Composite structure

1. A composite structure, comprising:

a panel formed from two or more plies of composite material, the panel having a surface formed with a step where the thickness of the panel changes; and

two or more stiffeners attached to the surface of the panel;

wherein a first one of the stiffeners is attached to the surface of the panel on a thicker side of the step, and a second one of the stiffeners engages and runs substantially parallel to at least part of the step.

2. The composite structure of claim 1 further comprising a cap attached to the surface of the panel on a thicker side of the step and attached to one of the stiffeners on a thinner side of the step.

3. The composite structure of claim 2 wherein the cap is formed from one or more plies of composite material.

4. The composite structure of claim 1 wherein the second one of the stiffeners which engages at least part of the step comprises a stiffening portion which extends away from the panel, and a foot which is attached to the surface of the panel on a thinner side of the step and extends away from the stiffening portion to a toe remote from the stiffening portion, wherein the toe engages at least part of the step.

5. The composite structure of claim 1 wherein the second one of the stiffeners which engages at least part of the step comprises a stiffening portion which extends away from the panel and engages at least part of the step.

6. The composite structure of claim 1 wherein the second one of the stiffeners which engages at least part of the step comprises a first part which is attached to the surface of the panel on a thicker side of the step; and a second part which is attached to the surface of the panel on a thinner side of the step whereby the second one of the stiffeners straddles the step.

7. The composite structure of claim 1 wherein the step is formed by the edges of one or more discontinued plies in the panel, and wherein the second one of the stiffeners engages the edges of discontinued plies.

8. The composite structure of claim 1 wherein the step is formed by the edges of one or more discontinued plies, and a filler between the second one of the stiffeners and the edges of the discontinued plies, wherein the second one of the stiffeners engages the filler.

9. The composite structure of claim 1 wherein the step extends at an angle of greater than 45° to the plies in the panel adjacent to the step.

10. The composite structure of claim 1 wherein a ramp is formed in the surface of the panel between the first one of the stiffeners and the step.

11. The composite structure of claim 1 wherein a ramp is formed in the surface of the panel, and at least part of one of the stiffeners engages the ramp.

12. The composite structure of claim 1 wherein the stiffeners are each formed from two or more plies of composite material.

13. The composite structure of claim 1 further comprising fasteners passing through the stiffeners and the panel.

14. A method of manufacturing the composite structure of claim 1 , the method comprising forming the panel with the step; attaching the stiffeners to the surface of the panel; and engaging at least part of the step with one of the stiffeners.

15. The method of claim 14 further comprising forming the step by discontinuing two or more plies in the panel, and machining the ends of the discontinued plies.

16. A method of transmitting load in the composite structure of claim 1 , the method comprising transmitting load between the step and the stiffener.