AM & FM differences...still learning after 27 years

I agree with you. There could be any number of reasons why AM is used for some aircraft speech comms (presumably some comms, perhaps data links is digital and they will be using different modulation).

Propagation through the atmosphere (and bouncing off objects!) is a difficult enough concept even without movement, so there may be all sorts of experience with moving aircraft. Even ultra weak AM signals transmitted at low power can be deciphered, who knows if FM receiver techniques have evolved to that level; it may be moot if digital modulation methods are more useful than AM speech for the aviation industry (I've no idea). Anyway, it's possibly a mix of technical reasons, legacy reasons, e.g maybe there's just not a significant enough benefit to switch, and legislation reasons too (available channels and their bandwidth).

I still quite like AM (I reluctantly recently purchased a VHF FM receiver, it is a lot more fun searching AM transmissions on HF; I have a decent receiver for that).. AM is still used worldwide by soldiers too, at HF and VHF. And they also use FM for different purposes. They have loads of experience with the technologies, and they know how to transmit for thousands of miles with AM (usually its sibling, i.e. SSB) using the atmosphere to bounce off (admittedly mostly on HF, so the comparison isn't exactly the same), with just a few Watts of power.

Well, I'd say it's a complex set of reasons ...

The first is probably for legacy reasons - it's not easy to force everyone to change-over to another system, especially one that is multinational. Aviation has been around for a long time, and the predominant technology at the time of conception was likely AM, so it's "stuck". That being said, FM has its benefits for lower-cost, higher-powered transmissions as linearity is not a major concern.

The second is likely to do with multiple signals simultaneously on the air (getting "stepped on"). AM modulation can handle multiple transmissions overlapping and still be intelligible. FM modulation, however, does one of two things - where the frequency is within "capture" range and the signal is strong, the demodulator tracks the stronger signal while rejecting the weaker one significantly (meaning you will hear just one, but not the other). Or, if the signals are very close together and similarly strong, it breaks down into a heterodyne "squeal" which is quite unpleasant to listen to but will also block out both transmissions without much way to identify each.

Because planes are in the air and frequencies are re-used across different airports and across jurisdictions, the possibility of having competing signals at vastly different strengths is more likely than for a ground-based system. Furthermore, VHF band transmissions can be vulnerable to very occasional summer-time sporadic-E tropospheric ducting events which increase the range of signals dramatically compared to the normal "limited-by-horizon" range.

Another issue may be related to how the signal-to-noise ratio of the demodulated output degrades as a function of input signal to noise ratio. AM generally works down to quite weak signals and tends to degrade "gracefully" into static. FM, on the other hand, tends to be very good up to a point and then fails more rapidly into white-noise static (as far as I can tell). Perhaps this gives some reassurance about the reliability of the communication channel.

Of course, in a perfect world, high-quality vocoder compression and a digital-radio with copious amounts of error correction will achieve further ranges with the same quality (think of digital mobile phones compared to analog mobile phones or digital radio versus analog radio, noting that in most countries, digital radio is being broadcasted at 1% to 10% of the power used by analog radio), but it's also a level of complexity that may make it more liable to failures, more difficult to service and a difficult proposition to get the world to change over to.

Definitely a good question to think of, as most land-based systems have long eschewed AM and FM for digital-modulations (e.g. BPSK/QPSK-variants) carrying error-corrected compressed digital audio (e.g. P25, TETRA, dPMR/DMR/NXDN/iDAS/MotoTrbo). Ironically, AM and FM mainly live on in the amateur radio world (where digital alternatives like D-STAR only have limited success) and in low-cost walkie-talkie (CB) type applications. For many radio planning authorities, they operate along the ethos of  "Bandwidth is a limited resource - spectral efficiency is king".

- Gough

Thanks - a good job explaining getting "stepped on"  :-)   I wonder if legacy is the main driver. There are lots of big aircraft , and also small aircraft and airports with existing equipment. Unless there is some huge advantage of one over the other , or governments intervention, its not likely to change. An example, the USA started a move in the late 60s and 70s to metric, useage driven by imported and exported equipment. I have 2 sets of wrenches, screwdrivers, allen wrenches, etc....one metric, one english/imperial. 

Interestingly enough, FM radios come in analog and digital flavors too. :-) 

Wide and narrow too.