Why do aircraft use AM modulating mode?

Question

As I understand it ( havent check it for years, Why do aircraft use AM Modulaton on there trancervers to comunicate betwen air craft and air to ground voice communication. I am aware they use other digitial devices for other comunication devices but at 115Mghz or close to that it would seem that FM would be more practical.

Answer 1

AM radios are easier to build. AM is for short range communications long range is done by satcom or HF SSB.

Answer 2

Why do aircraft use AM modulating mode?

AM is used because it does NOT exhibit "capture effect"

Capture effect is when two FM transmitters are transmitting on the same frequency, and if one of them is 3 or more db stronger that the other one, the stronger signal will capture the receiver and will be the only one heard.

With AM, both transmissions could be heard because there is NO capture effect. Very important in an emergency situation. An aircraft may have only one chance to transmit a "mayday" and nature of the emergency.

Answer 3

It's just that it's always been done that way.  Conversion to FM was considered at one point in time, but due to the cost and logistics of swapping, it was dropped.

FM modulation is not any more easy or difficult to do.

AM really has nothing to do with the range of the radio communications, it's the fact that it's VHF that determines what its range is (mainly line of sight).

In the early days of aviation VHF, the VOR navigation mode was a form of AM, and a com radio could be tuned there to listen for VOR, or tuned up higher for communications...which was influential in settling in AM mode.

Your right though, FM would have been a better mode had it been able to start that way due to its clarity and other reasons.