Boeing's 777X airliner will be a terrific plane. Can it gain ground that Airbus has lost? Photo: Boeing.

It's not every day that horrible news for one company results in a big hit to the stock price of its archrival -- but that's just what happened today to Boeing (NYSE:BA) stock.

On Wednesday, European plane-builder Airbus (OTC:EADSY) sustained a massive hit to its order book when Emirates canceled an order for 70 long-range Airbus A350-XWB wide-body jets. In response to the news, Airbus shares are down about 2.8% as of this writing -- but Boeing shares are down too, by 2.1%. Why?

Did Airbus lose an aerial dogfight?
Announcing the cancellation, Emirates said only that it is "reviewing our fleet requirements" -- and apparently deciding that these Airbus planes are no longer needed.

Deutsche Bank, however, sees something more significant afoot. According to Deutsche, the "launch of [Boeing's] 777X is clearly the motivator for the switch and the rationale is likely both a combination of size (777-9x) and range under hot operating conditions (777-8x has 1000nmi+ longer range for the same seat count vs. the A350-1000 or -900)."

To sum up, Deutsche thinks Boeing's 777X is quite simply a better plane than Airbus's A350-XWB -- at least for Persian Gulf weather. In fact, Emirates ordered 150 units of Boeing's newest wide-body in November, taking out an option to buy 50 more.

Those 50 "optioned" Boeings open up an intriguing possibility when viewed in the context of the 70 canceled Airbuses. What if Emirates was thinking ahead in November and ultimately decides to replace its canceled Airbuses with the Boeing 777X?

The impact on revenue
At a list price of $350 million for the new 777-8X (of which Emirates is buying 35 units), and $377 million on the 777-9X (115 units), Emirates is already on the hook for about $55.6 billion worth of Boeing planes. Adding the 50 optioned orders, and assuming Emirates buys them in the same proportions seen in its November order, could mean as much as $18.5 billion in new revenue for Boeing. (However, list prices are usually inflated, and the actual revenue Boeing gets could be far less than $18.5 billion.)

Still, in this best-case scenario, that $18.5 billion would be about 21% of Boeing's revenue for 2013. And if Emirates were to switch out all of its canceled A350s, replacing them with Boeings, revenue might swell 40% higher -- by $25.9 billion at list price, or 29% of annual revenues.

The impact on profit
This is pretty big news, any way you cut it. As for the impact on profit, data from S&P Capital IQ show that Boeing earns about a 5% net profit margin on its airplanes -- nearly twice what Airbus earns per revenue dollar. If Boeing succeeds in capturing the plane orders that Airbus is losing, we could be looking at anywhere from $925 million to $1.3 billion in extra profit for Boeing.

The risk
Make no mistake -- all of this is a "best-case" scenario for Boeing. If you own Boeing shares, you should curb your enthusiasm based on the fact that list prices almost always overstate the actual, real-world prices that airlines pay for their planes, especially when buying in bulk. And just because Airbus is losing orders from Emirates doesn't mean Boeing will win them.

Indeed, taking a dimmer view of the Emirates announcement than what Deutsche espoused, Royal Aeronautical Society fellow Howard Wheeldon commented that Emirates' cancellation "is about the industry, not the aircraft."

Quoted in The New York Times today, Wheeldon worried that "Emirates might be realizing that they looked a bit too far forward in terms of their expectations that traffic growth would continue on the same level as the past five years." If that's the case, it's entirely possible that Emirates' cancellations are just that -- business lost by Airbus, and not necessarily potential sales for Boeing.

The upshot for Boeing investors
Wheeldon's comments argue in favor of caution for Boeing investors today. Bad news for Airbus may not translate into good news for Boeing. And with Boeing shares trading for a heady 23 times earnings valuation, there's not a lot of room for error here.

In short, while selling Airbus seems a reasonable response to today's news, buying Boeing may not be.