Wikimedia Commons. When you used to talk to ATC, they referred to braking action as good, fair, poor, or nil. Compressed air or nitrogen is sometimes used. Variable pneumatic restrictor. As mentioned, it is imperative that the correct hydraulic fluid is used in each brake how do jet planes brake values. An expander tube brake is a different approach to braking vlues is used on aircraft of all sizes produced in the s—s.

Just remember that low numbers are bad, and high numbers are good. And how about when you ask tower for a braking action report? If you're talking to them, they're not going to use the numbers. They'll stick with the words that describe the braking action: good, medium, poor, and nil.

When it comes to slick runways, you still need to make good decisions about whether or not you should be diverting. But at least now the numbers used to report braking action are a little more straight forward. Instead of having to decipher a murky scale of How Do Jet Planes Brake Table 0 to , you only have to deal with 7 numbers.

And if you ask us, a little simplification goes a long way. Become a better pilot. Subscribe to get the latest videos, articles, and quizzes that make you a smarter, safer pilot. Colin is a Boldmethod co-founder, pilot and graphic artist.

He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ, and has directed development of numerous commercial and military training systems. You can reach him at colin boldmethod.

To: Separate email addresses with commas. Would You Go? All Videos. Planes Careers. Travis S. Colin Cutler Colin is a Boldmethod co-founder, pilot and graphic artist. Recommended Stories. High On Final? High on final? Need to clear an obstacle or lose altitude quickly? High bypass ratio engines usually reverse thrust by changing the direction of only the fan airflow, since the majority of thrust is generated by this section, as opposed to the core.

There are three jet engine thrust reversal systems in common use: [6]. The target thrust reverser uses a pair of hydraulically -operated 'bucket' type doors to reverse the hot gas stream. For forward thrust, these doors form the propelling nozzle of the engine.

In the original implementation of this system on the Boeing , [10] and still common today, two reverser buckets were hinged so when deployed they block the rearward flow of the exhaust and redirect it with a forward component. This type of reverser is visible at the rear of the engine during deployment. The clam-shell door system is pneumatically operated. When activated, the doors rotate to open the ducts and close the normal exit, causing the thrust to be directed forward.

The cascade thrust reverser is commonly used on turbofan engines. On turbojet engines, this system would be less effective than the target system, as the cascade system only makes use of the fan airflow and does not affect the main engine core, which continues to produce forward thrust. In addition to the two types used on turbojet and low-bypass turbofan engines, a third type of thrust reverser is found on some high-bypass turbofan engines.

Doors in the bypass duct are used to redirect the air that is accelerated by the engine's fan section but does not pass through the combustion chamber called bypass air such that it provides reverse thrust. During normal operation, the reverse thrust vanes are blocked. On selection, the system folds the doors to block off the cold stream final nozzle and redirect this airflow to the cascade vanes.

The cold stream system is known for structural integrity, reliability, and versatility. During thrust reverser activation, a sleeve mounted around the perimeter of the aircraft engine nacelle moves aft to expose cascade vanes which act to redirect the engine fan flow. This thrust reverser system can be heavy and difficult to integrate into nacelles housing large engines.

In most cockpit setups, reverse thrust is set when the thrust levers are on idle by pulling them further back.

Reverse thrust is always selected manually, either using levers attached to the thrust levers or moving the thrust levers into a reverse thrust 'gate'. The early deceleration provided by reverse thrust can reduce landing roll by a quarter or more. Once the aircraft's speed has slowed, reverse thrust is shut down to prevent the reversed airflow from throwing debris in front of the engine intakes where it can be ingested, causing foreign object damage.

If circumstances require it, reverse thrust can be used all the way to a stop, or even to provide thrust to push the aircraft backward, though aircraft tugs or towbars are more commonly used for that purpose.

When reverse thrust is used to push an aircraft back from the gate, the maneuver is called a powerback. Some manufacturers warn against the use of this procedure during icy conditions as using reverse thrust on snow- or slush-covered ground can cause slush, water, and runway deicers to become airborne and adhere to wing surfaces. If the full power of reverse thrust is not desirable, thrust reverse can be operated with the throttle set at less than full power, even down to idle power, which reduces stress and wear on engine components.

Reverse thrust is sometimes selected on idling engines to eliminate residual thrust, in particular in icy or slick conditions, or when the engines' jet blast could cause damage. Some aircraft, notably some Russian and Soviet aircraft , are able to safely use reverse thrust in flight, though the majority of these are propeller-driven. Many commercial aircraft, however, cannot. In-flight use of reverse thrust has several advantages.

It allows for rapid deceleration, enabling quick changes of speed. It also prevents the speed build-up normally associated with steep dives, allowing for rapid loss of altitude , which can be especially useful in hostile environments such as combat zones, and when making steep approaches to land. The Douglas DC-8 series of airliners has been certified for in-flight reverse thrust since service entry in Safe and effective for facilitating quick descents at acceptable speeds, it nonetheless produced significant aircraft buffeting, so actual use was less common on passenger flights and more common on cargo and ferry flights, where passenger comfort is not a concern.

The Concorde supersonic airliner could use reverse thrust in the air to increase the rate of descent. Only the inboard engines were used, and the engines were placed in reverse idle only in subsonic flight and when the aircraft was below 30, ft in altitude. The Lockheed C-5 Galaxy , introduced in , also has in-flight reverse capability, although on the inboard engines only.

The seal is only effective in the forward direction. When the piston is returning, or is fully retracted to the off position, fluid behind the piston is free to flow through piston head ports to replenish any fluid that may be lost downstream of the master cylinder. The aft end of the master cylinder contains a seal that prevents leakage at all times. A rubber boot fits over the piston rod and the aft end of the master cylinder to keep out dust. A parking brake for this remote reservoir master cylinder brake system is a ratcheting mechanical device between the master cylinder and the rudder pedals.

With the brakes applied, the ratchet is engaged by pulling the parking brake handle. To release the brakes, the rudder pedals are depressed further allowing the ratchet to disengage.

With the parking brake set, any expansion of hydraulic fluid due to temperature is relieved by a spring in the mechanical linkage. A common requirement of all braking systems is for there to be no air mixed in with the hydraulic fluid. Since air is compressible and hydraulic fluid essentially is not, any air under pressure when the brakes are applied causes spongy brakes. The pedals do not feel firm when pushed down due to the air compressing. Brake systems must be bled to remove all air from the system.

Brake systems equipped with Goodyear master cylinders must be bled from the top down to ensure any air trapped behind the master cylinder piston is removed. An alternative common arrangement of independent braking systems incorporates two master cylinders, each with its own integral fluid reservoir.

Except for the reservoir location, the brake system is basically the same as just described. The master cylinders are mechanically linked to the rudder pedals as before. Depressing the top of a pedal causes the piston rod to push the piston into the cylinder forcing the fluid out to the brake assembly. The piston rod rides in a compensator sleeve and contains an O-ring that seals the rod to the piston when the rod is moved forward.

This blocks the compensating ports. When released, a spring returns the piston to its original position which refills the reservoir as it returns. The rod end seal retracts away from the piston head allowing a free flow of fluid from the cylinder through the compensating ports in the piston to the reservoir. A common master cylinder with built-in reservoir is shown. Illustration A depicts the master cylinder when the brakes are off.

The compensating port is open to allow fluid to expand into the reservoir should temperature increase. In B, the brakes are applied. The piston rod-end seal covers the compensating port as it contacts the piston head. The parking brake mechanism is a ratcheting type that operates as described.

A servicing port is supplied at the top of the master cylinder reservoir. Typically, a vented plug is installed in the port to provide positive pressure on the fluid. In an independent braking system, the pressure applied to the brakes is only as great as the foot pressure applied to the top of the rudder pedal.

Boosted brake actuating systems augment the force developed by the pilot with hydraulic system pressure when needed. The boost is only during heavy braking. It results in greater pressure applied to the brakes than the pilot alone can provide.

Boosted brakes are used on medium and larger aircraft that do not require a full power brake actuating system. A boosted brake master cylinder for each brake is mechanically attached to the rudder pedals. However, the boosted brake master cylinder operates differently. A master cylinder for a boosted brake system augments foot pedal pressure with aircraft system hydraulic pressure during heavy braking. The initial movement closes the compensator poppet used to provide thermal expansion relief when the brakes are not applied.

As the pilot pushes harder on the pedal, a spring loaded toggle moves a spool valve in the cylinder. Aircraft hydraulic system pressure flows through the valve to the back side of the piston.

Pressure is increased, as is the force developed to apply the brakes. When the pedal is released, the piston rod travels in the opposite direction, and the piston returns to the piston stop. The compensating poppet reopens. The toggle is withdrawn from the spool via linkages, and fluid pushes the spool back to expose the system return manifold port. System hydraulic fluid used to boost brake pressure returns through the port.

Large and high performance aircraft are equipped with power brakes to slow, stop, and hold the aircraft. Power brake actuating systems use the aircraft hydraulic system as the source of power to apply the brakes. The pilot presses on the top of the rudder pedal for braking as with the other actuating systems. The volume and pressure of hydraulic fluid required cannot be produced by a master cylinder. Instead, a power brake control valve or brake metering valve receives the brake pedal input either directly or through linkages.

The valve meters hydraulic fluid to the corresponding brake assembly in direct relation to the pressure applied to the pedal. Many power brake system designs are in use. Most are similar to the simplified system illustrated in Figure A. Power brake systems are constructed to facilitate graduated brake pressure control, brake pedal feel, and the necessary redundancy required in case of hydraulic system failure.

Large aircraft brake systems integrate anti-skid detection and correction devices. These are necessary because wheel skid is difficult to detect on the flight deck without sensors. However, a skid can be quickly controlled automatically through pressure control of the hydraulic fluid to the brakes.

Hydraulic fuses are also commonly found in power brake systems. The hostile environment around the landing gear increases the potential for a line to break or sever, a fitting to fail, or other hydraulic system malfunctions to occur where hydraulic fluid is lost en route to the brake assemblies.

A fuse stops any excessive flow of fluid when detected by closing to retain the remaining fluid in the hydraulic system. Shuttle valves are used to direct flow from optional sources of fluid, such as in redundant systems or during the use of an emergency brake power source. An airliner power brake system is illustrated in Figure B. The orientation of components in a basic power brake system is shown in A. The general layout of an airliner power brake system is shown in B.

The key element in a power brake system is the brake control valve, sometimes called a brake metering valve. It responds to brake pedal input by directing aircraft system hydraulic fluid to the brakes. As pressure is increased on the brake pedal, more fluid is directed to the brake causing a higher pressure and greater braking action. A brake metering valve from a Boeing is illustrated in Figure The system in which it is installed is diagramed in Figure Two sources of hydraulic pressure provide redundancy in this brake system.

As in most brake control valves, the brake input shaft moves a tapered spool or slide in the valve so that it allows hydraulic system pressure to flow to the brakes. At the same time, the slide covers and uncovers access to the hydraulic system return port as required. A brake metering valve from a Boeing A machined slide or spool moves laterally to admit the correct amount of hydraulic system fluid to the brakes.

The power brake system on a Boeing The hydraulic supply pressure chamber is connected to the brake system pressure chamber by the movement of the slide, which due to its taper, unblocks the passage between these two.

As the pedal is depressed further, the valve slide moves farther to the left. This enables more fluid to flow to the brakes due to the narrowing shape of the slide. Brake pressure increases with the additional fluid. A passage in the slide directs brake pressure fluid into a compensating chamber at the end of the slide.

This acts on the end of the slide creating a return force that counters the initial slide movement and gives feel to the brake pedal. As a result, the pressure and return ports are closed and pressure proportional to the foot pressure on the pedal is held on the brakes. When the pedal is released, a return spring and compensating chamber pressure drive the slide to the right into its original position return port open, supply pressure chamber and brake pressure chambers blocked from each other.

The metering valve operates as described simultaneously for the inboard and the outboard brakes. Most brake control valves and metering valves function in a similar manner, although many are single units that supply only one brake assembly. The auto brake, referenced in the metering valve diagram, is connected into the landing gear retraction hydraulic line. Pressurized fluid enters this port and drives the slide slightly to the left to apply the brakes automatically after takeoff.

This stops the wheels from rotating when retracted into the wheel wells. Auto brake pressure is withheld from this port when the landing gear is fully stowed since the retraction system is depressurized. Many aircraft refine the feel of the pedal with an additional feel unit. The brake valve feel augmentation unit, in the above system, uses a series of internal springs and pistons of various sizes to create a force on the brake input shaft movement.

The request for light braking with slight pedal depression results in a light feel to the pedal and a harder resistance feel when the pedals are pushed harder during heavy braking. As can be seen in Figure 22, the brake metering valves not only receive hydraulic pressure from two separate hydraulic systems, they also feed two separate brake assemblies.

Each main wheel assembly has two wheels. The inboard wheel brake and the outboard wheel brake, located in their respective wheel rims, are independent from each other. In case of hydraulic system failure or brake failure, each is independently supplied to adequately slow and stop the aircraft without the other. More complicated aircraft may involve another hydraulic system for back-up or use a similar alternation of sources and brake assemblies to maintain braking in case of hydraulic system or brake failure.

NOTE: In the segmented rotor brake section above, a brake assembly was described that had alternating pistons supplied by independent hydraulic sources. This is another method of redundancy particularly suitable on, but not limited to, single main wheel aircraft. In addition to supply system redundancy, the brake accumulator is also an emergency source of power for the brakes in many power brake systems.

The accumulator is pre-charged with air or nitrogen on one side of its internal diaphragm. Enough hydraulic fluid is contained on the other side of the diaphragm to operate the brakes in case of an emergency. It is forced out of the accumulator into the brakes through the system lines under enough stored pressure to slow the aircraft. Emergency brake hydraulic fluid accumulators are precharged with nitrogen to deliver brake fluid to the brakes in the event normal and alternate hydraulic sources fail.

Some simpler power brake systems may use an emergency source of brake power that is delivered directly to the brake assemblies and bypasses the remainder of the brake system completely. A shuttle valve immediately upstream of the brake units shifts to accept this source when pressure is lost from the primary supply sources. Compressed air or nitrogen is sometimes used. A pre-charged fluid source can also be used as an alternate hydraulic source.

The parking brake system function is a combined operation. The brakes are applied with the rudder pedals and a ratcheting system holds them in place when the parking brake lever on the flight deck is pulled. This traps the fluid in the brakes holding the rotors stationary.

Depressing the pedals further releases the pedal ratchet and opens the return line valve. The parking brake lever on a Boeing center pedestal throttle quadrant. Some aircraft brake assemblies that operate on aircraft hydraulic system pressure are not designed for such high pressure. They provide effective braking through a power brake system but require less than maximum hydraulic system pressure.

To supply the lower pressure, a brake debooster cylinder is installed downstream of the control valve and anti-skid valve. Brake deboosters are simple devices that use the application of force over different sized pistons to reduce pressure. 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. Brake deboosters.

The spring in the debooster aids in returning the piston to the ready position. If fluid is lost downstream of the deboost cylinder, the piston travels further down into the cylinder when the brakes are applied. The pin unseats the ball and allows fluid into the lower cylinder to replace what was lost. Once replenished, the piston rises up in the cylinder due to pressure build-up. The ball reseats as the piston travels above the pin and normal braking resumes. This function is not meant to permit leaks in the brake assemblies.

Any leak discovered must be repaired by the technician. A lockout debooster functions as a debooster and a hydraulic fuse. If fluid is not encountered as the piston moves down in the cylinder, the flow of fluid to the brakes is stopped. This prevents the loss of all system hydraulic fluid should a rupture downstream of the debooster occur. Lockout deboosters have a handle to reset the device after it closes as a fuse.

If not reset, no braking action is possible. Large aircraft with power brakes require anti-skid systems. It is not possible to immediately ascertain in the flight deck when a wheel stops rotating and begins to skid, especially in aircraft with multiple-wheel main landing gear assemblies.

A skid not corrected can quickly lead to a tire blowout, possible damage to the aircraft, and control of the aircraft may be lost. 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 preset rate. When excessive deceleration is detected, hydraulic pressure is reduced to the brake on that wheel. To operate the anti-skid system, flight deck switches must be placed in the ON position. 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 taxi and ground maneuvering. Antiskid switches in the cockpit. There are various designs of anti-skid systems. Most contain three main types of components: wheel speed sensors, anti-skid control valves, and a control unit. These units work together without human interference. Some anti-skid systems provide complete automatic braking.

The pilot needs only to turn on the auto brake system, and the anti-skid components slow the aircraft without pedal input. Wheel speed sensors are located on each wheel equipped with a brake assembly. Each brake also has its own anti-skid control valve. Typically, a single control box contains the anti-skid comparative circuitry for all of the brakes on the aircraft. A wheel sensor left , a control unit center , and a control valve right are components of an antiskid system.

A sensor is located on each wheel equipped with a brake assembly. An antiskid control valve for each brake assembly is controlled from a single central control unit. Wheel speed sensors are transducers. They may be alternating current AC or direct current DC. The typical AC wheel speed sensor has a stator mounted in the wheel axle. A coil around it is connected to a controlled DC source so that when energized, the stator becomes an electromagnet.

A rotor that turns inside the stator is connected to the rotating wheel hub assembly through a drive coupling so that it rotates at the speed of the wheel. Lobes on the rotor and stator cause the distance between the two components to constantly change during rotation. This alters the magnetic coupling or reluctance between the rotor and stator. As the electromagnetic field changes, a variable frequency AC is induced in the stator coil.

The AC signal is fed to the control unit for processing. A DC wheel speed sensor is similar, except that a DC is produced the magnitude of which is directly proportional to wheel speed. The stator of an antiskid wheel sensor is mounted in the axle, and the rotor is coupled to the wheel hub spider that rotates with the wheel.

The control unit can be regarded as the brain of the anti-skid system. It receives signals from each of the wheel sensors. Comparative circuits are used to determine if any of the signals indicate a skid is imminent or occurring on a particular wheel. If so, a signal is sent to the control valve of the wheel to relieve hydraulic pressure to that brake which prevents or relieves the skid.

The control unit may or may not have external test switches and status indicating lights. It is common for it to be located in the avionics bay of the aircraft. A rack mounted antiskid control unit from an airliner. The Boeing anti-skid control valve block diagram in Figure 32 gives further detail on the functions of an anti-skid control unit. Other aircraft may have different logic to achieve similar end results. DC systems do not require an input converter since DC is received from the wheel sensors, and the control unit circuitry operates primarily with DC.

Only the functions on one circuit card for one wheel brake assembly are shown in Figure Each wheel has its own identical circuitry card to facilitate simultaneous operation. All cards are housed in a single control unit that Boeing calls a control shield. The converter shown changes the AC frequency received from the wheel sensor into DC voltage that is proportional to wheel speed.

The output is used in a velocity reference loop that contains deceleration and velocity reference circuits. The converter also supplies input for the spoiler system and the locked wheel system, which is discussed at the end of this section. A velocity reference loop output voltage is produced, which represents the instantaneous velocity of the aircraft. This is compared to converter output in the velocity comparator. This comparison of voltages is essentially the comparison of the aircraft speed to wheel speed.

The output from the velocity comparator is a positive or negative error voltage corresponding to whether the wheel speed is too fast or too slow for optimum braking efficiency for a given aircraft speed. The error output voltage from the comparator feeds the pressure bias modulator circuit. This is a memory circuit that establishes a threshold where the pressure to the brakes provides optimum braking.

The error voltage causes the modulator to either increase or decrease the pressure to the brakes in attempt to hold the modulator threshold. It produces a voltage output that is sent to the summing amplifier to do this.

A lead output from the comparator anticipates when the tire is about to skid with a voltage that decreases the pressure to the brake. It sends this voltage to the summing amplifier as well. A transient control output from the comparator designed for rapid pressure dump when a sudden skid has occurred also sends voltage to the summing amp.

As the name suggests, the input voltages to the amplifier are summed, and a composite voltage is sent to the valve driver. The driver prepares the current required to be sent to the control valve to adjust the position of the valve.

Brake pressure increases, decreases, or holds steady depending on this value. Anti-skid control valves are fast-acting, electrically controlled hydraulic valves that respond to the input from the anti-skid control unit.

There is one control valve for each brake assembly. A torque motor uses the input from the valve driver to adjust the position of a flapper between two nozzles. By moving the flapper closer to one nozzle or the other, pressures are developed in the second stage of the valve. These pressures act on a spool that is positioned to build or reduce pressure to the brake by opening and blocking fluid ports. An antiskid control valve uses a torque motor controlled flapper in the first stage of the valve to adjust pressure on a spool in the second stage of the valve to build or relieve pressure to the brake.

As pressure is adjusted to the brakes, deceleration slows to within the range that provides the most effective braking without skidding. The wheel sensor signal adjusts to the wheel speed, and the control unit processes the change.

Output is altered to the control valve. The control valve flapper position is adjusted and steady braking resumes without correction until needed. Anti-skid control valves are typically located in the main wheel for close access to hydraulic pressure and return manifolds, as well as the brake assemblies.

Two antiskid control valves with associated plumbing and wiring. It is essential that the brakes are not applied when the aircraft contacts the runway upon landing.

This could cause immediate tire blowout. A touchdown protection mode is built into most aircraft anti-skid systems to prevent this. Until the aircraft has weight on wheels, the detector circuitry signals the anti-skid control valve to open the passage between the brakes and the hydraulic system return, thus preventing pressure build-up and application of the brakes.

Once the squat switch is open, the anti-skid control unit sends a signal to the control valve to close and permit brake pressure build-up. As a back-up and when the aircraft is on the ground with the strut not compressed enough to open the squat switch, a minimum wheel speed sensor signal can override and allow braking.

Wheels are often grouped with one relying on the squat switch and the other on wheel speed sensor output to ensure braking when the aircraft is on the ground, but not before then. Locked wheel protection recognizes if a wheel is not rotating. When this occurs, the anti-skid control valve is signaled to fully open. Some aircraft anti-skid control logic, such as the Boeing shown in Figure 33, expands the locked wheel function. Comparator circuitry is used to relieve pressure when one wheel of a paired group of wheels rotates 25 percent slower than the other.

Inboard and outboard pairs are used because if one of the pair is rotating at a certain speed, so should the other. If it is not, a skid is beginning or has occurred. On takeoff, the anti-skid system receives input through a switch located on the gear selector that shuts off the anti-skid system.

This allows the brakes to be applied as retraction occurs so that no wheel rotation exists while the gear is stowed. 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.

Figure 35 shows a simplified diagram of the Boeing brake system with the auto brake valve in relation to the main metering valve and anti-skid valves in this eight-main wheel system. The Boeing normal brake system with auto brake and antiskid. It is important to know the status of the anti-skid system prior to attempting to use it during a landing or aborted takeoff. Ground tests and in-flight tests are used.

Built-in test circuits and control features allow testing of the system components and provide warnings should a particular component or part of the system become inoperative. An inoperative anti-skid system can be shut off without affecting normal brake operation. Ground tests vary slightly from aircraft to aircraft. Much of the anti-skid system testing originates from testing circuits in the anti-skid control unit.

Built-in test circuits continuously monitor the anti-skid system and provide warning if a failure occurs. An operational test can be performed before flight. A test is first done with the aircraft at rest and then in an electrically simulated anti-skid braking condition.

Some anti-skid control units contain system and component testing switches and lights for use by the technician. This accomplishes the same operational verification, but allows an additional degree of troubleshooting.

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