Date: 201410
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight phase: Parked
Aircraft component: Main Gear Wheel
Component problem: Malfunctioning; Design
Reporter organization: Air Carrier
Reporter function: First Officer
Human factors: Confusion
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: Pre-flight
Result: General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: During preflight inspection this morning; I checked the Main Landing Gear (MLG) wheel well. On the forward wall (firewall?) of the wheel well; adjacent to metal cylinders (I believe they are hydraulic system filters) on both the left and right side of the aircraft there was a large crack. If you pushed on the metal on either side of the crack; you could see that the metal was indeed separated.  I showed the Captain who then called Maintenance and grounded the aircraft. I wanted to bring this to your attention because this is the second crack that I have found in the last few months in exactly the same location! I regret that I don't remember the date or flight number from the first event; but on that one it was a Classic B737 in ZZZ. It seems that it may not be coincidental that the crack showed up in the same area; so I thought I should submit a report just in case you want to put something out to the Pilot Group so that First Officers can be vigilant for it during their preflight inspections.If it is determined that this is a problem: 1) Notify First Officers to be vigilant for cracks in this area.2) Consider moving to an industry standard walkaround inspection by a Pilot prior to every departure. Since the minimum turn times are now greater than 25 minutes; there should be time for a Pilot to do a walkaround on each leg. Pilots will be more likely to catch an anomaly than Ground staff; who is mostly looking for open panels and hatches. Cracks found in main landing gear well during preflight.

Callback: Reporter stated that all of the cracks he noticed were located in the same locations on the left and right sides of the main wheel well at the outboard side of vertical hydraulic filters. The location is an open area; no rivets or components touch the area. The cracks are consistently two inch to three inches long and vertically diagonal. He has found the same cracks in the same area on Classic B737s and now on the B737-700 and -800 aircraft. He was recently informed by Maintenance personnel the cracks are located on the vapor barrier area of the main wheel well; a non-structural and non-pressurized of the wheel well area below the forward pressure bulkhead. Maintenance had discussed the issue with Boeing and a special sealant was applied to the area and a permanent metal repair scheduled for a later date.



Synopsis: Pilot reported finding two to three inch diagonal cracks in the vapor barrier area at the forward side of the left and right Main Landing Gear (MLG) wheel well area just outboard of System 'A' and 'B' return filter modules. They have seen similar cracks on Classic B737s and now B737-700 and -800 aircraft.