The A320’s auto-trim is just that.
It’s a flight control system feature that automatically trims the Stabiliser throughout the flight.
It is available whenever the aircraft is operating in Normal or Alternate law.
In essence, when you use the sidestick to set a pitch, the auto-trim will trim the horizontal stabiliser so that the aircraft will maintain that pitch.
Let’s see how it works.
In the A320, the two elevators and the Trimmable Horizontal Stabiliser control the aircraft in pitch.
As you can see in the illustration above when you set a pitch using the sidestick you are using the ELACs (normal case) or the SECs (failure case) to position the elevator to the desired position.
These same computers ensure that the auto-trim runs automatically, trimming the horizontal stabiliser so that the aircraft will maintain the requested pitch with a neutral elevator.
While at first glance, this might not appear to be that big a deal, the auto-trim represents one of the most important differences between the A320 and conventional aircraft and has some interesting repercussions for pilots.
Let’s dig a bit deeper.
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Let’s assume you’re flying a conventional aircraft.
The first thing to understand is that a conventional aircraft is trimmed for a particular speed.
Let’s imagine that you’re in cruise, in trim and flying at a particular speed.
If the thrust is reduced, and no inputs are made on the stick, the aircraft will naturally pitch down and lose altitude as it seeks its previously trimmed speed.
This is a function of the natural stability of the aircraft and is a direct result of the CG being forward of the Center of Lift.
This nose-down tendency will eventually cause our conventional aircraft to return to its trim-speed.
In other words, it is “speed-stable.”
It’s a characteristic most pilots take for granted and highlights a huge difference between the Airbus and conventional aircraft.
In the A320, with the sidesticks at neutral, the flight control computers will maintain 1G.
So, in the A320, if we pull the thrust off as in the first case, the nose will not drop as it did in the conventional aircraft.
Instead, the auto-trim function will trim the aircraft to maintain the original 1G pitch.
This is pretty cool, but it’s important to realise that, although the A320 maintains the requested pitch, it will end up doing so at the expense of the aircraft’s speed.
In other words, the A320 is “pitch-stable.”
In fact, it’s so pitch-stable that eventually, the speed will bleed off, and if nothing is done, the aircraft will get dangerously slow.
This clearly illustrates the different stability characteristics of the Airbus and why it’s important to clearly understand, that this is not your previous aircraft.
When compared to a conventional aircraft, the Airbus displays radically different stability characteristics.
So, there you go.
We’ll continue to explore this concept throughout our collection of articles on the A320’s flight control system.
For now, you should understand that although the auto-trim is an amazing feature of the A320, it carries with it some interesting repercussions that every smart pilot should understand and appreciate.
Be kind, be smart, fly well.
Until next time…
“As you can see in the illustration above when you set a pitch using the sidestick you are using the ELACs (normal case) or the SECs (failure case) to position the elevator to the desired position.”
I’m not showing any illustration above.
Oh… the illustration is in the “previous” lesson…. I see…