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On February 17, 2021

Why should pilots apply reduced thrust takeoff?

This article will focus on a best practice to reduce costs for an airline: Reduced thrust takeoff. We will see why and how pilots should apply it.

What is reduced thrust takeoff?

Thrust required to take off an aircraft is often less than what may be provided by the engines, called full takeoff thrust, also known as TOGA. So, we can define a reduced thrust takeoff as a takeoff that uses less thrust than the engines provide.

What are the benefits of reduced thrust takeoff?

For financial and environmental reasons, it is hence an excellent practice to reduce this thrust at takeoff. Best in class airlines apply it consistently; let’s see why.

Maintenance cost savings

One of the main reasons to use the minimal thrust required at takeoff is to reduce maintenance costs. Regular reduced thrust takeoff applications yield significant cost savings for an airline.

By decreasing internal engine temperatures and the exhaust gas temperature (EGT) at takeoff, it reduces engine wear resulting in a longer engine on-wing life. Therefore, fewer maintenances are required as the engine may achieve more flight cycles between two maintenance actions.

The cost savings are quite dependent on the engine type and several other factors like labor cost and engine age, however according to the IATA, using reduced thrust instead of full thrust during takeoff saves about 150 US$ per engine and per takeoff.

Reduced thrust takeoff procedure does not reduce fuel consumption during the flight; on the contrary, it may even increase it. Nonetheless as the engine wear is reduced the engine remain more efficient longer. It is thus hard to conclude if it saves fuel in the long run.

Less emissions

From an environmental perspective, a reduced thrust takeoff also means fewer emissions.

As most emissions are not linear with thrust settings, a full thrust takeoff emits significantly more nitrogen oxides (NOx), black carbons (BC), hydrocarbons (HC), and carbon monoxyde (CO) than a reduced thrust takeoff. For instance, this study showed that using reduced thrust at takeoff instead of full thrust may generate fewer NOx emissions down to 47% and black carbons emissions down to 71%.

These emissions have two undesirable effects:

  • First, they contribute to global warming. NOx emissions, for example, are responsible for the formation of ozone, a greenhouse gas.
  • They also affect public health, being an important factor in the development of respiratory diseases like asthma.

It is not clear whether there is a CO2 emissions reduction, however. CO2 emissions are proportional to fuel consumption, and as we discussed previously, it is difficult to assess the fuel savings related to reduced thrust takeoff.

How can pilots reduce takeoff thrust?

Derate levels and assumed temperature

 There are two different methods to reduce takeoff thrust:

  • The derated thrust method

The derated thrust method consists of reducing the engine's rated thrust to a given level below full thrust. There may be one or more derate level on an engine. Therefore, the pilot should select one which provides enough thrust to takeoff.

  • The assumed temperature method

The assumed temperature method consists of entering a temperature, known as assumed temperature or FLEX temperature, in the Flight Management Computer (FMC) and is interpreted as the actual outside air temperature. The thrust is then electronically limited, as an engine may provide flat rated thrust up to a given temperature, then it decreases linearly with the temperature. As a result, the assumed temperature should be higher than the actual outside air temperature to reduce thrust effectively.

Both methods may be used together to provide the optimal thrust required to takeoff.

 A quick example of optimal thrust reduction parameters

Let's consider an engine with a flat-rated thrust of 10 000 daN and a single derate level of 8 000 daN.

The thrust required to takeoff is 6 600 daN, and the outside temperature is 20 °C.

To illustrate how to compute optimal derate level and assumed temperature parameters, let’s have a look at the following chart:

The thrust required is below the derated thrust; thus, it is possible to use it to take off. By selecting it, however, the engine still has more power than required to perform the takeoff. Here is where assumed temperature comes into play: as the outside air temperature is below the optimal assumed temperature of 38°C, one may use it to get the optimal thrust.

Hopefully, these performance calculations are computerized and available to the pilot inside their Electronic Flight Bag, and pilots may enter derate level and assumed temperature directly from their tablet. 

When is it possible to reduce thrust at takeoff?

As long as the required takeoff thrust is provided, considering the aircraft but also the runway length, there is not any limitations to take off with a derated thrust. It may even allow to take off with a higher takeoff weight in some cases. Whereas the assumed temperature method is forbidden where the runway is contaminated. Thus, the pilot should consider taking off with a derated thrust or a full takeoff thrust instead.

Improving reduced takeoff thrust application

To make sure more and more takeoffs are performed with a reduced thrust, the application of the practice should be regularly monitored by the airline. A thorough analysis is often led by the fuel team using a fuel efficiency software. Recommendations may be sent to the pilots to improve their takeoff through a dedicated mobile app.

Sources


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