Airbus MCDU Guide (4): Radio Navigation & Weight/Fuel Initialization Explained

August 30, 2025 Daniel W

PERF / Performance Settings

This section is very important, as it covers several critical phases of flight.

TAKEOFF

Takeoff Parameter Settings

The first values we need to pay attention to are THR RED / ACC (5L on the MCDU).

THR RED = Thrust Reduction Altitude. Above this altitude, the pilot can reduce engine thrust from TOGA or FLX/MCT down to CL.
THR ACC = Thrust Acceleration Altitude. This must be equal to or higher than the thrust reduction altitude. It represents the transition point from takeoff to climb. Usually, these two values are the same, but exceptions exist (e.g., due to airport noise abatement procedures).

The minimum for both must be: airport elevation + 400 ft.
In our example, we set both to 3000 ft, though this may differ in real-world operations.

TO SHIFT (Takeoff Runway Shift)

This value is used when the available takeoff distance is shorter than the actual runway length.
For example: if a runway is 3,600 m long but 600 m cannot be used, then 600 must be entered in TO SHIFT.
The suffix [M] indicates that the unit is meters.

FLAPS / THS (Trim of Horizontal Stabilizer)

The A320/A330 family offers five flap configurations for takeoff: 0/1/1+F/2/3/F.

Each flap setting has a corresponding maximum speed limit (VFE):

Flaps 1 → 230 kt
Flaps 1+F → 215 kt
Flaps 2 → 200 kt
Flaps 3 → 185 kt
Flaps FULL → 177 kt

For takeoff, Flaps 2 is generally recommended, as it usually gives the best performance.
Using 1+F may be slightly less efficient but has the advantage of automatic flap retraction.

Note: It is not possible to set Flaps 1 for takeoff. If the pilot tries, the aircraft will automatically use 1+F.

Good examples of Flaps/THS settings are:

2/UP0.4
1+F/DN0.1
3/UP0.6

Entry rules for FLAPS/THS:

The first character must be a number or the letter F.
Flaps and trim must be separated with a “/”.
After the slash, only UP or DN may be used, followed by a number.

FLEX TO TEMP

FLEX is a reduced-thrust takeoff setting designed to decrease engine wear and extend engine life.
It works by telling the FMGC (Flight Management and Guidance Computer) to assume the outside air temperature is higher than it actually is.

In simple terms: higher temperature = lower thrust, which is beneficial for the engines.

ISA (International Standard Atmosphere) at sea level = 15°C
OAT (Outside Air Temperature) = 28°C (example value here)
h = airport elevation (ft)

Thus, for this case, FLEX = 46°C.

ENG OUT ACC

This refers to the engine-out acceleration altitude (e.g., after one engine failure).
It is mainly for reference and is typically set at or above the highest of the THR RED / ACC altitudes.

V1/VR/V2 Speeds

V1 – Decision Speed
Before this speed, if an engine fails or another abnormality occurs, the pilot can choose to abort the takeoff, as there is enough runway remaining to stop safely.
VR – Rotation Speed
After reaching V1, the captain pulls back on the sidestick, raising the nose to begin liftoff.
Typical rotation pitch is around 10–15° nose up.
V2 – Takeoff Safety Speed
After rotation (VR), the aircraft should accelerate to V2 and then continue accelerating to the standard climb speed.
At V2, the aircraft is safely airborne and the landing gear may be retracted.

These speeds depend on runway length, aircraft weight, wind speed/direction, pressure, temperature, and flap setting.
Normally, they are calculated by a performance calculator.
Here, we set them arbitrarily to 140/142/144.

At this point, the MCDU INIT phase entries are complete, and the aircraft is ready for takeoff.
Before takeoff, the pilot must also set the desired climb speed and target altitude on the FCU (PRESEL values).
These will then display on the climb page.

Approach (APPR Phase)

After the cruise phase, the aircraft enters descent and approach.
At this point, the PERF page must be switched to APPR mode.

QNH → Destination airport pressure (provided by ATIS or ATC).
TEMP → Destination airport temperature.
MAG WIND → Wind speed and magnetic direction (note: referenced to magnetic north, not true north).
TRANS ALT → Transition altitude, typically 18,000 ft.
Vapp → Approach speed (automatically computed by the FMGS).
FINAL → Final approach segment.

MDA / DH

MDA (Minimum Descent Altitude): The lowest altitude for a non-precision approach.
DH (Decision Height): The height at which the pilot must make a decision to continue or go around during a precision (e.g., ILS) approach.

Key considerations for DH:

DH is a required decision point, but not the last possible chance to go around.
It marks when an action must begin (continue visually or initiate go-around), not the start of the decision process itself.
It represents the lowest altitude allowed for an ILS approach without visual references.
It is the absolute minimum before a visual reference must be acquired.
It does not cover all decision-making factors instantly—it is only one parameter.

Typical DH values range from 0–700 ft.

LDG CONF (Landing Configuration)

This defines the flap setting for landing.

On Boeing aircraft, flaps are defined in degrees (e.g., Flaps 30).
On Airbus aircraft, flaps are defined in “configurations” (e.g., Config 3).

By default, Airbus landings are performed in CONF FULL (full flaps).