Narrative:

During preflight inspection this morning; I checked the main landing gear (medium large transport) 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.

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Original NASA ASRS Text

Title: 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.

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.