What you'll learn:
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Reduced Acceleration Altitude (RAAL) involves starting the aircraft acceleration sooner, at a lower altitude than normal operations. Usually, aircraft ascend to a specific altitude before they start to retract flaps/slats and speed up to a faster, more efficient flying speed. When the acceleration starts at this lower height, the aircraft can switch to the efficient flight mode more quickly and reduce drag. This means it faces less resistance from the air, making it more efficient. In most cases, the altitude is reduced from 3000ft to 800ft. For lighter aircraft, it can go down to 400ft.
Figure 1: Reduce Acceleration Altitude (NADP 1 vs NADP 2)
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In red, the usual NADP1 procedure, where the aircraft accelerates at 3000ft
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In green, the perfect application of Reduced Acceleration Altitude, where the aircraft accelerates at 800ft. It is equivalent to NADP2 procedure.
Implementing RAAL offers several benefits:
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Fuel Efficiency: Reducing acceleration height can boost fuel efficiency based on the ability to adopt a clean configuration earlier in the flight. This involves retracting the aircraft's flaps and slats sooner than usual, reducing drag. By minimizing resistance from the air at an earlier phase, aircraft can achieve greater fuel efficiency, making for a smoother and more economical journey.
In 2024, the SkyBreathe® airline community saved more than 102,268 tons of fuel by applying the Reduced Acceleration Altitude best practice. That represents more than 322.145 tons of CO2 emissions prevented from going into the atmosphere.
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Reduced Emissions: With improved fuel efficiency comes a reduction in carbon emissions.
By embracing eco-flying techniques such as RAAL, the industry shows its commitment to innovation and progress toward achieving net zero CO2 emissions by 2050.
The chart below shows what are the typical fuel saved, depending on the aircraft type:
Figure 2: Typical fuel saved depending on the aircraft type by reducing acceleration height
The benefits than can be expected are about 16 kg per flight for a TurboProp up to 200 kg for a Long Range aircraft, which is non-negligible!
The 3 challenges of Reduced Acceleration Altitude
While the benefits are clear, integrating the RAAL eco-flying technique into standard aviation procedures requires overcoming various challenges, from technological adaptations to operational considerations.
Understanding and adhering to local noise abatement procedures is essential and can vary across different regions and airports. Pilots and airlines must be aware of specific noise abatement procedures at each airport and comply with them to minimize the impact of aircraft noise on surrounding communities.
RAAL is only applicable when noise abatement procedures (specified on the airport’s AIP) allow accelerating below 3000ft.
There are two noise abatement departure procedures, NADP1 and NADP2, used by aircraft during takeoff to minimize noise pollution. The choice between these procedures is specific to each airport. In the below figures, you will see what is the difference between NADP1 and NADP2.
Noise Abatement Departure Procedure 1
NADP1 focuses on noise reduction near airports, involving a steep climb and later acceleration. Many large airports, especially in Europe, prevent accelerating before 3,000 feet to mitigate the impact of aircraft noise on nearby communities.
Noise Abatement Departure Procedure 2
NADP2 allows for earlier acceleration, reaching optimal performance sooner than with NADP1 (acceleration from 800ft). RAAL fuel efficient procedure matches with NADP2.
Boeing claims that the fuel saved by flying an NADP2 procedure vs an NADP1 procedure is 67 kg on a Boeing 737-800 with winglets and 197 kg on a Boeing 777-200ER.
Not all aircraft types may experience the same level of benefit from RAAL. Factors such as aircraft size, engine type, and weight can influence the effectiveness of reduced acceleration procedures. Some aircraft may require longer distances to reach optimal climb performance, while others may benefit more from reduced acceleration height. Airlines need to consider the performance characteristics of their fleet and assess the feasibility and effectiveness of reduced acceleration altitude procedures on a case-by-case basis to advise pilots on its application.
Addressing these challenges requires collaboration and cooperation among stakeholders to work towards implementing the RAAL procedure more effectively. Airlines can leverage advanced fuel efficiency tools and technologies to integrate reduced acceleration height procedures into their flight management systems. This strategy can maximize environmental benefits while enhancing operational and cost-effectiveness.
How is RAAL implemented in airlines?
Pilots are the ones having authority to apply this fuel efficiency technique during the flight. You must be given access to relevant data so you can address any doubts about RAAL's safety and effectiveness, and adopt the technique when relevant.
Also, to be confident in the practice’s application, airlines should have clear SOPs for RAAL. This assures you that the airline has thoroughly evaluated the procedure and offers official guidance for its application.
Here’s an example of how a pilot app can help pilots adopt the technique:
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