多数情况下，刹车异响是正常的，不需要采取维护措施。但是通过避免不必要的快速刹车可以减少这些噪音。

波音公司也定期收到主轮舱噪音的报告，波音公司在737NG-FTD-29-03002中分享了有关该问题的一些信息。仅使用EMDP收放襟翼时飞机产生的异响，并且活门的突然转换经钢索反馈可能使刹车踏板有瞬间砰的轻微移动，机组有时能感觉到。波音认为这个现象是正常的，不需要采取措施，当使用EDP后流量足够大就不会出现这种情况。

作为隔离故障的一部分，客户可以更换以上提到的部件尝试减少这些噪音，但可能无法完成消除噪音。

DESCRIPTION:

when we change some hydraulic component(such as EMDP case drain filter element),we do all the task required,and bleed the hydraulic systems.when the airplane carry out the first flight,the crew tell us that there is abnormal nosie in the brake system.we encounter some airplane with the same condition.when we cycle the flight controls surface and cycle the braking several times,the abornal noise is gone.

DESIRED ACTION:

Q1:could you tell  us what is the root cause of this ?

Q2; is there any FTD or other experience for this isssue?how to solve this?

RESPONSE:

Boeing has reviewed the China Southern Airlines (GUN) GUN-GUN-20-0834-01C request and offers the following comments.

R1) Determination of Landing Gear noise/vibration root cause is a complicated issue and it is difficult to ascertain exact cause.  However some possible causes are that the vibration noise can be made during steering motion by high pressure/high flow rate hydraulic fluid in dynamic motion as it passes thru the steering metering valve to the steering actuators. When a steering command (via tiller or rudder pedal) is inputted into the system, the steering metering valve's control spool moves relative to the metering valve's control sleeve and opens up a hydraulic fluid path thru the swivel valve to the appropriate chambers inside of the steering actuators. The hydraulic fluid passing between the metering valve spool and sleeve internal components can generate noise.

Other potential root causes could include fluid flowing in and out of the valves and the brake heat stacks closing and opening as the pedals are applied and released.  The extent and severity of the noise (if it occurs) is often dependent on how rapidly the brakes are applied and the amount of pedal travel that is applied. Generally, noisy brakes are not a problem unless it is also associated with vibration within the brake pedals or airplane braking issues.  If either of these items occurs, we recommend replacing the brake metering valve that is making the noise.

It is also possible that hard and rapid brake pedal application can cause the Alternate Brake Source Select Valve (ASSV) and the Accumulator Isolation Valve (AIV) to shuttle, due to a momentary pressure drop of the "B" hydraulic system. When these valves shuttle, there is a very pronounced "bang-bang" in the main gear wheel well as these valves shuttle to their alternate modes and then shuttle back to their normal modes as the "B" system hydraulic system pressure recovers. This is especially likely when the "B" hydraulic system is only powered by the electric motor pump. 

These same two valves can also shuttle during hydraulic pump start up. As an example, if the "A" hydraulic system is turned on before the "B" hydraulic system, but the opposite was true during shutdown, then the ASSV/AIV will shuttle, accompanied by a loud bang.

In most cases, brake noises are normal and no crew or maintenance action is required.  However, as noted above, the noises can be minimized by avoiding unnecessary rapid applications of the brakes.

Also Boeing would expect that if there is a lot of air in the brake system, these noises might be more pronounced.   Some service history has confirmed that the noise can be caused by air trapped in the brake system. The extent of the noise from the normal brake metering valve area significantly ceased after bleeding the brake system. 

Another potential root cause involves hot antiskid valves suggest high internal bypassing of fluid. GUN might wish to review the shop bench test reports on such antiskid valves to see if there is a problem within the antiskid valves that may explain the heat. Boeing is not aware of antiskid valves causing noise with the airplane at rest. 

Finally airplanes have also been noted to have clunking noises in the wheel well when operating flaps/slats with only the EMDP are used. With both "A" and "B" systems pressurized only by the EMDPs, and the leading edge and trailing edge both requiring flow at the same time, system B pressure dips below the Alternate Source Selector Valve (ASSV) shuttle pressure and the ASSV automatically shuttles to provide system A pressure to the brakes. When the ASSV shuttles it provides higher pressure to the Accumulator Isolation Valve (AIV) and it shuttles to preserve the accumulator pressure. The shuttling of the ASSV and AIV under 3000 psi of hydraulic pressure creates a loud noise in some instances.

As part of troubleshooting, GUN can try to remove and replace the components listed above (steering valve, actuators) etc. to try to minimize/eliminate the noise but might not be able to completely eliminate it. 

R2) As discussed above, Boeing periodically has received notification of noise in the wheel well.   Boeing has shared some information on the issue in the following 737NG-FTD-29-03002 (included for reference).