Ask Captain Lim - What does the Boeing 777 Thrust Asymmetry Compensator do?

I have a question with reference to the Boeing 777.

I want to know about the mode of operation of the Thrust Asymmetry
Compensation (TAC) which is a special feature of the Boeing 777. As I understand, it is a system which helps the pilot to handle an engine failure during the critical stage of the flight, for example, an engine out after a V1 Speed on the Runway. Here are my questions:-

1. Is the LNAV button always engaged? As far as I know, there would definitely be a Flight Plan which would take care of this condition to safely guide the aircraft for a go-around and land at the same airport.

2. TAC will automatically control the rudder. So when an engine goes out, the
auto pilot adjusts the rudder to compensate. In this scenario, if the pilot initiates a turn, does the rudder needs to be applied? Will this have any consequences?

Also, if the plane is flying in the LNAV Mode and aircrafts makes a turn,
then the FMS would be giving the appropriate commands to maneuver the aircraft.

As TAC is a separate system from the FMS, would they not be receiving conflicting data? Which system has the priority?

3. FMS handles engine out conditions. So why one more system called
TAC. They are not found in any Airbus aircraft or in the B747-400. What makes the difference?

Please explain in detail how and in what different ways this situation is handled. How the systems interact?

Thank you, Warm regards,

Srihari J,
Software Engineer, Honeywell

Hi Srihari,

I believe you have some knowledge on the LNAV and the operation of the FMS. The TAC is unique in that is is only available on the Boeing 777. The TAC or Thrust Asymmetric Compensation system helps the pilot control the yawing effect of the Boeing 777 when it suffers an engine failure. When the system senses the thrust level on one engine differs by 10 % or more from the other engine, it automatically adds rudder pressure to minimize the yaw. After a few seconds, the TAC applies sufficient rudder to make it possible for the pilot or auto pilot to center the control column. The TAC can be manually overridden by making manual rudder input.

Now to answer your 3 questions:-

1. The LNAV is always engaged before the take off. If the auto pilot is
engaged after 200 feet above ground level, any yawing caused by an engine
failure would be taken care of by the auto pilot and the TAC. The
engine-out profile and route can also be programmed on the FMS to
enable the aircraft to land safely on the departure airport or at the
alternate airport.

2. If a pilot initiates a turn with the TAC whilst controlling an engine
failure, any rudder input will have no major consequence because the TAC is capable of maintaining a balanced turn without any rudder manual input.

3. You must be clear of the function of the FMS, the Flight Management
System and the TAC. The FMS can handle the engine-out performance. What it means is that it can provide VNAV and LNAV data to the pilot and auto pilot but it could not control the yawing of the aircraft caused by an engine failure. Only the TAC can control the yawing moment of an engine failure.

So this is a special system peculiar on the Boeing 777 and not on any of the
Airbus aircraft or Boeing 747-400. Safe flying,