Reducing aircraft engine thrust is primarily achieved through two sophisticated methods: derate and assumed temperature (also known as Flexible temperature or FLEX). These techniques allow operators to optimize engine performance, extend engine life, reduce fuel consumption, and minimize noise and emissions, especially during takeoff when full thrust may not be required.
Understanding Thrust Reduction Techniques
Aircraft often do not need the maximum available thrust, particularly during takeoff from long runways, with lighter loads, or in cool weather. Utilizing less than full thrust provides significant operational advantages.
1. Derate
Derate involves a pre-defined, fixed reduction in the maximum thrust an engine can produce. Essentially, it's like setting a lower, permanent limit on the engine's thrust capability. This reduction is typically programmed into the engine's Full Authority Digital Engine Control (FADEC) system and cannot be exceeded unless a different derate setting is selected or the derate is removed.
Key aspects of Derate:
- Fixed Reduction: Derate provides a specific, absolute thrust reduction (e.g., 22K, 24K, 26K lbs of thrust for an engine capable of 28K lbs).
- Purpose: It's primarily used for structural considerations, such as limiting the maximum thrust an airframe can safely handle, or for noise abatement procedures in sensitive areas.
- Engine Life: By preventing the engine from ever reaching its absolute maximum design thrust, derating significantly contributes to engine component longevity, reducing wear and tear.
- Selection: Pilots select a derate setting based on operational needs, which then becomes the new maximum thrust limit for that specific flight segment.
2. Assumed Temperature (Flexible Temperature or FLEX)
Assumed temperature, often referred to as Flexible temperature (FLEX), is a dynamic method of thrust reduction that "tricks" the engine's FADEC into believing the ambient air temperature is higher than it actually is. Since jet engines naturally produce less thrust in hotter air, inputting a higher assumed temperature causes the FADEC to command a lower thrust setting for takeoff.
Key aspects of Assumed Temperature (FLEX):
- Dynamic Reduction: Unlike derate, FLEX thrust is variable and calculated based on numerous factors, including aircraft weight, runway length, wind, and actual ambient temperature.
- Mechanism: The flight crew determines the lowest safe amount of thrust needed for takeoff. They then find the "assumed temperature" that would result in that specific thrust output. This assumed temperature (which is higher than the actual outside air temperature) is entered into the FADEC.
- Optimization: This method allows for fine-tuning thrust output to the absolute minimum required, optimizing performance for each unique takeoff condition.
- Benefits:
- Reduced Fuel Burn: Lower thrust settings consume less fuel.
- Extended Engine Life: Operating at lower thrust settings significantly reduces thermal and mechanical stress on engine components, leading to longer time on wing and lower maintenance costs.
- Lower Noise and Emissions: Less thrust means a quieter takeoff and reduced exhaust emissions.
Comparing Derate and Assumed Temperature
While both methods reduce thrust, they serve slightly different purposes and operate distinctly:
| Feature | Derate | Assumed Temperature (FLEX) |
|---|---|---|
| Nature | Fixed, pre-defined maximum thrust limit | Dynamic, calculated based on conditions |
| Control | Selects a specific thrust rating | Inputs an assumed temperature into FADEC |
| Primary Use | Structural limits, general engine life | Optimal thrust for specific takeoff conditions |
| Variability | Less variable (pre-set options) | Highly variable, optimized per takeoff |
| Can Be Used With | Can be combined with FLEX | Can be combined with Derate |
Why Reduce Thrust? (Benefits)
The practice of reducing thrust during takeoff, when full power isn't strictly necessary, offers several compelling advantages for airlines and aircraft operators:
- Enhanced Engine Longevity: Operating engines below their maximum capabilities reduces wear and tear on critical components, significantly extending the time between overhauls and lowering maintenance expenses.
- Improved Fuel Efficiency: Lower thrust settings translate directly into reduced fuel consumption during the takeoff roll and initial climb, contributing to operational cost savings.
- Noise Reduction: A quieter takeoff process is beneficial for communities near airports, helping airlines comply with stringent noise regulations.
- Lower Emissions: Reduced thrust also means lower exhaust gas temperatures and less fuel burned, leading to a decrease in pollutant emissions.
- Increased Safety Margin: While counter-intuitive, not using maximum thrust ensures that the full thrust capability is always available if an unexpected situation (e.g., wind shear, engine failure) requires it.
By strategically employing derate and assumed temperature, airlines can achieve a delicate balance between operational efficiency, environmental responsibility, and cost-effectiveness, all while maintaining the highest safety standards.
