The tests on a Eurofighter in Torrejón, the roar of which could be heard in Alcalá de Henares

Jet planes are anything but quiet, especially when they activate afterburner in their engines.

The CLAEX of the Spanish Air Force receives its two Pilatus PC-21s for testing
The aircraft and work of the CLAEX, the experimental center of the Spanish Air Force

This Monday, @lacajafantasma posted this video on Twitter showing a loud noise that could be heard from their home: "An incredible HUM sound was just heard in Alcalá de Henares! I recorded the moment it stopped. It had been going on for about 20 minutes nonstop."

Se acaba de escuchar un HUM increíble en Alcalá de henares! He grabado el momento en el que justo se corta. Llevaba unos 20 minutos sonando sin parar. pic.twitter.com/Et2YzNwrOg

— La Caja Fantasma (@lacajafantasma) February 9, 2026

Other residents in the area heard the same noise. What was unusual about the noise was that it sounded like a jet engine that wasn't moving, according to some witnesses. In response to the complaints, yesterday the Alcalá de Henares City Council issued a statement confirming that they had contacted the Torrejón Air Base "to determine the source of the noise heard on the nights of Monday, February 9th, and Tuesday, February 10th. This noise comes from tests of the new fuel being carried out on a Eurofighter aircraft."

The statement from the Alcalá de Henares City Council added: "As they explained, this operation must be carried out at night because sunlight could interfere with the infrared measurements. Furthermore, it must be done outside the hangars prepared for engine testing (which attenuate noise) because these hangars are not sized for this aircraft model. A final test is scheduled for Wednesday night, after which the trials will conclude."

Today the Spanish Air Force published a news item on its website reporting on this test: "During this week, tests are being carried out at Torrejón Air Base to verify whether the use of SAF biofuel emits less infrared radiation than conventional fuel conventional, which requires conducting the tests at night, thus avoiding solar radiation, and outdoors, which, combined with certain weather conditions, means that the noise produced can be perceptible from some neighboring population centers."

The Spanish Air Force news report includes this video showing a Eurofighter Typhoon from Wing 11, C.16-27 (11-27), based at Morón Air Base, activating its afterburner during this biofuel test. The report also includes two photos of the test, showing that the aircraft was anchored to the runway using the arresting hook located on its tail.

The Spanish Air Force added the following regarding this test: "One of the objectives includes evaluating the use of Sustainable Aviation Fuel (SAF) in aircraft propulsion systems. Since the first tests in 2022, with biojet blends, BACSI has accelerated the path towards a greener and more efficient future. Thus, in May 2023, the first results of SAF tests on military aircraft were presented during FEINDEF. In September 2024, the Spanish Air Force and Space Force once again demonstrated its leadership in innovation and commitment to sustainability by achieving the first supersonic flights with SAF, which were carried out with Eurofighter and F-18 aircraft."

In its statement, the Spanish Air Force notes: "In these test flights, SAF produced in Spain by Repsol was used, mixed at 30% with conventional fuel, and it was confirmed that the Air Force and Space Force could maintain its operational capabilities while reducing its dependence on fossil fuels. Since the end of last year, a modification to the regulations on the use of sustainable aviation fuels allows its use in these proportions, which provides our aircraft with clear advantages in interoperability missions with allied and friendly forces."

The Spanish Air Force points out that for these tests, "carried out in collaboration with the company ITP and the Polytechnic University of Madrid, a Eurofighter was used on the ground and consisted of three phases: a first phase in which the entire process was fine-tuned, another for the characterization of the conventional fuel called JP8, and a final phase in which the SAF characterization was carried out. These tests place the Air Force and Space at the forefront of research and experimentation in the use of this type of sustainable fuel, thus fulfilling a firm commitment made to the EU for its use in the flights of our aircraft".

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Photos: Ejército del Aire.