Takeoff Path and Takeoff Flight Path (Ref JAR/FAR)

Takeoff path extends from a standing start to a point at which the airplane is at a height:

• of 1500 ft above the takeoff surface, or
• at which the transition from takeoff to the en-route configuration is completed and final takeoff speed is reached,
• whichever point is higher.

Takeoff flight path begins 35 ft above the takeoff surface at the end of the takeoff distance.

The definitions of the takeoff path and takeoff flight path are used to establish performance requirements for multi-engine aircraft. Regulatory definitions assume that the aircraft is accelerated on the ground to Vef (engine failure speed), at which point the critical engine is assumed to be failed or inoperative for the remainder of the takeoff. Additionally, the aircraft must achieve V2 speed before reaching a height of 35 feet above the takeoff surface, and it must maintain a speed not less than V2 until it reaches a height of 400 feet above the takeoff surface.

Takeoff Segments and Climb Requirements

Within the context of the takeoff flight path, two primary requirements must be met, both predicated on the assumption of an engine failure occurring at VEF. Firstly, the aircraft must demonstrate the capability to achieve a minimum climb gradient. Secondly, it must ensure sufficient obstacle clearance throughout the climb phase.

When evaluating adherence to regulations, manufacturers may opt for either a continuous demonstrated takeoff climb or a segmented takeoff climb approach. The segmentation of the takeoff climb simplifies requirements and procedures, making them easier to comprehend and implement.

The takeoff flight path is divided into four segments, each with its distinct characteristics and requirements. After an engine failure at Vef, aircraft must fulfill minimum climb gradients as specified for different segments.

First Segment

• First segment of take-off flight path starts when takeoff is complete (aircraft is at 35ft with V2 speed and critical engine inoperative). 35 ft screen height is the start point of segment 1.
• Engine is at takeoff thrust setting.
• Objective at this point is to climb, as expeditiously as possible. Landing gear is retracted when positive climb is established, and once gear is up and locked then first segment is finished.
• Minimum climb gradient requirement for this segment is 0% for twin engine aircraft and 0.5% for quad engine aircraft.

Second Segment

• Second segment starts when landing gear is up and locked (Drag is reduced).
• Engine is at takeoff thrust setting.
• Minimum climb gradient requirement for this segment is 2.4% for twin engine aircraft and 3% for quad engine aircraft.
• Objective at this point is to retract the flap to reduce drag further, but flap retraction is not permitted below 400 ft. Therefore, aircraft is considered to be climbing at speed not less than V2 until 400 ft is reached. This marks the end of segment 2.

Third Segment

• Third segment starts at or above 400 ft and flaps/slats are retraction is considered in this segment. However, retraction of flaps/slats will increase the stall speed. Therefore, the aircraft must accelerate in this segment from V2 to the final take-off speed. The final take-off speed is intended to be the one engine inoperative best angle of climb speed (green dot speed for Airbus A320). Reaching this speed, thrust can be reduced to MCT.
• Minimum climb gradient requirement for this segment is 0%. Aircraft is in level flight to accelerate fast.

Fourth Segment

• Fourth segment starts when flaps/slats are retracted, and final segment speed is achieved.
• Engine is at maximum continuous thrust setting.
• In fourth segment aircraft climbs to 1500 ft which marks the end of takeoff flight path.
• Minimum climb gradient requirement for this segment is 1.2% for twin engine aircraft and 1.7% for quad engine aircraft.

Key points:

• In all the segments aircraft is considered out of ground effect.
• Segment 2 has most severe gradient requirement, climb limit weight ensures that the aircraft (in event of engine failure) is able to meet this requirement.
• Climb gradients are air based and therefore independent to the effect of wind.

Gross and Net Takeoff Flight Paths

Most runways are surrounded by obstacles that must be carefully considered before takeoff. It is crucial to ensure that there is a sufficient vertical margin between the aircraft and these obstacles in the takeoff flight path.

Gross Flight Path –

It is the takeoff flight path that is actually flown by the aircraft. In simple language it is what aircraft will do with an engine failed if flown according to the airplane flight manual.

Net Flight Path –

It is hypothetical path established for performance considerations of aircraft. Gross takeoff flight path minus a mandatory reduction gives net flight path. Flight manual performance data is all net data. This ensures that aircraft actual performance is better than expected.

Net Gradient = Gross Gradient – Gradient Penalty

Gradient Penalty is 0.8% for two-engine aircraft and 1% for four-engine aircraft.

Net Flight Path must meet all the climb gradient requirements of all four segments of takeoff flight path.

OBSTACLE CLEARANCE

Apart from climb gradient requirements we must also consider obstacle clearance requirements.

Operator must ensure that net take off flight path must clear all obstacle by a vertical margin of at least 35 ft. If this is not possible due to airfield topography, aircraft must turn away from obstacle and clear it by a horizontal distance.

– 90m + 0.125 D, or,
– half wingspan + 60m + 0.125 D (for aircraft with wingspan of less than 60 m)

D is the horizontal distance that aircraft has travelled from end of TODA.

Extra considerations when aircraft is clearing obstacle horizontally:

• turn increase the g load hence climb gradient is reduced and stall speeds are increased (allowance must be made for same)
• turns are not allowed below a height of half the wingspan or 50 ft whichever is greater
• up to 400 ft, bank angle may not be more than 15 degrees
• above 400 ft, bank angle may not be more than 25 degrees

Despite everything, obstacle clearance may sometimes require the second segment gradient to be greater than 2.4% and, consequently, the maximum takeoff weight may have to be reduced accordingly. This is a case of obstacle limitation.

Takeoff weight is calculated keeping all these obstacle considerations in mind is called obstacle limit weight.

Winds Considerations – In first section of this article we discussed about climb gradient requirements in different segments of take-off, these were the air gradients. For obstacle clearance, ground gradients and used and winds must be considered. [50% of headwinds and no less than 150% of tailwinds are used.]