Some Guy wrote:
I have no trouble receiving FM radio broadcasts on a small am/fm radio
I sometimes listen to while onboard commercial jet airliners (flying
at cruise altitude), but I never seem to be able to pick up AM radio
stations. It's just static across the AM band.

Any explanation for this?

Yes, there is. The AM cops have figured you out, since what you are
doing is illegal on commercial airliners. The FM cops are a little
slower, but they will pull the plug on you also, eventually.

Seriously, though, you are inside a metal cigar tube you call an
airplane, and you are being shielded by the body of the aircraft.
Although windows (portholes, not Gates), and the metal itself, don't
block out signals completely, you will see an effect from this (look up
"Faraday Cage" on google). AM broadcast is a very long wavelength
(hundreds of meters long) whereas FM broadcast is a smaller wavelength
(around 3 meters). If you were trying to throw a bunch of marbles
through an upstairs window, you would probably be able to do it. But if
you were trying to throw a bunch of beachballs through an upstairs
window, it wouldn't be as easy, right?

The aperture is the important issue. Although the airplane is not a
completely shielded RF-proof "screen room", it acts somewhat like one.
That is why avionics antennas are on the outside of the plane, not
inside. That is also why there is a teeny mesh grid in the door of your
microwave oven - they have to be that small to block the microwaves.

Using my example befo if you are throwing beachballs (AM broadcast),
or marbles (FM broadcast) or a handful of sand (microwaves), how small
would you want the window to be in order to block it?

OK, getting back to my first paragraph, if you are ever on a plane with
me, please let me know, so I can take the next flight. The local
oscillator of FM receivers is often on the same frequency as the VOR
stations that airplanes use to naviagate with, and can cause
interference. There are failsafe solutions that the pilot has, to deal
with loss of VOR coverage, but I don't want to depend on them because
you are listening to gangster rap at 32,000 feet. Get an iPod or something.

All the best,
Dave