B777 Auto Anti-Ice

[Subwoofer]

Howdy All - I hope someone can answer this for me.

In the 777 at least, maybe true of other aircraft as well, our EAI and WAI switches stay in Auto during flight. When ice is detected, the AI valves open and hot air flows. However, sometimes we'll see the EAI indication without WAI, so what determines whether the EAI, WAI, or both AI should turn on?

Thanks...
SW

[Ex Douglas Driver]

2x ice detectors vibrate at approx 40,000Hz, and these will vibrate at a lower frequency when they collect ice. A heater turns on when the probe vibrates less than 39,867Hz (according to the maintenance manual!). The heater stays on until the probe vibration returns to its usual value or the heater operates for 25 seconds. The usual time that the heater is on is 5 - 7 seconds, but if the heater is on for 25 seconds, the heater turns off and a fault condition is latched.

The detector has control logic that counts the number of heater on to off cycles.

If there are two or more cycles, and the airplane is in the air, the ice detector gives an engine icing signal to the ACIPS (Airfoil and Cowl Ice Protection System). The detector also gives an engine icing signal if the airplane is in the air and the heater is on for 15 seconds or more.

If there are ten cycles and the airplane is in the air, the detector gives a wing icing signal to the ACIPS.

The icing signals stay on until any of these conditions occur:
No ice for more than 3 minutes
The airplane is on the ground
The heater is on for 25 seconds.

[Subwoofer]

Wow, thanks Ex Douglas Driver! Just the answer I was looking for.