12/29/15
Overview
The ice detection system senses icing conditions during flight and gives the necessary warning to the flight crew. The ice detection system uses two ice detectors to sense icing conditions.
Ice Detector
The aircraft has two ice detectors, one on each side of the front fuselage below the flight compartment side windows. They are installed at FS239.85 and WL75.20. They are installed at FS239.85 and WL75.20. The left detector receives 28 VDC power from the L MAIN BUS and the right detector receives its 28 VDC from the R MAIN BUS. The ice detector uses an ultrasonic vibrating probe to sense if icing conditions are present. The probe is a nickel alloy tube installed on a strut at its midpoint (node) with 1 inch. exposed to the air. The oscillation frequency of the circuit is given by the natural resonant frequency of the sensor tube, which is tuned to 40 KHz. The ice detectors have internal thermal heaters.
System Operation
Theory
When the aircraft goes into an icing environment, ice collects on the probe. The ice causes the frequency of the probe to decrease. When the frequency decreases to a specified value, the ice detector de-ices the unit strut and probe through internal thermal heaters.
When the probe is de-iced, the probe temperature decreases quickly and can sense an icing condition again. This procedure is repeated while the ice detector stays in an icing environment. The ice detector keeps count of the number of icing/de-icing cycles of the sensor element.
When two consecutive probe icing/de-icing cycles are counted, an engine indicating and crew alerting system (EICAS) message is shown for 60 seconds. If one more icing/de-icing cycle occurs during that time interval, the 60 seconds period starts again.
Interfaces
Discrete outputs are supplied by the ice detector. They are directly connected to the stall protection computer (SPC), which uses them to calculate the advance of the stall protection trigger. The signals are also connected to the data concentrator unit (DCU) which uses them for the EICAS and synoptic page indication.
Indications
When ice is sensed, an ICE DETECTED caution message is shown on EICAS if the wing and engine anti-ice systems are set off. Also, in the same condition, an amber vertical ICE flag is shown to the left of the attitude ball on the primary flight display (PFD). If the wing and engine anti-ice systems are set on, an ICE DETECTED advisory message is shown on EICAS and the ICE flag is out of view. An ICE DETECTOR FAIL caution message is shown on the EICAS when the two ice detectors are failed. An ICE DETECTOR FAULT advisory message is shown on EICAS when one of the ice detectors is failed. When either ice detector is sensed failed, the flight crew must examine for visible moisture and for external temperature for icing condition detection. The DCU sends the status data to the maintenance diagnostic computer (MDC) to record ice detector failures into its memory.
Warning:
Be careful when you do work near the ice detectors. When the ice detectors are in operation, they become hot and can cause injury to persons.
The ice detectors do not operate correctly at airspeeds below 29.6 kts indicated air speed.
12/29/15
System Interface
The ice detection system interfaces with the following aircraft systems/components:
- Stall Protection Computer
- Engine Indicating and Crew Alerting System (EICAS)
- SYSTEMS TEST Control-Panel
- Maintenance Diagnostic Computer (MDC)
03/30/22
System Test
During system power-up, the ice detection system does a power-up built-in test (PBIT) of the ice detectors. The PBIT does an internal check of the electronics and the probe heater, as well as the external discrete ICE and STATUS signals. The STATUS signal is inverted to make sure that it is valid. This test can be started by the flight crew with the rotary test switch on the SYSTEMS TEST control panel set to the ICE DET position.
12/29/15
System Monitoring
The ice detectors have a continuous built-in test (CBIT) function that does an internal check of the electronics and the probe heater, as well as the external discrete ICE and STATUS signals. The CBIT is done during ice detector operation and alerts the flight crew of possible ice detector failure.
The EICAS messages that follow are related to the ice detection system
| EICAS MESSAGE(S) | LEVEL (COLOR) |
| ICE DETECTED | CAUTION (amber) |
| ICE DETECTOR FAIL | CAUTION (amber) |
| ICE DETECTED | ADVISORY (cyan) |
| ICE DETECTOR FAULT | ADVISORY (cyan) |
The ICE DETECTED advisory message is inhibited during takeoff and landing (below 400 ft).
Warning:
Activation of anti-icing systems can be a cause of engine thrust changes.
