Aircraft Wheels, Tyres and Brakes
The wheels and tyres of an aircraft support it when on the ground and provide it with a means of mobility for take-off, landing and taxiing.
The pneumatic tyres cushion the aircraft from shocks due to irregularities both in the ground surface and occasionally, lack of landing technique.
The main wheels, and in some cases nose wheels, house brake units which control the movement of the aircraft and provide a means of deceleration on landing.
Aircraft wheels are so designed as to facilitate tyre replacement. Wheels are classified as follows:
Loose and detachable flange
Loose and Detachable Flange Wheel
Wheels of this type, are made with one flange integral with the wheel body, and the other loose and machined to fit over the wheel rim.
The difference between the loose flange type and the detachable flange type is the method by which the removable flange is secured, the loose flange is retained by a locking device on the wheel rim, and the detachable flange is secured to the wheel body by nuts and bolts.
A detachable flange may be a single piece, or two or three pieces bolted together.
LOOSE FLANGE WHEEL
The Divided Wheel (Split Hub)
The divided wheel consists of two half wheels, matched up and connected by bolts which pass through the two halves, the bolts are fitted with stiff nuts, or, if one half of the wheel is tapped, each bolt is locked with a locking plate.
The wheel has two halves that are clamped together by bolts, nyloc nuts and washers.
This wheel is designed to be used with a tubeless tyre. A seal, incorporated at the joint, prevents abrasion between the two halves and provides an airtight joint.
When used with a conventional tyre, the wheel inflation valve is removed to enable the tube inflation valve to be fitted through the rim.
Prevention of Creep
When in service, the tyre has a tendency to rotate, creep (slippage) around the wheel . This creep, if excessive, will tear out the inflation valve and cause the tyre to burst.
Creep is less likely to occur if the tyre air pressure is correctly maintained, but additional precautions may be incorporated in the design of the wheel.
Methods of counteracting/monitoring creep are as follows:
Knurled Flange: The inner face of the wheel flange is milled so that the side pressure of the tyre locks the beads to the flange.
Tapered Bead Seat: The wheel is tapered so that the flange area is of greater diameter than at the centre of the rim. When the tyre is inflated, the side pressure forces the bead outwards to grip the rim.
Creep Marks: Creep can be detected by misalignment of two matched white lines one painted on the wheel and one on the tyre.
Aircraft wheels are either cast or forged, then machined and ground to the required finish. They are made of:
Magnesium alloy - Electron.
After initial machining has been carried out, an anti-corrosive treatment is applied:
Anodizing for aluminium alloy wheels.
Chromate treatment for magnesium alloy wheels.
A final finish using cellulose or epoxy resin paint is applied to each wheel.
Wheels for Tubeless Tyres
Wheels for tubeless tyres are similar in construction to non-tubeless but have a finer finish and are impregnated with Bakelite to seal the material. ‘O’ ring seals are used between the parts of the wheel to prevent leakage.
Unlike tubed wheels, the valve is built into the wheel itself and is thus not affected by creep though creep may still damage the tyre.
Under extra hard braking conditions the heat generated in the wheel, tyre and brake assembly could be sufficient to cause a tyre blowout, with possible catastrophic effect to the aircraft.
To prevent a sudden blowout fusible plugs are fitted in some tubeless wheels. These plugs are held in position in the wheel hub by means of a fusible alloy, which melts under excessive heat conditions and allows the plug to be blown out by the tyre air pressure.
This prevents excessive pressure build-up in the tyre by allowing controlled deflation of the tyre. They are made for 3 different temperatures, being colour coded for ease of identification:
Red - 155°C
Green - 177°C
Amber - 199°C
Aircraft wheels are fitted with pneumatic tyres which may be tubeless or have an inner tube. Tubes tend to be fitted to light and older aircraft.
Tyres are usually inflated with nitrogen which absorbs shock and supports the weight of the aircraft, while the cover restrains and protects the tube from damage, maintains the shape of the tyre, transmits braking and provides a wearing surface.
The tyre cover consists of a casing made of rubber which is reinforced with plies of cotton, rayon or nylon cords. The cords are not woven, but arranged parallel in single layers and held together by a thin film of rubber which prevents cords of adjacent plies from cutting one another as the tyre flexes in use.
During the construction of the cover, the plies are fitted in pairs and set so that the cords of adjacent plies are at 90 degrees to one another in the case of bias (cross-ply) tyres and from bead to bead at approximately 90 degrees to the centre line of the tyre in radial tyres.
To absorb and distribute load shocks, and protect the casing from concussion damage, two narrow plies embedded in thick layers of rubber are situated between the casing and the tread, these special plies are termed breaker strips.
The casing is retained on the rim of the wheel by interlocking the plies around inextensible steel wire coils to form ply overlaps, this portion of the cover is known as the bead.
The tyre manufacturers give each tyre a ply rating. This rating does not relate directly to the number of plies in the tyre, but is the index of the strength of the tyre.
For example, a 49 × 17 size tyre with a ply rating of 32 only has 18 plies.
The wire coils are made rigid by bonding all the wires together with rubber, to ensure a strong bond, each wire is copper plated. The bead coil is also reinforced by winding with strips of fabric before the apex and filler strips are applied. The apex strips, which are made of rubber and located by rubberized fabric filler strips, provide greater rigidity and less acute changes of section at the bead. They also provide a greater bonding area.
Finally, the bead portion is protected on the outside by chafer strips of rubberized fabric.
CONSTRUCTION OF A TYRE
The Regions of the Tyre
The tread of the tyre is situated in the crown and shoulder section, and it should be noted that the term ‘tread’ is applied irrespective of whether the rubber is plain and smooth, or moulded on a block pattern.
The most popular tread pattern is that termed Ribbed, which has circumferential grooves around the tyre to assist in water dispersion and to help prevent aquaplaning (hydroplaning). The grooves also help to improve traction and contact grip between the tread and the runway surface.
Not seen so frequently now, but still termed the all weather pattern, is the Diamond tread pattern.
Nose wheel tyres, particularly those fitted to aircraft with the engines mounted on the rear fuselage, may have a chine moulded onto the shoulder. This is to direct water away from the engine intakes and so prevent flameouts due to water ingestion.
A nose wheel tyre fitted to a single wheel installation will have a chine moulded onto both sides of the tyre.
REGION OF TYRES
An inner tube is manufactured by an extruding machine, which forces a compound of hot rubber through a circular die, thus producing a continuous length of tubing. The requisite length is cut off, the ends are then butt welded and a valve is fitted.
The tube is placed in a mould, inflated and vulcanized, so producing the finished tube to the required dimensions.
During braking, excessive heat is generated in some types of brake unit, which could cause damage to a standard base tube. Depending on the design of the wheel and the type of brake unit, the tube may have a standard, thickened, or cord reinforced base.
When renewing a tube it must be replaced by one of the same type.
The Inflation Valve
The tube is inflated through an inflation valve, in which the stem is attached to the rubber base by direct vulcanization, and the rubber is vulcanized to the tube, renewal of the inflation valve is not permitted.
Each inflation valve is fitted with a Schrader valve core which operates as a non-return valve. The valve core is not considered to be a perfect seal, therefore, the inflation valve must always be fitted with a valve cap, the valve cap also prevents dirt entering the valve. The older type of valve core has a spring made of brass, but the modern type is fitted with a stainless steel spring.
These tyres are similar in construction to that of a conventional cover for use with a tube, but an extra rubber lining is vulcanized to the inner surface and the underside of the beads. This lining, which retains the gas pressure, forms an gas tight seal on the wheel rim.
The gas seal depends on a wedge fit between the underside of the tyre bead and the taper of the wheel rim on which the beads are mounted. The inflation valve is of the usual type, but is fitted with a rubber gasket and situated in the wheel rim. The advantage of tubeless tyres over conventional tyres include the following:
The gas pressure in the tyre is maintained over longer periods because the lining is unstretched.
Penetration by a nail or similar sharp object will not cause rapid loss of pressure because the unstretched lining clings to the objects and prevents loss of nitrogen.
The tyre is more resistant to impact blows and rough treatment because of the increased thickness of the casing, and the lining distributes the stresses and prevents them from causing local damage.
Lack of an inner tube means an overall saving of approximately 7.5% in weight.
Inflation valve damage by creep (slippage) is eliminated.
The difference in landing speeds, loading, landing surfaces and landing gear construction of aircraft make it necessary to provide a wide range of tyre sizes, types of tyre construction and inflation pressures.
There are four main categories of tyre pressures, which are as follows:
Low Pressure: Designed to operate at a pressure of 25 - 35 psi (1.73 - 2.42 bar), used on grass surfaces.
Medium Pressure: Operates at a pressure of 35 - 70 psi, (2.42 - 4.83 bar) and is used on grass surfaces or on medium firm surfaces without a consolidated base.
High Pressure: Operates at a pressure of 70 - 90 psi, (4.83 - 6.21 bar) and is suitable for concrete runways.
Extra High Pressure: Operates at pressures of over 90 psi (some tyres of this type are inflated to 350 psi)(6.21 - 24.2 bar), the tyre is suitable for concrete runways.
The letters ECTA or Conducting are used to indicate a tyre that has extra carbon added to the rubber compound to make it electrically conducting to provide earthing (grounding) between the aircraft and ground.
The size of a tyre is marked on its sidewall and includes the following information:
The outside diameter in inches or millimetres.
The nominal width in inches or millimetres.
The inside diameter in inches.
The ply rating, the index of the tyre’s strength, is also marked on the sidewall. Normally it is shown as an abbreviation, e.g. 16PR, but occasionally it is shown in full as “16 PLY RATING”.
The speed rating of the tyre denotes the maximum rated ground speed in mph to which the tyre has been tested and approved. This is embossed on the sidewall of the tyre. The rating takes account of pressure altitude, ambient temperature and wind component, enabling the maximum take-off mass, MTOM, the tyres can sustain to be calculated.
Green or grey dots painted on the sidewall of the tyre indicate the position of the “awl” vents. Awl vents prevent pressure being trapped between the plies which would cause disruption of the tyre carcase if it was exposed to the low pressures experienced during high altitude flight.
A red dot or triangle indicates the lightest part of the tyre. If this is placed opposite the valve during tyre fitting then it assists in balancing the wheel assembly.
The letters DRR printed in the code panel and the words “REINFORCED TREAD” printed on the sidewall are indicative of the fact that the tyre has a layer of fabric woven into the tread which may become visible during normal wear. This layer must not be confused with the casing cords.
Tyres must be protected from excessive heat, dampness, bright sunlight, contact with oil, fuel, glycol and hydraulic fluid, all of these have a harmful effect on rubber. Oilskin covers should be placed over the tyres when the aircraft is to be parked for any length of time or during the periods when oil, fuel, cooling or hydraulic systems are being drained or replenished. Any fluid inadvertently spilt or allowed to drip on to a tyre must be wiped off immediately.
When tyres are first fitted to a wheel they tend to move slightly around the rim. This phenomenon is called ‘creep’ and at this stage it is considered normal. After the tyres settle down this movement should cease.
In service, the tyre may tend to continue to creep around the wheel. If this creep is excessive on a tyre fitted with an inner tube, it will tear out the inflation valve and cause the tyre to burst. Creep is less of a problem with tubeless tyres, as long as the tyre bead is undamaged and any pressure drop is within limits.
Creep is less likely to occur if the tyre air pressure is correctly maintained. To assist in this, tyre manufacturers specify a RATED INFLATION PRESSURE for each tyre. This figure applies to a cold tyre not under load, that is, a tyre not fitted to an aircraft. Distortion of the tyre cover when the weight of the aircraft is on it will cause the tyre pressure to rise by 4%. When checking the tyre pressure of a cold tyre fitted to an aircraft you should mentally add 4% to the rated tyre pressure.
During use, that is during taxiing, take-off or landing, the tyres will become heated. This can cause up to a further 10% rise in tyre pressure.
Correct Tyre Pressures
Tyres in use must be kept inflated to the correct pressures using nitrogen or other inert gas (with a maximum 5% oxygen content) as under-inflated tyres may move (creep) round the wheel, over-inflated tyres will cause other types of failure. It is estimated that 90% of all tyre failures can be attributed to incorrect gas pressure. Modern aircraft can even display tyre pressures on the electronic systems monitoring screen.
Aquaplaning is a phenomenon caused by a wedge of water building up under the tread of the tyre and breaking its contact with the ground.
Aquaplaning speed, in Nautical Miles per Hour, the speed that the tyre loses contact can be found by applying the formula:
AQUAPLANING SPEED = 9 √P (where P = the tyre pressure in psi)
AQUAPLANING SPEED = 34 √P (where P = the tyre pressure in kg/cm2, bar)
The possibility of aquaplaning increases as the depth of the tread is reduced, it is therefore important that the amount of tread remaining is accurately assessed. The coefficient of dynamic friction will reduce to very low values, typically 0, when aquaplaning.
When calculating take-off distance/obstacle clearance with increased V2 speeds it is important not to exceed the speed rating of the tyres fitted to the aircraft e.g. it may be necessary to reduce mass in order to satisfy mass, altitude and temperature (MAT) limits.
During servicing, tyre covers must be examined for cuts, bulges, embedded stones, metal or glass, signs of wear, creep, local sponginess, etc. The defects, which may make the cover unserviceable, should receive the following attention or treatment:
Cuts: Cuts in the tyre cover penetrating to the cords render the tyre unserviceable and must be repaired.
Bulges: These may indicate partial failure of the casing, if the casing has failed, i.e. the fabric is fractured, renew the cover.
Foreign Bodies: Embedded stones, metal, glass etc. These must NOT be removed but reported to maintenance staff, and the cuts probed with a blunt tool to ascertain their depth, repair or renewal of the cover is governed by the extent of the damage.
Wear: Pattern tread covers worn to the base of the marker grooves or marker tie bars for 25% of the tyre circumference, or plain tread covers worn to the casing fabric, must not be used.
Creep: Movement of the tyre round the wheel must not exceed 1 in for tyres of up to 24 in outside diameter and 11⁄2 in for tyres over 24 in outside diameter. If these limits are exceeded, the tyre must be removed from the wheel and the tube examined for signs of tearing at the valve, also examine the valve stem for deformation. If the tube is serviceable, the tyre may be refitted and creep marks re-applied.
Tread Separation and Tyre Burst
It is possible for a tyre to burst or the treaded portion to become detached from the tyre. This would result in a smaller footprint and an increased loading on the remaining tyres. There would also be a reduction in braking efficiency.
There is a risk of foreign object damage (FOD), with the possibility of damage to brake hydraulic lines and the ingestion of debris into the engine
Reduction of Tyre Wear
With the increased size of modern airports, taxi distances also increase, thus increasing the amount of tyre wear and risk of damage. To minimize tyre wear therefore, it is recommended that a speed of no more than 25 mph (40 kph) should be reached during taxi.
Over-inflation will cause excessive wear to the crown of the tyres whilst under-inflation is the cause of excessive shoulder wear.
In common with most braking systems, aircraft wheel brakes function by using friction between a fixed surface and a moving one to bring an aircraft to rest, converting kinetic energy into heat energy. The amount of heat generated in stopping a large modern aircraft, is enormous, the problem of dissipating this heat has been a challenge to aircraft designers and scientists for years. As progress has been made in this direction, so aircraft have got faster and heavier and the problem worse.
The ideal answer of course, would be to build runways of sufficient length, so that an aircraft would have no need to use its brakes at all, but the prohibitive cost of building runways 4 and 5 miles long makes it a non-starter.
The advent of reverse pitch on propeller driven aircraft and reverse thrust on jet engined aircraft, has provided a partial answer to the problem, but even with these, the need for normal braking still exists.
Plate or Disc Brakes
All modern aircraft now use plate brakes operated by hydraulic systems as their means of slowing down or stopping. This system uses a series of fixed friction pads, bearing on or gripping, one or more rotating plates, similar in principle to disc brakes on a car.
The number of friction pads and rotating plates that are used is a matter of design and wheel size, a light aircraft would be able to utilize a single plate disc brake whereas a typical arrangement on a large aircraft would be a multi-plate unit.
In this unit, the physical size of the braking area has been increased by employing multiple brake plates sandwiched between layers or friction material. In this sort of construction the rotating plates (rotors) are keyed to revolve with the outer rim of the wheel and the stationary plates carrying the friction material (stators) are keyed to remain stationary with the hub of the wheel. When the brake is applied hydraulic pressure pushes the actuating pistons, housed in the torque plate, squeezing the rotors and stators between the pressure plate and the thrust plate. The harder the brake pedal is applied the greater the braking force applied to the pressure plate by the pistons. The torque generated by the brake unit is transmitted to the main landing gear leg by a torque rod or ‘brake bar’.
The friction pads are made of an inorganic friction material and the plates of ‘heavy’ steel with a specially case hardened surface. It is this surface which causes the plates to explode if covered with liquid fire extinguishant when they are red hot. In the unfortunate event of a wheel or brake fire, the best extinguishant to use is dry powder.
Recent technological advancements in heat dissipation, have resulted in the design of the brake plates being changed from a continuous rotating single plate, to a plate constructed of many interconnected individual segments with the heat dissipation properties greatly improved, thus increasing brake efficiency.
Carbon is also used for manufacturing brake units because it has much better heat absorbing and dissipating properties. Carbon brakes are also much lighter than equivalent steel units. The disadvantage is their increased cost and shorter life, so they tend to be fitted only to aircraft where the weight saving is worth the extra cost, long haul aircraft, for example.
If the brakes become too hot, they will not be able to absorb any further energy and their ability to retard (slow down) the aircraft diminishes. This phenomenon is termed Brake Fade.
MULTI PLATE BRAKE UNIT
When the pilot releases the pressure on the brake pedals, the brake adjuster assemblies will move the pressure plate away from the stators and rotor assemblies, thus allowing them to move slightly apart. The internal construction of the brake adjuster assemblies allows them to maintain a constant running clearance when the brake is off thereby automatically compensating for brake wear.
If the return spring inside the adjuster assembly ceases to function, or if the unit is wrongly adjusted, then they could be the cause of a brake not releasing correctly. This is termed brake drag.
Brake drag will generate a lot of heat and can be responsible for Brake Fade occurring sooner than it otherwise would.
BRAKE ADJUSTER ASSEMBLY
Aircraft brakes are designed to give good retardation, while at the same time avoiding excessive wear of the brake lining material.
It is important that the thickness of the brake lining material is carefully monitored.
Too little brake lining material remaining may mean that the disc of a single disc brake system may become excessively worn or grooved, or that on a multiple disc brake, the remaining material overheats and erodes extremely fast.
There are several methods of determining the amount of brake lining material which remains on the brake unit, the following are just some of those methods.
On multiple disc brake systems, the most popular method of gauging the depth of brake lining material remaining is by checking the amount that the retraction pin (or the indicator pin, as it is sometimes called) extends from (or intrudes within) the spring housing with the brakes selected on.
Figure shows how a wear gauge can be used to check that the retraction pin has not moved too far within the spring housing.
An alternative method which can be used if no retraction pins are fitted to the system is that whereby the amount of clearance between the back of the pressure plate and the brake housing can be measured, once again with the brakes applied.
If the brake is a single disc unit, the amount of brake lining material remaining can be checked by once again applying the brakes and measuring the distance between the disc and the brake housing and ensuring that it is no less than a minimum value.
MEASURING BRAKE WEAR
Brake System Operation
Operation of the brake pedals on the flight deck, allows hydraulic fluid under pressure to move small pistons which, by moving the pressure plate, force the stator pads against the rotor plates, with the resultant friction slowing the plates down.
On a small aircraft the hydraulic pressure from the brake pedals may be enough to arrest its progress. On a large aircraft it is obvious that foot power alone will be insufficient, some other source of hydraulic power is required. This is supplied by the aircraft main hydraulic system.
Brake Modulating Systems
Optimum braking is important in the operation of modern aircraft with their high landing speeds, low drag and high weight, particularly when coupled with operation from short runways in bad weather. The pilot is unable to sense when the wheels lock and so the first requirement of a brake modulating system is to provide anti-skid protection.
Whenever braking torque is developed there must be only a degree of slip between the wheel and the ground, a skidding wheel provides very little braking effect. In all brake modulating systems the deceleration of the individual wheels is taken as the controlling parameter of braking torque.
A datum figure for wheel deceleration is selected which is known to be greater than the maximum possible deceleration of the aircraft - of the order of 18 ft/s2 (6 m/s2) - and when this datum figure is exceeded, brake pressure is automatically reduced or released.
The facility to “hold off” brake pressure in the event of a wheel bounce or to prevent brake operation before touchdown may also be built into the system.
Systems may be mechanical or electrical, mechanical systems have been in use since the early 1950s. Most aircraft use electrical or electronic systems.
Effects of Anti-skid Systems on Performance
An anti-skid system will reduce the braking distance on both take-off and landing. An inoperative anti-skid system will increase the take-off and landing distances required. Data will be available to determine the runway length required in the event of a rejected take-off.
Note: Take-off is prohibited with an inoperative anti-skid system on a wet runway
Mechanical Anti-skid Systems
The basic principle of these systems is the use of the inertia of a flywheel as a sensor of wheel deceleration.
A wheel directly driven by the aircraft wheel is coupled to the flywheel by a spring.
Any changes in aircraft wheel velocity cause a relative displacement between the flywheel and the driven wheel. This relative displacement is used as a control signal to operate a valve in the hydraulic braking system to release the brake pressure. The unit may be wheel rim or axle mounted.
Electronic Anti-skid Systems
The response rates of the flywheels used in mechanical systems are low when compared with electrical signalling and furthermore the modulation does not always conform to the true runway conditions.
It is also much easier to alter the response rates and system biases of electronic circuitry to suit different aircraft types, thus making it simpler to adapt the circuits to match the requirements of new aircraft types.
The electronic system gives approximately a 15% improvement over the mechanical unit with the advantage that it can be tested prior to use.
The electronic system comprises three main elements:
A sensor which measures wheel speed.
A control box to compute wheel speed information.
A servo valve to modulate brake pressure.
The basic control loop described above offers few advantages over a mechanical system except that the cycling rate is much improved. A system refinement is that of the Adaptive Pressure Bias Modulation Circuit.
This ensures that the brake pressure applied immediately after a wheel is released after an Anti-Skid Unit (ASU) operation, is lower than the pressure which was applied before the ASU operation preventing an immediate return to the conditions that caused the ASU to release the pressure in the first place.
The ASU has a number of important functions that may include.
This will prevent the brakes being applied before touchdown. The electronic anti-skid controller will monitor the wheel speed and air/ground logic. If no signal is received the brakes cannot be applied while the aircraft is airborne. On touchdown the wheels ‘spin up’ and apply a signal to the controller which will now allow the brakes to be applied.
The anti-skid controller will reduce the brake pressure to any wheel that it determines is approaching a skid by monitoring the deceleration rate of the individual wheels.
Locked wheel protection
If a wheel locks because of a wet patch, or ice, the anti-skid controller will release the pressure to that wheel completely until the wheel spins up again and the pressure will be re-applied.
Systems that have this facility will monitor aircraft velocity and wheel speeds of a complete bogie. If all braked wheels hydroplane and lock up, then the pressure to some of the wheels is released.
The method varies from aircraft to aircraft but typically, if all braked wheels lock then a number of brakes are released e.g. two wheels on a four wheel bogie would be released. The remaining pair will provide locked wheel protection. Subsequently, the hydroplaned pair will spin up and they will in turn provide locked wheel protection. If hydroplane conditions still exist the other pair will be released
To enable the pilot to have full control of the brakes for taxiing and manoeuvring, the anti-skid system is deactivated, either manually or automatically, when the aircraft has slowed down to below approximately 20 mph, it is assumed then that there is no further danger of skidding.
Aircraft Wheel Brake System
The brakes are powered by one of the aircraft hydraulic power systems (system 1) with automatic switch over to an alternate system (system 2) in the event of low system 1 pressure. When normal and alternate brake hydraulic sources are lost, an accumulator is automatically selected to maintain parking brake pressure.
The anti-skid valves receive hydraulic pressure from the normal brake metering valves or the autobrake valves with the anti-skid control unit providing electrical signals to the anti-skid valves to control braking during skid conditions. Wheel speed transducers mounted in the axle transmit wheel speed inputs to the anti-skid control unit.
Each wheel is provided individually with anti-skid protection when normal brakes are operative. When skidding is initially detected, the anti-skid controller commands the respective anti-skid valve to reduce brake pressure which protects the wheel from further skidding. Touchdown braking protection is provided by comparing wheel speed to IRS (inertial reference system) ground speed. During alternate brake operation anti-skid protection is provided to wheel pairs rather than individual wheels.
A brake torque sensor is provided at each wheel to detect excessive torque during braking to prevent damage to the landing gear (more a problem with CARBON brakes). When excessive torque stress is detected, a signal is sent to the anti-skid valve and brake pressure to that wheel is released.
TYPICAL BRAKING SYSTEM
This system permits automatic braking when using the normal brake system during landing rollout or during a rejected take-off (RTO). There are a number of levels of operation of the autobrake system:
The system is ready for use but certain conditions have to be met before the system will operate automatically.
A system that is armed may become activated once conditions have been met. It may be activated in a number of ways depending in aircraft type.
A system is operative if it is working as intended. An inoperative system will not accomplish its intended purpose and is not considered to be functioning correctly.
The autobrake system is not available when using the alternate brake system. Depending on the aircraft, three or five landing deceleration rates may be selected. Anti-skid protection is provided during autobrake operation.
Landing auto brakes are armed by selecting one of the deceleration rates on the autobrake selector. On touchdown with ground mode and wheel spin up sensed the brakes will be automatically applied and will provide braking to a complete stop or until the auto brakes are disarmed. The deceleration rate may be changed during autobrake operation without disarming by rotating the selector.
With RTO selected, maximum brake pressure will be applied automatically when all thrust levers are closed at ground speeds above 85 knots. Below 85 knots auto brakes are not activated. The landing auto brakes system disarms immediately if a fault occurs when the system is armed, the selector will move to the disarm position and a warning caption will be displayed.
Disarming will also occur if any of the following crew actions are taken during autobrake operation:
Advancing any thrust lever after landing.
Moving the speed brake lever to the DN (down) detent after speed brakes have been deployed on the ground.
Moving the autobrake selector to Disarm or Off.
The auto brakes are normally disarmed by the non-handling pilot or flight engineer as the aircraft speed reduces to approximately 20 knots.
The parking brake handle operates a shut-off valve in the return line to the reservoir from the anti-skid valves. To apply the parking brake depress the foot pedals, apply the parking brake lever, then release the foot pedals.
Hydraulic pressure is now trapped in the brakes because the return line from the anti-skid valves is closed. This will be capable of maintaining the brakes ‘on‘ for overnight parking if required.
PARKING BRAKE SELECTION
Brake Temperature Indicators
Larger aircraft types, (B747, B777, A340, A380 etc.) may be fitted with Brake Temperature Indicators.
Sensors are arranged to sample the temperature of the brakes of each individual wheel.
An indicator can be used to display the temperature of each pair of wheels as selected on the
system control panel.
The brake temperatures are constantly monitored by the system, if the temperature of any brake assembly rises above a predetermined level then a “HIGH TEMP” indicator light illuminates. Switch selection on the control panel will now enable the operator to locate the wheel brake or brakes which are triggering the alarm.
Should any brake temperature go above that level at which the High Temp warning light illuminates, then a brake “OVERHEAT” caption will come on.