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An aircraft capable of carrying passengers and cargo includes a fuselage, having a crown section and a keel, a first passenger cabin, having a first floor, located in a forward portion the fuselage, and a first cargo deck, located below at least a portion of the first passenger cabin. A split level

An aircraft capable of carrying passengers and cargo includes a fuselage, having a crown section and a keel, a first passenger cabin, having a first floor, located in a forward portion the fuselage, and a first cargo deck, located below at least a portion of the first passenger cabin. A split level cabin is located in an aft portion of the fuselage, the split level cabin including an upper second cabin having a second floor above the level of the first floor, and a lower third cabin beneath the upper second cabin and having a third floor below the level of the first floor. A second cargo deck is located in the aft portion of the fuselage and beneath at least a portion of the lower third cabin. The crown section has a substantially constant cross-sectional shape fore-to-aft above the first passenger cabin and the split level cabin, and the keel below the second cargo deck is lowered compared to the keel below the first cargo deck. A modified aircraft having a split level aft cabin and lowered keel, and a method for modifying an aircraft in this way are disclosed.

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1. An aircraft capable of carrying passengers and cargo, comprising: a fuselage, having a crown section and a keel;a main wing;a first passenger cabin, having a first floor, located in a forward portion the fuselage forward and over the main wing;a first cargo deck, located below at least a portion

1. An aircraft capable of carrying passengers and cargo, comprising: a fuselage, having a crown section and a keel;a main wing;a first passenger cabin, having a first floor, located in a forward portion the fuselage forward and over the main wing;a first cargo deck, located below at least a portion of the first passenger cabin;a split level cabin, located only in an aft portion of the fuselage behind the main wing, the split level cabin including an upper second cabin having a second floor above the level of the first floor, and a lower third cabin beneath the upper second cabin and having a third floor below the level of the first floor; anda second cargo deck, located in the aft portion of the fuselage and beneath at least a portion of the lower third cabin;wherein the crown section has a substantially constant cross-sectional shape fore-to-aft above the first passenger cabin and the main wing and the split level cabin, and the keel of the aft portion is lowered compared to the keel of the forward portion, the lowered keel extending linearly from a point aft of the main wing to a point of transition to an upswept undersurface of a tail portion of the aircraft. 2. An aircraft in accordance with claim 1, wherein the lowered keel transitions to an upswept undersurface of an aftmost portion of the fuselage, having an undersurface upsweep selected to enable aft body tailstrike margins suitable for takeoff and landing of the aircraft. 3. An aircraft in accordance with claim 2, further comprising an unpressurized tail cone section, attached to the aftmost portion of the fuselage, having a lower surface defining a continuation of the upswept undersurface of the aftmost portion of the fuselage. 4. An aircraft in accordance with claim 1, wherein the forward and aft portions of the fuselage are divided by one of an aft wheel well pressure bulkhead, a main wing center section, and an aft region of a wing-to-body fairing. 5. An aircraft in accordance with claim 1, further comprising an aerodynamic fairing transitioning between the keel of the forward portion of the fuselage and the lowered keel of the aft portion of the fuselage, the aerodynamic fairing comprising one of (i) a wing-to-body fairing, (ii) a main landing gear fairing, and (iii) a combined wing-to-body and main landing gear fairing. 6. An aircraft in accordance with claim 1, further comprising a passenger egress door in the third lower cabin, the passenger egress door having a ditching evacuation sill height at a level above a ditching waterline associated with a ditching condition of the aircraft. 7. An aircraft in accordance with claim 6, wherein the passenger egress door includes upper and lower portions that may be opened independently of each other. 8. An aircraft in accordance with claim 1, wherein the lower third floor is designed to act in tension to resist pressurization loads acting on the side walls of the fuselage when the fuselage is pressurized. 9. An aircraft in accordance with claim 1, wherein the second cargo deck is configured to accommodate standardized cargo containers, and further comprising first stanchions, supporting the lower third floor on a first side of the fuselage and second stanchions supporting the lower third floor on a second side of the fuselage; and wherein the second cargo deck accommodates the cargo containers between the first stanchions and the second stanchions. 10. An aircraft in accordance with claim 1, wherein the second cargo deck is configured to accommodate cargo containers selected from the group consisting of LD-1, LD-2, LD-3, LD-3-46, LD-3-46W, LD-3-45, and LD-3-45W containers. 11. A modified commercial aircraft, comprising: a crown section that is substantially identical to a crown section of a base aircraft;a main wing that is substantially identical to a main wing of the base aircraft;a forward fuselage portion, forward of and above the main wing, that is substantially identical to the base aircraft, having a first cabin with a first floor level;an aft fuselage portion having a lowered keel in comparison to the base aircraft, the lowered keel extending linearly from a point aft of the main wing to a point of transition to an upswept undersurface of a tail portion of the aircraft;an aft split level cabin only within the aft fuselage portion behind the main wing, including an upper cabin having a second floor above the level of the first floor, and a lower third cabin beneath the upper second cabin and having a third floor below the level of the first floor; andan aft cargo deck, within the lowered aft portion of the fuselage and beneath at least a portion of the lower third cabin;wherein the modified aircraft has a passenger capacity that is greater than a passenger capacity of the base aircraft, and a cargo capacity that is less than a cargo capacity of the base aircraft. 12. An aircraft in accordance with claim 11, wherein the lowered keel transitions to an upswept undersurface of an aftmost portion of the keel, having an undersurface upsweep providing an aft body tailstrike margin that is substantially identical to a tailstrike angular margin of the base aircraft. 13. An aircraft in accordance with claim 12, further comprising an unpressurized tail cone section, attached to the aftmost portion of the fuselage, the tail cone section being substantially identical to an unpressurized tail cone section of the base aircraft, and having a lower surface defining a continuation of the upswept undersurface of the aftmost portion of the fuselage. 14. An aircraft in accordance with claim 11, further comprising a modified fairing on the belly of the fuselage, the modified fairing defining a transition between the keel of the forward fuselage portion and the lowered keel of the aft fuselage portion, the modified fairing being at least one of a wing-to-body fairing, a main landing gear fairing and an air conditioning system covering fairing. 15. An aircraft in accordance with claim 11, wherein the aft cargo deck is configured to accommodate cargo containers selected from the group consisting of LD-1, LD-2, LD-3, LD-3-46, LD-3-46W, LD-3-45, and LD-3-45W containers. 16. A method for modifying a base aircraft comprising a main wing; a fuselage, including a crown section and a keel; a first cabin, having a first floor at a substantially constant level within a forward portion of the fuselage forward of and over the main wing; a forward cargo deck located below the first floor; and a base passenger capacity, a base cargo capacity, a maximum takeoff weight and a flight surface geometry, the method comprising: lowering an aft portion of the keel from a point aft of the main wing, linearly to a point of transition to an upswept undersurface of a tail portion of the aircraft, while keeping the crown section substantially constant fore-to-aft;providing a split level cabin only within an aft portion of the fuselage behind the main wing, the split level cabin including an upper second cabin having a second floor above the level of the first floor, and a lower third cabin beneath the upper second cabin and having a third floor below the level of the first floor; andproviding an aft cargo deck, within the lowered aft portion of the fuselage and beneath at least a portion of the lower third cabin;wherein the modified aircraft has a passenger capacity that is greater than the base passenger capacity, and a cargo capacity that is less than the base cargo capacity. 17. A method in accordance with claim 16, further comprising providing an upswept undersurface of an aftmost portion of the keel, having an undersurface upsweep selected to enable aft body tailstrike margins suitable for takeoff and landing of the aircraft. 18. A method in accordance with claim 16, further comprising providing an aerodynamic fairing transitioning between the keel of the forward portion of the fuselage and the lowered keel of the aft portion of the fuselage, the aerodynamic fairing comprising one of (i) a wing-to-body fairing, (ii) a main landing gear fairing, and (iii) a combined wing-to-body and main landing gear fairing. 19. A method in accordance with claim 16, further comprising providing a passenger egress door in the third lower cabin, the passenger egress door having a ditching evacuation sill height at a level above a ditching waterline associated with a ditching condition of the aircraft. 20. A method in accordance with claim 16, further comprising adapting the aft cargo deck to accommodate cargo containers selected from the group consisting of LD-1, LD-2, LD-3, LD-3-46, LD-3-46W, LD-3-45, and LD-3-45W containers.