jb747
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- Mar 9, 2010
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Trim. This mostly applies to non FBW aircraft...but includes everything from a Cessna to a 747.
When an aircraft is in trim, and in an otherwise steady state (i.e. level flight, not accelerating or decelerating, or turning), you can take your hands off the controls, and it will basically stay in the same position. The amount and type of trim available varies. Some aircraft are trimmed by putting small biases into the flight controls. Others have small tabs on the controls that have the effect of moving them slightly. Most aircraft can be trimmed in roll, pitch and yaw.
Any change to the state of the aircraft will result in a new trim requirement. Pilots are taught to hold the load in the controls until the aircraft reaches it's new 'steady state', and to then trim out any loads. You don't fly by trimming... So basically make whatever the change is, hold the load, trim it out...in that order. You don't trim into transient states, i.e. you don't trim in a turn.
In piston engined aircraft, there is always some interaction between the prop flow and the rudder, so invariably there is some yaw input required with any power changes. In jet aircraft (as long as all of the engines are running), there is very little rudder trim required.
Aircraft become a bit bent over their lives (and some start that way), so they will often need different roll and yaw trim at different speeds. Asymmetric loads (of either fuel or stores) also require roll trim, and again the amount will vary with speed 'cos the amount of lift being made by the trimmed surface varies with speed, but the load roll moment doesn't.
Military aircraft often have a 'coughed hat' on top of the joystick, and that puts the roll and pitch trim right under your thumb.
Pitch trim has to counter power settings from idle to full power, and speeds from the stall to the maximum, and any combination of the two. Flaps also affect the trim required. On most modern aircraft the entire horizontal tail moves for this trimming action...the upshot of which is that the trim could actually have more control authority than the elevator.
In the case of a 737/767, when on approach and steady state, if you initiate a go around, and push the power to TO/GA, the underslung engines have a very strong pitch couple that will drive the nose up. You can hold the load against it, but it's used by the pilots to get the nose moving. You then push and trim forward as the nose reaches your desired target attitude.
In the event of an engine failure, you need rudder to counter the yaw, and then trim to reduce the load on your foot (to zero). Any speed or power change will require rudder and trim. You should pretty well always be able to let go of the controls for a couple of seconds, without the aircraft changing attitude.
The FBW aircraft are simultaneously better and worse. Normally the FBW removes the need for any trim inputs from the pilots, as it sees the motion caused by the out of trim condition, realises it doesn't want it, and simply moves the flight controls to counter it. So, back to that go around, and you push the power up...and if you don't pull the nose up, it will stay right where it was. You do need rudder trim in the case of an engine out. So, they are better in that trim normally doesn't exist as something you need to do.
But they are worse in that they no longer have any access to roll trim, and in many modes even pitch trim. Normally this isn't an issue, but when the aircraft revert to lower flight control laws, they may cease trimming. Pitch control is given back in direct law, but you never gain it in roll. So, if your aircraft is in some way asymmetrically loaded, or has the RAT deployed, then there may be a substantial rolling moment that can't be readily trimmed out. The flight controls themselves can also cause this by not fairing to a load neutral situation on the law change.
How is this bad? Firstly it removes trimming from the top of a pilots' head. And normally that doesn't matter, but with a law reversion it assumes its old importance. Secondly, because an out of trim aircraft can be very hard (in some cases impossible) to fly, it means that all of your attention has to be locked into flying it - look away for a few seconds, and your aircraft will have moved away from where you wanted it. This assumes even more importance in the case of some system failures. We sometimes practice running into volcanic ash, and you lose all engines. You end up with simultaneous law reversions, depressurisation, and loss of most electronics. The bloke in the left seat has to keep all attention on just flying, especially as the now deployed RAT will be trying to make the aircraft roll left. If you make the mistake of watching the other guy in his attempts to manage the failures you can make the situation much worse.
