I will check my notes
Yes you’d better
OK – I shall try to set the 744 autobraking system straight!
The 744 Autobraking levels of 1, 2, 3, 4 and Max Auto are all selected levels of DECELERATION.
They equate to the following:
1 = 4.0 ft per second per second.
2 = 5.0 ft per sec/sec
3 = 6.0 ft per sec/sec
4 = 7.5 ft per sec/sec
MAX = 11.0 ft per sec/sec.
For example,
Landing speed of 130kts (219feet/sec). Autobrake “3” selected. (BSCU wants to “see” a decel of 6 ft/sec/sec)
Touchdown Speed= 130kts(219ft/sec)
after 1 secs = 126kts(213ft/sec)
after 2 secs = 122kts(207ft/sec)
after 4 secs = 115kts(195ft/sec)
after 8 secs = 101kts
after 12 secs = 87kts
after 16 secs = 72kts
after 20 secs = 58kts
after 24 secs = 44kts
after 28 secs = 30kts
after 30 secs = 23kts(39ft/sec)……… fastish taxi speed
Landing speed of 130kts (219feet/sec). Autobrake “MAX” selected. (BSCU wants to see a decel of 11 ft/sec/sec)
Touchdown Speed= 130kts(219ft/sec)
after 1 secs = 123kts(208ft/sec)
after 2 secs = 116kts(197ft/sec)
after 4 secs = 103kts(175ft/sec)
after 8 secs = 77kts
after 12 secs = 51kts
after 16 secs = 25kts (43ft/sec)………..fastish taxi speed
The Brake System Control Unit (BSCU) controls autobraking and receives aircraft deceleration data from the Inertial Reference Units.
When all arming conditions are met and the autobrakes are engaged, the BSCU will send a signal to the Autobrake Hydraulic Modules (one left and one right, located on the rear bulkhead of each wing landing gear wheel well) to port hydraulic pressure to the wheel brakes that will result in the selected deceleration rate. This signal will vary and be proportional to the amount of wheel braking required to achieve the decel rate.
i.e if you land and DO NOT select reverse and DO NOT deploy speed brakes, a greater proportion of wheel braking is required to achieve the selected decal rate. Accordingly, the BSCU will command the Autobrake Module to supply a larger amount of hydraulic press to the wheel units. The amount of hydraulic pressure supplied (and thus the actual amount of brake usage) will be increased so as to satisfy the BSCU that the aircraft is decelerating at the required rate.
Result = required decel rate but with hot brakes.
Conversely, if you land and select reverse, deploy speedbrakes, land on an upslope, land in quicksand, land and have all the passengers put their feet out on the ground or land and deploy a braking parachute(a la B52), then the amount of braking required from the wheel units to achieve the selected decel rate is reduced.
Result = required decel rate but with cooler brakes
Same as braking hard in your car. You can equate your reversers to “downshifting” in your car. If you are doing 100 km/h and shift back from 5th to 4th gear without touching the brakes or anything else, then you will slow down without using(or wearing out) your brake pads or discs.
Whilst all this is happening, the anti-skid will always override if ever needed to prevent a wheel lock-up (anti-skid operates completely autonomously).
If ever the pilot feels the need to take-over manual braking, whenever the output of either the left or right brake metering valves(i.e pilots pedals) reaches 750 PSI, the autobrakes are disarmed, allowing the pilot to control deceleration.
What’s that last selection?
Ah, yes RTO.
When arming conditions for RTO are met, (basically equates to T/O config + aircraft accel to greater than 80 kts and then thrust levers reduced to idle) then full Hyd press is supplied to the wheel brakes to stop the aircraft. No decel rate is considered – the maximum stopping force is commanded. Once again though, the anti-skid can interrupt pressure to a brake unit if a lock-up is sensed.
if memory serves, The 744 autobrake selector scale equates directly with the pressure supplied to the brakes. eg. AB1 = 1200psi, AB2 = 1600psi, AB3 = 2000psi to RTO = 3000psi. The rotary switch could just as easily read pressure rather than 1 - 5.
(ref AMM auto brake test)
No, no, no my friend. Do not confuse a “test” with the actual operation of the system. You are correct with a pressure limit, however it is an OR type system. For auto “3” it will be decel of 6ft/sec/sec OR a maximum of 1750PSI.
During a ground test of the autobraking system, it is possible to check the integrity of the system by installing a pressure guage on 1 brake unit of each truck and meeting the autobrake arming conditions. When the brakes are applied by the autobrake system the following pressure should be recorded on the brake gauges for each selected level of braking:-
1 = 1300PSI
2 = 1500PSI
3 = 1750PSI
4 = 2050PSI
MAX = 3000PSI
These pressures are also the MAXIMUM that can be applied at each level of braking during normal ops. Testing and certification data deem that the required decel level can be achieved during all normal conditions within the max pressure limit. But hey, if you want to land your Jumbo on a sheet of ice, don’t think that selecting MAX on your autobrake selector will guarantee that you’ll pull up. You can’t beat the laws of physics!
Bolty, I fear that all your recent time spent playing with the “Sky Skoda”, “The Hyundai of the Sky” , “The Runway Renault” has pickled your brain. Time to turn right when you walk out on the ramp and get re-acquainted with the “Queen of the Skies”
But then again, why believe me? From Mister Boeing:-
AUTOBRAKE SELECTOR MODULE
The Autobrake Selector Module commands a level of
deceleration for landing autobrake operation. The commanded rate of
deceleration and maximum available pressure increase at each switch
position from 1 thru MAX AUTO.
Landing Autobrake
The Autobrake Selector Module Switch will magnetically latch in a
landing autobrake position, when arming requirements are met, to
provide power and a commanded rate of deceleration to the
Autobrake System.
The Autobrake Card will command brake pressure by opening the
Solenoid Valve and will modulate the Control Valve to maintain the
selected deceleration to a complete stop, provided the average
velocity of the 4th, 5th, 6th and 7th fastest of the eight forward
wheels exceeds 60kts and stays above 30 kts for 3 sec, setting a
latch to allow full stop braking.
The wheel spin up latch is reset a air mode signal or system
disarmed or system turned off.
Full hydraulic system pressure is applied during RTO with no rate of deceleration computation.
Autobrake Card
The Brake System Control Unit (BSCU) contains a digital,
microprocessor controlled autobrake card which operates a solenoid
and control valve in the Autobrake Pressure Control Module.
Pressure Control
When the Apply logic is satisfied the Autobrake Card will command
the Control Valve to provide the following;
• Brake Fill
- An initial pressure spike of 1650 psi, for pilot feel.
• Followed by on-ramp pressure
- Pressure increase of 150 psi/sec for 2sec, followed by 450
psi/sec to autobrake selected pressure or deceleration rate
pressure.
• Maintain the selected deceleration to a complete stop
Operation
The module is connected to the Normal Braking System and meters
pressure to the brakes in response to brake system control unit
inputs.
• A solenoid shut off valve controls pressure into the Module.
• A control valve provides a controlled (metered) pressure output.
A pressure switch (750 +/- 50 psi) is installed on the Brake Metering
Valve side of the Shuttle Valve to provide an landing autobrake
disarm input to the Autobrake Card.
Test Application
The AUTOBRAKE APPLICATION test is run to verify proper
operation of the autobrake subsystem for all autobrake settings.
After all preconditions have been met, each arming level on the
switch may be tested separately by depressing START TEST.
• The test energizes the Autobrake Solenoid Valve and increase the
Command Valve pressure up to the set limit.
1=1300psi
2=1500psi
3=1750psi
4=2050psi
MAX=3000psi
• The pressure is maintained for 10 seconds, then reduced to
minimum pressure for 15 seconds.
• When the brakes are released the CMC message will no longer be
displayed and the autobrake switch will disarm.
The test is inhibited with airplane in motion.
Now how did this conversation start? Landing SOPS. What determines these aside from safety.........money!
The longest life of carbon brakes (reduced overhaul cost) can be achieved by using them at a highish temperature and using them for one longish/hard application rather then several short sharp ones. Just watch those F1 drivers jump on the anchors during the warm-up lap. It's mainly used these days to "warm-up" the brakes so as you can have your discs up to optimum operating temp to allow you to go deeper and harder into the first corner and steal a place. I digress........Best way to achieve optimum temps on landing is to let the brakes do a fair share of the work and you can ensure they do this by limiting the use of other deceleration devices (i.e idle/no reverse). Another way is to select a higher level of autobrakes to ensure the brakes are applied to ensure heat build-up to effective temps.
Clear as mud I'm sure. 