The FAA ranked the fires a "serious event" and "potentially hazardous situation which can cause serious injury and damage to (an) aircraft."
All of the fires were on scheduled flights, one of which was in flight and the other two occurred at the gate. More than 50 wires were burned on each plane and numerous circuit breakers were tripped
The safety alert affects all Boeing models 737-100 through 737-500, the so-called "Classic series," built from 1967 to 1999. More than 1,600 of the planes still remain in active service.
The alert says the malfunction occurred because of improperly installed clamps on air ducts. Operators inspected 279 Boeing 737 series planes, and 71 of those planes had incorrectly installed clamps.
Boeing has now issued new instructions for installation of the clamps that clarify the proper positioning to avoid future mishaps, the safety alert said.