According to the NTSB accident report on the crash of USAir Flight 427, all commercial aircraft have a crossover speed (the speed at which the maximum rolling force from the aircraft’s ailerons and spoilers is just sufficient to counter the rolling force generated by a full rudder hardover; above this speed, a rudder hardover can be weathered with sufficient yoke input, while below this speed, a rudder hardover will cause an immediate loss of control), which increases in step with the aircraft’s vertical load factor (so that the crossover airspeed at, say, 4 Gs will be higher than that at 1 G)1:
Several flight test conditions required the test pilots to maintain control of the airplane and, if possible, a constant (or steady) heading by using the control wheel to oppose full rudder surface deflections. These tests revealed that, in the flaps 1 configuration and at certain airspeeds, the roll authority (using spoilers and ailerons) was not sufficient to completely counter the roll effects of a rudder deflected to its blowdown limit. The airspeed at which the maximum roll control (full roll authority provided by control wheel input) could no longer counter the yaw/roll effects of a rudder deflected to its blowdown limit was referred to by the test group participants as the “crossover airspeed.”
The flight tests revealed that, in the flaps 1 configuration and at an estimated aircraft weight of 110,000 pounds, the 737-300 crossover airspeed was 187 KCAS at one G. At airspeeds above 187 KCAS, the roll induced by a full rudder deflection could be corrected by control wheel input; however, in the same configuration at airspeeds of 187 KCAS and below, the roll induced by a full rudder deflection could not be completely eliminated by full control wheel input in the opposite direction, and the airplane continued to roll into the direction of the rudder deflection. The flight test data also confirmed that an increase in vertical load factor, or angle-of-attack, resulted in an increase in the crossover airspeed.
... The M-CAB flight simulations indicated that, with a rudder deflected to its aerodynamic blowdown limit and in the configuration and conditions of the USAir flight 427 accident airplane, the roll could not be completely eliminated (and control of the airplane could not be regained) by using full control wheel inputs if the airspeed remained below 187 KCAS... To return the airplane to a wings-level attitude, the pilots had to avoid excessive maneuvering that would increase the vertical load factor, or angle-of-attack, and thus increase the crossover airspeed.
... One pilot described how the airplane would initially respond to aileron inputs and begin to roll out of the rudder-induced bank attitude and how, by pulling back on the control column and adding some vertical load factor, the recovery could be stopped and the airplane could hang in a sideslip bank. The test pilot said that he did not apply additional aft column inputs at these moments but that these inputs would have caused the airplane to “roll into the rudder.” The pilot concluded that “you can control roll rate with the control column.” The other Boeing test pilot said that, in referring to the control inputs required to perform a recovery from full rudder input, “there is some technique required between the G [normal load factor] and the roll.”
The flight test pilots affirmed that the Boeing M-CAB and computer simulation models incorporated the tradeoff between normal load factor and roll control but that the tradeoff occurred at a greater load factor in the simulator than in the airplane...
Boeing’s flight test pilots stated that, when they allowed the airspeed to increase to about 220 to 225 KCAS (sacrificing altitude as necessary to maintain airspeed), the airplane recovered easily. The pilots reported that, when they initiated the event at higher airspeeds, the airplane was easier to control and that recovery was accomplished with less roll... [pages 63-65 of the report/pages 87-89 of the report’s PDF file]
Boeing’s article defined crossover airspeed as the speed below which the rolling moment created by a full lateral control input will not overcome the roll effect from full rudder displacement. The article stated that “while the airspeed at which this occurs is variable, cross-over speeds exist on all commercial airplanes... ” [page 205/229]
... On the basis of the existing airspeed and the increase in vertical G load, by about 1903:02 the airplane would have been below the airspeed at which the roll controls (aileron and spoilers) could counter the effects of the fully deflected rudder (crossover airspeed). Thus, from that time onward, it would have been impossible for the flight crew to regain roll control without increasing airspeed and/or decreasing the airplane’s vertical G load. [page 256/280]
Given that an airplane’s ailerons and spoilers generally travel at the same airspeed as the rudder (and, thus, should experience matching increases in control authority with increasing airspeed), how can its lateral control authority surpass that of the rudder past a certain speed? Even though the aerodynamic forces generated by the ailerons and spoilers - and, thus, the control authority of same - increase as airspeed rises, shouldn’t this same effect cause the rudder’s control authority to increase as well, and stay ahead of the aileron/spoiler control authority? Why does the control authority of the lateral controls increase with airspeed faster than the rudder’s control authority? For that matter, is there any particular reason why the rudder’s control authority has to be greater than that of the ailerons and spoilers at low speeds? The rudder needs to have lots of control authority in order to be able to compensate for an engine failure just above V-1 - but why can’t the lateral controls have even more control authority, in order to be able to compensate for a rudder hardover at low speeds?
And why does an increase in vertical load factor cause the crossover airspeed to increase? For an airplane to experience a high vertical load factor, it has to be flying at an abnormally large angle of attack - shouldn’t this place the rudder further and further into the wake of the horizontal tail and aft fuselage, “blanking out” the rudder (reducing its control authority) and thus causing a decrease in the crossover airspeed?
1: For the 737, the crossover airspeed also depends on the aircraft’s flap setting, but that’s another question.