Oil Canning in a Fuselage — What You Need to Know

When you look closely at an aircraft’s fuselage, you’ll sometimes see a skin panel flex with a noticeable “pop.” This phenomenon is called oil canning, and while it looks dramatic, it’s a well-understood structural behaviour.

What Is Oil Canning?

Oil canning occurs when a fuselage skin panel temporarily buckles inwards or outwards between structural members. It behaves like the side of an old oil can — press it in, and it snaps back out.

On a 737 for example, it’s most commonly seen around:

• Lap joints

• Cargo door surrounds

• Fuselage over-bays between frames

• Large external doubler repairs

• Stringer or frame intersections

• Aft fuselage and crown skin areas

Why It Appears After a Pressurisation or “Fuse” Event

A “fuse event” — such as a pressurisation upset, valve opening, or rapid differential shift — redistributes loads through the fuselage. This can leave the skin with:

• Slight residual deformation

• Reduced stiffness in one bay

• A mismatch between adjacent stiffened and unstiffened areas

• Minor buckling during pressure cycles

Even though aircraft skins are designed for repeated cycling, a sharp pressure transient can cause local areas to relax into a slightly altered shape.

The Role of Repairs and Doublers

Repairs often stiffen one zone. When a doubler is installed, the surrounding skin can become the new “flex point,” leading to oil canning around the perimeter. This is normal — and expected — after structural work.

Is It Dangerous?

Generally, no. Oil canning is often cosmetic, and Boeing’s Structural Repair Manual (SRM) provides clear criteria for acceptability.

The condition is not a problem if:

• The panel does not remain permanently deformed

• There is no cracking

• Fasteners show no movement or fretting

• The deformation is elastic and repeatable

When Engineers Escalate It

Oil canning requires attention if you observe:

• A panel that stays “popped in”

• Sharp metallic popping noises during pressurisation

• Paint crazing or cracking around the flex area

• Smoking rivets or black dust

• Repeated worsening across pressure cycles

In these cases, a DVI and NDT (often eddy current) are performed to confirm structural integrity.

Which 737s Show It More?

• Classics (300/400/500): Most common

• Early NGs: Moderate

• MAX: Better stiffness and fewer occurrences thanks to improved manufacturing tolerances

Why It Matters

Oil canning is one of those maintenance realities the public rarely sees, but aircraft engineers monitor it closely. It can be entirely normal — but it can also provide early clues about local load transfer or stiffness changes in the fuselage.

Conclusion

Oil canning is not automatically a defect. It’s a known behaviour of pressurised fuselage structures, particularly on older or recently repaired 737s.

For airlines and maintenance organisations, recognising when it’s normal versus actionable is key to safe, efficient operation.

Stay Safe,

Craig.