Why Do Airliners Takeoff With Reduced Thrust?

The airliners we have today are highly overpowered as they are designed to safely take off with one engine inoperative. The main factor that makes it harder for an airplane to lift off the ground is its weight. The higher the weight, the higher the engine thrust or power that is required for a takeoff. Thus, when the actual weight of the aircraft is lower than the regulatory or performance-limited takeoff weight, the aircraft requires less thrust to get airborne. This reduction in engine thrust is known as flexible takeoff or assumed temperature takeoff.

The reduced takeoff thrust procedure increases engine durability, lowers maintenance costs, and increases engine reliability.

The difference between maximum structural takeoff weight and maximum performance limited takeoff weight

The Maximum Structural Takeoff Weight is a weight that is connected with the structure of the aircraft. It must never be exceeded in any circumstance. This weight is determined in accordance with inflight structure resistance criteria, the resistance of landing gear, and structure criteria during a landing impact with a vertical speed equal to -360 ft/min.

The Maximum Performance Takeoff Weight is purely performance weight. It is determined by checking whether the aircraft could meet the minimum climb gradient, runway length, and other factors that can degrade or improve the performance of the aircraft.

For instance, on a particularly cold day, an aircraft can perform better, which increases its performance takeoff weight. This weight has no connection to the structural weight. It can be higher or lower than the structural weight depending on the prevailing conditions. However, as said before, the structural takeoff weight is always limiting.

What is engine flat rating and how is thrust reduced for takeoff

The jet engines have two limitations: a pressure limit and a temperature limit. When the outside temperature is cold, the air is denser, and this means the engine generates its maximum thrust before reaching its temperature limits. In this condition, the engine is pressure or thrust limited. The opposite is the case when the outside temperature is high. The air is less dense, and the engine reaches its temperature limit before it reaches its full thrust or power. In this situation, it is said that the engine is temperature limited.

The engine manufacturers give a guarantee to the user or the operator that their engines can give out a constant amount of thrust up to a particular temperature. This thrust, called TOGA (Takeoff Go Around) thrust, is the maximum thrust the engines can generate without breaking something.

Once a temperature is reached, the engine can no longer generate the TOGA thrust, and it falls off to keep the engines from over-temping. This is the concept of flat rating. The engine thrust remains constant or “flat” until a limiting temperature is reached. There are many names for this flat rating temperature. Airbus calls it the reference temperature: tref.

The graph shows Thrust plotted against Outside Air Temperature (OAT). The Maximum Takeoff Weight (MTOW) in red is the performance-limited takeoff weight, and the actual Takeoff Weight (TOW) is in green.

What it shows is that the engine can give out the flat rated thrust until an OAT of tref is reached, at which point the thrust starts to drop. It also shows that a maximum performance limited weight can be accepted if the OAT does not exceed tref. It also shows that when the actual takeoff weight is lower than the maximum takeoff weight, a lower thrust can be accepted. This will be explained with a detailed example in the upcoming paragraph.

The following example is based on an Airbus A320 aircraft with CFM 56 engines which are flat rated to a temperature of 44 degrees Celsius (tref). The chart attached below is used by pilots to determine by how much the thrust can be reduced for a flight. This chart is for runway 19R in Bangkok airport.

On a particular day, say that the outside temperature is 30 degrees Celsius (circled in red). With this temperature, according to the table, the aircraft can take off, while meeting all the regulated performance standards with a full load of 84,400 kg with flaps at 2 (CONF 2). This 84,400 kg is what we call the Performance limited take-off weight. It has nothing to do with the structural integrity of the aircraft. It is a mass purely based on aircraft performance.

Now, let us say, when we got the final load figure at the gate, the actual mass of the aircraft dropped to 75,000 kg. At this lower mass, if we still opt to go for full-rated thrust, we will have a huge amount of excess thrust. As we do not need that excess thrust and as we also want to save the airline on engine maintenance, we try to find out at what outside temperature we are limited to a mass of 75,000 kg. Again, we look at the table. It says that at 56 degrees Celsius (circled in orange), we can carry 75,000 kgs onboard, with no issues.

What this implies is that if the outside temperature is 56 degrees Celsius, the aircraft can only take off with 75,000 kg. So, the pilot enters 56 degrees Celsius in the flight management computer. When entered, the aircraft believes that the outside temperature is 56 degrees Celsius, even though the actual temperature is 30 degrees Celsius.

Consequently, when the pilot pushes the thrust levers for takeoff, the aircraft limits the thrust, so it only develops the amount of thrust that it can generate with an outside temperature of 56 degrees Celsius. Remember that the engine has a tref of 44 degrees Celsius. And as 54 degrees Celsius is above that, the engine thrust is limited by the engine control computers to ensure that the engines do not exceed their maximum allowable exhaust gas temperature.

When is reduced thrust not an option?

Before we go into further details, it is vital to understand that the pilot always has the option to go full thrust even when the aircraft is on the takeoff roll. A common case is an engine failure situation. In this situation, a pilot may want full thrust to increase his/her margins. However, this is not necessary, as reduced thrust considers engine out performance. That is, an aircraft can safely continue the takeoff with one engine inoperative with reduced thrust.

The use of reduced thrust is prohibited when the runway is contaminated. A runway is considered contaminated when more than 25% of it is covered in water, ice, snow, slush, or frost. Most aircraft manufacturers do not provide data for reduced thrust takeoffs on contaminated runways. So, this is not possible due to the unavailability of data.

Source: simpleflying.com

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