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SPD-BRK

Assumed temp takeoff - V2

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Whenever I perform an assumed temperature takeoff, and commanded N1 changes from D-TO to CLB, V2 always falls within the orange low speed buffet range and A14X flashes on the speed window to indicate that the selected speed is too low. Obviously, the speed means very little because with the pitch mode in 'TOGA' the FD is commanding V2+20.

 

However, I have a friend who flies the real 738 and his company SOP calls for 'bug up' at thrust reduction alt (the queue for the PM to select the airspeed bug to flap up speed). He said that he's never seen the speed window indicating min speed reversion on takeoff.

 

Where am I going wrong?


Martin Neep

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Where am I going wrong?

 

This is assuming the FMS has been properly setup before takeoff.

 

Increase the thrust levers to 40% N1 and let the engines stabilize. Then press the TO/GA button (key command, clickspot or actual button). Ensure thrust has been set by 80 knots.

 

Rotate at VR and pitch up toward 15 degrees. Once airborne, follow the commands from the flight director. Raise the landing gear when a positive climb has been indicated on the baro altimeter.

 

Above 400 feet, select a roll mode (LNAV or HDG SEL)

 

At the thrust reduction altitude, select or verify that climb thrust has been set,

 

At the acceleration altitude, pitch down and increase speed to 250 knots. Retract the flaps on schedule. Select VNAV. Engage the autopilot after a roll mode and pitch mode are activated.

 

V2 always falls within the orange low speed buffet range and A14X flashes on the speed window to indicate that the selected speed is too low. Obviously, the speed means very little because with the pitch mode in 'TOGA' the FD is commanding V2+20.

 

I think you should monitor your airspeed and correct it if necessary.


Kenny Lee
"Keep climbing"
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The change to CLB has no effect on the minimum speed. The top of the lower amber bar is a function of weight - the more the higher it is. So if V2+20 is within the amber band, then your T/O performance calcs/speed is incorrect.


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Hi,

 

if you sent me your payload, number of passangers, metar, airport, runway, I can put it in my topcat and send you correct speeds. We can then compare if you have the same (plus minus)

 

Petr

 

 

 

Sent from my BlackBerry 9900 using Tapatalk


Best regards

Petr

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Sorry, I didn't explain it very well.

 

It's not that my actual airspeed is low, following the FD, current IAS is approximately V2+20.

 

The problem is that the airspeed bug, which is set to V2 prior to takeoff, falls within the low speed buffet margin, causing the A/T to display low speed protection.

 

I'll take a screenshot later when I get home that might help to explain it better.


Martin Neep

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Sounds like you might be in kg versus pounds.

 

I'm not using a 3rd party application to calculate V-Speeds. They're coming directly from the FMC based on actual weights.


Martin Neep

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I'm not using a 3rd party application to calculate V-Speeds. They're coming directly from the FMC based on actual weights.

 

That's not what he's talking about.

 

If the FMC is expecting LBS and you give it KGs, it will assume you're very light, and use very low (unattainable) speeds.


Kyle Rodgers

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I'm probably doing it wrong perhaps, but I always set the MCP IAS window to V2+20 (172 in your case) since I don't plan on going slower than that anyway. After that I'll engage one of the vertical modes (usually VNAV) and accelerate to 250kias unless otherwise stated in the flight plan.


Thanks

Matthew T Gardiner

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Hmmm. Did you maybe over-rotate and get slow?

 

Did you have F5 set for T/O? Your indicator is at about 7. . .

 

I'm probably doing it wrong perhaps, but I always set the MCP IAS window to V2+20 (172 in your case) since I don't plan on going slower than that anyway.

 

 

V2 goes in the window, and the box figures out the rest.


Matt Cee

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Hmmm. Did you maybe over-rotate and get slow?

 

Did you have F5 set for T/O? Your indicator is at about 7. . .

 

 

 

 

V2 goes in the window, and the box figures out the rest.

 

Makes sense. Thanks


Thanks

Matthew T Gardiner

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Where am I going wrong?

You're going wrong in that you haven't manually selected flaps up speed on the MCP at acceleration height, nor have selected a pitch mode (you're still on TO/GA), nor have armed a pitch mode (VNAV) on the ground. That, in combination with a heavily loaded aircraft, will result in the behaviour you're observing (disclaimer: I Am Not A Pilot).

 

Instead, do as the FCOM says (NP.21.29) and arm VNAV as part of your before start procedure. Alternatively (depending on the SOP you're following), you could engage LVL CHG at 1000' AGL and select flaps up speed manually at acceleration height.

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