Introduction
When an airline crew computes maximum allowable takeoff weight, they are not simply asking whether the airplane can lift off within the available runway. They are answering a series of parallel performance questions, each of which can independently constrain the answer. One of the most frequently limiting constraints—particularly on hot days or at high-elevation airports—is the second segment climb gradient requirement. This standard, codified in 14 CFR Part 25, Section 25.121, ensures that a transport-category airplane can continue climbing safely after a takeoff with one engine inoperative, at a gradient steep enough to clear obstacles and provide a meaningful safety margin.
Understanding the second segment is essential for Airline Transport Pilot (ATP) knowledge-test preparation and, more importantly, for real-world flight operations. The discussion below explains the four takeoff flight path segments, the specific requirements of the second segment, the aerodynamic reasons it is often the most limiting, and how flight crews and performance engineers use it to establish maximum allowable takeoff weight (MATOW).
The Four Takeoff Flight Path Segments
14 CFR Part 25 divides the one-engine-inoperative (OEI) takeoff flight path into four segments, each beginning and ending at a defined configuration change. Knowing the segments as a group helps frame why the second is uniquely demanding.
- First segment — From the point of engine failure (at or after VEF) to gear retraction. Gear is still extended, drag is high, and the required gradient is effectively zero (level flight is acceptable for some configurations). The airplane accelerates through V2 and begins to climb.
- Second segment — From gear retraction to the point where the airplane reaches 400 feet above the takeoff surface. Gear is up, flaps remain at the takeoff setting, and engines are at takeoff (or maximum continuous) thrust on the operative engines. This is where the minimum gradient requirement applies most critically.
- Third segment (acceleration segment) — From 400 feet to the point where the airplane accelerates to final segment speed and retracts flaps. Altitude is held essentially constant while the airplane accelerates. No specific gradient is required during this segment.
- Fourth segment (final segment) — From completion of flap retraction to 1,500 feet above the takeoff surface. A minimum gradient is required here as well, but the cleaner configuration and reduced weight make it less commonly limiting.
Second Segment Requirements Under 14 CFR 25.121
Section 25.121 establishes the minimum OEI climb gradients for each segment. For the second segment, the regulation requires the following minimum gross climb gradients (expressed as a percentage of horizontal distance covered per unit of altitude gained):
- Two-engine airplanes: 2.4%
- Three-engine airplanes: 2.7%
- Four-engine airplanes: 3.0%
These gradients may appear modest in raw percentage terms, but they must be achieved simultaneously with one engine producing zero thrust (or, in the worst case, producing drag), all remaining engines at their certified takeoff thrust limit, landing gear fully retracted, and flaps in the takeoff position—all while maintaining V2 airspeed. The climb gradient is a net value: it is the ratio of climb rate to forward speed, and any factor that increases drag or reduces lift reduces the gradient.
A gradient of 2.4% means the airplane climbs 2.4 feet for every 100 feet traveled horizontally. That translates to a climb angle of approximately 1.4 degrees. While small in absolute terms, producing this gradient with only one engine operating and the airplane at maximum weight is a stringent aerodynamic challenge.
Why the Second Segment Is Often the Most Limiting
The second segment is uniquely challenging for several interconnected reasons:
Configuration drag: Gear retraction eliminates a major drag source from the first segment, but the flaps remain extended at their takeoff setting. Takeoff flap positions are selected to maximize lift during the ground roll while accepting moderate drag. That elevated flap drag remains throughout the second segment.
OEI thrust deficit: With one engine inoperative, a twin-engine airplane has lost 50% of its installed thrust. A four-engine airplane loses 25%. The asymmetric thrust also requires rudder and aileron inputs to maintain coordinated flight, which increases induced drag slightly beyond what a simple thrust-loss calculation would suggest.
Altitude and temperature effects: Thrust from turbofan engines decreases as pressure altitude increases and as outside air temperature (OAT) rises. On a hot day at a high-elevation airport—a scenario called a hot and high environment—thrust loss can be substantial. Because the second-segment gradient depends on the thrust-to-weight ratio, reduced thrust directly shrinks the achievable gradient. Weight must be reduced to restore the gradient to the required minimum.
Duration of the segment: The second segment spans from gear-up to 400 feet above the runway. At high gross weights and low climb gradients, this can take a considerable horizontal distance, increasing obstacle exposure time compared to lower-weight operations.
How Second Segment Limits Takeoff Weight
Performance engineers and aircraft manufacturers provide climb-limited takeoff weight charts in the Aircraft Flight Manual (AFM) and in airline Operations Specifications. These charts are derived from the 14 CFR 25.121 second-segment requirement.
For a given airport, the crew inputs:
- Pressure altitude (or field elevation plus altimeter setting correction)
- Outside air temperature
- Takeoff flap setting
- Engine bleed and anti-ice configuration
The chart then returns the maximum weight at which the airplane can achieve the required second-segment gradient. If the runway-length-limited weight (the weight at which the airplane can accelerate-stop or accelerate-go within available distances) is higher than the climb-limited weight, the climb limit governs. The crew must use the lower of the two values as the MATOW.
A worked example: suppose runway analysis shows the aircraft can take off at 185,000 lb based on balanced field length, but the second-segment climb chart for that day's conditions of 30°C and a field elevation of 5,000 ft limits the airplane to 172,000 lb. The crew must use 172,000 lb. They cannot legally take off at 185,000 lb, even if the runway is long enough, because the aircraft would be unable to meet the 2.4% OEI gradient required by 25.121.
Derate, Assumed Temperature, and Climb Limit Interaction
Modern operations frequently use reduced-thrust takeoffs via engine derate or assumed temperature (flex thrust) techniques to extend engine life and reduce maintenance costs. These methods reduce the actual thrust used for takeoff, which reduces OEI climb performance. As a result, when a reduced-thrust takeoff is planned, the aircraft performance system must verify that the second-segment climb gradient is still achievable at the reduced thrust setting and the planned weight. If the second-segment gradient cannot be met at a given derate, the crew must either use a higher thrust rating or reduce weight further.
This interaction is a critical operational concept: the climb limit is thrust-dependent, and reducing thrust tightens the weight constraint. Crews must never assume that because a runway analysis allows a certain derate, the climb performance automatically approves the same derate at that weight.
Obstacle Clearance and the Net Takeoff Flight Path
14 CFR Part 25 and the associated operational regulations (principally 14 CFR 121.189 for air carriers) require that the net takeoff flight path—the actual flight path reduced by a gradient decrement (0.8% for two-engine, 0.9% for three-engine, 1.0% for four-engine airplanes)—clears all obstacles in the departure corridor by at least 35 feet vertically or 300 feet horizontally in the takeoff area, expanding to 200 feet laterally beyond. The second-segment performance feeds directly into the net flight path calculation. A better second-segment gradient provides more net-path margin over obstacles.
Key Numbers and Rules
- Second segment spans from gear retraction to 400 feet AGL
- Minimum gradients: 2.4% (twin), 2.7% (three-engine), 3.0% (four-engine) per 14 CFR 25.121
- Second segment is flown at V2 with takeoff flaps and OEI takeoff thrust
- Net flight path gradient decrement: 0.8% / 0.9% / 1.0% for two, three, and four-engine respectively
- Obstacle clearance on the net flight path: minimum 35 feet vertically
- Climb limit can be more restrictive than runway limit on hot, high days
- Reducing thrust (derate/flex) further tightens the climb weight limit
Common Test Traps
- Confusing segments: Many candidates mix up first and second segments. Remember: the first segment ends at gear retraction; the second begins there and runs to 400 feet. The gear-up transition is the key dividing event.
- Assuming runway length is the only limit: The ATP exam frequently presents scenarios where runway analysis permits a higher weight than the climb gradient analysis. The correct answer always uses the most restrictive (lowest) weight limit.
- Applying twin requirements to four-engine aircraft: The 2.4% figure is only for two-engine airplanes. Four-engine aircraft must meet 3.0%—a higher standard, because four engines provide greater average performance and the regulation demands more to reflect that capability.
- Ignoring the effect of derate on climb limits: A reduced-thrust takeoff that satisfies the runway analysis may still violate the second-segment gradient requirement. Always confirm the climb-limited weight is compatible with the chosen thrust setting.
- Forgetting the net vs. gross path distinction: The gradient values in 25.121 are gross (actual) climb gradients. The net flight path—used for obstacle clearance—subtracts the gradient decrement. Obstacle clearance must be verified against the net path, not the gross path.