B777 TAC fail rudder trim

[Milton2967]

I’m about to start a 777 course. I’ve been reading about TAC failure with an efato. This may seem an odd question but There’s no mention of which way to trim the rudder. Is it towards the failed engine or live engine. The rudder trim panel says nose l or r. Is this how you want the aircraft to move or where it is?

[Abbey Road]

Just to be sure, before beginning this 777 course, you do have a multi-engine rating, don't you? 🤔

Whether an aircraft has TAC or not, rudder trim is applied to zero the trim forces, after the rudder has been used to yaw the aircraft to where it needs be. As is done on any multi-engine aircraft with conventionally operating controls.

[awair]

B777 Engine Fail on Take-off

‘Simple failure’ - TAC should remain operational and will trim for you (10-13 units, depending on thrust/airspeed). Ideally your trainer will show this first and let you see how much trim has been applied.

Severe damage: engine indications may disappear, so TAC is automatically disabled in case this is only an indication failure and not an engine failure.

In this case 1) fly the aeroplane, 2) at a safe height/speed/attitude, apply the appropriate rudder trim…

Good luck, and enjoy.

[wiggy]

Milton2967

For your info TAC only backs up, somewhat slowly, what you should be doing with your feet in the event of an engine failure and as I recall it it deliberately leaves you slightly short of the required amount of trim. TAC inop or not don't throw away all you've learnt on any previous multi engine types you've flown, it still applies.

I'd ignore the nose L or R placard. On the T7, the rudder trim works in the same physical sense as any other trim control on any other aircraft...

If you are still in doubt have a think about how you trim in pitch (in terms of trim control movement) , apply that logic to rudder.

[tdracer]

Severe damage: engine indications may disappear, so TAC is automatically disabled in case this is only an indication failure and not an engine failure.


This was a major concern during TAC development - what we called "Adverse TAC". TAC is based on indicated N1 (since N1 is normally directly related to thrust). There are certain engine failure where N1 and thrust can go opposite ways (bird strike is big one - major fan damage due to a bird strike can actually unload the fan so it speeds up while thrust decreases - another is a sheared fan shaft forward of the N1 speed sensor so fan speed rapidly drops while the unloaded turbine speeds up giving a large increase in indicated N1).
Anyway, adverse TAC - the rudder going the wrong way with an engine failure - is potentially catastrophic. So there is a "real time engine model" constantly running in the Engine Data Interface Unit (EDIU) - if the engine parameter indications don't make sense to the EDIU (indication a faulty indication or severe engine damage) it disables TAC.
Also, TAC rudder authority is limited - for the same basic reason - limiting the ability of Adverse TAC to make a bad situation worse. So even with TAC active you'll likely need to apply some rudder.

[sudden twang]

Keep wings level if control wheel is canted to the right you need more right rudder and vice versa. The acid test that you are in trim is that the control wheel is level.