One of the most variable parts of airline flying is the takeoff. In contrast to general aviation takeoffs, which are always performed at full power and the same speed, airline takeoffs are calculated with exact precision that uses various power, trim, and pitch settings. This maximizes efficiency while abiding by strict climb criteria in 14 CFR Part 25.
The gist of these regulations is an unwavering commitment to ensure airliners can fly safely if an engine failure occurs on takeoff. This article briefly overviews the takeoff climb performance and requirements at the airlines. They are known as the four segments.
First segment
The first segment of an airliner's climb starts 35 feet above the runway at a variable speed between Vlof (the speed at which the aircraft first becomes airborne) and V2. The first segment ends after the landing gear is fully retracted, and the aircraft has achieved V2 speed.
The first segment of the climb is meant to ensure that a plane can climb away from the runway with the gear still extended in the event of an engine failure at or after V1. Accelerating to and maintaining V2 in this segment ensures that the aircraft is controllable and required climb gradients are met in the event of an engine failure.
Vlof, V1, Vr, and V2 are all determined by the conditions for the day. Aircraft weight, outside air temperature, runway slope, flap settings, and manufacturer limitations all play a part in determining the speeds for a takeoff. More than anything, the aircraft's weight and air temperature influence the amount of thrust needed for a launch. Heavier planes on warmer days require higher thrust values.
Second segment
The second segment of the climb is the most restrictive regarding performance limits. It mandates at least a 2.4% climb gradient (2-engine aircraft) is achieved while flying no slower than V2 with an engine failure. The second climb segment commences after the gear is fully retracted and ends 400 feet above the "takeoff surface." With the primary source of drag eliminated after gear retraction, the aircraft can climb at an increased rate, thus the higher gradient requirement.
Pilots want to fly a straight course down the departure corridor if an engine fails, but that isn't always practical if there is terrain off the departure end of the runway. The first turn after takeoff cannot be made until 400 feet above the takeoff surface, so it's the 2.4% gradient of climb that the FAA and airlines use as the standard to determine departure paths for engine failures at airports with surrounding terrain. This is also why the second segment of the climb ends at 400 feet.
Third segment
The third segment starts above 400 feet. The third segment allows for acceleration so that flaps and slats can be retracted. Many airline procedures require a climb to a higher altitude, such as 1,000 feet, before the aircraft accelerates beyond V2 in the event of an engine failure. Though the aircraft can accelerate in level flight during the third segment, regulations require it to be capable of climbing at a 1.2% gradient for the entire segment with a failed engine. The third segment is when passengers might notice reduced engine power as maximum continuous thrust is set.
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Fourth segment
The fourth and final segment of the climb starts when the flaps have been fully retracted and maximum continuous thrust is set. The plane is still required to be able to climb at a 1.2% gradient up to 1,500 feet above the takeoff surface. Once at this altitude, the fourth stage gives way to an en-route climb or an emergency return in the case of an engine failure.
This above is a general, quick overview. The regulations covering takeoff performance are vast and consider many topics not covered in this brief discussion (such as margins above stall speed, tire pressure limitations, etc.). A general takeaway is that takeoffs are complex, and there are quite literally hundreds of variables that are considered for each one. No two departures are the same, but the four segments of the climb are unwavering.