Why is horizontal stabilizer set to 4° up ( or any other setting other than neutral) in airbus aircraft for take off? Why can't we keep the trim neutral?
Airbus WILL take off ~safely~ regardless of the pitch trim, as long as trim is in the t/o green range.
Adjusting the trim to the correct position, however, depending on the location of the CG for that flight, insures the aircraft handles similarly between takeoffs and rotation will be more or less standard between flights. This means the pilot can make the same side-stick input on each takeoff and get the same 3deg/sec rotation.
If not properly trimmed, rotation would be faster/slower and the pilot would have to adjust for that at a critical phase of flight. Not doing a good rotation will impact t/o performance significantly.
Both landing and take-off are critical phases of flight, and it is standard procedure to configure the aircraft for those phases to reduce the workload on the crew.
On landing, aircraft are configured for a stabilized approach, which means that the aircraft can be flown to the runway with a minimum of configuration changes to flaps/slats, trim, power, and drag devices.
On take-off, the aircraft is configured for take-off, and trim is set so as to provide a slight nose-up. If the trim is set nose low, it requires a lot of pull to "unstick" the aircraft from the runway. If the trim is set too nose high, then the aircraft will want to nose up before the aircraft may be ready to fly, and then to continue a nose-up attitude after rotation, requiring more than a minimal effort to keep the a proper departure attitude.
Nose-up trim can induce what is called a trim stall, which is when the trim is set so far back that the aircraft will exceed the critical angle on take-off:
ELEVATOR TRIM STALL The elevator trim stall maneuver shows what can happen when full power is applied for a go-around and positive control of the airplane is not maintained. [Figure 4-8] Such a situation may occur during a go-around procedure from a normal landing approach or a simulated forced landing approach, or immediately after a takeoff. The objective of the demonstration is to show the importance of making smooth power applications, overcoming strong trim forces and maintaining positive control of the airplane to hold safe flight attitudes, and using proper and timely trim techniques.
Source: Airplane Flying Handbook p. 4-10
The idea is to trim the airplane for the initial climb phase, so it assumes the correct attitude and speed without further control input. The exact value depends on CG location, weight, flap setting, temperature and more.
Consider you take off in IMC with nose-down trim and get the least bit distracted. The aircraft is at a real risk to hit the ground.