[32起落架] 推拖飞机主轮有这种异响，别慌，正常的

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SR3-3857382607

SUBJECT: Query on Main Wheel Noise

DESCRIPTION:

China Southern Xinjiang Branch (XIJ) reported that recently they noted left main landing gear wheel noise during the datum airplane being towed. The voice was recorded in the attached Ref /A/ audio file. 

XIJ changed the No. 1 wheel and its associated wheel brake assembly P/N 2-1587-1, then the noise disappeared. 

XIJ inspected the removed wheel and the brake assembly. No any chafing trace was noticed, which could cause the noted noise. 

XIJ would like to know if Boeing has received the similar wheel noise reports from other operators and if Boeing can provide any comments or information about the cause of the noted noise. 

Desired Action

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1/ Please advise if Boeing has received the similar wheel noise reports from other operators.

2/ Please advise if Boeing can provide any comments or information about the cause of the noted noise. 

Your reply by 06 Apr 2017 (Seattle Time) would be appreciated. 

RESPONSE:

Boeing has listened carefully to the video and provides the following comments:

1/ Boeing occasionally receives reports of noises coming from the MLG area during ground operations. The causes of these noises can sometimes be difficult to determine. We have had reports of knocking/bumping sounds that were suspected to be due to improper shock strut or shimmy damper servicing (low strut/damper oil levels, excessive damper trapped air) and grinding type noises suspected to be due to improperly lubricated (dry) wheel bearings. Sometimes the noise complaints were accompanied by reports of aircraft vibration. The complaints usually disappeared after Operators conducted one or more of the following maintenance actions:

a) Complete shock strut complete servicing.

b) Shimmy dumper bleeding.

c) Torsion link apex pin retorque.

d) Main wheel replacement.   

2/ The noise on B-7XXX appears repeatable and is cyclic in nature relative to the wheels position. In other words, the noise changes as the wheel rotates but the same type of noise is heard at the same wheel rotational position each wheel revolution. The noises are neither a shock absorber/damper knocking/bumping or wheel bearing grinding noise as described in 1/ above. Rather, as it appears tied to the wheel's rotation (and the noise disappeared after #1 wheel/brake replacement), Boeing suspects the noise may have come from the #1 wheel hubcap's drive shaft that engages/drives the wheel speed antiskid transducer.

The hubcap's drive shaft has a two piece metal to metal spring-loaded ball joint to accommodate normal assembly tolerance miss-alignment between the hubcap and transducer installations. If the connection to the transducer has large free play, the transducer is loose and/or the ball joint has high misalignment and/or the ball joint spring is weak, when the hubcap is installed and the wheel rotates the two ball joint pieces can shift and hit each other (producing a noise similar to the one heard in the video).

If the #1 wheel/tire assembly was recently changed, it could be that the hubcap's drive shaft was misaligned relative to the wheel speed antiskid transducer. Then as the wheel/hubcap rotated that misalignment caused repeated metal to metal contact of the two piece ball joint as noted above. When the #1 wheel/brake was removed, the removal and re-installation of the hubcap onto another wheel changed the misalignment (and hence the noise).

At the next wheel change, Boeing recommends China Xinjiang Airlines does a DVI of the #1 wheel hubcap's drive shaft ball joint assembly and the mating #1 wheel speed antiskid transducer for any signs of individual component looseness/binding/damage or excessive clearance/wear at the shaft to transducer joint [ref AMM 32-45-11 SUBTASK 32-45-11-840-002 Step (1)(f)]. Replace any suspect part(s) per applicable CMM. If during the DVI any questionable items are identified, removal and inspection of the same axle/non-noise making hubcap installation for comparison purposes may be beneficial.

DESCRIPTION:China Southern Xinjiang Branch (XIJ) reported No.1 MLG wheel noise occurred again on B-7XXX airplane on 14 Apr 2017.  The attached Ref /D/ and /E/ two videos are the recorded noise with the hubcap installed and removed.XIJ advised that with hubcap removed, during the towing, the noise was still heard but less loud than with the hubcap installed. XIJ also advised that with No.1 wheel jacked and manually rotated, the noise was not heard.  XIJ replaced the brake assembly and the noise remained. After the wheel was replaced, the noise was gone. Then XIJ replaced the shimmy damper. XIJ removed and cleaned the bearing from the No.1 wheel. The bearing was in good condition and did not have evidence which could cause the noted noise. XIJ is planning to install the wheel on an airplane to check if the noise would occur again. On 17 Apr 2017, XIA initiated a 737NG fleet inspection of the MLG wheel noise and some airplanes have completed the inspection. XIJ provided the attached Ref /F/ audio and Ref /G/ video recorded on airplane B-1520 No.4 wheel and B-7995 No.1 wheel, which sounded the similar noise.RESPONSE AND/OR REQUIRED ACTION :1/ XIJ would like Boeing to analyze the attached Ref /A/ and Ref /B/ videos and advise what could cause the reported noise.   2/ XIJ also would like Boeing to advise what parts and evidence in the wheel should be inspected.3/ Any other Boeing comments on the reported MLG wheel noise would be appreciated. Your reply by 18 Apr 2017 (Seattle Time) would be appreciated.RESPONSE:Boeing has reviewed the required actions and provides the following response:1/ The REF/A/ and REF/C/ videos show that the noises are similar in nature. As previously stated in Boeing's REF/B -02B reply, the clicking/knocking noises could be due to shock strut (low oil/gas level), shimmy damper (low oil level, excessive damper trapped air, excessive free play at the apex joint), wheel (loose/improperly torqued wheel bearings, rotor drive keys contacting brake rotor inserts during rotation), hubcap (drive shaft drive shaft engagement to the wheel speed antiskid transducer) or transducer (excessive free play within axle). As the wheel/axle/inner cylinder/torsion links-shimmy damper installation comprises of several independent parts that move relative to/stop against each other under ground loads, some amount of noise associated with that motion can be expected.3/ We are unable to definitively explain the noise at this time.  As the noise occurs with and without the hubcap installed, it is possible that this noise is associated with movement of the brake rotors in the brake heat stack and how they interact with the brake's stators and the wheel's rotor drive keys (as opposed to the hubcap drive shaft to wheel speed antiskid transducer attach joint previously mentioned in REF/B/). As both areas are metal to metal contact, the clicking/knocking sound would be similar for both. The installed hubcap may be acting as an antenna and amplifying the wheel key to brake rotor contact noise (hence the noise being more pronounced when hubcap is installed versus removed). Additionally, noise due to wheel key to brake rotor contact could be more or less pronounced depending on the worn condition of the brakes, flatness of the rotor and stator plates, amount of wheel/rotor installed free play and the amount of hydraulic system return pressure that may be on the brakes (with the brakes released). Therefore Boeing suspects that the most likely cause of the noise on these aircraft is that brake rotor segment to wheel drive key radial installation free-play is allowing the brake rotor segments to shift/drop down slightly due to gravity as they rotate while the airplane is being towed slowly. Since China Southern has not discovered any obvious damage on wheels or brakes on airplanes where this noise has occurred, we suspect that the condition is acceptable.2/ In addition to the inspections already performed and those recommended in REF/B/, Boeing recommends the following additional checks to confirm no abnormal condition is causing the noises:  a/ Perform close examinations of the wheel and brake rotor drive lugs, keys and caps and the corresponding areas on the brake rotors to identify any unusual looseness or contact between surfaces/parts.    b/ Examine the wheel's balance weights and ensure that they are assembled and secured properly and are not contacting anything as the wheel rotates.  If nothing abnormal is found, Boeing's conclusion are as stated at the end of Answer #3 above.