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If VNAV PTH, contrary to VNAV SPD, maintains a given path angle, depending on the CI chosen, and not a given Speed, then in my mind winds shouldn't affect it, since they would only cause the Vertical Speed to be adjusted accordingly in order to stay on that "fixed" path (since winds will alter the Ground Speed) After all, VNAV PTH has a band of approx. +/- 12 kt (i.e. 24 kt in total) within which it does not "care" what your speed is. All it wants is to maintain the selected Path. Hence my question: Does one need to fill in the Forecast Descent Winds for a VNAV PTH descent?

Though I can clearly see and understand how they will affect a VNAV SPD descent. Because if your primary target is maintaining a given Indicated Airspeed, (be it Mach or IAS), then the winds definitely define where you are going to end up on the ground.

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  • $\begingroup$ Which aircraft? Idle path descent, or geometric path descent? $\endgroup$
    – Ralph J
    Nov 17, 2020 at 22:49
  • $\begingroup$ The B737-800 in particular and its VNAV PTH descent mode specifically. It only has one VNAV PTH descent, I believe it must be what you call "idle path descent" because it is carried out at idle thrust (as long as you stay within a broad band of airspeed) $\endgroup$ Nov 17, 2020 at 23:03

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Yes, a VNAV PTH certainly needs descent winds so as to calculate and construct the most efficient flyable PATH. The PATH as we know. is built backwards from the landing RWY threshold to Top Of Descent (TOD).

Before I continue, I must say that sometimes managing VNAV PTH can become an unnecessarily complex task, if that's where you're coming from. We always have the alternative of lower levels of automation.

Flyability of the PATH means that the airplane should be on the FMC calculated 3 dimensional PATH as well as the FMC calculated SPD at all times during descent, approach and landing. This depends on accurate depiction in the FMC of altitude and speed limitations, hopefully no changes to the lateral path, ie routing, and accurate wind data etc.

Small components of Head Wind or Tail Wind may not make it impossible to fly the PATH, but as the HW or TW component increases, it can make it very difficult to maintain and the SPD variations, especially in the upward direction and at altitude can be nerve-racking at times on certain types.

Try flying eastbound into TOKYO (Narita) on VNAV with 600+ kts of Ground Speed due 120+kts TW component without feeding the final cruise and descent wind data - guaranteed 'fun'n'games for the pilot and a possible violation. There's an 18,000 ft crossing restriction which, may become impossible to comply with depending on when the pilot catches on to the problem.

There is always recourse to the use of Speedbrakes if the airplane goes above the calculated profile, and Thrust if it goes below that profile. But the use of either of these indicates a deviation from the most efficient descent, and costs fuel.

In the ultimate analysis, @Stamatis Vellis, while one may not bother to put in any winds for a 40 minute flight at FL 250 with light winds aloft, why willingly transgress the VNAV Speed Band (referred by you as +/- 12 kts), waiting for the FMC to automatically 'flip' from VNAV PTH to VNAV SPD.

VNAV PATH also represents fuel savings - my take is that when a facility exists, why not use it? 2 minutes spent on putting in effective wind forecasts, can make a more optimized, fuel efficient flight, more comfortable speeds, lower workload, more headspace for the pilots to monitor and make better tactical decisions ALL of which has direct safety impact.

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  • $\begingroup$ It was not a matter of not being bothered to enter them, it was about understanding if and how the info influences a VNAV PTH descent. Now it is understood that the PATH chosen by entering a certain CI is not a fixed angle path for each CI entered but may vary even if the same CI is always chosen, depending on winds. Your explanation was quite clear. $\endgroup$ Nov 18, 2020 at 10:04
  • $\begingroup$ Hahaha @StamatisVellis $\endgroup$
    – skipper44
    Nov 18, 2020 at 11:40
  • $\begingroup$ Hahaha @StamatisVellis, I was talking about myself "not being bothered to enter them". $\endgroup$
    – skipper44
    Nov 18, 2020 at 11:47
  • $\begingroup$ You gave me the answer I was looking for, nonetheless :-) $\endgroup$ Nov 18, 2020 at 15:13

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