Flight-Testing of New Airbus A321XLR Gathers Momentum

The A321XLR’s test program will evaluate flight control system evolutions. (Image: Airbus)

Following the maiden flight of A321XLR development aircraft MSN11000, two more prototypes, currently in advanced stages of manufacture, will join the certification flight-test campaign.

Gary O’Donnell, Head of the A321XLR program provides an overview of what lies ahead in the run-up towards Type Certification: “Up until the end of this year, our focus is on completing the construction and then obtaining flight clearance for the remaining flight test aircraft. By the fourth quarter of this year, the three aircraft will be flying actively and we will have achieved a high level of production maturity.”

O’Donnell points out that there will actually be four flight test aircraft in the A321XLR development program. “The three -XLR new-builds are supported by an upgraded regular A321neo – MSN6839. This aircraft has already been fitted with several important new features designed for the -XLR.”

Once these development aircraft are all flying, the global flight testing will be fully underway. Concurrent with the flight testing is the ongoing ground lab testing campaign to finalize the serial standard modifications. “The completion of these activities and submission of all the final documents around the end of next year to the airworthiness authorities will allow us to validate and certify the complete aircraft. This will enable us to deliver to the airlines what they need on day one when the A321XLR enters service in 2024,” says O’Donnell.

“In parallel to entry into service, we have to build our full industrial system, including all jigs, all tools and processes, not only in every involved Airbus factory, but likewise in those of our extended industrial chain and at our suppliers around the world. We will also have to load them with the parts and materials. Overall securing the industrial system is a huge behind-the-scenes activity heavily involving especially our engineering, manufacturing and value chain teams,” notes O’Donnell.

“Our third pillar is to secure all the customer services documentation and ground support equipment, so the moment we hand over the aircraft to the customer it will be ready for use. And then – and only then – do we transition this project into ‘serial mode’ and hand it over to the bigger business.”

Philippe Pupin, who leads the flight test engineering team for the A321XLR program, and who was one of the crew members aboard the first flight of MSN11000 in June 2022, explains the rationale for the flight-testing phase.

“In order to become a long-range aircraft, the A321XLR needs to carry more fuel, which means increasing the A321’s maximum take-off weight. In turn this requires uprated landing gear and braking systems. However, since we are keeping the engine thrust unchanged, we have made some aerodynamic changes to retain our desired take-off performance. This has driven the physical modifications to the high-lift system – the slats and flaps – as well as reprogramming of the flight control system, all of which needs to be flight-tested and certified. In terms of flight hours of testing, the -XLR program stands somewhere in between a brand-new aircraft and a derivative. So, we have to ‘re-test’ virtually everything regarding aircraft design and flight physics,” he adds.

With the A321XLR, Airbus has also taken the opportunity to infuse some recent developments to the overall flight control system design – which was hitherto based on the original architecture for the A321 designed in the early 1990s. The aim is to enhance flight control design commonality across all programs – and further fulfils Airbus’ unified Fly-By-Wire architecture implementation.

Notable flight-physics-related changes on the -XLR (compared with today’s A321neo) include a simpler single-slotted inboard flap system (replacing the double-slotted inboard flap of the original A321’s wing); an electronically signaled “e-Rudder” (with related changes to the flight control computers); and uprated landing gear, wheels and brakes.

So as to gain an early head start for testing these features, several have been retrofitted into MSN6839 well before completion of the first new-build A321XLR – including the new inboard flap system. Having the latter means that MSN6839 is now aerodynamically equivalent to the -XLR once the landing gear is retracted. This has enabled it to perform ‘velocity minimum unstick’ (VMU) tests, the absolute minimum speed at which an aircraft can take off), for example. (VMU tests are determinants of the operational takeoff speeds used by airline pilots).

Functional testing is also needed to validate the operation of new non-flight-physics-related systems introduced on the -XLR. Two major ones include the new water & waste system, as well as the new fuel system elements (pumps and control systems, etc.) associated with the integral Rear-Center-Tank which can hold up to 12,900 liters of additional fuel.

Of the three new-build A321XLR test aircraft, the first two – MSN11000 (known as ‘FTV1’) and MSN11058 (‘FTV2’) are equipped with the full suite of flight-test instrumentation (FTI) and engineer interactive stations. Both FTV1 and FTV2 will feature a transferable water ballast system to ensure the change of CG during flight. Overall, they will focus on the aircraft’s technical systems, updated flight controls and handling and performance. The only major physical difference between these aircraft is their respective engine type: CFM LEAP-1As for FTV1 and P&W GTF engines for FTV2.

With the flight test program still in its early days following the first flight, FTV1 had already completed the following: flight-envelope opening; flight control laws clearance; rotation law evaluation and angle-of-attack (AOA) protection tuning; high speed performance flights, anemometry calibration, fuel and landing gear system ground testing, and some autopilot tests.