Enligt NTSB-olycksrapporten om kraschen av USAir Flight 427 , har alla kommersiella flygplan en överfartshastighet (den hastighet vid vilken maximal rullande kraft från flygplanets flygplan och spoilers är bara tillräcklig för att motverka den vridkraft som genereras av en fullständig roderöverhängning, över en sådan hastighet kan en roderhäftning förväxlas med tillräcklig okångning, medan under denna hastighet kommer en roderhäftning att medföra en omedelbar förlust av kontrollen), vilket ökar i steg med flygplanets vertikala belastningsfaktor (så att korsningen med 4 Gs blir högre än den vid 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]
Med tanke på att flygplanets flygledare och spoilers i allmänhet reser i samma flyghastighet som rodret (och därmed skulle uppleva matchande ökningar i kontrollmyndigheten med ökande flyghastighet), hur kan dess sidostyrningsmyndighet överträffa det av roret över ett visst fart? Även om de aerodynamiska krafterna som genereras av ailerons och spoilers - och därmed samma kontrollmyndighet - ökar som flyghastigheten, bör inte samma effekt leda till att rederens kontrollmyndighet ökar lika bra och fortsätter framför aileron / spoiler kontrollmyndighet? Varför ökar kontrollmyndigheten för sidokontrollerna med flyghastigheten snabbare än roderens kontrollmyndighet? För den delen är det någon särskild anledning till varför rederens kontrollmyndighet måste vara större än den hos ledare och spoilers med låga hastigheter? Roret behöver ha mycket kontrollmyndighet för att kunna kompensera för motorfel strax över V-1 - men varför kan inte sidostyrningarna ha ännu större kontrollmyndighet för att kunna kompensera för ett roder hårdvara vid låga hastigheter?
Och varför ökar den vertikala belastningsfaktorn att crossover-hastigheten ökar? För att ett flygplan ska kunna uppleva en hög vertikal belastningsfaktor, måste den flyga vid en onormalt stor angreppsvinkel - borde det inte placera roret längre och längre in i kölvattnet på den horisontella svans- och ryggkroppen, "släcka" roder (reducerar sin kontrollmyndighet) och därigenom orsakar minskning i crossover-flyghastigheten?
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1 : För 737 beror också korsningshastigheten på flygplanets flikinställning, men det är en annan fråga .