As U.S. legacy airlines renew their fleets, determining how to integrate them and ensure consistency becomes a challenge. Flight operations, maintenance, ground-handling and passenger experience all come into play. This quest for consistency is a balancing act, one in which airlines weigh the “seamless” benefits of retrofitting older aircraft against potential payoffs.

One way to reduce retrofits, of course, is to purchase aircraft that segue well with the present fleet. The analysis starts up front. “You want to make sure that the aircraft you're buying has cockpit technology that's common, to a large degree, to the fleet that you're flying,” says Ron Baur, United Airlines' vice president of fleet. From an operational perspective, there's a balance carriers seek to strike, one “between maintaining commonality with the existing fleet while still making the airplane advanced enough to take advantage of new things” such as Required Navigational Performance, he says. The aim is to rationally render the aggregate fleet “as efficient as possible,” says Baur.

Type training thrives on commonalty, drawing its sustenance from consistency. United is the first U.S. network carrier to fly the Boeing 787, which has a common type rating with the 777. One way Boeing does that is “through software, and how images are collected and displayed to the pilot,” Baur says. In terms of crew scheduling, the airline's ultimate goal is to have pilots who can fly the 787 one day and the 777 the next—seamlessly. The operational challenge with large, existing fleets is to ensure that new aircraft “integrate well with the current fleet,” says Baur.

Interestingly, the bigger the fleet, the easier the integration becomes, contends Adam Pilarski, senior vice president for aviation advisory company Avitas. “That's part of the advantage of big airlines,” he says. From a training perspective, “if you have a small airline that is buying five of this and three of that, you have problems.”

Purchase a substantial fleet of new aircraft and he insists that—comparatively, at least—integration issues will be “no big deal.” It is a classic case of economies of scale, buttressed by a concerted effort to make sure everyone in the airline is properly prepared to accept that new airplane type.

Melding an aircraft such as the 737-900ER into an extensive fleet of shorter Boeing narrowbodies is one thing; assimilating a 787 is something else. Once the decision to bring such an aircraft onboard is made, no one consideration trumps another. “They're all important,” asserts Baur. That's where United's Service Ready Team comes into play. For the carrier's 787 induction, that team is comprised of 150-175 people. He says thousands of tasks accompany service launch, from provisioning for parts and spares “to painting a new stripe on the [tarmac] to tell the pilot where to stop” at a given gate.

That is precisely the challenge facing American Airlines as it prepares to take delivery of its first Boeing 777-300ER. At 242-ft., 4-in. nose-to-tail, it is the planet's longest commercial airplane. “There are certain gates that can handle it, and certain gates that can't,” says Jim Butler, American's managing director for commercial planning and performance. He says every area of the company is engaged in a coordinated effort to usher in the airplane.

“There's a structured process,” one that gets deep down into the weeds of the endeavor, adds Peter Warlick, American's managing director of fleet. It entails everything from proper provisioning for parts “to having the right solution to clean the seats.”

As American prepares to bring on new Airbus A319s to help replace venerable MD-80s, precise parts-provisioning is particularly important, especially during the initial integration phase. That means reviewing the retirement schedule for older aircraft in light of the effect of capacity, routing and scheduled maintenance activities at both line and base stations, says Matt Pfeifer, American's managing director for operations strategic planning. Forecasting component removals and replacements for the MD-80 is pegged to its lengthy historical record, dating from when it was introduced at American in the early 1980s.

“For the new fleets,” says Pfeifer, “we will take delivery of a small number of spare engines to cover premature removals for fairly rare items, such as bird strikes.” These powerplants will be sited such that maintenance can dispatch them quickly to where they are needed, because the location of these types of removals “is very unpredictable,” he says.

Metering the inventory of new components with the induction of the aircraft themselves is a matter of delving into the details of the delivery schedule, mapping the routes new aircraft will fly and accounting for the mean time between unscheduled removals and scheduled removals. This is an industry metric that Pfeifer says, “give[s] a fair level measure of failure rate at the component level.”

Integrating a new airplane, in short, demands flexibility, especially when it comes to provisioning for new aircraft spares where the carrier has scant experience operating the type.

Even when an airline has extensive history with an overall species of airliner, sometimes it can suddenly find itself with an excess of a particular part predicated on past experience.

When US Airways prepared to take delivery of A330-200s in 2009, it evaluated parts-stocking for the sub-fleet (it had been flying the A330-300 for almost a decade). The -200s featured a new business-class seat in the carrier's Envoy cabin. “Our experience with previous generations of business-class seats showed that leg-rest failures were relatively common as a result of heavy use . . . or damage,” says Kevin Brickner, the airline's vice president of technical services.

Based on that experience, US Airways ordered 11 leg-rest assemblies and distributed them to each station served by the A330-200. As things developed, expected demand for the part never materialized—at least not to the degree anticipated. “The reliability of our new seat has been strong,” says Brickner, “and we have not experienced the level of leg-rest replacement that we forecast.”

There is a fundamental, widely applicable, lesson here that is underscored by the leg-rest example. “The idea is to make sure we have the right parts in the right place,” says Brickner. But the concomitant imperative is to do so it in such a way that “we don't overspend, so that we don't have too many parts in the system.” The principle still applies when integrating new aircraft.

Airlines are loathe to scrimp on anything that contributes to first- and business-class accommodations and services. According to the International Air Transport Association, the 8% of its member-carriers' traffic in 2011 pegged to premium passengers accounted for 27% of revenues. Consequently, the lion's share of customer-facing fleet-integration initiatives occur when new aircraft with updated premium cabins are introduced, and those in legacy aircraft must be updated to maintain the homogeneity of the overall premium product.

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.