Autobrake systems on aircraft provide automated deceleration functions during the take off and landing phases of flight. On sophisticated commercial aircraft, this automation reduces the workload for the flight crew while also ensuring the braking power applied is at a smooth and sufficient rate.
The deceleration and braking power applied depends on the required stopping distance for the aircraft while also taking into consideration the thermal limits of the brakes.
If excessive braking power is applied, this may cause the brakes to overheat; which can lead to additional time required prior to the next departure of the aircraft for cooling purposes.
In more extreme circumstances, overheating of the brakes can lead to a brake fire.
On the 737/747/757/767/777/787 aircraft, there is an autobrake system located beside the flap indicator and landing gear. These series of switches allows the flight crew to pre-select the desired braking setting for the aircraft upon landing.
For the take off phase, there is also an option known as RTO – short for Rejected Take off. Upon selecting this system, the autobrake system activates the hydraulically powered maximum braking setting.
While the RTO setting is armed via selecting the autobrake switch to the respective position, it can only be engaged if:
- Groundspeed is at or above 85kts
- Idle or Reverse Thrust has been selected
- The aircraft is not airborne – this is confirmed via the WOW (Weight on Wheels) system
- Aircraft is below the calculated V1 speed
If these conditions are met, RTO mode will be engaged and maximum braking power will be applied. If spoilers/speed brakes are armed, they will also be deployed upon activation of the RTO setting.
Airbus Autobrake System
On Airbus aircraft, there is no dedicated switch for the RTO setting. Alternatively, the “AUTOBRAKE MAX” setting is selected prior to take off. This setting performs the same function as the RTO setting found on Boeing autobrake systems.
Another difference to the Airbus autobrake system and take off procedures is that the speed brake lever is ARMED for departure. On Boeing aircraft, speed brakes are not armed for departure.
In order for RTO mode to be activated on Airbus aircraft, the same 4 conditions stated above are required in order for the system to engage.
In relation to cooling, Airbus aircraft have a customer option in which brake fans can be installed. The brake fans can be activated by the flight crew after landing.
Brake fans are activated if an amber indication appears on the brake fan panel. This amber indication will extinguish once the brakes have been cooled to a sufficient temperature.
However, this optional equipment comes at an additional cost. Many operators opt for no brake fans for cost-saving purposes.
Autobrake Deceleration Rates
The different autobrake settings found on aircraft correspond with various deceleration rates for the aircraft. The deceleration rates are typically measured in Metres/Second (M/S) or in Knots/Second.
With most autobrake systems, these parameters range from 3-6 Knots/Second or 5-10 Metres/Second.
The required autobrake setting for landing can be determined via calculations made on the Electronic Flight Bag (EFB) system and by also assessing the runway length, conditions (WET/DRY/ICY etc) and the landing weight.
The autobrake system upon landing is typically used in conjunction with reverse thrust and spoilers/speed brakes.
Reverse thrust and the speed brake system assist in decelerating the aircraft. This is especially important for leveraging the autobrake setting required; higher settings can be avoided in order to reduce wear and tear and to also reduce brake temperatures.
On the rollout following landing, the autobrake system is usually disengaged at 80 knots ground speed in which the PNF (Pilot Not Flying) will provide an aural callout of “MANUAL BRAKING”.
This informs the Pilot Flying (PF) that the autobrake system should be disengaged. The autobrake system can then be disengaged via the pilot pressing the toe brakes located on top of the rudder pedals simultaneously.
Following this input, the autobrake panel should provide an indication of no active autobrake setting.