Stall Warning System

Dornier

• Two vane-type angle of attack sensors

• To ensure accurate AOA sensing in ice conditions, the vane and the sensor case are heated by individual 28 VDC electric heater elements

• The sensor has a failure detection facility which can detect the following faults:
• vane heater failure
• resolver failure
• failure in the electronics section
• power supply failure
• ARINC failure (inhabit stick shaker and pusher)

Challenger

• Transducer on each side of fuselage

• Consists of an externally mounted trailing vane assembly connected by a shaft to an internally mounted potentiometer (5-15 VDC)

• Voltage will vary in proportion to displacement of trailing vane

• Signals then transmitted to SPS computer

*Trailing vanes protected against ice with heating elements

Lateral Accelerometers

• Monitor aircraft skid or sideslip and send signals to the corresponding left and right channels of SPS computer.

• Signals are used to compute compensated AoA values for the differing left and right trailing vane angles produced by maneuvers involving aircraft skid or sideslip

• If SPS detects a difference in the two accelerometer signals equivalent to more than 0.03 G of lateral acceleration, both STALL PROTECT FAIL lights come on flashing

• If the signals differ by more than 3.5 degrees of AoA at predetermined lateral acceleration limits, the STALL PROTECT FAIL lights come on flashing

Altitude Transducers

• Provide signals to SPS for altitude compensation

• Connected to the left and right static systems

• Convert the static pressures to electrical signals

• As transducers signal increase, computer will set up progressively lower AoA trip points for the stick shakers/pusher subsystems

• If a sensor or any part of the altitude compensation circuitry should fail, 2 ALT COMP FAIL lights are illuminated on the glareshield

Flap Position Transmitters

• Installed on each of the inner flap hinge assemblies

• As flap angle increases, the SPS computer uses the signals to compute progressively lower AoA trip points for the stick shaker/pusher

Dornier

Data Aquisition Unit (DAU)

• Two channels in each DAU: Channel A and Channel B

• LH AOA Sensor
• Channel A of DAU 1
• Channel B of DAU 2

• RH AOA Sensor
• Channel A of DAU 2
• Channel B of DAU 1

• The DAUs compare AOA measures to pre-defined threshold levels
• Flap position
• Icing status
• Relationship between measured AOA and true AOA

• In the event of an AOA sensor failure, the associated stick shaker and the stick pusher are inhibited.

• The onside AOA values are then used for the other stick shaker instead of the average values.

Challenger

Stall Protection System (SPS)

• Powered with 28 VDC

• Right and left channels receive inputs from their respective AoA transducers, flap position transmitters, altitude transducers, and lateral accelerometers

• Required to compute AoA trip points for auto-ignition, the associated stick shaker, aural warning horn and stick pusher.

Dornier

Stick Shaker

• The shaker consists of a DC electric motor that drives an unbalanced mass through a reduction gear

• The unit is attached the control column

• When the stick shaker is activated, the unbalanced mass rotates and causes the control column to vibrate

• An extension to the spring has a small hammer attached

• The vibrations cause the hammer to strike a bell installed immediately above the shaker

• The flight crew is thus provided with an aural warning of an impending stall

Challenger

Stick Shaker

• Bracket-mounted on each control column

• When AoA reaches the shaker trip point, shaker will begin slightly vibrating, which will gradually become a continuous vibration as the AoA approaches the stick pusher trip point

• The shaker vibration is loud enough to constitute an aural warning

• Will disengage auto-pilot

Stall Protection Warning Horn

• Horn sounds when the aircraft AoA has reached the stick pusher trip point

• If there is signal loss from one SPS computer channel, or if the stick pusher is disabled, the horn indicates corrective action is required

Auto Ignition

• To prevent engine flameout at aircraft AoAs, which subject the engine intakes to turbulent airflow from the wings

• SPS computer automatically activates continuous operation of both igniters in both engines until the aircraft attitude is corrected

• The auto-ignition AoA trip points decrease with higher flap settings and increased aircraft altitude sensed by the altitude compensation system

Dornier

Stick Pusher

• When both DAU 1 and DAU 2 thresholds are reached
• DAU energizes a relay
• The contacts of the two relays form a series circuit line to stick pusher control unit
• A third relay is controlled by a FWC (fault warning computer) which monitors conditions
• All three relays must energize to activate the stick pusher

• Stick pusher activates the elevator movement to nose down

• Crew can override the stick pusher by pulling on the column

Function

• Installed in the rear fuselage

• Under normal conditions the spring is compressed and the piston rod extended

• The piston rod is locked in this position by a motor brake and a mechanical lock

• If the stick pusher is activated, brake is released and motor starts piston rod is initiated through the reduction gearing mechanical lock is released and allows the spring to retract the piston rod into the pusher housing

• The pusher is equipped with three integral micro switches 2 of the switches act as limit switches and stop the motor at the travel limits. The third switch controls the following EICAS caution messages:
• Pusher fail message if the pusher does not activate when it receives the activate signal
• Pusher reset fail message if the pusher does not reset when it receives the reset signal

Challenger

Stick Pusher

• Once trip point is activated, push signals from the right channel of the SPS computer will push the stick to an equivalent pilot effort of 80 pounds
• The torque limiter

• 2 red warning lights, STALL PUSH, are located on each side of the glareshield and will come on flashing when a stick push is signaled

• STALL PUSH lights flash

• When SPS computer detects that the aircraft AoA has decreased five degrees below the stick pusher trip point, stick pusher is de-activated

Stick Pusher

Function

The pusher consists of an electric motor coupled through an electromagnetic clutch and torque limiter to a capstan driving the R/H forward elevator quadrant.

Dornier

System Tests

• Two test switches are installed for testing the stick shaker and stick pusher circuits on the ground

• If the LH or RH test switch is pressed the associated stick shaker is activated

• The stick shaker is deactivated when the test switch is released

• An automatic power-up test is performed by the fault warning computer

• The test is only performed on the ground and checks the integrity of the shaker and pusher circuit breakers and the pusher control relay interfaces

• If the test fails, an appropriate message is displayed on the electronic indicating, caution and advisory system EICAS

System Inhibition

• DAU 1& 2 receive MLG weight on wheel (WoW) signals from a proximity switch
• Inhibit the stick shaker on ground
• Inhibit the stick pusher on ground and for 2mins after the main wheels leave the ground

• DAU 1 & 2 receive signals from radar altimeter system
• Inhibits the stick pusher when the aircraft radar altitude below 100ft

Challenger

System Test

• The TEST switch, if held, will test the respective channel of the stall protection computer and cause the SPS test indicator needle to sweep and trigger auto-ignition, stick shaker, STALL PROTECT FAIL lights, STALL PUSH lights and aural warning.

• To test the stick pusher, both TEST switches must be held simultaneously, and then both SPS test indicators will sweep together. At the end of the sweep the pusher will be activated.

• The stick pusher can only be tested with weight-on-wheels.

System Inhibition

• Weight-on-wheels (WOW) information which, when activated, inhibits the stall protection system except for the test function.

• Outputs from each channel of the computer include an activation signal to the stick shaker and auto ignition, and a contributing signal for the stick pusher.

• The two channels of the computer are monitored by a comparator circuit and must be in agreement for stick pusher operation.