AIRCRAFT ELECTRIC ANTI-ICING

ELECTRIC ANTI-ICING

The pitot heads (tubes) installed on almost all aircraft that may possibly encounter icing are electrically heated. These heaters are so powerful that they should not be operated on the ground because, without an adequate flow of air over them, there is a possibility that they will burn out. Their operation is monitored in flight by indicator lights or watching the ammeter. These heaters require enough current that the ammeter will deflect noticeably when the heater is on. A heated pitot tube, shown in Figure 5.5, prevents ice from plugging the entry hole by warming it with an electric heater built inside the pitot tube housing. Static ports and stall warning vanes on many aircraft are also electrically heated. The static port on some of the smaller aircraft are not heated, but if there is no provision for melting the ice off of this vital pressure pickup point, the aircraft should be equipped with an alternate source valve. This valve allows the pilot to reference the flight instruments to a static source inside the aircraft (nonpres­surized) if the outside static port should become covered with ice.

Large transport aircraft that have flush toilets and lavatories have electric powered heating ele­ments to prevent the drains and water lines from freezing.

Windshields and cockpit windows are electrical­ly heated to prevent ice obstructing the vision of the pilot and the co-pilot. There are two methods of heating these components. One method uses a conductive coating on the inside of the outer layer of glass in the laminated windshield, shown in Figure 5.6, and the other method uses tiny resis­tance wires embedded inside the laminated windshield. It is heated by electric current flowing through a conductive film on the inside of the outer layer of glass.

The windshield of a high-speed jet aircraft is a highly complex and costly component. For all of the transport category aircraft, these windshields must not only withstand the pressures caused by pressurization and normal abuse and flight loads, but they must also withstand, without penetration, the impact produced by a four-pound bird striking the windshield at a velocity equal to the airplane's design cruising speed. For a windshield to be this strong, it is built as a highly complex sandwich, with some of the business jet windshields about an inch and a half thick, made of three plies of tempered glass with layers of vinyl between them. The inner surface of the outer ply of glass is coated with a conductive material through which electric current flows to produce enough heat to melt off any ice that forms on the windshield. There are temperature sensors and an elaborate electronic control system to prevent these windshields from becoming overheated. The windshields are heated not only to prevent ice, but to strengthen them against bird strikes. When the windshield is heated, the vinyl layers are less brittle and will withstand an impact with much less chance of penetration than they will when they are cold.

The engine intakes of some turboprop aircraft are anti-iced by using electric heating elements which prevent ice build-up.

AIRCRAFT THERMAL ANTI-ICING

Thermal Anti-icing

Heated air can be directed through a specially designed heater duct in the leading edge of the wing, as shown in Figure 5.1, and the tail surfaces to heat this portion of the airfoil and prevent the formation of ice. This air can be heated in reciprocating engine aircraft by using combustion heaters or heater shrouds around the engine ex­haust system.

Most aircraft that use the thermal anti-icing systems today are turbine powered, and it is a very simple matter to use some of the engine's heated compressor bleed air to heat the leading edges and prevent the formation of ice.

The Boeing 727 takes bleed air from the two outboard engines and directs it through the wing anti-icing control valves to a common manifold and then out into the wing leading edge ducts. As can be seen in Figure 5.2, the two inboard leading edge flaps and eight leading edge slats are protected with this hot air. These portions of the wing are protected from overheating by overheat sensor switches. If they sense an overheat condi­tion, they turn on an overheat warning light and close the anti-icing valves, shutting off the flow of hot air into the ducts. When the duct temperature drops to an allowable range, the overheat light will go out, and hot air will again flow into the duct.

Figure 5.2: Typical wing thermal anti-icing system (Boeing 727)

Turbine engines are susceptible to ice damage if chunks of ice form on some of the exposed portions of the engine which can break off and be sucked into the engine's compressor. The engine intakes of most turbine engines are heated by compressor bleed air being circulated around the intake area of the engines preventing ice from forming, as shown in Figure 5.3. Most large turbine engines have hot compressor bleed air directed through the inlet guide vanes, the engine bullet nose, and through the oil cooler scoop for the constant speed drive, as well as for the inlet duct for the centre engine.

The Boeing 727, shown in Figure 5.4, has the centre engine's air intake at the rear top of the fuselage and, because of this, some of the anti ­icing hot air is ducted to the upper VHF radio antenna to prevent ice forming on it and breaking off to be ingested into the centre engine.

AIRCRAFT ICE CONTROL SYSTEMS

  ICE CONTROL SYSTEMS

There are two types of ice control systems used on aircraft: anti-icing and de-icing systems. Anti ­icing systems prevents the formation of ice, while the de-icing systems allow ice to accumulate, then it is removed. This discussion will concern itself with onboard or in flight de-icing and anti-icing. In flight anti- and de-icing method will be described now while ground de-icing and anti-icing will be covered later. Each de-icing or anti-icing system may use several different methods to remove the ice. Anti-icing may be done by heating the surface or the component with hot air, engine oil, or using electric heating elements

AIRCRAFT ICE AND RAIN PROTECTION(CONTD)

Present day all weather flying requires measures against ice/rain/mist that may affect safety and performance of flight. There are different methodologies used in these measures. The previous week introduced the methods mostly used. This week illustrates these in detail.

ICE REMOVAL PROCESSES

ICE REMOVAL PROCESSES:

 Ice may be removed by:

·         De-icing methods

·         Anti-icing methods.

  De-icing: In these methods, ice is first allowed to deposit up to a certain extent. Then system works to melt ice. System remains again ‘off’ for the ice to accrete until it de-ices again. This cyclic process continues as long as the system is made ON.

 Anti-icing: In these methods, system is ON in a known icing condition and system works not to allow any ice accretion.

  De-icing/Anti-icing Methods: There are quite a number of methods/processes employed for ice protection, such as:

Fluid method: Uses Ethylene Glycol, Isopropyl alcohol, Distilled water

Pneumatic method: Boots at the leading Edge of the wings, and stabilizers inflate alternately to break up the accumulated ice.

Thermal/Hot air method:

·         Ram air heat heated by engine exhaust,

·         Bleed air directly used by the system

 Electrical Method: Electrical heater coil installed at icing surfaces

Figure 4.5: Typical ice removal method.

Normally thermal/pneumatic/bleed air method is used in wing, tail plane and fin anti-icing while electrical method is used in windshield, windows, radomes and different probes. Liquid e.g. alcohol is used for ice removal and as rain repellant in the windshields and windows. As an example, Figure 4.5 illustrates a method of ice removal from wing main plane and engine leading edges by hot air method (anti-icing). In this system, hot air from aircraft pneumatic system is ducked span-wise along the inside of the leading edges of the wing and engines and distributed between double thickness skins. Further illustrations will be given in the next week i.e. week # 05 on de-icing and anti-icing methods.

AIRCRAFT ICE AND RAIN PROTECTION

NEED FOR ICE AND RAIN PROTECTION

The flight operation in present day is all-weather flying. To fly in all weather conditions, it is necessary:

To take protection against ICE build up that may affect safety and performance

To take protection against RAIN / MIST which may impair visibility?

Aircraft must be provided with:

·         Ice detection equipment

·         Ice protection equipment

 

 SOURCES OF ICE

Source of ice is the water in the atmosphere. Water may exist in the atmosphere in three forms:

• Invisible vapor
• Visible liquid particles or moisture
• Ice.

Invisible vapor and visible liquid particles in atmosphere may condense into ice on aircraft surfaces when they are at sub-zero temperatures. Ice already formed in the atmosphere may deposit on the aircraft surfaces.

 AREAS TO BE PROTECTED

Areas/Locations where ice is most likely to form and which are to be protected are:

• Wing leading edges
• Stabilizer leading edges
• Fin leading edges
• Wind shield
• Radome
• Stall warning probes/angle of attack sensors
• Pitot tubes
• Antennas
• Drain masts
• Engine air intake
• Propellers

 EFFECTS OF ICE FORMATION

Ice formation in the aircraft aerodynamic surfaces affects the performance and safety of the aircraft as a result of:

• Loss of lift due to change in wing section
• Increase in drag due to rough surface and friction over the wing upper camber
• Decrease in propeller efficiency due to change in blade profile
• Loss of control preventing control surface movement
• Increased load and wing loading Loss of inherent stability due to C.G changed because of weight of extensive deposition of ICE

 METHOD OF ICE DETECTION

  When an aircraft flies through icing weather condition, ice accumulates in ice detecting equipment. This equipment may be a probe/head for giving an electrical/electronic warning signal (light) in the cockpit or simply a visual indicator to be seen by a light from the cockpit when ice-accretion occurs on it.

 Pressure sensor method: These consist of a short stainless steel or chromium plated brass tube, which is closed at its outer end and mounted so that it projects vertically from a portion of the aircraft known to be susceptible to icing. Four small holes are drilled in the leading edge of this tube and in the trailing edge are two holes of less total area than those of the leading edge (see figure 4.1). A heater element is fitted to allow the detector head to be cleared of ice.

Figure 4.1: Pressure sensor method of ice detection.

Electro Mechanical Method: This consists of a serrated rotor, incorporating an integral drive shaft coupled to a small ac motor via a deduction gearbox, being rotated adjacent to a fixed knife-edge cutter (Figure 4.2). The motor causing is connected via a spring – tensioned toggle bar to a micro switch assembly.

The electrical motor continuously drives the serrated rotor on the detector head so that its periphery rotates within 0.050mm (0.0002 inch) of the leading edge of the knife-edge cutter. The torque therefore required to drive the rotor under non-icing conditions will be slight, since bearing friction only has to be overcome. Under icing conditions, however, ice will accrete on the rotor until the gap between the rotor and knife-edge is filled, where upon a cutting action by the knife edge will produce a substantial increase in the required torque causing the toggle bar to move against its spring mounting and so operate the micro-switch, to initiate a warning signal.

Hot Rod Method: This consists of an aluminum alloy oblong base (called the plinth): on which is mounted a steel tube detector mast of aerofoil section, angled back to approximately 30° from the vertical, mounted on the side of the fuselage, so that it can be seen from the flight compartment windows. The mast houses a heating element and in the plinth there is a built-in floodlight.

The heating element is normally off and when icing conditions are met ice accretes on the leading edge of the detector mat. The flight crew can then observe this. 

During night operations the built-in floodlight may be switched on to illuminate the mast. By manual selection of a switch to the heating element the formed ice is dispersed for further observance (see figure 4.3).

5 Ultra sonic vibrator method: This ice detector senses the pressure of Icing conditions and provides an indication in the flight compartment that such conditions exist. The system consists of a solid-state ice detector and advisory warning light. The ice detector is attached to the fuselage with its probe protruding through the skin Figure 4.4. The ice detector probe (exposed to the air-stream) is an ice-sensing element that ultrasonically vibrates in an axial mode of its own resonant

Restraining and securing loose freight

Restraining and securing loose freight: 

When a containerized system for loose equipment is not available, loads will have to be carried loose, but such loads must still be restrained from moving about the aircraft during flight. They may be carried in a) a cargo bay beneath the passenger floor or b) baggage area on the main deck. In any case, there must be restraining and securing system. Following is a description of a main deck restraining/securing system.

Main Deck Restraint: Main deck restraining system is basically a net wall that can be removed to enable loading and unloading. The wall is made up of nylon webbing formed into a net and lashed to posts which are attached to the floor and ceiling sockets. The lashing posts are generally hollow aluminium tubes. At the top of the post, there is a spring-loaded plunger which can be retracted to remove the post from the ceiling socket. The base of the post is then simply lifted out. A locking pin on the spring-loaded plunger must be released to enable the plunger to be withdrawn. The nets are attached to shackles, which are fixed to strong points on the aircraft fuselage.

Cargo retention equipment

Cargo retention equipment

Containers: Baggage/cargo retention system uses box shaped containers of which are designed to suit to the contour of the aircraft fuselage to maximize the capacity of the freight bays. Containers can be made of various materials, such as:

• Alloy honeycomb
• Fibre glass
• Fibre board

Containers come in various sizes and shapes to enable the maximum usable area. Containers can also be specialized for use in the carrying of specific loads. See Figure 3.8.

Most containers are recognized by code. Codes are given according to size. An international agreement was reached to enable various aircraft manufacturers to make the freight bay sizes compatible with various container sizes at present in use. Obviously the width of the cargo bay will determine the sizes of containers that can be loaded, but by using various guides on the cargo floor more than one size of container can be carried. Shape and size of containers are to suit the contour of the lower deck and upper deck.

See Figure 3.9 illustrating containers designed for lower deck freight bay. Passenger carrying aircraft uses freight bay in the lower deck. The identifying letters for the lower deck containers are LD. Thus LD2, 3, 4, 5, 6 and 8 are all lower deck containers but vary in size.

Upper deck (area above the floor) is used as freight bay in the freight carriers. Containers for these aircrafts are specially designed to maximize the space available. These containers have the contoured ends on the upper surface, although in some cases rectangular containers are used. 

Pallets: Pallets are often used where many small loose items such as suitcases or packages are available for loading. Pallets are flat panels with seat track around the circumference. To restrain the load, ropes, nets or shackles are secured over the load with the help of the seat track.

There are two basic pallet constructions: a) Solid pallet b) Core pallet

(i) Solid Pallet: The solid pallet is constructed of a solid sheet of aluminium alloy.  A frame, which contains seat tracks, is mounted all round the edge of the sheet.

(ii) Core Pallet: Core pallets are sandwich panels made from aluminium alloy sheet laminated to a core of honeycomb material. The core can also be made of balsa wood or rigid vinyl foam. The aluminium skin is much thinner than that used in the solid pallet. See Figure 3.10

PASSENGER COMPARTMENT ESCAPE FACILITIES

PASSENGER COMPARTMENT ESCAPE FACILITIES

The passenger compartment is provided passenger/crew doors and emergency exit doors.

Wide body aircraft has forward, mid and aft passenger/crew doors on both sides and two emergency over wing doors for emergency exit only. Normal doors are also used as emergency exit. Each door is fitted with an automatically deployed and inflated escape slide to facilitate the emergency evacuation of all passengers and crew in the shortest possible time. Modern aircraft has all slides of the inflatable type, using cylinder-stored compressed gas.

The escape slides are mounted on the door structure, armed/disarmed by the door emergency escape slide release mechanism, and activated by the door opening process. In a typical modern aircraft, the FWD doors, MID and AFT passenger/crew doors are each fitted with a dual-lane escape slide or an optional slide/raft allowing for two abreast evacuation and the emergency exit doors are each fitted with a single lane escape slide. The door-mounted girt bar retaining mechanism controls actuation and deployment of the escape slides. All doors are equipped with an emergency escape slide warning system. When the emergency control handle is placed in the ARMED position and the interior control handle is lifted to open a door, an electrical warning system on the door is activated providing audio and visual warning signals. See Figure 3.7.

The emergency doors have their own emergency slide release mechanism. The escape slide release mechanism functions to activate emergency opening of the door and initiate the escape slide deployment process. The mechanism is manually operated by the two-position Emergency Control Handle (ECH). The ECH is connected to the door locking mechanism so that the handle can only be repositioned when the door is closed and locked. A safety pin secures the ECH in the DISARMED position only. The ECH is connected by rods to the stop lever and the girt bar control mechanism.

CABIN SAFETY EQUIPMENTS

CABIN SAFETY EQUIPMENTS

There is various cabin safety equipment located throughout the aircraft compartment. This safety equipment includes:

A. Torches and flashlights: The hand held torches and flashlights are installed as a movable light source. They are used if bad light conditions occur during an on-board emergency. The flashlights are battery operated and you can easily replace the batteries.

B. Crash/Emergency Axe, Crow bar: The crash/emergency axe is used to cut through light structures, panels and windows to get access or exit in an emergency. The insulated handle is resistant to high voltages.

In an emergency the crowbar can be used to open doors that do not move freely, or remove panels for access or exit.

C. Emergency Location Transmitter/ Beacon: The emergency location transmitter /beacon has:

(i)                 a water-activated battery,

(ii)               a self erecting antenna,

(iii)             a lenyard,

(iv)             a special water bag,

(v)               an anti-freeze bat,

(vi)             a wet indicator.

D. Protective Gloves: The protective gloves are made of materials that are resistant to heat. They are supplied for use in fire emergencies and to handle overheated equipment.

E. Safety Instruction Card: A safety instruction card is put in the rear pocket of each passenger seat. The card shows the passenger, with the aid of illustrations:

(i)                 take-off and landing procedures,

(ii)               emergency exit and escape slide locations,

(iii)             emergency landing procedures,

(iv)             use of oxygen mask,

(v)               floor level escape route markings,

(vi)             use of life vest.

F. Escape slides and ropes/life lines, rafts:

Escape slide: Escape slide offers a means of escape from an aircraft usually under emergency conditions, when the normal means of evacuation, such as stairs, are not available.

Some escape slides are inflatable and some are non-inflatable. Both are stowed in their appropriate stowage compartments but operational aspects are different.

Non-inflatable escape slide: A webbing harness behind the centre roof panels in the forward vestibule retains the escape slide packs. Each pack consists of a slide assembly, a telescopic anchorage bar, handles and restraining tapes.

The slide assembly is fabricated from a continuous filament nylon material, made in to two single thickness side panels and double-thickness centre panel. The centre panel incorporates a foam rubber pad at one end. See Figure. 3.5    

Inflatable escape slide: In many types of aircraft, escape chutes or slides are of the inflatable type as shown in Figure 3.7 The slide is packed in a valise and comprises two parallel anti-static neoprene proofed fabric tubes linked and spaced apart by transverse tubes, The slide path is an anti-static neoprene proofed fabric sheet which links the two parallel tubes and merges into the head and foot transverse tubes. A transverse tube at the foot of the slide provides cushioning when the slide is at a high angle and assists persons evacuating to regain their feet when the angle is moderate. The upper transverse tube supplements the sill of the aircraft doorway assists with boarding and ensures that the slide when inflated is at right angles to the fore and aft line in the aircraft.

Compressed air for inflating the slide is stored in a cylinder on the underside of the slide. The cylinder is fitted with a valve and operating head connected by a flexible hose to an inflator through which the compressed air is admitted to the slide. An attachment bar at the end of the slide secures it to the aircraft. In the unlikely event of the slide failing to inflate it can still be used un-inflated. For this purpose suitable handholds are provided open each side of the slide at its lower end any by means of which ground personnel can stabilize the lower end during operation. The slide packed in its valise is normally stowed adjacent to the aircraft doorway. The arrangement is one whereby an attachment bar is secured at its lower end to a pivot attachment on the cabin floor.

The upper end of the attachment bar is secured to a quick-release fitting on the cabin wall. To operate the slide the upper end of the attachment bar is released from its attachment and the entire valise pivoted to the left until it lies on the floor and across the aircraft doorway. The free end of the attachment bar is then engaged with another fitting on the cabin floor. Removing a strip, which secures the flaps of the valise, opens the valise. As the operator pulls the operating cable and pushes the valise out of the doorway the slide rapidly inflates.  It is necessary is some other types for life rafts to be carried on board. A typical life raft shown in the ready for use condition is illustrated in Figure 3.6. It consists of two buoyancy chambers with an inflatable boarding ramp, an automatically erected canopy and a manually inflated floor. It is fitted with stabilizing water pockets, and other associated equipment such as lifelines, position light, paddles etc. Rafts can be stowed in a pannier located in the aircraft structure (normally a wing) or in a fabric valise stowed in the fuselage. Inflation is by CO₂ gas and is carried out automatically when the raft is released. Dinghies are small boat like rafts that may be installed in the cabin as survival equipment.

Escape Rope: Escape ropes are used by the cabin crew for evacuation during emergencies. It is made of Nylon. It has a handle to grip and through out its length there are Nylon moulding knot for easy descending of the cabin crews. It is installed above the side windows of the cockpit and beside the slide.

Rafts: Some slides /chutes are configured to use as rafts after deployment. They are slide rafts. A slide rafts is stowed in its valise adjacent to the passenger, over-wing and emergency doors/exits. After deployment, it is disconnected from the girt bar to float on seawater to be used as a raft.

It consists of two buoyancy chambers with an inflatable boarding ramp, an automatically erected canopy and a manually inflated floor. It is fitted with stabilizing water pockets, and other associated equipment such as lifelines, position light, paddles etc.

Rafts can be stowed in pannier located in the aircraft structure (normally a wing) or in fabric valves stowed in the fuselage. Inflammation is by CO₂ gas and is carried out automatically when the raft is released. Figure 3.6 illustrates a raft.

G. Portable oxygen: A typical portable oxygen set consists of an alloy steel light weight oxygen cylinder fitted with a combined flow control/reducing valve and a pressure gauge, a breathing mask with flexible connecting tube and a carrying strap. The charged cylinder pressure is usually 1800 ib/in². The capacity of the sets varies but a size commonly used Contains 120 litres. Depending on the type of set, it is normally possible to select at least two rates of flow. Normal and High.  With some sets three flow selections are possible i.e. 'Normal' 'High and Emergency which would correspond to flow rates of 2.4 and 10 litres/ minute with endurance under these flow rates of 60, 30 and 12 minutes respectively for a cylinder of 120 litre capacity. Portable oxygen bottles are located in stowage compartment of each passenger compartment as well as cockpit compartment.