A quick Google turned up a published article answering precisely this question (Lynch, 2008). The abstract states:

Reports and photographs claiming that visual observers can detect the curvature of the Earth from high mountains or high-flying commercial aircraft are investigated. Visual daytime observations show that the minimum altitude at which curvature of the horizon can be detected is at or slightly below 35,000 ft, providing that the field of view is wide (60°) and nearly cloud free. The high-elevation horizon is almost as sharp as the sea-level horizon, but its contrast is less than 10% that of the sea-level horizon. Photographs purporting to show the curvature of the Earth are always suspect because virtually all camera lenses project an image that suffers from barrel distortion. To accurately assess curvature from a photograph, the horizon must be placed precisely in the center of the image, i.e., on the optical axis.

Note that the given minimum of 35,000 feet (10.7 km) is a plausible cruise altitude for a commercial airliner, but a passenger window probably won't give the necessary 60° field of view.

Lynch, D. K. (2008). Visually discerning the curvature of the Earth. Applied Optics, 47(34), H39-H43.

A quick Google turned up a published article answering precisely this question (Lynch, 2008). The abstract states:

Reports and photographs claiming that visual observers can detect the curvature of the Earth from high mountains or high-flying commercial aircraft are investigated. Visual daytime observations show that the minimum altitude at which curvature of the horizon can be detected is at or slightly below 35,000 ft, providing that the field of view is wide (60°) and nearly cloud free. The high-elevation horizon is almost as sharp as the sea-level horizon, but its contrast is less than 10% that of the sea-level horizon. Photographs purporting to show the curvature of the Earth are always suspect because virtually all camera lenses project an image that suffers from barrel distortion. To accurately assess curvature from a photograph, the horizon must be placed precisely in the center of the image, i.e., on the optical axis.

Note that the given minimum of 35,000 feet (10.7 km) is a plausible cruise altitude for a commercial airliner, but you probably shouldn't expect to see the curvature on a typical commercial flight, because:

1. 10.7 km is the bare minimum for seeing curvature, so the apparent curvature will be very slight at this altitude.
2. 10.7 km is near the upper end of the usual range of commercial cruise altitudes. Many flights won't get this high, and very few will go significantly higher.
3. A passenger window may not give the necessary 60° field of view, especially if you're over the wing.
4. As the quotation states, you need a nearly cloud-free horizon to perceive the curvature.

Lynch, D. K. (2008). Visually discerning the curvature of the Earth. Applied Optics, 47(34), H39-H43.

A quick Google turned up a published article answering precisely this question (Lynch, 2008). The abstract states:

Reports and photographs claiming that visual observers can detect the curvature of the Earth from high mountains or high-flying commercial aircraft are investigated. Visual daytime observations show that the minimum altitude at which curvature of the horizon can be detected is at or slightly below 35,000 ft, providing that the field of view is wide (60°) and nearly cloud free. The high-elevation horizon is almost as sharp as the sea-level horizon, but its contrast is less than 10% that of the sea-level horizon. Photographs purporting to show the curvature of the Earth are always suspect because virtually all camera lenses project an image that suffers from barrel distortion. To accurately assess curvature from a photograph, the horizon must be placed precisely in the center of the image, i.e., on the optical axis.

Note that the given minimum of 35,000 feet is a plausible cruise altitude for a commercial airliner, but a passenger window probably won't give the necessary 60° field of view.

Lynch, D. K. (2008). Visually discerning the curvature of the Earth. Applied Optics, 47(34), H39-H43.

A quick Google turned up a published article answering precisely this question (Lynch, 2008). The abstract states:

Reports and photographs claiming that visual observers can detect the curvature of the Earth from high mountains or high-flying commercial aircraft are investigated. Visual daytime observations show that the minimum altitude at which curvature of the horizon can be detected is at or slightly below 35,000 ft, providing that the field of view is wide (60°) and nearly cloud free. The high-elevation horizon is almost as sharp as the sea-level horizon, but its contrast is less than 10% that of the sea-level horizon. Photographs purporting to show the curvature of the Earth are always suspect because virtually all camera lenses project an image that suffers from barrel distortion. To accurately assess curvature from a photograph, the horizon must be placed precisely in the center of the image, i.e., on the optical axis.

Note that the given minimum of 35,000 feet (10.7 km) is a plausible cruise altitude for a commercial airliner, but a passenger window probably won't give the necessary 60° field of view.

Lynch, D. K. (2008). Visually discerning the curvature of the Earth. Applied Optics, 47(34), H39-H43.