The National Highway Traffic Safety Administration is alerting fleet operators that certain 1991-92 Monon trailers may be prone to cracking in the upper coupler plate. NHTSA's Office of Defects Investigation found that there is potential for the trailer king pin to separate from the trailer flooring framework due to fatigue cracks that may develop and propagate through a section of the upper coupler plate in 28-foot-long dry freight trailers manufactured by Monon in 1991 and 1992. ODI has found that cracks can originate in the welds connecting the trailer flooring frame members in an area near the king pin cross block support. If these types of cracks are not detected and corrected, NHTSA says that they can propagate and eventually allow the trailer king pin to separate from the trailer.NHTSA says normal vehicle inspections are unlikely to detect this particular crack condition during the initiation or early-stage phases of crack propagation. A section of the trailer flooring must be removed to effectively detect cracks originating near the omitted weld in the trailer ribs in the early stages of propagation.Removing a section of the trailer flooring components is not practical for normal inspections so affected vehicle operators are not likely to be aware of a crack condition until the crack has propagated to the outside of the trailer flooring (on the upper coupler plate) where the crack can be detected by visual inspection.Because of Monon's bankruptcy filing in 1997, ODI was not able to obtain actual production records, but has estimated the population of affected vehicles, based on phone interviews with former employees, to be 4,000 (2,000 manufactured in both 1991 and 1992).ODI recommends that owners of affected trailers obtain professional engineering advice regarding appropriate repair methods and specifications to assure that any indicated repairs are performed in a reliable and effective manner.