Thunderbirds F-16 crashed after misaligned engine vanes led to blade fracture

Pilot ejected as jet lost thrust during formation flight practice over California desert.
Investigators have attributed the loss of a Lockheed Martin F-16CM from the US Air Force Thunderbirds aerobatic team to engine compressor blade failure arising from the airflow disruption caused by misaligned variable vanes.

Three vanes in the fourth and fifth stages of the Pratt & Whitney F100-PW-229 powerplant were misaligned, and the irregular airflow increased vibratory stress on the fifth- and sixth-stage compressor blades.

The USAF investigation board states that two blades – one from each of the fifth and sixth stages – separated during flight, causing “catastrophic” engine damage and a significant loss of thrust.

This made sustained flight “impossible”, it adds. The pilot successfully ejected and the aircraft – with tail number 92-3881 – crashed near Trona in California.

The F-16 had been part of a six-ship Thunderbirds formation undertaking a practice flight out of Nellis air force base in Nevada on 3 December last year.

After reaching the practice airspace west of Nellis, four aircraft from the formation commenced a loop with the mishap aircraft – one of the two solos – breaking away into a 360° right turn with the intention of rejoining once the loop was complete.

But as the F-16 was accelerating to rejoin the formation, the engine experienced an unrecoverable stall at 391kt with the jet about 2,500ft above ground.

The pilot testified that the aircraft felt as if it “stopped mid-air”, and they initially attempted to climb to gain altitude for a diversion to the Naval Air Weapons Station at China Lake.

Unable to maintain airspeed in level flight, however, the pilot turned south to fly down a valley, preserving altitude above terrain. One of the other F-16s in the formation took up a chase position to provide support.

With engine thrust low, the pilot switched to secondary engine control – away from the primary full-authority digital control – but the substantial increase in fuel flow caused the engine to flame out.

The pilot managed to relight the engine but it only delivered idle power and did not response to commands for higher thrust.

Some 4min 21s after the initial stall the pilot ejected 467ft above the ground, and the aircraft struck flat desert terrain about 5s later.

“The [pilot]…exhausted all options to restart the engine and safely recover the aircraft prior to running out of airspeed and altitude,” says the inquiry. “Ejecting was ultimately the only option which provided a favourable outcome.”

While the inquiry has been unable to determine the cause of the misaligned vanes, it points out that there had been four occasions since October 2021 in which the engine had undergone inspection to check for possible damage.

These examinations were carried out in line with technical order guidance on foreign-object damage, and found no defects, but the borescope inspection prescribed did not enable a full check of certain core areas, including the fifth and sixth compressor stages.

Inspection of these stages requires the engine to be removed from the aircraft to allow a “more invasive teardown” of the core, the inquiry says.

“The core of the engine is only inspected during scheduled maintenance when a core component reaches its overhaul threshold, leaving time for potential damage to progress undetected,” it adds.

It says the Thunderbirds F-16 loss marked the third instance of fifth- or six-stage compressor blade fractures attributed to misaligned variable vanes in the fourth or fifth stages.

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