The FZ1073 upset reconstructed from open data

On 30 September 2026 a Boeing 737 MAX 8 (flydubai FZ1073, A6-FKF) went into a steep spiral dive over northern Saudi Arabia after a struggle on the flight deck, and landed at Tabuk with its rudder missing. Public reports describe a loss of 14,000 ft in under 30 s and a peak of Mach 0.96. We reconstruct the upset from two public Automatic Dependent Surveillance-Broadcast (ADS-B) feeds and the ERA5 reanalysis, and check every derived quantity against the airspeed, Mach number and wind that the aircraft transmitted through Mode S Enhanced Surveillance before and after the dive: over 164 points the reconstructed Mach number has a spread of 0.006 against the transmitted one, close to its resolution, and a systematic difference below 0.003. Each altitude segment is converted into the Mach number needed to fly it; jumps that would need more than Mach 1 or 3 g are rejected. What remains supports the 14,000 ft in 29 s, at a mean Mach number of 0.86, with a steep phase of about 200 m/s that needs Mach 0.91 on average and 0.95 over its first 10 s, beyond the limit of the ADS-B vertical-rate field. At the bottom of the pull-out the aircraft flew at Mach 0.88 (0.85 to 0.91) and 438 kt calibrated (422 to 454), against a maximum operating speed of 340 kt: the design dive Mach number, at least 0.87 by regulation, was likely reached in the steep phase, and the design dive speed at the bottom. A point-mass model, calibrated on the phugoid that followed and constrained by the surrounding data, fills the gap with 288 compatible histories: a bank near 90 degrees or a push to low load factor, a flight-path angle near -46 degrees, a peak near Mach 0.95 (0.91 to 1.00), and a pull-up of at least 2.7 g with the wings still banked. These are predictions, fixed before recorder data. A drift model places the rudder within about a kilometre of 29.82 N, 38.33 E. Changes in v4: a new section reconstructs the control histories compatible with the data inside the 30 s where the record is sparse (point-mass model with the OpenAP polar of the 737 MAX 8, calibrated on the observed phugoid; population sampling under twelve constraints; two seeds and two sensitivity runs), with a table of predictions to be tested against recorder data; the debris paragraph treats the rudder as the whole surface that the photographs show missing, under two release hypotheses; the viewer flags follow the consistency chain. Earlier versions: v3 corrected three statements, v2 restated the validation statistics. This record contains the preprint, the analysis and simulation scripts, derived data, the accepted histories and an interactive viewer. Raw ADS-B files are not redistributed; the scripts retrieve them from the original providers. Code: MIT licence. Preprint, derived data and figures: CC BY 4.0.