD

Posted: Wed Feb 06, 2019 6:23 am
by BoeingGuy
BA777FO wrote:
Couple of reasons:

When in cruise, if you hit Alt inhibit (the Altitude selector knob on the MCP) you'll commence a descent (as long as the MCP alt is lower than your current altitude). This would be when ATC clear you down prior to your T/D point for example.

Opening the speed window (pushing the speed knob on the MCP) gives the pilot control over the speed and the aircraft will descend at the speed in the speed window rather than following the FMC-computed path.


Another reason is if you get off your VNAV decent path by more than about 200 feet. Another reason is when you do a step climb.

What you describe pushing the MCP speed switch and overriding the VNAV calculated speed will NOT change you from VNAV PTH to VNAV SPD. That’s not correct. That’s called Speed Intervention. The autothrottle will stay in the SPD mode and VNAV will stay in VNAV PTH.

Re: Transition from VNAV PTH to VNAV SPD

Posted: Wed Feb 06, 2019 6:56 am
by BA777FO
From our FCOM:

"In a VNAV descent after the T/D VNAV PTH changes to VNAV SPD during speed intervention. In all other phases, the pitch mode does not chnage with speed intervention."

Obviously an RNAV approach is flown in VNAV PTH with the speed window open but I was correct above. Sorry for not being specific enough with regards to phase of flight.

The deviation from the calculated Vnav path to change from PTH to SPD is 150ft.

Re: Transition from VNAV PTH to VNAV SPD

Posted: Wed Feb 06, 2019 9:57 pm
by BoeingGuy
BA777FO wrote:
From our FCOM:

"In a VNAV descent after the T/D VNAV PTH changes to VNAV SPD during speed intervention. In all other phases, the pitch mode does not chnage with speed intervention."

Obviously an RNAV approach is flown in VNAV PTH with the speed window open but I was correct above. Sorry for not being specific enough with regards to phase of flight.

The deviation from the calculated Vnav path to change from PTH to SPD is 150ft.


What model is that? 777-300ER? I’ve done speed intervention in the simulator on approach in VNAV on some other models and it hasn’t changed to VNAV SPD. I’ll try it next time I’m in there.

And the threshold has been about 200 feet on the models I’ve worked with. I don’t know the 737 or 747 as well.

Re: Transition from VNAV PTH to VNAV SPD

Posted: Thu Feb 07, 2019 9:57 am
by BA777FO
BoeingGuy wrote:
BA777FO wrote:
From our FCOM:

"In a VNAV descent after the T/D VNAV PTH changes to VNAV SPD during speed intervention. In all other phases, the pitch mode does not chnage with speed intervention."

Obviously an RNAV approach is flown in VNAV PTH with the speed window open but I was correct above. Sorry for not being specific enough with regards to phase of flight.

The deviation from the calculated Vnav path to change from PTH to SPD is 150ft.


What model is that? 777-300ER? I’ve done speed intervention in the simulator on approach in VNAV on some other models and it hasn’t changed to VNAV SPD. I’ll try it next time I’m in there.

And the threshold has been about 200 feet on the models I’ve worked with. I don’t know the 737 or 747 as well.


It's on all models of the 777. FCOM explicitly states 150ft deviation for VNAV PTH to transition to VNAV SPD too.

It's after T/D but prior to approach, it's designed to stay in VNAV PTH for the approach so you can fly an Rnav approach with the speed window open but gives you extra control for the initial phases of descent.

https://1drv.ms/u/s!AirWxewqfGFCbGjbF-nSmb-OmPo

Hopefully that image helps? If it works? Sorry it's sideways!

Re: Transition from VNAV PTH to VNAV SPD

Posted: Thu Feb 07, 2019 4:01 pm
by Acey559
In the 757/767 it will switch to VNAV SPD if it gets too fast/slow while on the path (generally about 10-15 knots). The aircraft generally attempts to perform an idle descent but this doesn’t always work out. Occasionally we will get too fast and get a “drag required” message; and if we get too slow the engines will spool up to get back to target airspeed.

I haven’t flown for three weeks so I may be a little off on the exact verbiage.

Re: Transition from VNAV PTH to VNAV SPD

Posted: Thu Feb 07, 2019 11:24 pm
by BoeingGuy
Acey559 wrote:
In the 757/767 it will switch to VNAV SPD if it gets too fast/slow while on the path (generally about 10-15 knots). The aircraft generally attempts to perform an idle descent but this doesn’t always work out. Occasionally we will get too fast and get a “drag required” message; and if we get too slow the engines will spool up to get back to target airspeed.

I haven’t flown for three weeks so I may be a little off on the exact verbiage.


That’s true on all Boeing models. It’s similar to what the BA777FO and I were discussing. If you get off path or off speed by a certain threshold in descent, VNAV PTH will revert to VNAV SPD.

I’m curious if the OP understands the difference between VNAV SPD and VNAV PTH. VNAV SPD is a speed-through-elevators mode like FLCH and TO/GA (TO or GA on the 757/767).