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Wind effect on descent

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Wind effect on descent

Old 16th Jun 2021, 07:21
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Wind effect on descent

Hi there,

I was looking through the diagram below and was puzzled. The diagram shows that with a stronger headwind and a fixed path, aircraft speed will reduce and vice versa. Shouldn't the speed increase with a stronger headwind component on descent?


extricate is offline  
Old 16th Jun 2021, 08:10
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When descending with a headwind, the aircraft will be able to achieve a steeper descent path over ground (flight path angle) than in zero wind or tailwind conditions, because ground speed reduces with increasing headwind, while rate of descent remains constant.

If the FMS is programmed with say no wind, it will calculate a somewhat shallow descent path. If you then try to follow the calculated descent path in a headwind situation, the airspeed will reduce, unless thrust is added.

Hope that makes sense.
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Old 16th Jun 2021, 08:22
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Presume that you are flying a fixed path with flight idle thrust way above minium drag speed?
The stronger headwind puts you below the ideal flight path and by reducing the airspeed and changing the position on the L/D curve will allow the aircraft to continue the ideal descent path.
(with fuel savings at the expense of time).
No doubt someone else can explain it with clearer terms as I am from the generation of barber pole descents, with PPL application when necessary, split arse visual circuits and fully established by 400ft.
(PPL - poor planning lever- air brakes) Although my first kite had airborne engine reverse.
blind pew is offline  
Old 16th Jun 2021, 08:28
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FMC calculates TOD based on predicted wind, weight and required flight path/speed constraints. We hit TOD and close the TLs.

Generally, ROD = Deficit of Power / weight.

Given that this is intended to be an idle descent, deficit of power is determined by drag. Drag in turn is dependant on air speed. Thus ROD depends on air speed. We are probably well above Vmd therefore fly faster = more ROD, fly slower = less ROD

If this is a path descent and HW is higher than forecast, then your GS will be slower than usual. The planned ROD will tend to leave you low on the profile.

But if we (or the APFD) are pitching for path we will raise the nose, therefore we will slow down.

As we slow down, energy dissipation decreases (less drag) therefore our ROD decrease. (There is also some short duration trading of kinetic Energy for Gravitational Potential energy). But ultimately a lower descent rate, associated with a lower speed, can correct the geometry of the path.

At high speed energy dissipation approximates the cube of speed (power required = drag x tas, and we are up on the V squared bit of the drag curve). Thus speed changes are very powerful (badum, tschhh, I’n here all week) in modifying descent rate).

Hope that helps.

ps it may be easier to visualise of you think about a stong tailwind.
Capt Pit Bull is offline  
Old 16th Jun 2021, 20:25
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^---- The assumption that this is based on idle thrust, explains it. (Also, that these are long term steady state conditions, and not effects of gusts.) Another way of putting it: More headwind means the constant path relative to the ground, yields a shallower path relative to the airmass. This equates to a shallower descent angle (lower descent rate), which means (1) more thrust required at the same airspeed, OR (2) same as before thrust required (= same as before thrust available) at a lower airspeed.
​​​​​​
Since thrust available is fixed at idle, only option 2 is possible. ​​​​​​Put another way, the increase of TR due to descent angle, is offset by the decrease due to loss of airspeed. The final value stays the same, and matches the TA.

Last edited by Vessbot; 16th Jun 2021 at 20:57.
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