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"We are the protagonists of our stories called life, and there is no limit to how high we can fly."

Type rated on A330, B747-400, B747, B757, B767, B737, B727. International Airline Pilot / Author / Speaker. Dedicated to giving the gift of wings to anyone following their dreams. Supporting Aviation Safety through training, writing, and inspiration.

Wednesday, October 18, 2017

B777 Hight Lift Control System


The HLCS extends and retracts the leaning edge devices and trailing edge flaps. The flaps have six position: Up, 5, 15, 20 (takeoff), 25 and 30 (landing). Hydraulic or Electric motors on the PDU, turn flap torque tubes, that operate transmission assemblies, which use a ballscrew and gimbal to extend and retract the flaps. Sounds logical. 

Leading edge devices include seven slats and a Krueger flap on each wing. The Krueger flap seals the gap between the engine strut and inboard slat. The slats have three positions: Cruise (up), Takeoff (sealed, or mid), and landing (gapped, or fully extended). 

The HLCS works in three modes:
  1. Primary
  2. Secondary 
  3. Alternate
Primary is the fly-by-wire closed loop hydraulic system, where the flaps move from up to 30 with detents at the 1 and 20 positions. The Flap/Slat Electronic Units (FSEUs) we'll call the communication system, receive and transmit data and other systems add airspeed and hydraulic data through the busses for high lift protection that will help with high lift protection. 

With the primary system operating then FSEUs operate the autoslat priority valve for autoslat extension if plane gets close to a stall. 

If the FSEUs find a fault in the primary system it will automatically default to the Secondary mode. While the secondary mode is Electric, pilot control is still operated like the primary; however, electric motors on the flap and slap PDUs move the flap and slat mechanisms. 

Alternate mode is independent of the FSEUs and use electric motors. Pilots select the Alternate mode with the flaps arm switch and can extend or retract the flaps with the selector. While these switches utilize the secondary/alternate control relays for the flaps and slats in the same way as the secondary mode, the alternate mode uses flap and slat limit switches to limit the flaps to 20 degrees and the slats to the sealed position. 

Flap Slat Indications: 

Learn the difference between displays 
depending upon the flaps system used. 

Enjoy the Journey!
XOX Karlene 


  1. Hello, I wonder what is the source for the image of three-position slats? I'd like to use them in my work and want to cite it properly.

    1. Archernar, I believe I took a photo out of my Boeing Marketing book. I think. I would have to look when I get home (I'm out of the country). I don't think it's something that was open publication, but I found it at a used bookstore and saw the diagram and took a photo with my phone. I'll check when I get home and see if I could write you a reference.

  2. Thank you very much! It would be great.

    1. Great. How about you email me.


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