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. 

Note that in the Airbus philosophy, a flexible takeoff can not be cumulated with a derated takeoff. 

1 Standard after 1998, option before 1998 2 Option soon available on A320 family 3 For A340-500/-600, two supplementary derate levels: 32% and 40% 

D. EN ROUTE LIMITATIONS 

1. EN ROUTE FAILURE CASES 

In flight, engine or pressurization failures are potential problems, which must be carefully studied before operating a new route. Their occurrence seriously impact on flight altitudes and, therefore, become very constraining over mountainous areas. 

In case of an engine failure during flight, the remaining thrust is no longer sufficient to balance the drag force and to maintain an adequate cruise speed. The thrust necessary to fly at the initial altitude suddenly becomes greater than the available thrust delivered by the engines pushed at their Maximum Continuous Thrust (MCT) rating. The only solution is to then descend to a more appropriate flight altitude, where the available thrust can equal the required thrust, thus allowing the aircraft to level off. 

In case of an in-flight cabin pressurization loss, descent is also necessary. It is not dictated by a performance constraint, but by the oxygen system constraint. Indeed, at the initial cruise altitude, the rate of oxygen in the air is insufficient to allow crewmembers and passengers to breathe normally. This is why the installation of an oxygen system is required. As the necessary oxygen quantity must be quite significant to supply the entire cabin, its flow rate is limited to a maximum duration. So, a new flight altitude, where oxygen is no longer required must be reached, before a certain time limit. 

The descent cannot be always operated in the same conditions, since, aircraft are sometimes over-flying mountainous areas. This is why, in these particular cases, a route study is necessary to evaluate whether or not an acceptable escape procedure is possible when a failure occurs at the worst moment during flight. If it is possible, it must be clearly defined and indicated to the pilots. If it is not possible, a new route must be found. 

Any route study must be done in accordance with airworthiness requirements, detailed in the following sections. 

2. ENGINE FAILURE(S) 

2.1. General Definitions 

2.1.1. Drift Down procedure 

In case of an engine failure over a mountainous area during the climb or cruise phase, the Obstacle Strategy or Drift Down Strategy (Figure D1) should be applied. This procedure consists in: 

Selecting Maximum Continuous Thrust (MCT) on the remaining engine(s). 

Decelerating to green dot speed. 

Climbing or descending at green dot speed until reaching the drift down ceiling1. 

Figure D1: Drift Down Procedure (Climb and Descent) 

Green dot speed, indicated by a green circle on the primary flight display (PFD), represents the best lift-to-drag ratio speed, where aerodynamic efficiency is maximum. As a consequence, the drift down strategy is the procedure enabling the highest possible altitude to be achieved versus the distance covered. 

2.1.2. Gross and Net Drift Down Flight Paths 

JAR 25.123 Subpart B FAR 25.123 Subpart B 

2.1.2.1. Gross Drift Down Flight Paths 

The Gross Drift Down Flight Path is the flight path actually flown by the aircraft after engine failure (Figure D2). Regulations require that operators be provided the with drift down performance information, as stated below: 

“JAR/FAR 25.123 

(a) For the en-route configuration, the [gross drift down] flight path must be determined at each weight, altitude, and ambient temperature […]. The variations of the weight along the flight path, accounting for the progressive consumption of fuel […] by the operating engines, may be included in the computation. The flight paths must be determined at any selected speed, with: 

The most unfavourable centre of gravity 

The critical engine inoperative” 

2.1.2.2.  Net Drift Down Flight Path 

The Net Drift Down Flight Path represents the Gross flight path minus a mandatory reduction (Figure D2). 

1 Drift down ceiling = maximum altitude that can be flown at green dot speed (level off) 

“JAR/FAR 25.123 

(b) The one-engine-inoperative net flight path data must represent the actual climb performance diminished by a gradient of climb of