The 737 fuselage is the aircraft's pressure vessel and structural backbone, and inspectors examine it for cracks, corrosion, and metal fatigue that can compromise safety

A close-up inspection of a Boeing 737 fuselage — the tube-shaped body that holds passengers, cargo, and fuel — looks for damage invisible from the ground. Inspectors use borescopes, ultrasonic testing, and visual examination to find metal fatigue cracks, corrosion pitting, and delamination in the aluminum skin. These defects matter because the fuselage must hold cabin pressure at 35,000 feet; a crack that starts small can grow during flight cycles and eventually fail.

The 737 fuselage is built from aluminum alloy sheets riveted and welded together. Over time, the repeated stress of pressurization, depressurization, and temperature swings causes the metal to weaken. Cracks often start at rivet holes or weld seams, where stress concentrates. Corrosion — from moisture, salt air, or hydraulic fluid — eats into the metal from the outside or inside. Inspectors must catch these problems before they reach a critical size.

Key Takeaways

  • The 737 fuselage is an aluminum pressure vessel that must withstand repeated cycles of pressurization and depressurization, making metal fatigue and corrosion the primary safety concerns.
  • Inspectors use borescopes, ultrasonic testing, and visual examination to find cracks, corrosion pitting, and delamination that are too small or hidden to see with the naked eye.
  • Cracks typically start at rivet holes and weld seams, where metal stress is highest, and can grow during flight if not repaired.
  • Airlines follow Federal Aviation Administration (FAA) inspection schedules that increase in frequency as an aircraft ages, with older 737s inspected more often than newer ones.
  • Repair methods range from sealing small corrosion spots to replacing entire fuselage sections, depending on the defect size and location.

Where cracks and corrosion typically form on the 737 fuselage

Fatigue cracks most often appear at rivet holes in the fuselage skin, especially around the cabin windows and door frames. These areas experience the highest stress during pressurization. The rivets themselves can loosen over time, creating small gaps where cracks initiate. Weld seams — where fuselage sections are joined — are another common failure point because the welding process changes the metal's strength and can leave tiny defects.

Corrosion typically starts on the exterior where rain, salt spray, and dirt accumulate, but it also grows inside the fuselage where moisture from the cabin air condenses. Galley areas, lavatories, and cargo holds are particularly vulnerable because water sits longer there. Corrosion under insulation (CUI) is especially dangerous because it hides from view until an inspector removes the insulation to look. A 737 that has flown coastal routes or in humid climates for many years may have significant corrosion that younger aircraft do not.

How inspectors examine the fuselage up close

Visual inspection is the first step: an inspector walks the fuselage exterior and interior with a flashlight, looking for discoloration, pitting, or visible cracks. Paint bubbling or peeling signals corrosion underneath. For areas the eye cannot reach — inside the fuselage walls, behind insulation, or deep in rivet holes — inspectors use a borescope, a thin camera on a flexible probe that transmits images to a screen.

Ultrasonic testing measures metal thickness by bouncing sound waves through the aluminum. If corrosion has thinned the skin below a safe limit, the ultrasonic reading will show it. Eddy current testing uses electromagnetic fields to detect cracks too small to see, even with a borescope. These tools can find defects before they are visible, which is why close-up inspection catches problems early.

Inspectors also tap the fuselage with a small hammer and listen to the sound. Solid metal rings clearly; corroded or delaminated metal sounds dull or hollow. This straightforward technique, called tap testing, can reveal hidden damage under paint or insulation.

FAA inspection schedules and how they change with aircraft age

The Federal Aviation Administration (FAA) sets mandatory inspection intervals for the 737 based on flight hours, flight cycles (takeoffs and landings), and calendar time. A new 737 might undergo a basic visual inspection every 400 to 600 flight hours. As the aircraft ages, inspections become more frequent and more thorough. A 737 with 20,000 or more flight hours may require detailed fuselage inspections every 100 to 200 hours.

Airlines also follow manufacturer-recommended maintenance programs, which often exceed FAA minimums. Boeing publishes service bulletins that direct airlines to inspect specific areas if a problem has been found on other 737s. For example, if cracks appear on multiple aircraft at a particular rivet row, Boeing may issue a bulletin requiring all 737s to inspect that location. These bulletins can trigger inspections sooner than the regular schedule.

Repair methods for fuselage damage

Small corrosion spots — less than a few inches across — are often cleaned out and sealed with epoxy or a metal-filled compound. The corroded area is scraped away, the surface is primed, and the sealant is applied. This repair is quick and keeps the aircraft in service.

Larger corroded areas or small cracks may require a patch. Technicians cut out the damaged section, fit an aluminum patch over the hole, and rivet or bond it in place. The patch must overlap the damaged area by a set amount (typically several inches) to may support the repair is strong enough. After patching, the area is primed and painted to prevent new corrosion.

Extensive damage — a large crack, widespread corrosion, or damage near a critical joint — may require replacing an entire fuselage section. This is a major repair that takes weeks and costs hundreds of thousands of dollars. The aircraft is taken apart at a predetermined break line, the damaged section is removed, and a new or salvaged section is installed and riveted back together. After section replacement, the fuselage must be pressure-tested to confirm it still holds cabin pressure safely.

Why fuselage condition affects aircraft value and service life

The fuselage condition is one of the first things a buyer or lessor checks when evaluating a used 737. An aircraft with a clean fuselage inspection history — few repairs, no major corrosion — is worth more and can command higher lease rates. An older 737 with extensive corrosion repairs or a history of major fuselage work is worth less because future repairs are more likely and the aircraft may not be economical to operate much longer.

Airlines use fuselage condition to decide when to retire an aircraft. A 737 with a structurally sound fuselage can fly safely for 25 to 30 years or more. But if corrosion is widespread and repairs are becoming frequent, the airline may decide it is cheaper to retire the aircraft and buy a newer one than to keep repairing it. Fuselage inspection records are part of the aircraft's maintenance history and follow it through its entire service life.

Common fuselage problems found on older 737s

The original 737 Classic series (built in the 1980s and 1990s) has experienced widespread corrosion in certain areas, particularly around the cargo door and in the lower fuselage where moisture collects. Some aircraft have required major corrosion repairs or even fuselage section replacement. The 737 NG (Next Generation), introduced in 1997, has better corrosion resistance due to improved materials and design, but older NGs are now starting to show corrosion as they approach 20 years in service.

Structural cracks have also been found on high-cycle 737s — aircraft that have flown many short routes with frequent pressurization cycles. A 737 that has flown 50,000 short flights experiences more stress cycles than one that has flown 50,000 hours on long routes. This is why flight cycle limits are part of the inspection schedule. Some 737s have been retired not because they ran out of flight hours, but because they reached their cycle limit and further operation would require expensive reinforcement or replacement.

Frequently Asked Questions

How often does a 737 fuselage get inspected?

Inspection frequency depends on the aircraft's age and flight history. Newer 737s may be inspected every 400 to 600 flight hours; older aircraft are inspected every 100 to 200 hours. Airlines also follow Boeing service bulletins, which can require additional inspections if a problem has been found on other 737s. The exact schedule varies by operator and regulatory authority.

Can a 737 fly safely with a patched fuselage?

Yes. Patches are engineered repairs that restore the fuselage to its original strength if done correctly. The patch must overlap the damaged area by a set amount and be riveted or bonded securely. After repair, the fuselage is pressure-tested to confirm it holds cabin pressure. Thousands of 737s fly safely with multiple patches.

What does corrosion under insulation mean?

Corrosion under insulation (CUI) is metal damage that grows hidden beneath the fuselage insulation blanket. Moisture gets trapped between the insulation and the aluminum skin, causing corrosion that spreads unseen. Inspectors must remove insulation in suspect areas to find it. CUI can be extensive by the time it is discovered, which is why regular inspection of insulation areas is important on older aircraft.

Why do some 737s get retired before they reach their flight hour limit?

Aircraft can reach a flight cycle limit before a flight hour limit. A cycle is one takeoff and landing; an aircraft that flies many short routes accumulates cycles faster than one flying long routes. When a 737 reaches its cycle limit, further operation requires expensive structural reinforcement or replacement. At that point, airlines often retire the aircraft rather than invest in major repairs.

How much does a fuselage section replacement cost?

A fuselage section replacement can cost several hundred thousand dollars and takes several weeks. The exact cost depends on which section is damaged, whether a new or salvaged section is used, and the labor rates at the repair facility. This is one reason airlines carefully monitor fuselage condition and perform preventive repairs before damage becomes severe.