Thoughts on the F-35B fighter crash

The recent Lockheed-Martin F-35B Lightning II (STOVL variant) accident at Fort Worth (see video) is, of course, subject to detailed investigation. Any speculation, of course, is just that, but there are some possible pointers in the video which may be worthy of comment.

The aircraft performs a low-speed, near-vertical, landing, and touches down with sufficient descent-rate to experience a bounce on landing. Following the bounce, it pitches rapidly nose down, strikes the ground, and falls over onto its side. The engine remains running, and thrust from the deflected rear nozzle drives the aircraft around in a circular motion on the ground. Towards the end of the sequence, the rotation of the aircraft on the ground reverses, suggesting that the fan may have re-engaged. The pilot ejects, and there is no fire.

What could have caused this mishap? The initial pitch down is immediate and quite rapid, suggesting a large thrust imbalance rather than an intentional manoeuvre. This might be caused by a fan system failure, noting that single point failure of the clutch, the gearbox, the drive-shaft, or of the fan itself could cause such a thrust imbalance. Of these, the clutch and gearbox appear most significant, given the apparent re-engagement of the fan, inferred from the reversal of the motion of the aircraft.

ASTOVL aircraft in jet-borne flight are vulnerable to single-point failures of this sort, as are helicopters. The cause of this particular accident should be relatively rapidly identified, given the apparently successful ejection by the pilot, the relatively limited damage to the aircraft, the available video imagery, and the lack of a post-crash fire.

Aspects of particular interest are likely to be the nature and cause of the initial failure; whether there was any adverse interaction between the propulsion control system and the flight control system; whether, and why, the fan re-engaged; and why the engine continued to run throughout the sequence, rather than being shut down automatically or through pilot control.

Creating a supersonic stealthy vertical take-off fighter is an extremely difficult task, and years of studies —and a healthy handful of initialisms and acronyms — paved the path to today’s F-35B. Jim Smith describes his role on the ASTOVL project and the challenges it faced here.

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