13.15 Ice and Rain Protection Flashcards
What effect does icing on forward facing areas of an aircraft cause?
- Decrease in lift (loss of streamline flow of air around the leading edges and top surfaces)
- Increase in drag (rough surface increases skin friction)
- Decrease propeller efficiency (change in blade shape)
- Loss of control (restriction of control surfaces)
- Increase in weight (CofG affected as well)
- Blockage of pitot static heads
- Obscured windows and windshields
What is supercooled water?
Water is present in liquid form at a temperature below the freezing point of 0°C
What is collection efficiency?
Measure of how much ice an aerofoil collects compared with the moisture content of air, droplet size and airspeed
For a small radius leading edge, what can the collection efficiency be?
95%
Why might the horizontal stabiliser have a higher collection efficiency than the wing?
It has a smaller radius than that of the wing
Where will Glaze icing or Runback icing occur?
In very dense clouds
Water particles are large and only slightly supercooled
or
In supercooled rain where the water droplets are of a large size
Temperatures range from 2°C to –10°C
What does glaze or ‘runback’ ice consist of?
Transparent or translucent coating of ice with a glossy surface appearance
Why is glaze or ‘runback’ ice considered the most dangerous?
Much greater spoiling effect on the airflow over the various surfaces
How will rime ice appear?
Light texture having a white crystalline appearance
Describe hoar frost?
Occurs on a surface which is below the freezing point of 0°C
Forms in clear air, water vapour being converted directly into ice crystals forming a white feathery coating
Occurs on the surface of wings where integral fuel tanks are fitted
What has research shown about the relative stalling speed of propeller driven aircraft that are affected by icing?
30% above the normal stalling speed
What is the difference between anti–icing and de–icing?
Anti–icing – prevents the formation of ice
De–icing – removes ice after formation
What are the four methods used for aerofoil ice protection?
Thermal
Electrical
Pneumatic
Fluid
How is bleed air delivered to the leading edge of the wing for anti–icing purposes?
Perforated air duct called a piccolo, or spray tube
What pressure is the ice protection system air regulated to?
18–20 psi
What is the period for which the tail ice protection will be active for, after the crew push the switch marked TAIL?
2 – 2.5 minutes
Overheat switches for ducting which are restricted on the ground, are restricted at what temperature and why?
90°C
No cooling airflow from aircraft movement, means components are at risk of overheating
What is the purpose of the amber VALVE light in the wing anti–icing system?
While the valve is moving between positions (fully open and fully closed)
What indication is there that a wing anti–icing system is turned on?
White ON light
What are the three occasions where you would receive an amber VALVE caption for the wing anti–icing system?
Normal operation when the valve is motoring between stops
When there is a discrepancy between the valve position and the switch position
During test
What causes the anti–ice valve, of the wing anti–icing system, to close during ground test and where is it located?
Ground overheat sensing switch just outboard of each anti–ice valve
Sensing 90°C approximately
What are spray mats also known as?
Heater mats
Where are continuously heated breaker strips fitted and why?
Between intermittently heated area
Ensures that small areas of ice removed are small enough not to cause a hazard to the aircraft
How is the heater mat constructed to provide insulation between mat and aircraft structure, and to provide better transference of heat to the outside where ice forms?
Outer glass fibre layer is thinner than the inner layer
How is a heater mat protected from rain or hail erosion and impact damage from runway debris?
Covered by an erosion resistant coating
Why is the heater mat coating electrically conductive?
Electrically bonds the outer surface of the mat to the aircraft structure to provide a discharge path for static charges
Which areas of the heater mats are heated continuously?
Extreme leading edge of an aerofoil
Breaker strips between intermittently heated areas
How are de–icer boots held tight to the aircraft when the system is switched off, or during the ‘off’ cycle for that particular boot?
Vacuum pressure
Why is estane used as an outer ply on pneumatic de–icer boots?
Resistance to abrasion and deterioration
Electrically conductive to provide a path to dissipate static electricity
What would be considered repairable damage to de-icer boots?
Cuts and tears less than 0.75 in (19 mm) long
Scuff damage to an area of 4.5 in x 9.5 in (114 mm x 241 mm)
What might be the cause of sluggish inflation and deflation?
Obstruction in certain pipelines
Sticking solenoid valves
What might the fluid method of anti–icing also be known as?
Chemical method
What are the advantages of fluid ice protection systems?
- Does not require a large amount of electrical power
- Does not use bleed air from the engines
- Light in weight, excluding fluid
- Porous stainless steel or titanium leading edge can resist damage by hail, stones and birds
- Distributor is installed flush with the aerofoil leading edge so there is no aerodynamic penalty on the aircraft when compared with pneumatic boods
- Natural runback of the de–icing fluid gives ice protection behind the leading edge
- Only switched on when icing is encountered and doesn’t require the pilot to estimate thickness
How many pumps are used for fluid ice protection systems and how many are used during priming?
Both are used for initial priming
After 3 minutes, one stops, the other pump supplies the complete system
What is the purpose of proportioning units in the fluid ice protection system?
Installed at appropriate locations to send the correct amount of de–icing fluid to the individual porous distributor panels
How is the fluid ice protection system primed if icing is likely to occur during a flight?
Maintained fully primed by running the pump for approximately 30 seconds every 90 minutes
What must be carried out after a distributor panel has been installed as part of the fluid ice protection system?
Check should be make to ensure that a smooth and continuous profile is maintained both chordwise and spanwise
Test flight must be carried out to check the stall characteristics of the aircraft
When painting an aircraft which incorporates a fluid ice protection system, what must you do?
Mask the distributor panels with non–adhesive material attached to the panels at the edges
What are the four types of ice detector?
Ice formation spot light
Serrated rotor ice detector head
Vibrating rod ice detector head
Hot rod ice detector head
What happens on a serrated rotor ice detector head under no icing conditions?
No opposing torque
Switch contacts remain open
Which detector measures torque on a rotor as ice builds up?
Serrated rotor ice detector head
What is the resonant frequency of the vibrating rod ice detector head?
Approximately 40 kHz
What happens to the vibrating rod as the ice builds up (vibrating rod ice detector head) and what does this cause?
Probe frequency decreases
Detector circuit compares this with reference oscillator, at pre–determined frequency change the ice detector is activated
Red ice warning light on the flight deck is illuminated and probe heater removes the ice, the probe returns to detector mode and the red light turns off
What is the secondary benefit of heating a windscreen for de–icing?
Heating the vinyl interlayer makes the window more resilient
Better able to withstand possible collision with birds
How are windscreen heat sensing elements usually fixed to the windscreen?
Embedded within the panel
Held against the window by a rat trap tension spring
When the windscreen heating is switched on, why does it take 2 to 4 minutes to reach maximum value?
Prevent thermal shock to the window which might cause it to fail
In windscreen heating, what does the primary sensing element maintain the temperature of the windscreen at?
40°C
In windscreen heating, what is the name of the secondary sensing element, what temperature does it regulate the windscreen heat to, and how is this indicated?
Overheat sensing element
Higher temperature of 55°C
Overheat light on the control panel
What is the effect on maintenance due to the fact that the resistance of windscreen heating elements changes with age (usually increases)?
Must be connected to a different transformer to supply a different voltage
Resistance of the windscreen is checked against a chart
A replacement windscreen requires the same procedure
What checks might be required if a heated windscreen is replaced?
Resistance test
Functional check
Compass swing
Pressure test
During testing, if the window is selected on and no heat is felt and no lights illuminate, what might be the cause?
May be caused by high ambient temperature
What are the four methods of rain removal?
Windscreen wipers
Blowing it off using engine bleed air
Windshields which have a hydrophobic coating
Chemical rain repellent
Why is pneumatic rain removal system better than windscreen wipers?
Tendency of the airflow to reduce the wiper blade pressure on the window
Difficulty in achieving fast enough wiper action to clear the water during heavy rain falls
When can rain repellent be used and why?
In conjunction with windscreen wipers only to one windscreen at a time
As the fluid ‘foams’ on the window for a short time after application unless wiped
How much rain repellent is applied to a windscreen when the switch is pressed?
5cc
What must be done if rain repellent is inadvertently applied to a dry windscreen?
Washed off with water otherwise it will dry within a few hours
What should be done before operating windscreen wipers on a dry windscreen?
Wet windscreen prior to operation
Why should rain repellent not be applied to a dry windscreen?
It will form a gum
How is wiper blade tension checked?
With an accurate spring balance
Attached to the end of the wiper arm and held at 90°
Force required usually about 10 lbs
Force can be adjusted with the blade tension bolt
What produces the changeover of power supply for a waste water drain mast?
Weight on wheels switch
What are the power supplies of a waste water drain mast heater?
115v AC
28v DC
Where might ice form in an engine?
Engine intake
Inlet guide vanes
Nose cone
Leading edge of the nose cowl
First stage stator blades
In engine thermal ice protection, what illuminates the green HEAT light?
Lower low pressure switch at 5 psi
In engine thermal ice protection, what illuminates the amber HIGH PRESSURE light?
Upper high pressure switch at 40 psi
In engine thermal ice protection, when does a relief valve vent to atmosphere and why?
If differential pressure exceeds approximately 5 psi
Protect the nose cowl from excessive pressures
What might some of the large high by–pass engines use instead of thermal anti–icing for the nose cone?
Small cone of soft rubber mounted slightly offset on the front of the nose cone
Vibrates sufficiently to shake loose any ice that forms
When can normal engine intake icing be expected where visible moisture is present?
At temperatures of +8°C to +10°C
At what total air temperature will the engine anti–icing system come into operation when the push switch is in the ON position?
Below 5°C
What conditions will cause the engine intake anti–icing to stop operating?
System switched off
TAT above 7°C
Propeller rpm below 600
Timer control failure
What gives mixed ice (glime) its opaque appearance?
Air is trapped in the frozen ice
Why is rime ice rougher than other forms of icing?
Particles of ice will remain separate and air will be trapped between them thus the ice will be of a light texture
What are the dangers when dry icing is encountered?
Will not adhere to the aircraft’s surface but can accumulate in auxiliary cooling ducts and engine air intakes
What is cold soaking?
Lower fuel temperatures at take–off result in even lower fuel temperatures after a period of flight
Sub–zero fuel touching the wing upper surface keeps the wing temperature under 0°C
Which component cycles power to the intermittently heated areas and also monitors temperature sensor function?
Controller
Which deicing system uses inflatable rubber boots?
Pneumatic de–icing system
Why are the pneumatic boots held fully deflated by the vacuum supply?
Minimising aerodynamic drag
Describe what happens when the ejector flow control valve has its solenoid deenergised?
Main poppet is closed and air will not come out of the valve
Air is sent to the ejector which creates the necessary vacuum for the boots
Air is vented to atmosphere
Describe what happens when the ejector flow control valve has its solenoid energised?
Solenoid poppet releases the pressure in the chamber above the main poppet
Main poppet moves upward and closes the vent poppet
Pressure switch checks the de–icer boot pressure which is connected to the timer unit
What happens when ‘heavy’ is selected on the control panel for the pneumatic de–icer boots?
Period between inflation cycles changes from 3 to 4 minutes to 1/2 minute
What is the principle of operation of the fluid ice protection systems?
A fluid with a low freezing point may be used to keep an aerofoil free from ice
What type of fluid is used in the fluid ice protection systems?
Glycol based fluid
How many heating elements are in a windscreen and how many sensing elements?
Two sensing element for automatic control
One heating element
In a windscreen heating element, what happens when the sensing element reaches its predetermined temperature, eg 40°C?
The current through the sensing element balances a wheatstone bridge
Control unit and power control relays are de–energised, switching off the heating supply
What two things happen when the primary sensing element of the windscreen heating circuit fails?
Temperature is now controlled at a higher temperature, eg 55°C
Overheat light on the control panel is indicated
Why are the pilot’s and co–pilot’s windscreen wiper systems run by separate motors?
Ensures that clear vision is maintained through one of the windows should one system fail
State the probes and sensors that require to be heated?
Pitot probes
Total air temperature probes
Angle of attack sensors
Static vent plates
Engine EPR probe
How are the various probes and sensors which require heating powered?
115v AC
