The wing spar of the Model 18 was fabricated by welding an assembly of tubular steel. The configuration of the tubes in combination with drilled holes from aftermarket STC modifications on some of these aircraft have allowed the spar to become susceptible to corrosion and cracking while in service.  This prompted the FAA to issue an Airworthiness Directive in 1975, mandating the fitting of a spar strap to some Model 18s. This led, in turn, to the retirement of a large number of STC-modified Model 18s when owners determined the aircraft were worth less than the cost of the modifications. The corrosion on unmodified spars was not a problem, and occurred due to the additional exposed surface area created through the STC hole-drilling process. Further requirements have been mandated by the FAA and other national airworthiness authorities, including regular removal of the spar strap to allow the strap to be checked for cracks and corrosion and the spar to be X-rayed . In Australia, the airworthiness authority has placed a life limit on the airframe, beyond which aircraft are not allowed to fly.   
Turboliners arrived on the Detroit run on April 10, 1975. Additional equipment allowed Amtrak to add a round-trip in late April; the arrival of a third trainset in May made Chicago–Detroit the "first all-turbine-powered route". After one year of operation, ridership on the corridor had increased by 72 percent. The fixed capacity of 292 passengers on an RTL trainset proved an impediment; Amtrak could not add capacity when demand outstripped supply. Amtrak replaced one of the trainsets with a conventional locomotive hauling then-new Amfleet coaches in 1976; Turboliner service ended altogether by 1981 as more Amfleet equipment became available.