VNAV SPD/PTH Descent?
Join Date: Jan 2014
Location: UK
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Not sure on the nature of the flying you do/intend to do but while VNAV is complicated I find it becomes much less so (as with anything) when you fully understand the principles. Appreciate that's what you are trying to do on here wth the question but in reality the FCOM's are your best friend. If it's for flight sim and you only require a basic knowledge fair enough but don't be surprised if you find yourself asking 'what's it doing now?'...regularly!
Join Date: Apr 2008
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Maybe it is just best to think of the VNAV path as a sort of glideslope but calculated for an idle descent based on the winds placed into the FMC.
Imagine if the glideslope for the airport coincided with the VNAV path and there is no wind forecast for today. What happens if you intercept the glideslope 100 miles back(if possible) on the descent. You have captured it and are maintaining it at lets say 290 knots. How about manual flying today.
Pitch will be used to maintain the GS/path. If there is an unforecast headwind, the speed slows down as you pitch up to maintain the glideslope or path. In my aircraft type at 15 knots slow on the VNAV path, the autothrottle wakes up and adds thrust to get the target speed back.
What if you get a tailwind on the glideslope. The and are still following the GS/path the airspeed increases. If you want to maintain the target speed, extend speedbrakes.
If ATC askes you to expedite your descent rate while on the glideslope, speedbrake don't work. You have to go down below the glideslope. Same with the VNAV path. Typically you are on autopilot. So you open the speed window and immediately VNAV path changes to VNAV speed and you increase the speed to increase the rate of descent. You see your VNAV path indicator move up just like the GS needle moves up. You are below the path. Or maybe ATC wanted a lower rate of descent so you did the opposite and went above the GS/path in VNAV Speed.
ATC cancels their request so you change your pitch as required and when the VNAV path deviation indicator comes back to near center(which happens to coincide with the GS), close the speed window and VNAV Path appears.
In VNAV speed, the path is no longer considered, you may get closer or farther away from it as the winds change on the descent(or imagine while flying through a CB).
Imagine if the glideslope for the airport coincided with the VNAV path and there is no wind forecast for today. What happens if you intercept the glideslope 100 miles back(if possible) on the descent. You have captured it and are maintaining it at lets say 290 knots. How about manual flying today.
Pitch will be used to maintain the GS/path. If there is an unforecast headwind, the speed slows down as you pitch up to maintain the glideslope or path. In my aircraft type at 15 knots slow on the VNAV path, the autothrottle wakes up and adds thrust to get the target speed back.
What if you get a tailwind on the glideslope. The and are still following the GS/path the airspeed increases. If you want to maintain the target speed, extend speedbrakes.
If ATC askes you to expedite your descent rate while on the glideslope, speedbrake don't work. You have to go down below the glideslope. Same with the VNAV path. Typically you are on autopilot. So you open the speed window and immediately VNAV path changes to VNAV speed and you increase the speed to increase the rate of descent. You see your VNAV path indicator move up just like the GS needle moves up. You are below the path. Or maybe ATC wanted a lower rate of descent so you did the opposite and went above the GS/path in VNAV Speed.
ATC cancels their request so you change your pitch as required and when the VNAV path deviation indicator comes back to near center(which happens to coincide with the GS), close the speed window and VNAV Path appears.
In VNAV speed, the path is no longer considered, you may get closer or farther away from it as the winds change on the descent(or imagine while flying through a CB).
