Airbus is on the verge of securing certification for its A321XLR, the campaign for which has been particularly focused on safety given the substantial technical modifications introduced during development of the long-range aircraft.
With the window for third-quarter delivery open, Airbus has been looking to ensure operators are fully informed of the changes implemented on this latest A321neo variant, as it awaits final approval from the European Union Aviation Safety Agency (EASA).
“[It’s] not signed before it’s signed,” says Airbus chief executive Guillaume Faury, who in early July noted that the approval was nevertheless “very close to happening”.
“Each and every document, justification, answer to questions, everything has to be completed before it goes from ‘zero’ to ‘one’,” he says.
Spanish flag-carrier Iberia is poised to become the first operator to accept the XLR, supplanting sister airline Aer Lingus, and intends to put the twinjet into service on routes to Boston from mid-November, and Washington Dulles in January 2025.
MATURITY CAMPAIGN
Airbus has already undertaken route-proving flights and evaluated the XLR during a maturity campaign.
It simulated in-service conditions last October with one of the prototypes, MSN11080, conducting a flight of more than 6h carrying 200 people – including 167 of its own employees. The same aircraft had carried out a 10-day series of operational tests the previous month, comprising around 100h of flying in various climates, to check technical reliability.
The XLR’s long range means the airframer is placing emphasis on both crew and passenger comfort, and the simulation involved measuring such aspects as noise, vibration and cabin temperature.
Several design changes marked the transition from the basic A321neo to the XLR variant – the most significant of which is the 12,900-litre integrated rear centre fuel tank, and the associated revised fuel system, which gives the twinjet its range of around 4,700nm (8,700km).
Combined with the standard centre tank and wing tanks – and an optional 3,120-litre forward auxiliary tank – the rear tank takes overall fuel capacity to nearly 39,750 litres.
But the tank’s size and location have presented safety challenges similar to those faced two decades ago when Airbus developed the long-range A340-500. The four-engined type featured a 20,000-litre rear centre tank but, while European regulators were satisfied, their US counterparts required a more robust design to protect against fuel spillage during an accident.
There is less contrast between the opinions of EASA and the US Federal Aviation Administration (FAA) over the XLR – a reassuring situation given the customer base for the model that exists on both sides of the Atlantic.
“Airbus has worked closely with both EASA and the FAA since the start of the XLR development and we have aligned over time on modifications to satisfy their new requirements,” says the airframer.
EASA has led the certification effort, concentrating on three specific safety aspects of the tank design.
With the tank integral to the aft fuselage structure, EASA’s initial concern in early 2021 centred on protecting passengers from the effects of an external pool fire.
Although aluminium alloys have fire-resistant properties if thick enough, it stated, an aluminium skin provides “very limited protection”, and the fire-protection function is primarily provided by insulation material.
Airbus is thermally insulating the cabin floor directly above the tank, to provide passenger comfort against the cold fuel. But doing so with materials that meet fire-protection regulations, the airframer found, was not feasible because compliant insulation panels do not fit the space between the floor and the tank.
VENTILATION REQUIREMENTS
Protective materials could not jeopardise ventilation needs for the tank, nor could insulation be permitted to block fuselage decompression panels, potentially leaving a substantial proportion of the area above the tank unprotected from burn-through.
EASA, as a result, required the lower half of the fuselage to be resistant to penetration by an external pool fire, buying time for cabin evacuation.
Aircraft equipped with standard fuel tanks or auxiliary tanks are considered to have sufficient protection against explosion from an external ground fire. Certification regulations require demonstration that, in the event of system failure, the fuel temperature will remain safely below auto-ignition point, for example, while fuel-tank vent systems must prevent explosions for at least 2min 30s.
But EASA states that none of these adequately covers ignition risk in the XLR’s rear tank in the event of a fuel-fed ground fire. This prompted the regulator to demand, in 2022, that the design must prevent ignition of vapour – due to hot surfaces – occurring inside the tank, and allow sufficient time for evacuation.
The FAA’s own requirement on the matter – published in May this year – remarks that Airbus should show that the design prevents ignition of vapour in the tank during at least 5min of exposure to ground fire, a timeframe consistent with previous studies on passenger survivability during fuselage burn-through.
Airbus’s modifications to meet the special conditions have included installing a protective tank liner, resistant to damage and spillage, similar to the solution it previously derived for the A340-500. The XLR will also feature an extended and reinforced belly fairing.
“Our flight-test aircraft were modified during the flight-test campaign to embed new features and validate [them],” says Airbus. “A high volume of testing has been completed on each feature to ensure that it addresses the requirements by both authorities.”
Airbus says the features are already embedded in customer aircraft on its final assembly line.
The tank’s location, behind fuselage frame 47, just aft of the wing, makes it potentially vulnerable in the event of a survivable crash.
EASA’s guidance for crashworthiness of the tank, which it published in 2022, states that consideration should be given to vertical descent rates up to 25ft/s – with a slight reduction of 23ft/s for the tank itself, given the cushioning effect of engines and landing-gear.
It says the tank should be installed in an area of the fuselage unlikely to fail or rupture in a survivable accident, and different impact conditions – including various aircraft weights and configurations, with possible loss of engines or landing-gear – should be evaluated to show the tank will not spill hazardous quantities of fuel.
SAFETY FACTORS
Sliding along the ground, adds EASA, should not result in temperatures in the tank increasing to the point of fuel or vapour ignition. It says parameters to be assessed include specific forward speeds and up to 20° yaw.
Airbus says the extended belly fairing includes new materials to increase the rear centre tank’s protection under specific crash scenarios, including a gear-up landing.
“In contrast, the belly fairing on today’s A321neo, which contains no [rear tank], primarily serves as an aerodynamic shape,” the airframer adds. This means operators of the standard A321neo are able to repair the fairing autonomously.
But the fairing on the XLR will have “limited” potential for repairability, owing to its safety role in shielding the tank, says Airbus, adding: “This leads to specific sections of the belly fairing being classed as a replacement item – to be replaced rather than repaired in the event of damage.”
Airbus lists the XLR as having three weight variants – two with a maximum take-off weight of 101t and a third at a lighter 92.5t. To support the higher weight and long-range capability, the XLR’s development has resulted in new main landing-gear and new flaps, as well as high-capacity water and waste systems.
All these technical updates have to be accommodated in revised documentation with instructions for continued airworthiness on the XLR’s various structures, systems and components. Airbus says this documentation must be completed by entry into service.
Airbus had previously acknowledged that the protective fuel tank liner would carry a weight penalty, but it has sought to minimise the impact on range and believes it will meet commitments to customers.
“When we launch a new programme we always take development buffers, provisions – and you have risks and opportunities in your performance, in your range, your speed,” says Faury.
“The outcome of the XLR development is that it is an excellent aircraft that matches – if not exceeds – expectations of the customers.”
Airbus has obtained highly-positive feedback over the XLR’s capabilities during the certification flights, he adds, and the airframer is satisfied.
INTERIOR CONFIGURATION
But Faury points out that the XLR’s ability to encroach upon long-haul routes means range targets are complicated by greater variations of interior configuration, as carriers seek to equip the single-aisle cabin with features typically associated with twin-aisle models.
“One of the challenges we have at the moment is that the XLR is a victim of its own success,” he claims.
“The airlines are so satisfied with the aircraft to do long range, with a single aisle, that they put very complex, heavy and sophisticated cabins that have a negative impact on the performance, because they are much more complex and heavy than what was [considered] when we launched the XLR.”
Faury says the airframer has been able to tailor the aircraft with “very nice cabins, better than we thought when we discussed with the first customers”, even as he accepts that the XLR has “taken a hit” from the weight of the crash-resistance modifications and the customer configuration requirements.
“But that’s okay,” he states. “That’s a good price to pay for the aircraft to be successful.”
