There is an inverse relationship betweeen price and yield.  So as confidence builds, and prices are bid up, yields fall.  Bonds also don't trade as much , so indicated bid/ask is sometimes very different from where decent volume can clear.

currently, AC trading in line with comparable rated bonds.  For those that can have access to bond pricing , just keep an eye on it.   The bonds trading down would possibly portend very nasty ramifications for equity and vice versa.   

last point is lower pricing and higher yields would drive up servicing costs if debt needs to be rolled at higher yield.  Ultimately, that has a ramification for equity holders since higher interest payments eats into cash flow.  

the other way Debt issues hurt equity is thru multiple.  If amount of leverage rises and associated yields rise (servicing cost), the multiple on the firm typically falls, which impacts the equity.  

that is why the inverse during an industry upturn generates so much appreciation in the share price.  Debt falling, rating rising, interest costs falling, operating leverage rising and so much value accrues to the equity owner.  Unfortunately, AC and the airlines facing the inverse for the time being...