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gruesen

777 auto throttle logic question...

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For you triple 7 captains out there...

 

While in TOGA mode, will the auto throttle logic prevent the plane's speed from going beyond the flaps schedule during normal ops? I've notice in the sim that after takeoff the plane will accelerate past the point at which I need to retract flaps 1, up, etc.... Is this normal? Is it up to me to keep this from happening?

 

Thanks in advance!


                                                                                                              Michael Gruesen

   

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The autothrottle TOGA will command maximum thrust to the EICAS limit (THR REF). The airplane accelerates so it can meet its speed schedule. For example if you have a fix that requires you to be at 230 knots 10nm after takeoff then the autothrottle will accelerate to mantain that airspeed. So if the autothrottle has to comply with the speed schedule, then yes you have to retract the flaps.


Rem Yang
Flying is Fun!
 

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The question is will the AT allow VMO exceedance for the current config? I have found the answer to be yes in the sim.

 

I thought it would be smarter and pitch for speed (VMO - 5 kts) or go to SPD mode until config permits further acceleration.

 

Best regards,

Robin.

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While in TOGA mode, will the auto throttle logic prevent the plane's speed from going beyond the flaps schedule during normal ops? I've notice in the sim that after takeoff the plane will accelerate past the point at which I need to retract flaps 1, up, etc.... Is this normal? Is it up to me to keep this from happening?

 

Hi Michael,

 

TOGA sets a constant thrust.  You should have VNAV (and LNAV or Heading Select) armed in the MCP (autopilot) before takeoff.  On takeoff, you need to pitch the aircraft up to maintain the target speed shown in pink, upper left on the PFD.  If you climb more slowly the speed for the current flap setting can and will be exceeded, since the thrust is constant.  The Flight Director (visible on the PFD) will give vertical guidance.  As you pass the retraction speed for the current flap setting, you should retract the flaps to the next setting.  The pink target speed will increase.  Repeat until flaps are fully retracted.  Above 400 feet you can engage the autopilot and VNAV will pitch the aircraft up to do the same thing.

 

At a preset altitude the engine control will switch to climb, climb-1 or climb-2, depending on the thrust setting you have set in the CDU (FMC screen).  (You should choose a lower climb thrust unless the aircraft is very heavily loaded, or it will pitch up excessively.  The 777 has very powerful engines.)  The engines will switch to a somewhat lower thrust.  But they will still maintain a constant thrust so you, or the MCP, still need to maintain an appropriate nose-up attitude to maintain the proper speed and prevent overspeeding.

 

Only when you approach the altitude set in the MCP (or the altitude limit of the next waypoint if lower) will the autothrottle switch to Speed mode and manage thrust to hold the target speed shown in pink in the PFD.

 

Hope this helps!

 

Mike


 

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Helps a lot! Thanks for the replies!

 

Then, in the sim, when the trip planned requires relatively little fuel and payload, expect the triple 7 to climb like a rocket and flaps retraction to happen quite quickly unless a pretty generous derated takeoff setting is used.. I suppose realistically there aren't too many circumstances in which this would happen for the 777, but when it does, you guys that drive this thing (more so you freighter guys) must have a complete ball during takeoff! Do you tell any passengers to please hold on to the bar before you hit the gas??

 

Thanks again for the help! I'll practice derated takeoffs and following the reference cross bars on the PFD as needed while avoiding tail strikes.


                                                                                                              Michael Gruesen

   

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TO/GA maintains a given thrust setting. If the pilot engages TO/GA but doesn't effectively start to climb, then the aircraft WILL exceed maximum speeds.

 

Then, in the sim, when the trip planned requires relatively little fuel and payload, expect the triple 7 to climb like a rocket and flaps retraction to happen quite quickly unless a pretty generous derated takeoff setting is used.. I suppose realistically there aren't too many circumstances in which this would happen for the 777, but when it does, you guys that drive this thing (more so you freighter guys) must have a complete ball during takeoff! Do you tell any passengers to please hold on to the bar before you hit the gas??

 

I'm not a pilot, but I can tell you that yeah, at low weights and high power settings the aircraft will rocket to the sky, just like the 757 does. Even if you apply derated thrust, assumed temperatures or both, at low weights the 777 is a rocket.

 

As you know, during the initial climb after take-off, the target is speed and pitch will be modified so as to maintain the target speed (V2+15 usually). However, pitch is modulated up to a maximum of 25º, even if that results in acceleration beyond the target speed. And the 25º pitch limit is to keep it comfortable for everyone on board (and for other reasons like, it's easier to level off from a 25º than from a 35º nose-up attitude, not overshooting a low initial cleared altitude, visibility over the nose...)


Jaime Beneyto

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Then, in the sim, when the trip planned requires relatively little fuel and payload, expect the triple 7 to climb like a rocket and flaps retraction to happen quite quickly unless a pretty generous derated takeoff setting is used.. I suppose realistically there aren't too many circumstances in which this would happen for the 777

 

Hi, Michael,

 

Yep!  It does climb like a rocket when moderately loaded unless serious thrust derate is used.  There are some relatively short -200LR flights in real world -- Atlanta to LAX and Toronto to Victoria BC for example (both on way to Sydney Australia).  PAX might be pretty full but not a lot of fuel.

 

Mike


 

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visibility over the nose

 

Not a concern. Pitch limit is due to pax comfort and controllability without requiring fighter-pilot reactions and hard control inputs. The aircraft is designed to be flown by pilots with average ability on an average day.

 

Best regards,

Robin.

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