An Airbus A321XLR is currently on a 13-hour test flight touring Europe from the skies.

Airbus took its prototype A321XLR on a 13 hour test flight this week covering nine separate countries and drawing a large XLR in the sky over the Bay of Biscay. While the flight and special sky art was exciting to follow on Flightradar24, the crew was busy monitoring the aircraft’s systems. For this flight in particular, the pilots and flight test engineers were checking that the unique modifications to the A321’s fuel systems performed as they should during an extended flight.

What makes the Airbus A321XLR fuel system unique?

To further extend the range of the A321 from the A321LR’s 7,400 kilometers to 8,700 km for the A321XLR Airbus added a permanent rear center fuel tank integrated directly into the fuselage. It’s this range that truly sets the A321XLR in rare air, bringing single-aisle economics to routes that were previously only serviceable with a widebody aircraft, like the A330 or 787. As lead flight test engineer Jim Fawcett explains of the fuel tanks, “We’ve made some modifications on this aircraft and it was important to check that those work correctly over the full flight envelop of the aircraft.”

To accomplish this test, the A321XLR was fueled to maximum capacity and flown all the way down to the minimum allowable fuel level before landing. This gave the flight test team plenty of time—more than 13 hours—in the air to study the fuel system. Fawcett reports, “all of the transfers worked correctly, all of indications to the crew were good, all of the transfer rates were nominal, so it was very important for us to perform a long flight to check that.”

In addition to testing the actual operation of the fuel system over such a long flight, engineers were validating the thermal model of the system within the A321XLR. The air at cruising altitude is very cold, regularly reaching temperatures of -60℃ or less and that affects the temperature of the aircraft and the fuel inside. Ensuring that the aircraft’s system designed to keep the fuel above critical temperatures is operating correctly is an important step toward certification.

Putting the XLR in A321XLR

With a 13 hour test flight, there’s room for other activities and Airbus took advantage of the time in the air to do a little sky writing. 90 minutes into the flight, the aircraft flew over the Bay of Biscay and drew a 500 km wide, 150 km tall “XLR” in the sky. Impressively, and unlike nearly all other pieces of sky art created by large transport aircraft, the Airbus pilots flew the entire drawing by hand. The entire XLR logo took just over three hours to complete at 27,000 feet.

Missing a bit of the X

For this test flight, Airbus decided to not use the A321XLR ADS-B transponder, instead broadcasting only in Mode S. ADS-B is not required in all jurisdictions yet and Airbus decided to test the Mode S transponder as some customers may opt not to include ADS-B when they take delivery. Fawcett says the long flight using Mode S only validates that “the aircraft still works correctly and is well-integrated with the air traffic control system.”

That’s certainly important for Airbus to test, but it makes the aircraft much more difficult to track over open water. With ADS-B, the aircraft sends Flightradar24 all the data we need to plot the aircraft on the map. When the aircraft broadcasts Mode S only, we need to use a technology called Multilateration, which requires signals from the aircraft reach four of our terrestrial receivers to determine the position of the aircraft. The lower left of the “X” was just outside our MLAT coverage area at that altitude.

What’s next for the Airbus A321XLR flight test program?

The A321XLR is currently expected to enter service in 2024. Ahead of certification in late 2023 or early 2024, the flight test aircraft will have a full schedule of system checks. In early 2023, Airbus will send the A321XLR for cold weather testing, which will be the aircraft’s first intercontinental flight.