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1.1% for two-engined aeroplanes 

1.6% for four-engined aeroplanes.” 

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

0.5% for four-engined aeroplanes.” 

Net Gradient = Gross Gradient – Gradient Penalty 

Figure D2: Gross and Net Drift Down Flight Paths (Climb and Descent) 

2.1.3. Takeoff Alternate Airport 

JAR-OPS 1.295 Subpart D FAR 121.617 Subpart U 

If an engine failure occurs during the takeoff phase, the preferred option is generally to turn back and land at the departure airport. When the landing requirements are not met, for meteorological or performance reasons, it is necessary to plan a takeoff alternate airport, which shall be located within: 

One hour flight time at a one-engine-inoperative cruising speed in still air for twin engine aircraft. 

Two hour flight time at a one-engine-inoperative cruising speed in still air for four engine aircraft. 

When it is not possible to return, the flight must be pursued to the takeoff alternate airport, and the en route configuration1 achieved after a maximum of 10 minutes from the brake release point. As a result, the drift down climb phase starts at the end of the takeoff flight path. To reach the takeoff alternate airport, the obstacle clearance must be ensured, in accordance with the following paragraph: 

2.2. En route Obstacle Clearance – One Engine Inoperative 

2.2.1. Lateral Clearance 

Obstacle clearance must be ensured throughout the route, in case of an engine failure. The problem is to clearly identify which obstacles must be cleared. Regulations indicate which obstacles must be taken into account: 

“JAR-OPS 1.500 

(c) The net flight path must permit the aeroplane to continue flight from the cruising altitude to an aerodrome where landing can be made […] clearing […] all terrain and obstructions along the route within 9.3 km (5 nm) on either side of the intended track”  (d) […] an operator must increase the widths margins […] to 18.5 km (10 nm) if the navigational accuracy does not meet the 95% containment level2.” (Figure D3). 

Note that the FAR regulation is quite similar, except that it requires a lateral margin of 5 statute miles on each side of the intended track. Moreover, it stipulates that a “different procedure” approval is needed, when the aircraft is further from the nearest approved radio navigation fix than from the critical obstruction it has to pass over. 

To carry out a detailed route study (engine failure case), a topographic map shall be used and the highest obstacles inside the required corridor width determined. 

Another, less time consuming, but less accurate method, consists of using the published Minimum Flight Altitudes which already account for a margin of 2,000 feet on the obstacles (refer to the “Minimum flight altitude” section of this chapter). 

2.2.2. Vertical Clearance 

Vertical clearance shall always be understood as a margin between the net flight path and the obstructions. The en route net flight path shall be determined from the Aircraft Flight Manual, and must take into account the meteorological conditions (wind and temperature) prevailing in the area of operations. Moreover, if icing conditions can be expected at the diversion level, the effect of the anti-ice system must be considered on the net flight path. 

Any route study should be conducted by checking one of the following two vertical clearance conditions. When Condition 1 cannot be met, or when it appears to be too penalizing in terms of weight, a detailed study must then be carried out based on Condition 2. 

2.2.2.1. Condition 1 : 1,000 feet clearance margin 

“JAR-OPS 1.500 

(b) The gradient of the net flight path must be positive at at least 1,000 ft above all terrain and obstructions along the route.” (Figure D4) 

A-Methodology, in case of an Engine Failure in Climb 

Determine the location of the start of the en route flight path in the worst conditions. 

From a topographic map select, in the regulatory corridor, all the constraining obstacles that must be cleared during the climb phase. Plot these obstacles on a graph, with their distance from the start of the en route flight path (horizontal axis) and their height (vertical axis). 

From the AFM, determine the climb net flight path for a conservative weight (for instance, use the maximum certified takeoff weight), and for conservative meteorological conditions. Plot it on the previous graph.