When the novel coronavirus swept through the travel industry in March, grounding flights and stalling airplane orders, TransDigm didn’t wait for a congressional bailout in order to keep itself solvent. It didn’t have to. On March 23, with a green light from Treasury Secretary Steven Mnuchin, the Fed announced that it would buy as much corporate debt as necessary — in the form of bonds — to reassure companies that they could raise money they needed to ride out the pandemic. Within a few weeks, the bond market had nearly regained the ground it lost, spurring a debt boom. U.S....