Assumed Temperature Method (ATM)
This method achieves a takeoff thrust less than the maximum takeoff thrust by assuming a temperature that is higher than the actual temperature. The thrust reduction authorized by most regulatory agencies is limited to 25% below the maximum takeoff thrust.
The primary thrust setting parameter (N1) is not considered a limitation. If conditions are encountered during the takeoff where additional thrust is desired, such as windshear, the crew should not hesitate to manually advance thrust levers to maximum takeoff thrust.
The assumed temperature method of computing reduced thrust takeoff performance is always conservative and provides performance equal to or better than the performance obtained if actually operating at the assumed temperature. This is because the true airspeed of the airplane is lower than would be the case if the actual temperature were equal to the assumed temperature.
737 Flight Crew Training Manual
Do not use the ATM if conditions that affect braking such as a runway contaminated by slush, snow, standing water, or ice exist, or if potential windshear conditions exist. ATM procedures are allowed on a wet runway if suitable performance accountability is made for the increased stopping distance on a wet surface.
Note: An increase in elevator column force during rotation and initial climb may be required for ATM takeoffs.
Fixed Derate
This method uses a takeoff thrust less than maximum takeoff thrust for which complete and independent performance data are provided in the AFM. Derated thrust is obtained by selection of a fixed takeoff derate in the FMC.
The fixed derate is considered a limitation for takeoff. Thrust levers should not be advanced beyond the fixed derate limit unless conditions are encountered during the takeoff where additional thrust is needed on both engines, such as windshear. A thrust increase following an engine failure could result in a loss of directional control.
Note: Although fixed derate takeoffs are permitted on wet or contaminated
runways, provided takeoff performance accounts for runway surface
conditions, they are not recommended if potential windshear conditions
exist.
Combination Fixed Derate and ATM
This method uses a takeoff thrust less than the fixed derate takeoff thrust by first selecting a fixed takeoff derate from the FMC. This derate takeoff thrust is then further reduced by assuming a temperature that is higher than the actual temperature. In this case, the thrust reduction authorized by most regulatory agencies is limited to 25% below the derated takeoff thrust.
Improved Climb Performance Takeoff
When not field length limited, an increased climb limit weight is achieved by using the excess field length to accelerate to higher takeoff and climb speeds. This improves the climb gradient, thereby raising the climb limit and obstacle limited weights. V1, VR and V2 are increased to maintain consistent performance relationships. V1, VR and V2 must be obtained from dispatch or the airport analysis.
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3.16 FCT 737 (TM) October 31, 2006
737 Flight Crew Training Manual
Low Visibility Takeoff
Low visibility takeoff operations, below landing minima, may require a takeoff alternate. When selecting a takeoff alternate, consideration should be given to unexpected events such as an engine failure or other non-normal situation that could affect landing minima at the takeoff alternate. Operators, who have authorization for engine inoperative Category II/III operations, may be authorized lower alternate minima.
With proper crew training and appropriate runway lighting, takeoffs with visibility as low as 500ft/150m RVR may be authorized (FAA). With takeoff guidance systems and centerline lighting that meets FAA or ICAO criteria for Category III operations, takeoffs with visibility as low as 300ft/75m RVR may be authorized. Regulatory agencies may impose takeoff crosswind limits specifically for low visibility takeoffs.
All RVR readings must be equal to or greater than required takeoff minima. If the touchdown or rollout RVR system is inoperative, the mid RVR may be substituted for the inoperative system. When the touchdown zone RVR is inoperative, pilot estimation of RVR may be authorized by regulatory agencies.
Adverse Runway Conditions
Slush, standing water, or deep snow reduces the airplane takeoff performance because of increased rolling resistance and the reduction in tire-to-ground friction.
Most operators specify weight reductions to the AFM field length and/or obstacle limited takeoff weight based upon the depth of powdery snow, slush, wet snow or standing water and a maximum depth where the takeoff should not be attempted.
Slush or standing water may cause damage to the airplane. The recommended maximum depth for slush, standing water, or wet snow is 0.5 inch (12.7 mm) on the runway. For dry snow the maximum depth is 4 inches (102 mm).
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