Again, US Airways' experience with the A330-300/-200s is illustrative. The airline's new -200s came with new Panasonic eX2 IFE systems. Customers were so taken by the system that US Airways installed the eX2, as well as its new Envoy Suite, on older A330-300s. “From the customers' perspective, we made the product look identical,” says Brickner. On-demand entertainment and a fully lie-flat bed are standard—regardless of the age of US Airways' Airbus widebodies.

Passengers appear to be demanding the same sort of consistency when it comes to connectivity. Brickner says Gogo Wi-Fi was first installed on US Airways' A321s and is being rolled out on the rest of its narrowbody Airbuses. Inflight Wi-Fi is “becoming more of an expectation,” says Kerry Hester, US Airways' senior vice president of customer experience. It is a matter of consistency, which is buoyed by the ATG-4 (air-to-ground) iteration of the system that Brickner says offers as much as treble the bandwidth of previous systems.

And it is a matter of putting money where there is profit potential. Hester says US Airways “really takes seriously our responsibility of being good stewards of our shareholder's capital.” Determine the demand, calculate the return on investment and “focus on the areas that our customers value most,” he says.

Certainly, customers value Boeing's new Sky Interior cabin on the newest 737 NGs, an interior one industry source says a large majority of customers are choosing. It features new overhead bins, sidewalls, ceiling panels and passenger service units. While American says passengers love the new layout, Pilarski contends that carriers probably will not budget to bring the interiors of their legacy fleets up to Sky standards. “I don't think they will spend money on all the planes and put lipstick on a pig,” he says.

While United is installing larger, all-new overhead bins in its A319/A320 fleet, it is not retrofitting its 737NGs with Sky. That is “too cost-prohibitive,” says Baur. Except for the premium cabin, “usually, the benefit you get from [enhancing] the passenger experience won't offset that cost,” he says.

The new 737-800s American is receiving are outfitted with Sky, but “we're not going back to our current 737s and retrofitting [it] into to the aircraft,” says Warlick. While the move would beget consistency of product, it would be inconsistent with reasoned return on investment.

That is not the case with the business-class cabin, however. Alice Liu, American's managing director of onboard product, says the company is planning not just to retrofit its Boeing 777-200s to match many of the elements of its coming 777-300ERs, but also looking to upgrade the business-class cabin of its 767-300s “so that we will have fully-flat, direct-aisle-access seats.” This is an important move toward harmonizing the airline's premium accommodations.

Pampering premium passengers begins with affording them space. In American football vernacular, that means blocking and tackling, getting the basics right and ensuring that the delivery is consistent—at least where the diameter of the fuselage permits. American's A321s, slated to come on line next year, will usher in a tri-class transcontinental product with 10 fully-flat first-class seats. All come with direct-aisle access. The business-class configuration calls for 50% direct-aisle accessibility.

Despite the call for commonality of passenger experience, integration still gives way to economics. As U.S. legacy network carriers merge new fleets with old, the fundamental laws of airline physics remain consistent.

However, the energy, motion and force exerted on airlines is making them scrutinize the basic business case of upgrades and modernization. In cases like RNP, where airlines can quickly realize a return on investment through big fuel savings, the upgrades will come more quickly. The pressure for airlines to keep up with customers' increasing demands for mobility may be one of the most fluid forces to watch.