330 Derated Takeoff

[Down Three Greens]

It is a minimum control speed issue. New control speeds for what is a essentially a procedure that tells the aircraft it has a different rated engine fitted to the aircraft.

Minimum Control Speeds with Derated Thrust

A given derate level corresponds to the basic maximum thrust reduced by a given percentage. Therefore, the new maximum available thrust at any point of the takeoff flight path is cut back, compared to the non-derated thrust. New minimum control speeds (VMCG, VMCA) can then be established, as per JAR/FAR 25.149. A reduction in the minimum control speeds sometimes generates a takeoff performance benefit (higher MTOW) when taking-off on a short runway. Indeed, the decision speed V1 is the maximum speed at which it is still possible to reject the takeoff and stop the aircraft within the runway limits. Nevertheless, V1 must be greater than VMCG, and the Accelerate Stop Distance is often the most constraining limitation on a short runway. A reduction of the VMCG can then permit a reduction of the ASD for a given takeoff weight, and lead to better takeoff performance when the
MTOW without derate is ASD/VMCG limited.

Derated Takeoff Procedure

Derated takeoff is not available for all Airbus aircraft models. It is basic on all A330 and A340 models, but doesn’t yet exist on the other Airbus aircraft types WRONG...now out of date...see later post...Available on A320 series too!!!. .When derated takeoff is available, 6 certified levels exist, ranging from (TOGA- 4%) to (TOGA–24%) with a constant four percent increment (4%, 8%, 12%, 16%, 20% and 24%)3. This means that the AFM must contain a set of performance data for TOGA, and a set for each derate level (TOGA - X%). To carry out a derated takeoff, the actual takeoff weight and speeds have to be checked against the Maximum permissible takeoff weight computed for the given derate level (specific RTOW chart or equivalent computerized system). The derate level must then be entered in the MCDU (Multipurpose Control and Display Unit) during the takeoff preparation phase. At the brake release point, the thrust throttles must be pushed to the FLX position.

Important: When a derated takeoff is carried out, TOGA thrust must never be
selected until the aircraft is airborne and above the minimum flap retraction speed (“F” speed). The reason for this is that performance calculations are made for minimum control speeds, different from the ones of TOGA.