Order backlogs are usually a good thing. So Embraer's (NYSE:ERJ) announcement over the weekend that it has $9.9 billion worth of firm jet orders waiting to be delivered was enough to get most investors excited. From homebuilders to semiconductor stocks, bookings for future deliveries matter, and they truly are a decent gauge for near-term fiscal performance.

But if you consider that these are mostly commercial airplane orders that we're talking about, that smile can easily turn upside down and let disillusionment kick in. Air carriers have been known for putting off orders, being bad credit risks, or even going kaput.

The four leading aircraft manufacturers feel the same pain even if they come from all over the world. Canada's Bombardier, France's Airbus, Brazil's Embraer, and our own country's Boeing (NYSE:BA) know the finicky ways of commercial aviation and military contracts.

For Embraer, at least it can take heart that one of its largest customers is the steadily growing JetBlue (NASDAQ:JBLU). Of the 383 jets on order with Embraer, 100 of those will belong to JetBlue -- as Embraer begins delivery of its new 190 family of flying machines this year. Of Embraer's airline orders, 44% are for the new 190 class of commercial aircraft. If things go well with the 190, JetBlue has an option for a hundred more jets.

Yes, it is encouraging to see Embraer's jets in demand. Yet it's also important to remember that everything from soaring jet-fuel costs to the threat of terrorism can trip up the struggling airline industry. And if the carriers take a fall, the manufacturers have little choice but to fall right behind them.

Related links:

Longtime Fool contributor Rick Munarriz doesn't have a fear of flying -- or landing. He does not own shares in any of the companies mentioned in this story. The Fool has a disclosure policy. He is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early.