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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?

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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

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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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Bug up should be before 2000ft. Probably around 1000ft or 1500ft. In the "Manual (Non-V-NAV) departure" operation, the Pilot (usually the not flying pilot) will select "Bug Up" that is, move the MCP Speed window up so that the "bug" (the magenta selector which shows what MCP speed is selected on the PFD) points to the little green "Up" (In your picture this is at around 219 knots).

 

If you depart in V-NAV, the FMS will basically do this for you.

 

Always accelerate before removing any flap. Only touch flaps when passing the green flap markings on the speedtape in the PFD (marked -5 -1 -up etc)

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Bug up should be before 2000ft. Probably around 1000ft or 1500ft. In the "Manual (Non-V-NAV) departure" operation, the Pilot (usually the not flying pilot) will select "Bug Up" that is, move the MCP Speed window up so that the "bug" (the magenta selector which shows what MCP speed is selected on the PFD) points to the little green "Up" (In your picture this is at around 219 knots).

 

'Bug UP' height depends on the type of departure profile flown....the MCP SPD Bug is 'wound' to 'UP' on the PFD SPD Tape after the selection of LVL CHG as the active AFDS Pitch Mode by the PNF...dependant on operator this normally happens at:

 

NDAP1 - 3000ft aal (On an NDAP1 the AP can be engaged after N1 is annunciated at 1500aal which is the Thrust Reduction Height)

 

NDAP2 - 1000ft aal & held at Flap UP until 3000ft aal (On an NDAP2 the AP can go in at 1000ft aal)

 

These heights can vary from operator to operator & the engagement of an AP at this height it will also ensure the AFDS Pitch Mode switches from TOGA to LVL CHG...much easier for a single FS pilot to manage.

 

Always accelerate before removing any flap. Only touch flaps when passing the green flap markings on the speedtape in the PFD (marked -5 -1 -up etc)

 

Ahhh I do love a good old ""FS'ism"....also illustrated in the YT Vid posted further up this thread.

 

The 73NG Flap Retraction Sequence differs from that used on say a B744 & it is wrong to assume that just because the PFD & SPD Tape/Flap Indications 'look' the same as the 744 then the same method is used.

 

It is not.

 

In an NG after take off maintain a minimum of V2 + 15 knots during the initial climb (marked on the SPD Tape by a white BUG)...at light gross weights a higher speed (up to V2 + 25) may be flown, to synchronize F/D pitch commands and avoid objectionable body angles.

 

20 degrees nose up pitch or body angle should not be exceeded, also limit the bank angle to 15 degrees until reaching V2 + 15 & do not use V/S until the flaps are UP & the lights are out.

 

Just to clarify messrs Boeing & latterly PMDG gave us a natty little table to follow.....

 

PMDG-NGX-FCOMv1 NP.21.46 (pp122) ....

 

So, during a Flap 5 take off & after 'Bug Up' & with the SPD increasing once past the white V2 +15 bug on the PFD SPD Tape select the Flaps to "1".

 

Passing the green "1" bug on the PFD SPD Tape with the airspeed increasing select the Flaps to UP.

 

With the Flaps UP (also check lights out) & when above any NDAP limiting altitude (think NDAP 2 & 3000ft) 'bug up' to 250kts or the ECON CLB SPD if ATC allow - then select VNAV.

 

Many operators do not allow the selection of VNAV before this point as they have NGs of differing AFDS capability within their fleet.

 

This ensures that the same proceedure is used whether the aircraft has had FMS upgrades (U10.8A etc) or not.

 

Readers might also like to review the NG Flap Extension Schedule as highlighted later on in the FCOM in another natty little table provided by messrs Boeing & latterly PMDG:

 

PMDG-NGX-FCOMv1 NP.21.68 (pp144) ....

 

(Disclaimer...of course I am not a RW Commercial Pilot so I might also be talking out of my 'hole'...but I do love reading & learning RW procedures from the manuals...makes my FS experience more real for me)

 

Right...that's my useful input for the month of January 2013 completed.

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As a real NG pilot, I can safely say you know what you are talking about.

 

Just a little extra on the acceleration altitude. As most operators now calculate performance with EFBs and not the charts or old runway analysis tables, then as rightly mentioned acceleration altitudes can vary between operators depending on what obstacle database they use and how they calculate their performance, usually 800'-1000' I believe. Generally, a normal departure will use the same acceleration altitude everywhere except for specific local requirements or the standard noise abatement NADP 1,2,A or B. However, EO acceleration altitudes can vary greatly, are weight, configuration and runway specific and depend on obstacles and terrain.

 

Regarding VNAV, we arm VNAV and LNAV for departure on the ground. LNAV automatically kicks in at 50' RA and VNAV just above 400' RA. I haven't used 'manual' accelerator since flying the classic, except for practice or in the sim.

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So the question remains, when reaching for the MCP to set 'bug up', is it normal to see AXXX indicating low speed protection BEFORE you reselect the bug?

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So the question remains, when reaching for the MCP to set 'bug up', is it normal to see AXXX indicating low speed protection BEFORE you reselect the bug?

No.

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The 73NG Flap Retraction Sequence differs from that used on say a B744 & it is wrong to assume that just because the PFD & SPD Tape/Flap Indications 'look' the same as the 744 then the same method is used.

 

Wow, I had no idea that the flap schedule is used differently. What about on the approach, you wouldn't select a speed slower than the manoeuvring speed for the current flap setting, right?

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Wow, I had no idea that the flap schedule is used differently.

 

It works every time except flaps 1 take-off.

 

What about on the approach, you wouldn't select a speed slower than the manoeuvring speed for the current flap setting, right?

 

Right.

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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?

 

I think that what you described occurs on the NGX regardless of whether you select assumed termperature reduced thrust or not. The reason that the bugged V2 speed is in the amber band is that the top of the amber band (when flaps are extended) shows the speed where you have 1.3 g's (which is 40 degrees of bank angle) maneuver capability to stick shaker. V2 speed only provides 30 degrees of bank maneuver capability to stick shaker, which would be in the middle of the amber band.

 

Now, the question is whether you should ever see the amber band and V2 at the same time (as well as get an underspeed limiting indication on the mode control panel). According to page 10.11.9 of Vol 2 of the FCOM, the amber band should be inhibited on takeoff until the first flap retraction. So, you should never see what you are seeing because you would select a higher speed before retracting the flaps, and the amber band should not appear until you retract the flaps.

 

It would be nice if a real world pilot would confirm this, but I suspect that the NGX is not true to the airplane in this regard.

 

Don Stimson

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I have never thought about the amber band on take off in the real aircraft and the FCOM explanation, which I had obviously forgotten, explains why :blush: I'll have a look on my next T/O but I am certain NGX is incorrect. However, in hindsight it is obvious that the amber band should be inhibited given that airspeed is low on takeoff and as the yellow is a place where the speed should never be during normal operations.

 

There is also a lot information on this in the FCTM, General Chapter on Maneuver Speeds and Command Speed.

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