Brakes
The brakes are steel, actually an alloy called "Cerametalix(R) friction brake material" with versions made by either Goodrich or Honeywell. From 2008 the 737 NG will be available with carbon Duracarb(R) brakes which will be supplied by Goodrich and Messier-Bugatti. These will be 300kgs lighter and last twice as long.
The brake pressure gauge merely shows the pressure of the air side of the accumulator and should normally indicate 3000psi. The normal brake system and autobrakes are powered by hydraulic system B. If brake pressure drops below 1500psi, hydraulic system A automatically provides alternate brakes which are manual only (ie no autobrake) and the brake pressure returns to 3000psi. Antiskid is available with alternate brakes, but not touchdown or locked wheel protection on series before the NG's.
If both system A and B lose pressure, the accumulator isolation valve closes at 1900psi and you are just left with residual hydraulic pressure and the pre-charge. The gauge will indicate approx 3000psi and should provide 6 full applications of brake power through the normal brake lines (so full antiskid is available) As the brakes are applied the residual pressure reduces until it reaches 1000psi at which point you will have no more braking available.
If the brake pressure gauge ever shows zero, this merely indicates that the pre-charge has leaked out, normal and alternate braking are unaffected if you still have the hydraulic systems (see QRH). The accumulator also provides pressure for the parking brake.
Note that on the 737-1/200, hydraulic system A operates the inboard brakes and system B operates the outboard brakes. Both brake pressures are indicated on the single hydraulic brake pressure gauge.
Brake Pressure Indication (psi) Condition
3000 Normal.
3000 No hydraulics, minimum 6 applications of brakes available with accumulator.
1000 No hydraulics, accumulator used up.
Zero No pre-charge, normal braking available with hydraulics.
Brake Accumulator
Brake Wear Pin
Autobrakes
Autobrake Selector
Max Pressure at Brakes (PSI)
Deceleration Rate (ft/sec²)
1
1250
4
2
1500
5
3
2000
7.2
Max
3000
12 (below 80kts)
“
“
14 (above 80kts)
RTO
Full
Not Controlled
There is an "on ramp" period where autobrake pressure is applied over a period of time. Approximately 750psi is applied in 1.75 sec, then the pressures above are reached in another 1.25sec for autobrakes 1, 2, or 3 and approx. 1.0 sec for autobrake MAX.
Notice from the table above that autobrake Max does not give full brake pressure. For absolute maximum braking on landing, select autobrake Max to assure immediate application after touch down then override with full toe brake pressure.
Using high autobrake settings with idle reverse is particularly hard on the brakes as they will be working for the given deceleration rate without the assistance of full reverse thrust.
To cancel the autobrake on the landing roll with toe brakes you must apply a brake pressure in excess of 800psi (ie less than that required for autobrake 1). This is more difficult on the NG's because the feedback springs on the brake pedals are stiffer. Autobrake can also be cancelled by putting the speedbrake lever down or by switching the autobrake off. I would advise against the latter in case you accidentally select RTO and get the full 3000psi of braking!
Occasionally you may see the brakes (rather than the cabin crew!) smoking during a turnaround. This may be due to hard braking at high landing weights. But the most common reason is that too much grease is put on the axle at wheel change so that when the wheel is pushed on, the grease is deposited inside the torque tube; when this gets hot, it smokes. It could also be contamination from hydraulic fluid either from bleeding operation or a leak either from the brakes or another source.