The autobrake, the components which are located on center panel of the Main Instrument Panel (MIP), is designed as a deceleration aid to slow an aircraft on landing. The system uses pressure, generated from the hydraulic system B, to provide deceleration for preselected deceleration rates and for rejected takeoff (RTO). An earlier post discussed Rejected Takeoff procedures. This post will discuss the autobrake system.
LEFT: Ryanair B737-800 - autobrake set, flaps 30, spoilers deployed, reverse thrust engaged (photograph copyright Pierre Casters).
The autobrake selector knob (rotary switch) has four settings: RTO (rejected takeoff), 1, 2, 3 and MAX (maximum). Settings 1, 2 and 3 and RTO can be armed by turning the selector; but, MAX can only be set by simultaneously pulling the selector knob outwards and turning to the right; this is a safety feature to eliminate the chance that the selector is set to MAX accidently.
When the selector knob is turned, the system will do an automatic self-test. If the test is not successful and a problem is encountered, the auto brake disarm light will illuminate amber.
The autobrake can be disengaged by turning it to OFF, by activating the toe brakes, or by advancing the throttles; which deactivation method used depends upon the circumstances and pilot discretion. Furthermore, the deceleration level can be changed prior to, or after touchdown by moving the autobrake selector knob to any setting other than OFF. During the landing, the pressure applied to the brakes will alter depending upon other controls employed to assist in deceleration, such as thrust reversers and spoilers.
The numerals 1, 2, 3 and MAX provide an indication to the severity of braking that will be applied when the aircraft lands (assuming the autobrake is set).
In general, setting 1 and 2 are the norm with 3 being used for wet runways or very short runways. MAX is very rarely used and when activated the braking potential is similar to that of a rejected take off; passenger comfort is jeopardized and it is common for passenger items sitting on the cabin floor to move forward during a MAX braking operation. If a runway is very long and environmental conditions good, then a pilot may decide to not use autobrakes favouring manual braking.
Often, but not always, the airline will have a policy to what level of braking can or cannot be used; this is to either minimize aircraft wear and tear and/or to facilitate passenger comfort.
The pressure in PSI applied to the autobrake and the applicable deceleration is as follows:
• Autobrake setting 1 - 1250 PSI / 4 ft per second.
• Autobrake setting 2 - 1500 PSI / 5 ft per second.
• Autobrake setting 3 - 2000 PSI / 7.2 ft per second.
• Autobrake setting MAX and RTO - 3000 PSI / 14 ft per second (above 80 knots) and 12 ft per second (below 80 knots).
To autobrake will engage upon landing, when the following conditions are met:
(i) The appropriate setting on the auto brake selector knob (1, 2, 3 or MAX) is set;
(ii) The throttle thrust levers are in the idle position immediately prior to touchdown; and,
(iii) The main wheels spin-up.
If the autobrake has not been selected before landing, it can still be engaged after touchdown, providing the aircraft has not decelerated below 60 knots.
To disengage the autobrake system, any one of the following conditions must be met:
(i) The autobrake selector knob is turned to OFF (autobrake disarm annunciator will not illuminate);
(ii) The speed brake lever is moved to the down detent position;
(iii) The thrust levers are advanced from idle to forward thrust (except during the first 3 seconds of landing); or,
(iv) Either pilot applies manual braking.
The last three points (ii iii and iv) will cause the autobrake disarm annunciator to illuminate for 2 seconds before extinguishing.
It is important to grasp that the 737 NG does not use the maximum braking power for a particular setting (maximum pressure), but rather the maximum programmed deceleration rate (predetermined deceleration rate). Maximum pressure can only be achieved by fully depressing the brake pedals or during an RTO operation. Therefore, each setting (other than full manual braking and RTO) will produce a predetermined deceleration rate, independent of aircraft weight, runway length, type, slope and environmental conditions.
Autobrake Disarm Annunciator
The autobrake disarm annunciator is coloured amber and illuminates momentarily when the following conditions are met:
(i) Self-test when RTO is selected on the ground;
(ii) A malfunction of the system (annunciator remains illuminated - takeoff prohibited);
(iii) Disarming the system by manual braking;
(iv) Disarming the system by moving the speed brake lever from the UP position to the DOWN detente position; and,
(v) If a landing is made with the selector knob set to RTO (not cycled through off after takeoff). (If this occurs, the autobrakes are not armed and will not engage. The autobrake annunciator remains illuminated amber).
The annunciator will extinguish in the following conditions:
(i) Autobrake logic is satisfied and autobrakes are in armed mode; and,
(ii) Thrust levers are advanced after the aircraft has landed, or during an RTO operation. (There is a 3 second delay before the annunciator extinguishes after the aircraft has landed).
Preferences for Use of Autobrakes and Anti-skid
When conditions are less than ideal (shorter and wet runways, crosswinds), many flight crews prefer to use the autobrake rather than use manual braking, and devote their attention to the use of rudder for directional control. As one B737 pilot stated - ‘The machine does the braking and I maintain directional control’.
Anti-skid automatically activates during all autobraking operations and is designed to give maximum efficiency to the brakes, preventing brakes from stopping the rotation of the wheel, thereby ensuring maximum braking efficiency. Anti-skid operates in a similar fashion to the braking on a modern automobile.
Anti-skid is not simulated in FSX/FS10 or in ProSim737 (at the time of writing).
To read about converting an OEM Autobrake Selector navigate to this post.