Boeing (NYSE: BA) shares recovered slightly on Wednesday following a small hit Tuesday after cracks similar to those that ruptured the roof of a Southwest (NYSE: LUV) 737-300 last Friday were discovered in five more of the airline’s planes of that same model.

The stock was recently up 0.8%.

The Federal Aviation Administration issued an emergency airworthiness directive on Tuesday, requiring emergency electromagnetic inspections on all Boeing 737-300s, -400s and –500s – about 175 planes in the U.S.  Foreign airline regulators are likely to issue similar rules for their carriers.

Emergency inspections of these older-model 737s come at a critical time for Boeing, which has seen its stock rise about 13% in 2011 and is taking great pains to show that it can deliver on its next-generation aircraft promises, despite notorious past delays in its 787 Dreamliner program.

Boeing has booked 850 orders for the 787, which already is three years behind schedule. Further delays earlier this year pushed back the delivery date for the first planes to the third quarter of this year.

Since one-third of the parts of the mostly composite-material jet are made in Japan, the possibility of supply chain disruptions in the coming weeks loom large as the humanitarian and nuclear disaster in that country continues to play out.

And then there’s the issue of the composite fuselage itself.  Critics of the 787 program, including a former senior Boeing engineer, have said that unlike cracks in traditional metal, cracks in the plane’s carbon fiber, reinforced-plastic fuselage are invisible.

It’s a pretty good bet that Boeing doesn’t want the current talk about “surprise” cracks in the metal 737s to revive debate on whether its innovative use of composites is tough enough to go the distance.

As of this writing, Susan J. Aluise did not hold an interest in any of the stocks mentioned here.