Operating an axial turbine off-design creates high residual swirl at the runner discharge. Below the runner hub, a helical precessing vortex core (corkscrew vortex rope) forms inside the draft tube cone at 0.2 to 0.4 times runner rotational frequency ($f_{rope} approx 0.3 f_0$). If this hydraulic excitation matches the acoustic natural frequency of the water column or powerhouse concrete penstock structure, violent hydraulic resonance shocks occur, shattering powerhouse windows, cracking concrete foundations, and causing severe turbine shaft fatigue. Aeration injection through the runner hub center is mandatory to cushion the vortex core.

High-specific-speed Kaplan turbines ($n_q > 220$) operate with critical Thoma cavitation coefficients $sigma_{crit} > 0.65$. Civil engineering developers attempting to minimize powerhouse excavation costs often set the runner centerline above or level with tailwater level ($z_s ge 0$). At full load, static pressure on the blade suction side drops below water vapor pressure ($P_{vap}$). Microscopic vapor cavities implode violently against the stainless steel runner blades at 1,000 MPa micro-jet shock pressures, gouging out spongy holes, shearing blade trailing edges, and destroying runner discharge rings within 3,000 operating hours.

Kaplan double regulation relies on an exact mathematical relationship (the 3D conjugate on-cam curve) between wicket gate servomotor stroke and runner blade pitch angle. If linkage play, digital governor sensor calibration drift, or sticky pilot valves cause "off-cam" operation, flow enters the runner blades with severe angle-of-attack separation. Efficiency drops by 12% to 20%, massive draft tube flow separation occurs, and heavy hydraulic unbalance forces bend the main drive shaft.

Unlike Francis turbines (which have runaway speed ratios of 1.6 to 1.8), axial Kaplan turbines have runaway speed multipliers reaching 2.8 to 3.2 times synchronous speed if governor trip occurs and blades open. Centrifugal hoop stress scales with the square of RPM ($S propto n^2$), meaning generator rotor poles experience nearly **10 times normal operating centrifugal tensile stress**. If the generator rotor, pole dovetails, and damper windings are not rated for full Kaplan runaway RPM, the generator rotor can literally burst through the stator housing.

Traditional Kaplan hubs contain several thousand liters of hydraulic mineral oil to lubricate internal blade trunnion bearings and operating crossheads under static head pressure. High-frequency blade angle adjustments cause elastomeric blade trunnion lip seals to wear against silt and debris. Seal failure leaks high-pressure hydraulic oil directly into the river discharge, causing severe environmental contamination, EPA fines, and immediate powerhouse shutdown. Modern retrofits mandate water-filled hubs or non-toxic synthetic ester bio-lubricants with mechanical face seals.