Someone please tell me WHY, if true, there is an "Automatic" position on the Pitot Heat Switch on this, or any aircraft.
Avionics maintenance engineer here, specializing in air data systems.
The "automatic" mode is used to prevent electrically-heated sensors (pitot probes, AOA vanes, TAT probes etc.) from being switched "on" when the aircraft is on the ground.
The system is activated by the weight-on-wheels sensors, permitting electrical current flow to the probe heating elements only after the aircraft is airborne.
Without the cooling effect provided by airflow, the pitot probes in particular can reach a temperature of several hundred degrees in very short order, which can quickly cause the heating elements to burn out (at the very least), to say nothing of possible damage to surrounding aircraft structure.
A manual mode must be available, however, as there are occasions where it would be necessary to power the heaters on the ground - i.e. on a cold day where frozen precip (snow, sleet, freezing rain) is present.
A significant amount of electrical current flows through the heating elements when they are activated. If sensors detect no (or inadequate) current flow in a situation where the heater
should be on, it will trigger a "fail" annunciator or EICAS message for the associated probe.
Many "vintage" Boeing aircraft (737, 727 etc.) had a switch-selectable ammeter so that the crew could actually see the amount of electrical current being delivered to each probe's heating element. More modern airframes depend solely on annuciators to detect heater failure.
However, none of these monitoring systems can detect a situation where the heater might be working perfectly, and yet unable to keep up with ice accretion in a particular environmental scenario due to a design flaw.
JR Barrett