Aircraft Brakes System PDF

MM3531 Aircraft Systems Hydraulic and Pneumatic Power Systems  Official (Closed), Non-Sensitive Scope Brake System Describe the types and construction of aircraft brake assembly. Describe the operations and component...

MM3531 Aircraft Systems Hydraulic and Pneumatic Power Systems  Official (Closed), Non-Sensitive Scope Brake System Describe the types and construction of aircraft brake assembly. Describe the operations and components of an independent brake, booster brake, power brake and emergency brake systems. Describe the anti-skid system components and explain its operating principles for the various functions when the aircraft is on ground or in the air. Describe the indication and warning system. Describe the auto brakes system. Page  2  Official (Closed), Non-Sensitive Hydraulic Power Operation of bicycle brakes Page  3  Official (Closed), Non-Sensitive Hydraulic Power Operation of bicycle brakes Page  4 Hydraulic Power Page  5  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System All modern aircraft are designed to operate at higher speed and higher payload hence brakes are equipped in each of the main wheel to slow and stop the aircraft promptly during landing and taxying. In the brake system, the pilots activates the brakes in the main wheels which are mechanically and/or hydraulically linked, by pushing on the top of the rudder pedal located in the flight deck. The operation of the brakes involves transformation of the kinetic energy of motion during landing and taxying to heat energy by friction. Page  6  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Types and Construction of Aircraft Brakes Disc brakes are generally used on modern aircraft and multiple-disc brake consists for use with a power brakes control valve or boost master cylinders. The brakes assembly consists of various component such as pressure cylinder, pistons, reline indicator, pressure plate, rotors and stators. Upon pushing the rudder pedal, hydraulic pressure will extend the piston causing the stationary plate (stator) and a rotating assembly (rotor) which rotates with the turning wheel hub assembly to be compressed. This creates friction and heat and slow the rotation of the wheel. Page  7  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Brakes Actuating Systems There are three basic actuating system: 1. An independent system which is not part of the main aircraft hydraulic system. 2. A booster system that uses the aircraft hydraulic system intermittently only when required. 3. A power brake system uses the main aircraft hydraulic system as it source of pressure. Page  8  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Brakes Actuating Systems The independent system consists of a reservoir, brake pedals, brake assemblies and master cylinders. When the pilot pushes the top of the rudder pedal which is mechanically connected to the master cylinder, the piston inside the cylinder will extend and pressurised the hydraulic fluid leading to the brakes assembly. Upon release of the pedal, the return spring will retract the piston back to its original position. Page  9  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Brakes Actuating Systems A boosted brakes actuating system is used on medium size aircraft that requires more braking force than can be applied by the independent master cylinder as the pressure delivered to the brakes is proportional to the foot pressure applied on the top of the rudder pedal. The boosted brake master cylinder increases the pressure applied on the brakes by the pilot with the main aircraft system pressure. When the brakes are applied, the piston rod which is mechanically connected to the pedal moves the piston rod (downwards) resulting in pressurised fluid supplied to the brakes. As the pilot step on the pedal harder, the spring-loaded toggle will move the spool valve (downwards) and the aircraft system pressure will flow through the spool valve to the rear end of the piston. This increase pressure supplied from the main system increases force to apply the brakes. Page  10  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Brakes Actuating Systems The power brakes actuating system is used on large sized aircraft with the source of pressure supplied from the aircraft main hydraulic system once brakes are applied. The brake assemblies demand higher pressure and higher volume because large aircraft operates at higher speed and higher payload hence a power brakes control valve (also known as brakes metering valve) is used instead of a master cylinder. The brakes power control valve receives the brake pedal input either directly or through linkages and the valve meters hydraulic fluid to Emergency Air Cylinder the corresponding brakes assembly in direct relation to the pressure applied to the brake pedal. Page  11  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Brakes Actuating Systems The power brakes actuating system receives the pressurized fluid from the main hydraulic system through a check valve. The function of this check valve is to prevent the loss of brake system pressure in the event of main hydraulic system failure. An accumulator is installed that stores fluid under pressure to operate the brakes for a few brakes cycles if hydraulic system is lost. The pilot would have to press on the brake pedal harder when the more pressure at the brakes assembly is required. Emergency Air Cylinder Page  12  Official (Closed), Non-Sensitive Hydraulic Power Accumulators The accumulator is a sphere or cylinder type which is divided into two chambers. One chamber contains fluid at system pressure, while the other chamber is charged with nitrogen or air. These accumulators may be a main system accumulator and an emergency system accumulator. The functions of an accumulator is to: 1. Dampen pressure surges in the hydraulic system caused by actuation of a unit and the effort of the pump to maintain pressure at a preset level. 2. Provide complement to the power pump when a few subsystem are operating at the same time 3. Store and provide power for the limited operation of a hydraulic unit when the pump is not operating. 4. Compensate for small internal or external leaks Page  13 For Training Purpose Only  Official (Closed), Non-Sensitive Hydraulic Power Accumulators There are generally two types of accumulators used in aircraft: 1) Spherical 2) Cylindrical Spherical Type A synthetic rubber diaphragm, or bladder, is installed in the sphere to create two chambers. Pressurized hydraulic fluid occupies the upper chamber and nitrogen or air charges the lower chamber. A screen at the fluid pressure port keeps the diaphragm, or bladder, from extruding through the port when the lower chamber is charged, and hydraulic fluid pressure is zero. Spherical type accumulators usually cost less than piston type and hold their gas pre-charge longer. It is more popular in the industrial hydraulic application. One disadvantage is that failure tends to be sudden owing to rupturing of the bag or diaphragm. Page  14 For Training Purpose Only  Official (Closed), Non-Sensitive Hydraulic Power Accumulators Cylindrical Type Cylindrical accumulators consist of a cylinder and piston assembly. The internal piston separates the fluid and air/nitrogen chambers. In one end cap, a hydraulic fitting is used to attach the fluid chamber to the hydraulic system. In the other end cap, a filler valve is installed to perform the same function as the filler valve installed in the spherical accumulator. Many modern aircraft hydraulic systems employed piston-type accumulators because they require less space than an equivalent spherical accumulator. A loud Hammering noise in systems having an accumulator usually indicates an insufficient per-load in the accumulator. Page  15 For Training Purpose Only  Official (Closed), Non-Sensitive Hydraulic Power Working principle of an Accumulator 3 2 1 V2 – V3 = working volume Page  16 For Training Purpose Only  Pre-charge 1000PSI Air Hydraulic Piston Type Accumulator Fluid Side Nitrogen Cart Operation of hydraulic pump 3000PSI 3000PSI Air Hydraulic Side Piston Type Accumulator Fluid 2000PSI Hydraulic System Failure Air Hydraulic Piston Type Accumulator Fluid Side For Training Purpose Only  Pre-charge 1000PSI Air Hydraulic Piston Type Accumulator Fluid Nitrogen Cart Side Operation of hydraulic pump 3000PSI 3000PSI Air Hydraulic Side Piston Type Accumulator Fluid Hydraulic System Failure 2000PSI Air Hydraul Piston Type Accumulator ic Fluid Side For Training Purpose Only  Official (Closed), Non-Sensitive Hydraulic Power Accumulators installed into the system When there is a hydraulic pressure from the upstream, the accumulator will be charged to system pressure. In the event of hydraulic system failure, the accumulator pressure will supply the brake temporary. Page  9 For Training Purpose Only  Official (Closed), Non-Sensitive Hydraulic Power Sizing of Accumulator Now, assuming isothermal process, P1V1 = P2V2 = P3V3 P1 P1 V2 = x V1 and V3 = x V1 P2 P3 working volume VW = V2 - V3 P1 P1 VW = [ − ] V1 P2 P3 VW The accumulator size, V1 = P1 P1 ( )−( ) P2 P3 VW Similarly, for isentropic process, V1 = 1 1 P1 γ P ( ) − ( 1 )γ Page  10 P2 P3 For Training Purpose Only  Official (Closed), Non-Sensitive Hydraulic Power Sizing of Accumulator: Sample Calculation Page  21 For Training Purpose Only  Official (Closed), Non-Sensitive Hydraulic Power Sizing of Accumulator: Sample Calculation Page  22 For Training Purpose Only  Official (Closed), Non-Sensitive Hydraulic Power Sizing of Accumulator: Sample Calculation (c) 210 bar 3 2 114.18 bar 1 70 bar V2 – V3 = working volume 6.6 litres (d) accumulator size, V1 = VW 6.6 1 1 = 1 1 P1 γ − ( P1 ) γ 70 1.4 70 1.4 ( ) ( ) −( ) P2 P3 114.18 210 = 26.51 litres Page  23 For Training Purpose Only  𝑃𝑃𝑃𝑃1=70 bar 𝑉𝑉𝑉𝑉1= 26.51 litres Pre-charge Air Hydraulic Piston Type Accumulator Fluid Nitrogen Cart Side 𝑉𝑉𝑉𝑉3=4 litres 210 bar 𝑃𝑃𝑃𝑃3=210 bar Air Hydraulic Operation of Side Fluid hydraulic pump 𝑤𝑤𝑤𝑤= 6.6 Litres 𝑉𝑉 Extension of 2 actuators To actuators Air 𝑉𝑉𝑉𝑉2=10.6 litres Side Hydraulic Fluid 𝑃𝑃𝑃𝑃2=114.18 bar For Training Purpose Only  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Emergency Brake System In a event of total hydraulic failure, the pilot can operate a pneumatic valve on the instrument panel at flight deck to direct the compressed nitrogen in air bottle to the brakes system. Emergency Air Cylinder Page  25  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Parking Brake The parking brakes are set (On) with the rudder pedals and a ratcheting system holds them in place when parking brake lever on the flight deck is pulled. The release of parking park is performed by depressing the pedals further inwards to the release the ratchet. Page  26  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Brake Deboosters There are some aircraft brake assemblies provides effective braking with a lower pressure than aircraft hydraulic system pressure. Therefore a brake debooster is installed downstream of the control valve and anti-skid valve. Brake debooster valve use the application of force over different sized piston to reduce pressure. Note: Pressure = Force/Area High-pressure hydraulic system input pressure acts on the small end of a piston. This develops a force proportional to the area of the piston head. The other end of the piston is larger and housed in a separate cylinder. The force from the smaller piston head is transferred to the larger area of the other end of the piston. The amount of pressure conveyed by the larger end of the piston is reduced due to the greater area over which the force is spread. The volume of output fluid increases since a larger piston and cylinder are used. The reduced pressure is delivered to the brake assembly. Page  27  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Anti-skid system The challenge for pilots during braking of an aircraft, is to establish from the flight deck when a wheel stops rotating and begins to skid and especially in aircraft with multiple-wheel main landing hear assemblies. A skid not corrected can result in a tyre blowout which can lead damage to aircraft and loss of control of the aircraft. The anti-skid system not only detects wheel skid, it also detects when wheel skid is imminent. It automatically relieves pressure to the brake pistons of the wheel in question by momentarily connecting the pressurized brake fluid area to the hydraulic system return line. This allows the wheel to rotate and avoid a skid. Lower pressure is then maintained to the brake at a level that slows the wheel without causing it to skid. Maximum braking efficiency exists when the wheels are decelerating at a maximum rate but are not skidding. If a wheel decelerates too fast, it is an indication that the brakes are about to lock and cause a skid. To ensure that this does not happen, each wheel is monitored for a deceleration rate faster than a pre-set rate. When excessive deceleration is detected, hydraulic pressure is reduced to the brake on that wheel. Page  28  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Anti-skid system To operate the anti-skid system, flight deck switches must be placed in the ON position. After the aircraft touches down, the pilot applies and holds full pressure to the rudder brake pedals. The anti-skid system then functions automatically until the speed of the aircraft has dropped to approximately 20 mph. The system returns to manual braking mode for slow taxy and ground manoeuvring. There are three basic components of an anti-skid, they are the wheel speed sensors, anti-skid control valve, and a control unit. Page  29  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Wheel Speed Sensor (Anti-Skid System) An alternating current (AC) tachometer generator is mounted in the axle and as the wheel rotates, the wheel hub spider will rotate with the wheel. The sensor will measure wheel rotational speed and senses any change in speed. This rotation of the wheel develops a output voltage which is directly proportional to the angular velocity of the wheel. The output AC voltage is converted to DC voltage before it enters the control box. Page  30  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System The Control Box (Anti-Skid System) The main functions of the control box are to generate electric signal to usable by the anti-skid control valve to control the following: 1. To prevent a brake pressure being applied prior to touchdown. 2. The brakes pressure to prevent a skid during landing deceleration. Page  31  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System The Control Box (Anti-Skid System) Anti-skid Control Valve 1. To prevent a brake pressure being applied prior to touchdown. When the aircraft is in the air prior to touch down, the squat switch is closed. This cause the locked-wheel detector to sends a signal into the amplifier, which the control valve will allow the flow of fluid from the brakes to the system return manifold. This prevents the application of brakes. Anti-skid control Box Page  32  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Touch down protection Aircraft in flight Aircraft landing gear extended Aircraft landing gears touched down landing gear retracted Wheels are not rotating. on the runway. Squat switch located at the landing The landing gear wheels are able to gear is in the close position ie rotate. aircraft is airborne. This prevents the application of brake when the pilot depress the brakes pedal. Brakes pressure gauge indicates zero. 2 3 1 0 Brakes Pressure Gauge Page  33  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Touch down protection (Malfunction) Aircraft in flight Aircraft landing gear extended Aircraft landing gears touched down landing gear retracted Wheels are not rotating. on the runway with brake pedal The pilot depress the brakes pedal. depressed upon landing on runway. Brakes pressure gauge indicates The landing gear wheels will not brakes pressure. rotate. Wheels are not rotating. Tyres will likely burst upon contacting the runway surface 2 3 Brakes Pressure 1 Gauge 0 Page  34  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System The Control Box (Anti-Skid System) Anti-skid Control Valve 2. The brakes pressure to prevent a skid during landing deceleration. The moment the plane touches down, the squat switch opens to the locked wheel arming circuit. When the wheel speed reached 20 mph, the wheel sensors generates voltage high enough to cause the locked-wheel detector to remove the touchdown control signal from the amplifier and the anti-skid Anti-skid control Box control valve to allow full pressure to go the brakes. Page  35  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System The brakes pressure to prevent a skid during landing deceleration. Aircraft landing gears touched down The landing gear wheels are able to on the runway. rotate and the wheel speed generated in Squat switch is the open position the wheels registers a speed of 20mph. The landing gear wheels are slowly The anti skid system is armed. rotate and gain speed. The anti-skid control valve will allow full pressure to go the brakes if brake pedal is applied. Page  36  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Anti-skid Control Valve The Control Box (Anti-Skid System) 2. The brakes pressure to prevent a skid during landing deceleration. When the aircraft is on ground and the wheels are turning more than 20 mph, the skid detector and modulator provides signal for the amplifier. The deceleration threshold of around 20 Anti-skid control Box ft/𝑠𝑠2 , with a wheel speed that is at least 6mph below the speed of the aircraft, is programmed into the skid detector circuit as a reference. Any time a wheel decelerates at a rate greater than this threshold value, a rate signal is sent to the amplifier and then to control valve to dump the brake pressure. Page  37  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System The brakes pressure to prevent a skid during landing deceleration. The landing gear wheels are able to The anti-skid detector circuit is The aircraft will slow down rotate and the wheel speed programmed with a deceleration gradually with every application of generated in the wheels registers a threshold of around 20 ft/𝑠𝑠2, with a brakes without the aircraft speed over 20mph. wheel speed that is at least 6mph skidding. below the speed of the aircraft as a reference. The pilot applied brakes and aircraft starts slowing down with the wheel decelerates, at a rate 2 3 2 3 greater than this threshold value, a 1 1 rate signal is sent to the amplifier 0 0 and then to control valve to dump the brake pressure. This release Brakes Pressure Gauge the brakes applied to prevent the aircraft from skidding. Page  38  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Anti-skid Control Valve The Control Box (Anti-Skid System) 2. The brakes pressure to prevent a skid during landing deceleration. When all the wheels are turning at less than 20 mph, the locked wheel arming circuit becomes inoperative, giving the pilot full control for low speed taxying and parking. Anti-skid control Box Page  39  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System The brakes pressure to prevent a skid during landing deceleration. The aircraft will slow down The aircraft will slow down and the gradually with every application of wheel speed is less than 20mph. brakes without the aircraft skidding. The locked wheel arming circuit becomes inoperative, giving the pilot full control for low speed taxying and parking. Page  40  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Anti-Skid System Return Reservoir Return Return Supply Electric Hydraulic Pressure Pilot Brakes Pressure Anti-skid Electrical signal Pump input via pedal Solenoid valve Electrical signal Electrical Electrical Pressure signal Anti-skid Wheel brakes signal Wheel Sensor control box Anti Skid System Page  41  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Anti-skid system performs four functions 1. Normal Skid Control It controls comes into play when wheel rotation slows down but has not come to stop. During the slowing down happen, the wheel sliding action has just begun but has not reached a full scale slide. In this situation, the skid control valve removes some of the hydraulic pressure to the wheel. This permits the wheel to rotate a little faster and stop its sliding Page  42  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Anti-skid system performs four functions 2. The locked wheel skid control It causes the brake to the fully released when its wheel locks. A locked wheel easily occurs on a patch of ice due to lack of tyre friction with the surface. It will occur if the normal skid control does not prevent the wheel from reaching a full skid. To relieve a locked wheel skid, the pressure is bleed of longer than normal skid function. This is to give the wheel time to regain speed. The locked wheel skid function. This is to give the wheel time to regain speed. The locked wheel skid control is inoperative during aircraft speed less than 20mph. Page  43  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Anti-skid system performs four functions 3. The touchdown protection The circuit prevents the brakes from being applied during the landing approach even if the brakes pedals are depressed. This prevents the wheels from being locked when they contact the runway. The wheels have chance to begin rotating before they carry the full weight of the aircraft. Two conditions must exist before the skid control valves permit brake application. (Without them the skid control box will not send the proper signal to the valve solenoids). The squat switch must signal that the weight of the aircraft is on the ground. The wheel generators sense a wheel speed of over 20mph. Page  44  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Anti-skid system performs four functions 4. The fail-safe protection The circuit monitors operation of skid control system. It is automatically returns to the brakes system to full manual in case of system failure. It is also turns on the warning light. Page  45  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Indication and Warning System (Anti-skid system) The operation of the anti-skid system is controlled by a two-position switch. A anti skid inoperative warning light illuminates when the system is switched off or if there is a system failure. A self system test is incorporated in the anti-skid control box to test the functionality of the components in the system. If any component is not operative, the warning light will also illuminate. When the anti skid inoperative light illuminates, the pilot can disable the anti-skid system without affecting the normal braking. Page  46  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Anti-Skid Control Valve (Anti-Skid System) The anti-skid control valve is hydraulically located in the pressure line between the brakes assemblies and brakes control valve, with a line connecting the control valve to the system to the return manifold. For normal operation of the brakes when no skid condition, the anti-skid control valve acts as a passage to allows brakes fluid to flow pass into and out of the brake assemblies from the brakes control valve. TO BRAKES ASSEMBLIES Page  47  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Anti-skid solenoid Anti-Skid Control Valve (Anti-Skid System) 1. If a skid develops, signal from the control BOX is sent to the anti-skid control valve solenoid. 2. This signal energised the coil on the armature of the flapper valve, causing the flapper to move to the left. 3. This cause the fluid to flow more to the right of the flapper, i.e more pressure drop. This cause the P1 is greater than P2. 4. The spool valve thus move to the right, TO BRAKES ASSEMBLIES causing the pressure line to be blocked, while the brake pressure line connects to the return line which permits the wheel to rotate a little faster and stops its sliding. Page  48  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Anti-Skid Control Valve (Anti-Skid System) When the flapper is centred between When the armature of the flapper valve is energised, the nozzles, the pressure drop the flapper moves over and restricts the flow through across the two orifice is the same the orifice 𝑶𝑶𝟏𝟏 while increasing through 𝑶𝑶𝟐𝟐. The and 𝑷𝑷𝟏𝟏is equals to 𝑷𝑷𝟐𝟐 increased pressure drop across 𝑶𝑶𝟐𝟐 causes 𝑷𝑷𝟏𝟏 to be greater than 𝑷𝑷𝟐𝟐 Page  49  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Auto Braking System Aircraft equipped with auto brakes typically bypass the brake control valves or brake metering valves and use a separate auto brake control valve to provide this function. In addition to the redundancy provided, auto brakes rely on the anti-skid system to adjust pressure to the brakes if required due to an impending skid. The auto brake system allows the pilot to select the deceleration rate after aircraft touch down which is controlled automatically. When the aircraft touches down with auto brake system armed and the thrust levers at idle, the system will direct the correct amount of pressure to the brakes assemblies to achieve the desired deceleration rate. The brakes pressure will decreased automatically to compensate for the deceleration caused by the thrust reversers and speed brakes. Page  50  Official (Closed), Non-Sensitive Hydraulic Power Aircraft Brakes System Auto Braking System The auto brake system will disengage if any of the following happens: 1. The pilot moves the selector switch to DISARM or OFF position. 2. The pilot uses manual braking. 3. The thrust lever are advanced. 4. The speed brake level is moved to the DOWN detent Page  51