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set speed changing after going into level change

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If this is the logic of the 747 that is fine. I change the speed manually as the airplane is in a mode like vnav/lnav. I dial the speed to a lower number than I am currently flying, and then press level change. Level change resets my speed from the last number I just set, to the current speed of the aircraft. is that by design? Thanks - David Lee

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If you're in VNAV, how are you even setting the speed since the speed window is blank when VNAV is engaged. As far as I'm aware, you hit FLCH, the present speed shows up in the window, and you dial it down to whatever speed you want.

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My books say...

If VNAV was engaged before the FLCH button was pushed (MCP window closed), the
MCP speed display shows the present FMC target speed. If VNAV
was not engaged, the display shows the higher of present airspeed or
present MCP speed. Selection of a different speed is possible after
FLCH engages.

It's not clear what happens if FLCH is selected when the A/P is in VNAV with speed intervene.

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2 hours ago, Qavion2 said:

My books say...

If VNAV was engaged before the FLCH button was pushed (MCP window closed), the
MCP speed display shows the present FMC target speed. If VNAV
was not engaged, the display shows the higher of present airspeed or
present MCP speed. Selection of a different speed is possible after
FLCH engages.

It's not clear what happens if FLCH is selected when the A/P is in VNAV with speed intervene.

In the last PMDG 747 version, and every other Boeing PMDG models, if I manually have a speed set in the window then go to level change, it keeps the number in the window not reset it to the current speed - David Lee

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Sorry, David, I can't test this at the moment.

I did find more information on it, however.

 

FLCH push....

1) when IAS/MACH window blank, IAS/MACH window opens to FMC
target speed, if valid. If not valid, IAS/MACH window opens to current
speed
2)when IAS/MACH window open, displays command speed

3) when changing from TO/GA to FLCH, IAS/MACH window displays
highest value of current airspeed or selected speed

 

I wish these manuals were written so "normal" people can understand them

:blink:

 

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13 hours ago, 777200lrf said:

If this is the logic of the 747 that is fine. I change the speed manually as the airplane is in a mode like vnav/lnav. I dial the speed to a lower number than I am currently flying, and then press level change. Level change resets my speed from the last number I just set, to the current speed of the aircraft. is that by design? Thanks - David Lee

I believe that FLCH is primarily a level-changing command, as the acronym implies. When you press it, you are telling the aircraft "I want to descend (or climb) NOW". It gives priority to the level change. If you open the speed window, and dial in a lower speed, and immediately press FLCH, you are asking it to do mutually contradictory things. It's the old problem of "you can't go down and slow down" (at the same time).

I think that the proper technique would be to activate speed intervention by opening the MCP speed window, dial in your desired lower speed and WAIT. Thrust will reduce and the aircraft's speed will roll back. Once you reach the desired (lower) speed, THEN press FLCH, and your descent will immediately commence, with FLCH using primarily pitch, (and secondarily thrust), to hold your desired speed in the descent.

This is exactly what VNAV does when your flightplan calls for a lower speed in descent than your programmed cruise speed. In this case, you will see a decel point appear on your ND several miles before your TOD, (or before a waypoint that has a speed restriction).

FLCH is more useful in a climb, than in a descent. It is primarily a pitch mode, in which the aircraft will pitch to maintain the current IAS or Mach, with power being added as necessary (up to the current calculated limit) to keep the climb going. If you dial in a higher speed in the climb, pitch will reduce while the speed builds up, (and rate of climb reduces), once the speed is achieved, pitch will again be held  constant, while power is used to keep the climb going. It is easy for pitch to directly control airspeed in a climb, because the aircraft is working against the force of gravity, which acts as a brake of sorts.

In a descent, the opposite situation holds true. The force of gravity is working WITH the aircraft, tending to cause it to accelerate.

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Quote

The behavior of the aircraft is correct. Please consult the FCOMs.

Actually, I was :laugh: I didn't fully understand them.

 

In my other favourite sim, during VNAV speed intervention followed by FLCH, the aircraft will speed up/slow down to achieve the speed set in the window (if the speed was different from the MCP window speed). It will not sync with the current speed (unless that is already the target speed). I don't know what David is seeing in v3

 

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On 2/26/2017 at 11:14 PM, Qavion2 said:

My books say...

If VNAV was engaged before the FLCH button was pushed (MCP window closed), the
MCP speed display shows the present FMC target speed. If VNAV
was not engaged, the display shows the higher of present airspeed or
present MCP speed. Selection of a different speed is possible after
FLCH engages.

It's not clear what happens if FLCH is selected when the A/P is in VNAV with speed intervene.

When you go from VNAV to FLCH, the target speed that the FMC in VNAV was commanding is displayed in the IAS/Mach window when selecting FLCH, whether the window was closed (FMC controlling speed) or open during speed intervention. During speed intervention, the target speed is controlled directly by the pilot instead of the FMC, so the speed would not change if you were to select FLCH from VNAV with speed intervention.

Also, FLCH is used more, and therefore more useful, during descent,rather than climb. Most operators stay in VNAV during takeoff, climb, cruise, and most of the descent until ATC starts yanking our chain in the terminal area, and then the economy of VNAV is countered by turn left, turn right, slow down, and speed up by ATC, in which case at some point we transition to FLCH (usually below 10,000), when the economy aspects are overshadowed by ATC requirements to fit into the flow of traffic.

There is a ratio of about 60/40 when you are trying to slow down and go down at the same time. FLCH will go more for speed first (60%) then it will for descent (40%) and then once on speed continue the descent at the new speed and the descent rate will then increase.

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Thanks, Alexander.

David, could you clarify your issue?

Thanks,

Cheers

 

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