Function of the Pitot-Heat system

The pitot heat system prevents icing of the pitot tube and (on larger aircraft also) the static port. It is one of the few anti-icing systems found on virtually every PPL training aircraft (C172, PA-28, DA40, etc.).

Why critical: Iced pitot → the ASI shows a wrong or no airspeed reading. Consequences:

With pitot blocked, static open → the ASI behaves like an altimeter: increases with climb, decreases with descent — independent of actual speed.
With pitot and drain blocked → ASI stuck at zero or fixed value.
With static port blocked → altimeter, VSI and ASI all malfunction.

How it works

Construction:

Inside the pitot tube an electrical heating element (resistance wire) heats the air inside the tube and the outer wall.
Power: typically 50–200 W in light aircraft, more in larger aircraft.
Voltage: 14 or 28 V DC, depending on the electrical system.

On/off switch:

Cockpit switch "PITOT HEAT" or "PITOT STATIC HEAT".
On some aircraft: separate switch or combined with AoA heat.
Indication: warning lamp (green = on) and possibly an ammeter.

Ground test (pre-flight):

Briefly switch on,
Carefully (it gets hot!) check by hand that the pitot tube becomes warm,
Then switch off — leaving it on without airflow can cause overheat damage.

When to switch on?

General rule — switch pitot heat on:

In visible moisture and outside air temperature +10 °C or below (rule of thumb, varies by AFM),
In flight in actual or forecast icing,
In flight in clouds, even when the temperature is above 0 °C — pitot icing can occur from evaporative cooling even at +5 °C,
For IFR flights in IMC routinely,
Many AFMs recommend switching pitot heat on before every flight just after engine start, so any failure is detected early.

Important: consult the AFM/POH of the specific type — manufacturers give exact instructions.

Failure modes

No electrical current (blown fuse, broken heating element):

Pitot heat light does not illuminate,
Ammeter shows no current draw.

Heating power insufficient:

In severe icing even an active heater cannot melt the ice.
Action: exit icing conditions (climb, descend, turn back).

Overheating:

If pitot heat remains on without airflow (on ground) the tube can be damaged.
Therefore: on the ground switch pitot heat on only briefly for testing.

Pre-flight: pitot tube inspection

Remove the pitot cover (a common cause of erroneous ASI after take-off!),
Check the pitot opening is free of insects/debris,
Outer cap undamaged,
Drain hole (small hole on the underside) clear.