737-200 brake pressure accumulator

[md-100]

Hi

Can anyone tell me how the 737-200 brake pressure accumulator work?

[CaptainSandL]

You have two brake accumulators on the -200 (just one on all later series) one for inboard brakes & one for outboard brakes. They store enough energy for six full brake applications - even if all hydraulic power is lost. If you should get into that situation just apply the brakes and hold them on, don’t cycle or pump the brakes because you only get six applications. The accumulators also dampen pressure surges and assure instantaneous flow of fluid to the brakes regardless of other hydraulic demands.

[CaptainSandL]

PS As to how they physically work, they hold a charge of air pressure which will act as the force on the hydraulic fluid to power the normal brakes when hydraulic pressure is lost. This means that they only replace the hydraulic pressure (pumps), so if the hydraulic quantity (fluid) is lost they cannot work. This should not happen because of standpipes & hydraulic fuses, etc.

S&L

As ever, there is more info & photos to be found on a certain website.

[westhawk]

A hydraulic accumulator consists of a pressure vessel containing two chambers separated by a sealed movable piston or a diaphram. One chamber is pre-charged with nitrogen at aproximately 1/3 to 1/2 of hydraulic system pressure depending upon the specific application. The other chamber is available to be filled with fluid from the hydraulic pressure manifold. In general, hydraulic accumulators store hydraulic fluid under pressure by allowing hydraulic fluid to enter the "wet" chamber through a check valve at system pressure, thereby moving the piston or diaphram and compressing the nitrogen pre-charge in the "dry" side to the system pressure and reducing the volume of the nitrogen, rather like compressing a spring. The hydraulic fluid is prevented from escaping the chamber by the check valve if system pressure drops below the accumulator pressure. In this way, the fluid is trapped under pressure and will maintain this pressure and volume until needed. In the case of hydraulic system failure, including loss of all reservoir fluid, this fluid under pressure is available to be used as a backup source of hydraulic power to the brakes within the limits of the accumulator's capacity. When needed, the fluid is forced out of the chamber by the pressurized nitrogen in the "dry" chamber pushing the piston or diaphram over as fluid flows out of the "wet" side until empty of fluid. As long as the accumulator is servicable and the fluid lines from the accumulator to the brake valves and on to the brakes are intact, emergency wheel braking should be available.

Hope this helps,

Westhawk

[gimpgimp]

The 737 classic accumulator can be seen in the wheel well and has a pressure gauge installed next to it. The pre-charge is 1000psi There is also a pressure transmitter attached which sends a signal to the cockpit gauge. It is a black cylinder about 5 inches long. Occasionally on a cold morning it sticks (even after hydraulic pressurisation) the cockpit Brake Pressure gauge needle can be seen to be below 2800psi just out of the green. A gentle knuckle tap or two during the walk around on the afore mentioned cylinder will see a movement on the gauge in the cockpit into the green giving correct reading. This can prevent unnecessary delays finding a GINGER BEER (engineer) who knows this little technical titbit.