External Doubler: Puzzlement

[Wingtip46]

As you approach the main entrance door of a commercial airliner, one often views a large external doubler about the upper corners of the doorway cutout.

I had once stumbled upon the reason why, but no longer remember why or the source of information.

Of course, the doorway cutout requires reinforcement, but why external? I often thought it was an add-on post- manufacture, and wondered why the doubler could not be incorporated internally.

But there is a JUST reason to install externally; balance load distribution about main skin (???) Just what is that reason ???

Puzzled,
Wingtip46

 

[737eng]

Most of these doublers are probably installed due to damage or cracks in the corners. The cut-outs are designed with internal doublers (bear straps, etc...), however, due to fuselage bending and the increased stress at the corners, the skin and/or bear straps usually crack at the corners, thus resulting in the need for repair doublers. These repair doublers generally can only be installed on the external side due to space constraints. In addition, if there is a known issue with the skin/doubler cracking at the corner, there is probably a Service Bulletin or similar manufactures document or operator document which installs these doublers as a post-production mod in order to reduce the stress in these corner. There is also jetway rash which may have induced damage.