The only way is up: Flight-testing the A321XLR gathers pace

During the coming weeks, following the recent 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 programme 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.”

He points out that there will actually be four flight test aircraft in the A321XLR development programme. “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 finalise the serial standard modifications [see our previous article about A321XLR lab test means activities]. 

“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.
 

Securing industrial maturity

“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.”
 

Flight testing scope overall rationale

Philippe Pupin, who leads the flight test engineering team for the A321XLR programme, 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 programme 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 programmes – and further fulfils Airbus’ unified Fly-By-Wire architecture implementation.

Caption:The A321XLR’s test programme will evaluate flight control system evolutions.
 

Testing the -XLR's new flight-physics features

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 signalled “e-Rudder” (with related changes to the flight control computers); and uprated landing gear, wheels and brakes.

 Caption: Closeup view showing the new inboard single-slotted flap of the A321XLR. 
 

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-Centre-Tank which can hold up to 12,900 litres of additional fuel.
 

Flight-test assignments – FTV1 & FTV2

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. 

Jean-Philippe Cottet, head of Flight Tests says: “We are certifying the two engine types in parallel on the A321XLR in the same timescale. It's a first in our history, since in the recent past we have been certifying engine variants sequentially one after the other.”

However, for test scenarios where the engine-types are not themselves the focus, FTV1 and FTV2 can perform a similar scope of flight testing: “We can easily interchange between them. For example, for evaluating the A321XLR’s handling qualities we have more than 100 flights to perform, and only a few of these tests are specific to FTV1 or FTV2 – since the majority can be performed by either aircraft, ” he adds.

At the time of writing, with the flight test programme 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.

 

New-generation high-capacity Flight-Test Instrumentation (FTI)

Cottet points out that the value of having prototypes is only fully realised by their capability to provide the best results and recorded data to the teams for analysis and certification: “We are using a new flight test instrumentation in these aircraft which is able to record approximately for over 20 hours. We need this capacity – as we have some missions that require us to record large amounts of data not only during flight, but also pre- and post-flight – ie. on the ground.”

Caption: MSN11000 features a new-generation of flight-test instrumentation.
 

“Our on board FTI suite can record and process as much as 80,000 lines of data,” he notes. “This is fed by more than 1,000 physical measurement transducers installed throughout the aircraft, and whose wiring inside the aircraft can be seen to converge towards the Flight Test Engineer console and the telemetry equipment.”
 

Passenger cabin elements and route proving - FTV3

The third new-build aircraft – MSN11080 (FTV3) – which is powered by CFM LEAP engines, is currently being fitted with a ‘lighter’ FTI installation since its duties will focus more on maturity testing of the passenger cabin interior elements and also route-proving for customers.

Pupin explains: “When you transform a medium to short range aircraft into a very long range aircraft, very special care needs to be taken regarding the cabin comfort, both in terms of thermal comfort over an 11 hour flight, and also in terms of the level of noise. We are therefore fitting an upgraded cabin into the -XLR which is closer to what is offered in our standard Long Range aircraft.”

As well as showcasing the -XLR's interior, FTV3’s duties will also focus on demonstrating the aircraft’s operation on the expected route scenarios for its customers, especially so-called ‘demanding’ ones to validate the aircraft’s operation at the extremities of take-off weights, range, runway constraints, ground temperatures and weather conditions etc..

“We need to demonstrate the aircraft from multiple types of runways which would be experienced by the pilots, and so we plan to invite our customers as early as possible to fly with us – as they did on the A330neo. I'm sure we will learn a lot, so this campaign will ensure we will deliver the product which we have promised to our customers,” concludes Cottet.