There are many reasons for the financial demise of Spirit Airlines Inc., the discount airline with the bright yellow planes, which sought Chapter 11 bankruptcy protection on Monday.

The biggest was the pandemic, which halted flying and forced airlines to rack up debt. A resulting labor shortage increased the costs of pilots and mechanics. Another blow to Spirit was RTX Corp.’s defective engine part. which grounded part of Spirit’s fleet of some 210 Airbus SE narrowbody aircraft.