Why do aircraft have a crossover airspeed, and why does it increase at higher vertical load factors?

Question

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.

Answer 1

how can [an airplane’s ailerons and spoilers] lateral control authority surpass that of the rudder past a certain speed?

This depends on the wing's lift coefficient. At a higher lift coefficient the lower aileron can not add the same amount of lift that it could at a lower lift coefficient. While the raised aileron on the opposite side will still reduce lift locally, the lowered aileron becomes less effective in raising lift as the lift coefficient increases. A higher lift coefficient causes a higher suction peak near the leading edge and puts more stress on the boundary layer, and adding more of the same will become harder as the wing approaches stall conditions.

Another factor is adverse yaw which increases with lift coefficient. This adverse yaw will add to the yawing moment of the hard-over rudder and will increase sideslip, which in turn will produce more rolling moment from dihedral against the aileron effect. As adverse yaw dies down with lower lift coefficient, so does sideslip angle and the ailerons gain control power.

Note that aeroelasticity will lower the effectiveness of all control surfaces as speed increases. Details depend on the torsional stiffness of the fuselage (which will be twisted by the deflected rudder) and of the wing (which will warp against the aileron deflection). Depending where stiffness is higher, the crossover speed and the eventual roll authority at speeds above it might vary.

why does an increase in vertical load factor cause the crossover airspeed to increase?

Because a higher load factor needs a higher lift coefficient when flying at the same speed.

Answer 2

This is an addendum to @Peter's accepted answer, which is the correct answer as far as the B737 is concerned.

Crossover speed is not an industry standard nomenclature. I suspect this term as it's defined here is restricted to the B737 program. Not every aircraft has a "crossover speed" in its current definition.

1. B737 with single rudder PCU

As noted in the NTSB accident report, almost all power actuated Part 25 aircraft have redundant control paths, with two or more independent PCUs. In the case of B737, it has a single PCU (albeit powered by dual redundant hydraulic systems), plus a standby PCU that is normally unpressurized along with the standby hydraulic system. Unless the aircraft detects dual hydraulic failure, reversion to the standby system is a manual process.

In the case of USAir 427 and Eastwind 517, it appears that the root cause of the accidents lies with a jam inside the PCU, which may cause a rudder reversal opposite to the direction of command. Once the rudder reversal is triggered, it feels more like a runaway to the actuator load limit (i.e. the blowdown limit).

Had the rudder been designed with redundant control paths, this rudder runaway would not have occurred due to the other actuators fighting the jammed one.

2. Rudder jam with redundant control paths

However, rudder jam (not runaway) could still occur with redundant active-active control paths. This is something that aircraft manufacturers must consider when certifying for 14 CFR 25.671(c). Manufacturers must show that the aircraft is capable of continued safe flight and landing with the rudder jammed.

But the position at which the rudder is jammed must lie somewhere the pilot (or automatic control) has previously commanded. As you can imagine, commanding rudder to full travel (i.e. the blowdown limit) is exceedingly rare, and having a jam there would be extremely improbable over the lifetime of the aircraft. Other factors also prevent rudder jamming at its blowdown limit; for example, many aircraft have rudder travel limiter or rudder control limiter.

Therefore, we typically do not need to consider rudder jammed full for certification. The exact position for jam consideration has been an active topic and been subject to an ARAC task. Generally, the jam position is maneuver and flight phase dependent.

To summarize, due to the varied nature of rudder jam, there may not be a single crossover speed (in the B737 definition), nor does every aircraft have such a speed.

Answer 3

This seems to be a question of aileron blanking vs rudder. Higher AoA will blank out upward raised aileron. This is what happens at lower speeds and greater G "vertical loading" with elevator. Possibly contributing would be that the elevator is also deflecting airflow diagonally across the rudder, increasing its effective length.

Although rudder is blanked at higher AOA, the effect may be greater on the ailerons.

This could be tested in a wind tunnel for possible consideration of re-design. Larger ailerons or dual rates for slower speeds may provide a solution, as well as evaluation of the rudder(s).

Post Script: essentially the aircraft went into a cross controlled elevator aft stall. Perhaps newer technology (such as modifying a set of wing spoilers into clamshells) to back up the rudder function would help.