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R  probe heaters, and the probe biasing magnetic. The probe vibrates 
R  axially at its natural frequency of 40 KHz. As ice accretes on the 
R  sensing element, the mass loading affect causes the natural frequency 
R  to drop. A 133 Hz drop is correlated to 0.020 plus or minus 0.005 in. 
R  (0.508 plus or minus 0.127 mm) of ice and is considered as the 
R  detection trip point. When the trip point is reached, the probe is 
R  deiced by means of internal heaters to prepare for subsequent 
R  detections. 
R  (2) Controller (electronic part) 
R 
R  The controller portion of the ice detector contains the electronics 
R  necessary to drive and monitor the sensing element, control the 
R  input/output signals, provide the built-in test capabilities and

 

R  convert the input power to the necessary voltages required by the 
R  controller electronics. 
R  (a) Microcomputer 
R 
R  The microcomputer section consists of a microcontroller and 
R  asssociated components to provide the required decision processes 
R  of the ice detector. 
R  A watchdog timer circuit is used to monitor the correct operation 
R  of the microcontroller function. A time out of this circuit will 
R  cause the microcontroller to enter the hardware reset state: this 
R  enables the FAULT output (FAULT indication) and disables the ICE 
R  and SEVERITY outputs. 
R  (b) Probe oscillator 
R 
R  The probe is driven at its resonant frequency by a closed loop 
R  oscillator circuit. This circuit consists of a drive coil and a 
R  feedback coil. These coils are mounted concentric with the 
R  unexposed half of the probe tube. The drive coil produces a 
R  magnetic field which causes the magnetostrictive probe tube to 
R  elongate. The relaxation of the tube is picked up by the feedback 
R  coil which drives the input of the oscillator operational 
R  amplifier setting up the resonant circuit. 
R  (c) Heater control 
R 
R  The heater control consists of a silicon controlled rectifier 
R  with zero cross switching that controls the flow of current 
R  through the strut and probe heaters. 
R  D. Detector Inputs 
R  (1) Test inputs 
R 
R  To perform initiated test, the test input must be activated for at 
R  least 500 ms. The test input can be initiated several times between 
R  energization and de-energization of the aircraft electrical circuits. 
R  (2) Reset inputs 
R 
R  The reset input is activated to reset the severity counter and cut 
R  off the SEVERITY signal. 
R  In the ice detection system, the reset input receives the wing anti 
R  ice valve status. When this valve is open, the SEVERITY signal is 
R  reset and its processing is inhibited as long as the wing anti ice is 
R  supplied from the aircraft.

 

R  **ON A/C 001-049, 051-099, 101-149, 151-199, 201-299, 301-399, 
R  E. Lighted Icing Indicator
 A lighted icing indicator is installed in lieu of one winshield center
 panel retainer bolt. It is visible by both pilots. It provides a visual
 indication when icing conditions are encountered.
 Its illumination is controlled by a switch INT LT/ICE IND & STBY COMPASS
 (2LF) for night visibility. 
R  **ON A/C 401-499, 
R  E. Detector Outputs 
R 
R  Each ice detector generates three signals: 
R 
R  (1) ICE signal 
R 
R  The ice detector sends the ICE signal when a thickness of 0.5 mm of 
R  ice is accreted on its sensing element. This signal is maintained for 
R  60 seconds. If new ice detections occur within 60 seconds, the ICE 
R  signal is maintained for 60 seconds after the last detection. 
R 
R  (2) SEVERITY signal 
R 
R  The ice detector generates the SEVERITY signal when a number of ten 
R  elementary ice detections is reached. 
R  The threshold for the SEVERITY signal corresponds to approximately 5 
R  mm of ice accreted on the most critical protected surface of the 
　
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