General layout of Boeing 747 showing doors, engines and main equipment bay.

Modified Boeing 747-200, called E4B, cargo door open and ladder propped on sill.

Pan Am Boeing 747 loading cargo into forward cargo bay.

Pan Am 103 reconstruction showing area of destruction which matches UAL 811 below.

UAL 811, the almost victim which came back to tell what happened, door opened in flight.

Air India 182 reconstruction showing vertical tearing of fuselage skin.

TWA 800 showing door in pieces.

The door seal is a fabric impregnated with a rubber/silicone substance that is shaped like an angled T. The top of the T is press fitted into an aluminum c channel riveted to the door frame. The free flange of the T is compressed against an aluminum V type channel called a seal depressor on the fuselage.
Air pressure trying to blow this seal out pushes it harder against the V channel. Cargo containers often damage the seal depressors, especially in the top corners and the seals let a lot of air pass. It is not a safety concern other than it reduces fuel efficiency by requiring more bleed air from the engines to maintain a cabin altitude.

Boeing 747 nose picts right side cargo door
811 picture
AD Airworthiness Directive 79-17-02
Airworthiness Directive Amendment to 89-05-54 amending 88-12-04
Airworthiness Directive 88-12-04 Boeing 747
More pictures of UAL 811 cargo door hole
Cargo door accidents

Comment: Noses and doors. Description of cargo door seal from major airline lead mechanic. 10 Aug 97