Engine loss after takeoff roll and manual trim

[737Andi]

Hi,

I'm a flight simmer who wants to dig a bit deeper into the systems of the A320.

I saw 2 videos with an engine failure directly after takeoff roll and I was wondering why the pilot is manually trimming the side rudder to compensate the engine loss.
Shortly after that he engages the autopilot.
Now my question...why can he not engage the autopilot instantly without manually trimming and let the system do the side trim automatically?

Thanks!

Best regards Andi

[john_tullamarine]

Moved from Tech Log. Questions might be the more appropriate forum ?

[FlyingStone]

It's generally bad idea on any aircraft to engage the autopilot without aircraft being in trim.

[Uplinker]

The AP might not accept a very out of trim aircraft, and might give it straight back to you.

You don't need this and the cavalry charge attention getter on top of an engine failure, so best to trim the rudder before AP engagement. You might need to hold the trim switch over for about 12-15 seconds. :ok:

[Mad (Flt) Scientist]

Additionally, depending on the aircraft, it might not actually have the AP in command of the yaw axis - in some cases the AP only directly controls the pitch and roll axes (through elevator and aileron servos) - when the AP is engaged the yaw axis is left to the yaw damper, which handles any turn coordination requirements - and is almost certainly not expecting to have to deal with a significant asymmetry.

[deltahotel]

Indeed. Certainly on the 757/767 the rudder is a responsibility of the pilot. On a 3 channel ils the ac takes over the rudder at about 1500’

[Uplinker]

OP specifically asks about A320. Airbus A320 family autopilots do control the rudder.

As a matter of fact, in the event of an engine failure, the Airbus FBW puts in a certain amount of rudder for you. Not the whole amount - you still need to apply rudder pedal input and trim it before engaging the AP.

[Denti]

Tried it once in the simulator to engage the autopilot without trimming. Not nice at all, the autopilot will not necessarily disconnect, but it won't keep a straight and level path either and since it moves the trim very slowly and rudder authority is limited, chances are high you are far outside your obstacle one engine out path, or even crashed if you engaged it too early. So center the beta target, trim out the aircraft, engage the autopilot. Oh, and don't forget to aim for the correct pitch, don't sink, keep on the correct track. Not really rocket science, but nice training.

Hard to do it at home in a less complete (hardware wise) simulator environment.

Just for comparison, the 737NG can have a rudder servo, but doesn't help at all in the OEI take off case. During approach it will take over roughly around 1400 feet AGL (after the autopilot confidence test at 1500') and keep engaged throughout the rest of the flight and rollout, or go around, but will drop out once a different lateral mode is engaged. Could be similar to the 757/767 although it needs only two autopilots for that (and CAT IIIb).

[deltahotel]

Denti. Yes, like the 75/76 which will work with 2 or 3 autopilots. I have no 737 experience so always good to improve breadth/depth of knowledge.

The point I was trying to make to help the OP (obviously not well enough) is that a lot of ac won’t do all the rudder work for you (on my ac none of it until 1500’ on approach) so the pilots need to sort that out themselves before presenting a well trimmed ac to the AP.

HtH

[Denti]

Denti. Yes, like the 75/76 which will work with 2 or 3 autopilots. I have no 737 experience so always good to improve breadth/depth of knowledge.

The point I was trying to make to help the OP (obviously not well enough) is that a lot of ac won’t do all the rudder work for you (on my ac none of it until 1500’ on approach) so the pilots need to sort that out themselves before presenting a well trimmed ac to the AP.

HtH
Always good to get a different perspective. Although, the A320, the aircraft the OP was asking for, actually uses the rudder servo throughout its flight envelope when on autopilot. Still need to trim it out first if in a high power situation (like the usual V1 cut), but once trimmed, engage the autopilot and that takes care of (nearly) everything.

That said, the funny thing about the 737NG fail operational (most are fail passive) is that it gives a LAND 3 indication with just two autopilots, and even in single engine flight.