Cardinal,
Radio propagation is mainly a matter of frequency. Modulation (FM/AM) is fairly neglectable here. What we commonly call FM radio, are the freqs. in the 88 - 108 MHz range. Remember that aviation communication takes place in the 118-137 MHz range, and thus will be subject to similar propagation conditions than FM radio, i.e. line of sight. AM broadcast radio takes place in the 0.5 - 1.7 MHz range, therefore propagation is different here. It's just a matter of frequency... basically.
Someone correct me if I'm wrong, but the reason we use AM in aviation ( I believe ) is that you need much less separation between channels. Channel spacing in aviation is currently as low as 8.33 kHz using AM, as opposed to 50 kHz used for FM broadcast.
In a nutshell:
High frequencies ( 65+ MHz ) propagate nearly in "line of sight", FM is used where quality is important (e.g. FM radio broadcast), AM is used where quality is less important, but many channels are needed (e.g. aviation comm, AM radio...).
Hope that was more or less correct,
greetings...
Luxflyer, 5 am local
