When flydubai flight FZ1073 landed at Tabuk on September 30, the first images to circulate were of the tail. A large section of the rudder on the Boeing 737 Max 8 was missing, torn away somewhere over northern Saudi Arabia during a descent that, by every account, took the aircraft far beyond anything it was designed to experience in service.
The UAE, as the aircraft's State of Registry, is leading the investigation through a Public Prosecution team appointed by the Attorney-General, working alongside the GCAA and with regional and international authorities. The GCAA has described the event as a security incident that was brought under control. As that work continues, attention across the industry has also turned to the aircraft itself, and to what its survival has revealed about the strength of a modern narrowbody.
The figures in circulation vary, as you would expect a week after an event of this kind. Flight tracking suggests the aircraft lost around 17,400 feet in 92 seconds and was at one point pitched around 39 degrees nose down, when a normal descent is flown at around three degrees. The same reporting suggests a peak speed of around Mach 0.96, set against a published maximum operating speed of Mach 0.82, and estimates that the recovery from the dive loaded the airframe to around 3g.
The certified limit for the 737 Max is 2.5g. The flight recorders will settle the precise numbers, and they may well differ from what has been reconstructed from tracking data. Even allowing for a generous margin of error, the aircraft was flown deep into territory that no airline pilot will ever see in a career, before being brought back under control and landed safely.
No new aircraft is ever flown to breaking point before it enters service, and that is exactly why engineers are paying such close attention to FZ1073. Structural integrity is established through analysis, modelling and ground testing, built around two figures. The limit load is the highest load an aircraft is expected to meet in its entire service life, which for a transport category jet such as the 737 is 2.5g. The ultimate load is the limit load multiplied by a safety factor of 1.5, and the structure must carry it for at least three seconds without failing. Manufacturers demonstrate this on a static test airframe in a hangar, with hydraulic rigs bending the wings upwards until the required numbers are reached.
Speed is treated in a similar way. Above the maximum operating speed sits a design dive speed, which test pilots approach with great care during certification flying, and above that sits a further margin within which the structure must remain free of flutter, the self-reinforcing vibration that can destroy a control surface in seconds. Everything beyond the tested points is understood through prediction, extrapolation and engineering judgement. The industry has every reason to trust those margins, and only very rarely does it see them exercised in a single, sustained and violent event in the real world.
On the reported numbers, FZ1073 exceeded its maximum operating speed by a wide margin, sustained a steep, high-energy dive, and was then pulled out at a load above its limit load and within its ultimate load. The wings, fuselage, engine mounts, horizontal tail and vertical fin all appear to have held. The component that failed was the rudder, a relatively light hinged surface on a long trailing edge, which is precisely the kind of structure most exposed to flutter and aerodynamic overload at extreme speed. That is the working assumption across the industry among those who have studied the photographs, and investigators will look for any separated parts, since the fracture surfaces will show which of the two it was.
From what we can see, the aircraft behaved very much as its designers would have predicted. The structure was taken past its limit load, stayed within its ultimate load, and survived. The part that failed is the one an engineer would have expected to go first, and its loss did not spread to the fin or the rest of the tail.
For Boeing, the regulators and the wider industry, the data from A6-FKF will be among the most useful structural records in recent memory. Accident investigators usually work with recordings from aircraft that did not survive. Here they will have a complete record of a modern narrowbody operating at speeds and loads beyond its envelope, matched to a physical airframe that can be inspected in detail. Strain, deformation, fastener condition, the fin attachments and the rudder fracture surfaces can all be compared directly with the predictions made during certification, and the findings will inform structural modelling well beyond the 737 programme.
Whether this particular aircraft flies again is a separate question, and some engineers have already expressed doubt. An airframe taken past its limit load requires extensive inspection, and on a four-year-old narrowbody the cost of that work may make retirement the sensible commercial decision. That judgement belongs to flydubai, Boeing and the civil aviation authority.
The 737 Max carries a history the travelling public has not forgotten, and much of what has been said online has moved quickly towards praise for the aircraft's resilience. Some of that commentary has run ahead of the facts, including claims that the aircraft should by rights have broken apart. Engineers will recognise what actually happened: The airframe performed within the margins that certification is designed to guarantee, and those margins proved deep enough to absorb an event far outside normal operation.
The author is an aviation analyst. X handle: @AlexInAir.