Why are 737-200 engines more susceptible to separation?

Question

It seems the 737-200 had an issue with separation of engines from the wings. Considering that the engines are mounted completely under the wings, there should be more attaching points than a modern jet engine the question is why is this aircraft so prone to this sort of incident? Is it something to do with the airflow and the forces associated to it? Or is it purely a manufacturing defect?

Source: https://www.flightglobal.com/news/articles/nationwide-737-engine-loss-inquiry-finds-fatigue-crack-in-mount-220017/

Snippet from the article:

Fatigue failure of aft cone bolts has been attributed to a number of engine-separation incidents involving 737-200s – among them a US Air aircraft at Deptford in December 1987, a United Airlines jet at Chicago in January 1989, and a Delta Air Lines aircraft at Dallas in January 1992. All three lost their right-hand engines on take-off.

Answer 1

In general, jet engines are mounted under wings using only a few attachment points- Boeing uses cone pins in 737-100s and 200s (these were changed into fuse pins from 737-300s).

Older 737s with cone pins (the SA 737 is a 200) has a history of failure and almost all the the cases being blamed on the fatigue and manufacturing defect of the aft bolt (which usually resulted in over-stressing and failure of the two front bolts). I've just listed some of the incidents and the probable causes (from NTSB):

Incident: January 03, 1986 involving BOEING 737-2H4, registration: N86SW

Probable Cause: ...EXAMINATION OF THE CONE BOLT REVEALED THAT IT FAILED AS A RESULT OF FATIGUE, MOST PROBABLY DUE TO IMPROPER INSTALLATION OF THE BOLT, SPECIFICALLY, THAT IT WAS UNDER TORQUED WHEN THE OPERATOR RE-INSTALLED THE ENG.

Incident: December 05, 1987 involving BOEING 737-2B7, registration: N319AU

Probable Cause: ... THE AFT MOUNT CONE BOLT FOR THE #2 ENG HAD FAILED FM FATIGUE THRU THE THREAD RELIEF UNDERCUT RADIUS. FATIGUE CRACKS HAD INITIATED ON DIAMETRICALLY OPPOSITE SIDES OF THE RADIUS.

Incident: January 20, 1989 involving BOEING 737-201, registration: N242US

Probable Cause: PREVIOUS DAMAGE TO THE AFT CONE (ENGINE MOUNTING) BOLT, WHICH RESULTED IN MISMATCHED SURFACES BETWEEN THE BOLT AND ISOLATION MOUNT, LOSS OF TORQUE DURING CYCLIC LOADING OF THE MOUNTING BOLT, AND SUBSEQUENT FATIGUE FAILURE OF THE BOLT

Incident: January 07, 1992 involving BOEING 737-232, registration: N322DL

Probable Cause: THE FAILURE OF THE AFT CONE BOLT AS RESULT OF PREEXISTING FATIGUE CRACKING DUE TO IMPROPER MAINTENANCE, AND THE FAILURE OF THE SECONDARY SUPPORT STRUCTURE AS A RESULT OF LOADS THAT EXCEEDED THE CAPACITY OF THE ATTACHING HARDWARE AND THE CRUSHABLE HONEYCOMB CORE.

The incident involving the Nationwide 737-230 was also traced back to fatigue of the engine bolt:

The right-hand engine separated from the aircraft due to the failure of the aft cone bolt as a result of a pre-existing fatigue crack which was most likely caused by incorrect installation of the cone bolt.

No investigation has pointed to any problem with the airflow or associated forces.

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In a related note, 747s, which use fuse pins also had a number of incidents/accidents caused due to failure of the pins and Boeing changed the design quite a few times.

Answer 2

When Boeing first designed the 737, their focus was to reduce weight by having a short and simplified landing gear. This affected how the JT8Ds were fitted to the wings (directly underneath, no pylons) and introduced the design and associated issues that @aeroalias has explained so well.

It was only when the 737-300 was being designed to have the larger (read quieter and more efficient) CFM engines that the engine position was changed (from directly under wing to slightly forward of it), and allowed Boeing the opportunity to do away with the cone pins.