777 FBW control law
 

Greetings,

Can anyone enlighten me on the following questions.

1.) Is there such logic like Primary law, secondary law, direct law.?

2.) Is there a point on the approach phase, say 100AGL that the
FBW computer goes to direct-law?

thanks. :ok:

That sounds like Airbus terminology.

The 777 has Normal, Secondary and Direct flight control modes.

It has 4 actuator control electronics (ACEs), and 3 primary flight computers (PFCs).

NORMAL MODE.

The ACEs receive pilot control inputs and send them to the PFCs. The PFCs verify the signals and info from other systems to compute control surface commands. These commands are returned to the ACEs. The ACEs then send enhanced signals to the flight control surface actuators.

With the A/P engaged, the A/P sends commands to the PFCs in a similar manner to pilot control inputs. The pilot controls move to indicate what the A/P is doing.

Normal mode also offers envelope protection for:-

-stall
-overspeed
-roll envelope bank angle

SECONDARY MODE.

When the PFCs can no longer support normal mode due to faults or lack of reqd. info from other systems, secondary mode is activated.

The ACEs still receive pilot control input, and send this to the PFCs. But now the PFCs use simplified computations to generate control surface commands. These are sent back to the ACEs, and are then sent to the control surface actuators as in normal mode.

Secondary mode affects handling qualities. The elevator and rudder are more sensitve at some airspeeds.

The following are inop in secondary (and direct) mode:-

-A/P
-Auto speedbrakes
-Envelope protection
-Gust suppression
-Thrust asymmetry compensation
-Yaw damping (degraded or inop)

DIRECT MODE.

When all 3 PFCs fail, the ACEs go to direct mode. The PFCs no longer generate control surface commands. Pilot inputs received by the ACEs go directly to the control surface actuators. Handling qualities are similar to secondary mode.

Direct mode can also be manually selected (secondary cannot).

Finally there is mechanical back up in the event of total electrical shut down. Cables run to the stabilizer and selected spoilers to enable straight and level flight until electrics are restored.

if I am not wrong, tail strike protection, amidst those you listed, is also inop in secondary mode.

I believe tail strike protection is applicable to certain variants only, on the -300ER.

I also missed in direct mode, the manual rudder trim cancel switch is inop.

Tail strike protection is also available on the 777-200LR.