The recent video posted by Stephenof the Global 7500 Max KE braking test raised the question of why reverse thrust was not used during the test on a dry runway.
As a Global 7500 pilot myself, I have been wondering about this rule a few weeks ago since I couldn’t remember what it was based on. So here is my take on it.
It is important to note that most in-service RTOs (rejected takeoff) utilize reverse thrust if available, but in order to certify a transport-category airplane, compliance with 14 CFR 25.109, paragraph (f) is required. (click)
A good Video of an Easy Jet RTO can be found here
This regulation precludes the use of thrust reversers on dry runways when calculating the accelerate-stop distance. This means that the demonstration required by paragraph (i) cannot be conducted with reverse thrust on a dry runway.
The rationale behind this differentiation is that the regulations are written this way in order to demonstrate that the aircraft can safely stop under the worst conditions. This includes a flight test demonstration of the maximum brake kinetic energy accelerate-stop distance with no more than 10% of the allowable brake wear range remaining on each of the airplane’s wheel brakes.
Boeing has a nice short video that explains this topic well and can be found around 1:10. Although the aircraft in the video is a “bit” larger than the Global 7500, the regulations would still apply to the 7500 as well. (Video here)
Additionally, when it comes to wet runways, the regulations state that the use of reverse thrust is taken into account when calculating the accelerate-stop distance.
I believe that should address the question. However, I am open to further discussion on the topic.
