Braking During Rejected Takeoff

Making the go/no-go decision to continue a takeoff or reject it is not new. With the advent of jet engine technology for transport aircraft in the 1950s, more crews were faced with the decision to either continue or reject a takeoff more often than in previous aircraft types. Crews continue to face the same go/ no-go decision points; however, with the reliability of aircraft, it is not often that crews reject a takeoff, especially in the latter part of the takeoff where the decision is most crucial. Although crews routinely train for these scenarios, it is crucial to emphasize the importance of making the decision and rapidly transitioning the aircraft to stop.

The decision-making process to continue a takeoff or reject it due to a fault or malfunction is in recurring training and often centers around the discussion of the low- or high-speed regime. Generally speaking, once the aircraft enters the high-speed regime, the reject decision should be based on an increasing level of criticality or the aircraft’s ability to take flight. Although it sounds simple, the decision must be well thought out before entering the runway.

Once the decision has been made to reject a takeoff, a quick transition must be executed as several key systems must be employed. Although these tasks are simple, crews may forget how easily they can become difficult when they were not expecting to have to stop and are most likely startled. Also, there may be a transfer of crew duties depending on who was flying the aircraft.

Various aircraft flight manuals have different terminology, but the procedures are roughly the same. A quick and simultaneous response is required by the crew, often beginning with reducing the power to idle to stop the acceleration forward. Next, speed brakes are often extended, either manually or automatically. Lastly, the aircraft brakes are applied. The utilization of the aircraft braking system is the most crucial component in the rejected takeoff scenario. A quick and proper application of braking is needed to validate the assumption factored into the takeoff planning.

Once the aircraft is transitioned into a reject configuration, it is not uncommon for a rejected takeoff to lead to a runway excursion/ overrun. This issue is due in part to the crew’s perception that they have sufficient runway available to slow and do not initially apply maximum braking. As the aircraft enters the end of the runway, crews attempt to apply maximum braking to stop, only to have poor braking due to rubber deposits, as they are now in the touchdown zone of the opposite runway. This situation is preventable by applying maximum braking from the initiation of the reject procedure.

Instantaneous with the application of the aircraft’s brakes is a rise in the brake energy. However, the brake energy utilized during the aircraft taxi from ramp to runway is often overlooked. A taxi consisting of multiple slopes, long distances, and multiple stops can already have started the increase of brake energy before a rejected takeoff. To mitigate this threat, crews should consider the taxi route in their takeoff planning and use proper taxiing techniques, such as not riding the aircraft brakes.

As the energy increases in the brake system, it begins to transfer outward toward the tire. One of the most potentially dangerous situations would be a tire exploding; therefore, a fuse plug feature is standard practice. The fuse plug functions as a pressure relief device to deflate the tire before the energy reaches the point of a tire explosion. The deflating tires will pose additional challenges for maneuvering the aircraft, especially given that not all fuses will melt at the same time due to differential brake heating. Also, although fuse plugs are excellent protection against damage from a rejected takeoff, they do not prevent all hazards. Existing hydraulic leaks in or around the brakes could quickly result in a fire.

Here are some practical steps to ensure you are utilizing the aircraft’s brakes in the best way during a rejected takeoff:

• Thoroughly assess the environment and aircraft limitations for the takeoff. How close to maximum gross weight is your aircraft? Is there a long taxi? What is the runway condition (e.g., dry, wet, icy?), and how might it be during all phases of a ground roll from a rejected takeoff?
• Conduct a thorough crew briefing to ensure everyone has the same mental image of their roles and responsibilities during a rejected takeoff. The procedures must be carried out instantly, and any delay decreases your safety margin of available runway.
• At the aircraft, position the seat to allow for maximum braking. Maximum braking must be obtained by “standing on them,†which means the ability to completely depress the top of the rudder pedals, ideally without having to release them.
• Once the decision to reject has been made, quickly transition the aircraft. Deploying the speed brakes will increase the vertical wheel load and make braking more effective. Studies have shown that speed brake deployment is more critical than thrust reverser deployment.
• Do not assume that you have more than enough runway available, and do not stop braking in an effort to reach the end of the runway sooner.
• Continue the maximum braking until the aircraft has slowed to a safe taxi speed, attempting to make this reduction in speed in one single braking application. Once slowed, attempt to vacate toward a designated hot brakes area or a location away from other aircraft.

A rejected takeoff is an event that is not outside the odds of occurring in a crew’s career. However, it does not occur often enough to always be at the forefront of the mind. Given this infrequency, crews must continue to take the opportunity during crew briefings to reinforce the duties and procedures of a rejected takeoff. Furthermore, crews must be mindful of the after-effects of a rejected takeoff as they may not yet be out of harm’s way.