Anti-Skid & Auto-Brakes - Aircraft Brakes - Airframes & Aircraft Systems #23

Summary

This lesson delves into the intricacies of brake modulating and automatic braking systems in modern airliners, highlighting their significance for safe and efficient landings. The script explains how anti-skid systems prevent wheel lock, ensuring optimal braking, and details the components of electronic anti-skid systems, including sensors, control units, and servo valves. It also covers the auto brake system's capabilities, such as providing maximum braking during rejected takeoffs and adjusting deceleration rates, emphasizing the system's swift response and precise braking application.

Takeaways

• 🛫 **Brake Modulation Systems**: Modern airliners are equipped with brake modulating systems, commonly known as anti-skid systems, which are crucial for optimal braking during landing, especially on short runways and in adverse weather conditions.
• 🔒 **Skid Prevention**: The primary function of these systems is to prevent wheel skidding by ensuring there is only a small degree of slip between the wheel and the ground, which is essential for effective braking.
• 📉 **Deceleration Control**: Brake modulating systems control the deceleration of individual wheels, using a selected datum figure for wheel deceleration that exceeds the maximum possible rate of the aircraft.
• 🔧 **Mechanical vs. Electronic Systems**: While mechanical systems have been used since the 1950s, most modern aircraft utilize electronic or electrical systems for more precise control.
• 🚫 **Touchdown Protection**: Anti-skid systems include touchdown protection to prevent brake application before the aircraft has touched down, ensuring safety and proper braking only after landing.
• 🔄 **Adaptive Pressure Bias Modulation**: Electronic anti-skid units adapt brake pressure to prevent immediate return to skid conditions, maintaining optimal braking efficiency.
• 🔒 **Locked Wheel Protection**: In case of wheel lock due to wet or icy conditions, the system releases brake pressure to the affected wheel until it spins up again, then reapplies pressure.
• 🔍 **Break Torque Sensor**: Sensors detect excessive torque during braking to prevent damage to the landing gear, especially with carbon brakes, by reducing brake pressure when needed.
• ✈️ **Automatic Braking Systems**: Modern aircraft feature automatic braking systems that can be selected for landing rollout or rejected takeoff, providing quick reactions and precise braking force for the required deceleration.
• 🔄 **Selectable Deceleration Rates**: Pilots can select different landing deceleration rates using controls like rotary switches or push buttons, with specific settings for maximum braking during rejected takeoffs.

Q & A

What is the primary purpose of a brake modulating system in aircraft?

The primary purpose of a brake modulating system, also known as an anti-skid system, is to prevent skidding by ensuring there is only a small degree of slip between the wheel and the ground during braking.

Why is optimum braking crucial for modern aircraft?

Optimum braking is crucial for modern aircraft due to their high landing speeds, low drag, and high weight, especially when operating from and into short runways in bad weather conditions.

How do mechanical and electrical brake modulating systems differ?

Mechanical systems have been in use since the early 1950s, while most modern aircraft use electrical or electronic systems, which offer more advanced control and reliability.

What is the role of the anti-skid unit in an electronic anti-skid system?

The anti-skid unit in an electronic anti-skid system measures wheel speed, computes wheel speed information, and modulates brake pressure to prevent skidding and optimize braking.

What are the three main functions provided by the anti-skid unit?

The three main functions provided by the anti-skid unit are touchdown protection, skid prevention, and locked wheel protection.

How does the touchdown protection feature work in an anti-skid system?

Touchdown protection prevents the brakes from being applied before the aircraft touches down by monitoring wheel speed and air-ground logic. Brakes can only be applied once the aircraft is on the ground and the wheels have spun up.

What is the purpose of the adaptive pressure bias modulation circuit in electronic anti-skid units?

The adaptive pressure bias modulation circuit ensures that after a skid, the brake pressure applied to the wheel is lower than the pressure that caused the skid, preventing an immediate return to skid conditions.

How does the locked wheel protection system respond to a wheel lock during braking?

If a wheel locks due to a wet patch or ice, the anti-skid controller releases the brake pressure to that wheel completely until it spins up again, then reapplies the pressure.

What is the function of a break torque sensor in an aircraft's braking system?

A break torque sensor detects excessive torque during braking to prevent damage to the landing gear strut or strut mountings, especially with carbon brakes.

How does the auto brake system differ from the anti-skid system in its operation?

The auto brake system automatically applies the brakes after touchdown and decelerates the aircraft at a predetermined rate, whereas the anti-skid system modulates brake pressure to prevent skidding and optimize braking.

What happens if the anti-skid system is unserviceable during auto brake operation?

If the anti-skid system is unserviceable, the auto brake system is disabled to prevent potential safety issues during braking.

Outlines

00:00

🛫 Introduction to Brake Modulating and Anti-Skid Systems

This paragraph introduces the brake modulating and automatic braking systems found on modern airliners. It emphasizes the importance of optimum braking for aircraft with high landing speeds, especially during operations on short runways in poor weather conditions. The primary function of a brake modulating system, also known as an anti-skid system, is to prevent skidding by ensuring there is only a small degree of slip between the wheel and the ground. The system automatically adjusts brake pressure based on the deceleration of individual wheels. The paragraph also discusses the evolution from mechanical to electronic systems and outlines the three main components of an electronic anti-skid system: a sensor for measuring wheel speed, an anti-skid unit (ASU) for processing this information, and a servo valve for modulating brake pressure. The ASU performs three key functions: touchdown protection to prevent brake application before landing, skid prevention by monitoring and modulating brake pressure, and locked wheel protection which releases brake pressure to a locked wheel until it spins up again. Additionally, the paragraph mentions the role of brake torque sensors in detecting excessive stress during braking.

05:05

🛬 Demonstration of Anti-Skid and Auto Brake Systems in Action

The second paragraph provides a demonstration of how the anti-skid system operates during the approach and landing phases of flight. It explains that the touchdown protection feature prevents brakes from being applied while the aircraft is still airborne. Upon landing, once the ASU receives an on-ground signal and the wheels have spun up, braking can commence. The anti-skid system modulates the brakes for optimum braking, and if any wheel locks, the locked wheel protection system releases pressure until the wheel spins up again. The paragraph also covers the auto brake system, which can be selected for operation during landing rollout or a rejected takeoff. This system reacts quicker than a pilot and applies the correct amount of braking for the required deceleration rate. Different aircraft types may have different methods for selecting deceleration rates, such as rotary selector switches or push buttons. The auto brake system is disarmed under certain conditions, such as when the aircraft slows below a certain speed, or if manual braking is applied, or if there is a system fault. The paragraph concludes with a note on how the auto brake system can be disarmed by the pilot or automatically under specific circumstances.

10:07

🔐 Conclusion on Anti-Skid and Auto Brake Systems

The final paragraph concludes the lesson on anti-skid and auto brake systems. It reiterates the benefits of a modern anti-skid system, which not only optimizes deceleration but also includes features like touchdown protection to prevent premature brake application and locked wheel protection to release pressure on wheels that lock due to slippery conditions. The auto brake system, when armed before landing, automatically applies the brakes after touchdown to decelerate the aircraft at a pre-set rate. In the event of a rejected takeoff, the system is designed to stop the aircraft in the shortest distance possible. The paragraph ends with a reminder of the importance of these systems in ensuring safe and efficient aircraft operations.

Keywords

💡Brake Modulating Systems

Brake modulating systems, often referred to as anti-skid systems, are crucial for modern aircraft due to their high landing speeds and the need for efficient braking, especially on short runways. These systems automatically adjust brake pressure to prevent wheel lockup and ensure maximum braking efficiency. In the script, it is mentioned that 'the primary requirement of a brake modulating system is to provide skid prevention', highlighting their importance in maintaining control during landing.

💡Optimum Braking

Optimum braking is a concept that refers to the ideal balance between braking force and tire-road friction to achieve the shortest stopping distance without causing the wheels to lock. The script emphasizes the importance of optimum braking for aircraft, stating that 'optimum braking is important in the operation of modern aircraft with their high landing speeds, low drag, and high weight'.

💡Anti-Skid Systems

Anti-skid systems are a type of brake modulating system designed to prevent the wheels from locking during braking, which can reduce braking efficiency and control. The script explains that these systems are necessary because 'a skidding wheel provides very little braking effect', and they work by modulating brake pressure based on wheel deceleration.

💡Deceleration

Deceleration is the rate at which an object slows down, measured in units of distance over time squared. In the context of the script, deceleration is a key parameter for brake modulating systems, as they use it to control the braking torque and prevent skidding. The script mentions that 'a datum figure for wheel deceleration is selected which is known to be greater than the maximum possible deceleration rate of the aircraft'.

💡Touchdown Protection

Touchdown protection is a feature of electronic anti-skid systems that prevents the brakes from being applied before the aircraft has touched down. This is a safety feature to ensure that the brakes are only used when the aircraft is on the ground. The script describes how 'touchdown protection prevents the brakes being applied before touchdown', and it is activated once the aircraft's wheels spin up after landing.

💡Locked Wheel Protection

Locked wheel protection is a function of anti-skid systems that detects when a wheel has locked due to excessive braking and releases brake pressure to that wheel to allow it to spin up again. This is crucial for maintaining control and preventing damage to the landing gear. The script states that 'locked wheel protection...releasing the pressure to any wheel that locks because of hitting a very slippery patch on the runway'.

💡Electronic Anti-Skid Unit (ASU)

The Electronic Anti-Skid Unit, or ASU, is a component of the anti-skid system that processes wheel speed information and controls the brake pressure to prevent skidding. It performs functions like touchdown protection, skid prevention, and locked wheel protection. The script explains that 'an electronic anti-skid system comprises three main elements: a sensor which measures wheel speed, a control box known as an anti-skid unit or ASU to compute wheel speed information, and a servo valve or anti-skid valve to modulate brake pressure'.

💡Auto Brake System

The auto brake system is an automated system that applies the optimal amount of braking force after landing or during a rejected takeoff, based on pre-selected deceleration rates. It reacts faster than a human pilot and ensures consistent deceleration. The script mentions that 'most modern aircrafts have an automatic braking system which can be selected to operate during landing rollout or during a rejected takeoff'.

💡Rejected Takeoff (RTO)

A rejected takeoff refers to the decision made by the pilot to abort a takeoff after the initiation of the takeoff roll. The auto brake system can be set to provide maximum braking in such scenarios to bring the aircraft to a stop quickly. The script explains that 'the rejected takeoff or RTO selection provides maximum braking on an Airbus'.

💡Brake Torque Sensor

A brake torque sensor is a device that detects excessive torque during braking, which can cause damage to the landing gear. It is part of the safety measures in anti-skid systems to protect the aircraft's structural integrity. The script states that 'a break torque sensor is provided at each wheel to detect excessive talk during braking to prevent damage to the landing gear strut or strut mountings'.

Highlights

Brake modulating and automatic braking systems are crucial for modern airliners, especially for high-speed landings.

Anti-skid systems, also known as brake modulating systems, are designed to prevent wheel lock and skidding.

Optimum braking is essential for aircraft with high landing speeds, low drag, and high weight, particularly on short runways.

Pilots cannot sense wheel lock, hence anti-skid systems automatically modulate brake pressure to prevent skidding.

Deceleration of individual wheels is the key parameter for controlling braking torque in anti-skid systems.

Mechanical anti-skid systems have been in use since the 1950s, but most modern aircraft use electronic or electrical systems.

Electronic anti-skid systems significantly reduce the stopping distance required for high-speed aircraft.

An electronic anti-skid system consists of a sensor, control unit, and servo valve to manage wheel speed and brake pressure.

The anti-skid unit provides touchdown protection to prevent brake application before landing.

Adaptive pressure bias modulation ensures optimal braking by adjusting brake pressure after a skid.

Locked wheel protection releases brake pressure to a locked wheel until it spins up again.

Break torque sensors detect excessive stress during braking to prevent damage to the landing gear.

Anti-skid systems are automatically deactivated at low speeds, transferring full braking control to the pilot.

Automatic braking systems can be selected for landing rollout or rejected takeoff, providing quick and precise deceleration.

Auto brake systems react faster than pilots and apply the correct braking force for the required deceleration rate.

Boeing and Airbus use different mechanisms to control auto brake systems, with Boeing using a rotary selector and Airbus using push buttons.

Auto brake systems disarm automatically if there's a system fault or if manual braking is applied.

The auto brake system provides maximum braking pressure when all thrust levers are closed at specific ground speeds.

In the event of a rejected takeoff, the auto brake system stops the aircraft in the shortest possible distance.

The auto brake system is disarmed by the pilot tapping the brake pedals when the aircraft's speed is under control.

Transcripts

in this lesson the brake modulating and

automatic braking systems as fitted to

most modern airliners will be described

brake modulating systems and normally

refer to as anti-skid 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 and into short runways in

bad weather the pilot is unable to sense

when the wheels lock and so the primary

requirement of a brake modulating system

is to provide skid prevention

we're never braking torque is developed

they must only be a small 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 four wheel deceleration

is selected which is known to be greater

than the maximum possible deceleration

rate of the aircraft when this datum

figure is exceeded brake pressure is

automatically reduced or released

systems maybe mechanical or electrical

mechanical systems have been in use

since the early 1950s most aircraft

today use electrical or electronic

systems

anti-skid systems have a huge effect on

the distance required to bring an

aircraft to a stop from high-speed the

required stopping distance increases

considerably if the anti-skid system is

unserviceable

an electronic anti-skid system comprises

three main elements a sensor which

measures wheel speed

troll box known as an anti-skid unit or

a su to compute wheel speed information

and a servo valve or anti-skid valve to

modulate brake pressure

the anti-skid unit provides three

important functions these are touchdown

protection

Skitch prevention

and locked wheel protection

touchdown protection prevents the brakes

being applied before touchdown the

electronic anti-skid unit 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 control unit which

will now allow the brakes to be applied

this does not however imply that you

should attempt a landing with the brake

pedals depressed it is a safety backup

system only

prevention the anti-skid control unit

will reduce the break pressure to any

wheel that it determines is approaching

a skid by monitoring the deceleration

rate of the individual wheels

it will then modulate the pressure to

ensure optimum braking most electronic

anti-skid units have an adaptive

pressure bias modulation circuit this

ensures that with the pilot applying a

constant pressure on the pedals the

brake pressure applied immediately after

a wheel is released following a skid is

lower than the pressure which caused the

skid

this prevents an immediate return to the

skid conditions that cause the anti-skid

unit to release the pressure in the

first place

if the wheel locks because of a wet

patch or ice for instance the anti-skid

controller will release the brake

pressure to that wheel completely until

the wheel spins up again then the

pressure will be reapplied

this feature is usually disabled at low

speed

a break torque sensor is provided at

each wheel to detect excessive talk

during braking to prevent damage to the

landing gear strut or strut mountings

this is more of a problem with carbon

brakes

when excessive talk stress is detected a

signal is sent to the anti-skid valve

and brake pressure to that wheel is

reduced

to enable the pilot to have full control

of the brakes for taxiing and

maneuvering on some aircraft types the

anti-skid system is automatically

deactivated when the aircraft has slowed

down to below approximately 10 knots

when it is assumed that there is no

further danger of skidding

to summarize the electronic anti-skid

system here is a demonstration of how it

will operate during approach and landing

on approach with the gear selected down

and anti-skid switched on the touchdown

protection system will not allow the

brakes to be applied

on touchdown once the ASU receives an

on-ground signal and the main wheels

have spun up to about 80 knots

touchdown protection is disabled and

braking can commence

the anti-skid system will now modulate

the brakes to produce optimum braking

should any wheel lock for any reason

then the locked wheel protection system

will completely release the pressure to

the locked wheel until it spins up again

as the aircraft slows through about 20

knots the locked wheel protection system

is disabled

finally on some systems as the aircraft

slows through about 10 knots the entire

anti-skid system is disabled and braking

is completely in the hands or more

correctly the feet of the pilot

most modern aircrafts have an automatic

braking system which can be selected to

operate during landing rollout or during

a rejected takeoff

the auto brake system is usually only

usable when the brakes are being

supplied from their normal hydraulic

source it is not normally available when

using the alternate brake system

the main advantages of such a system are

that it will react more quickly than the

pilot and will also constantly apply the

correct amount of braking to provide the

required rate of deceleration

depending on the aircraft type a number

of landing deceleration rates may be

selected the Boeing system shown here

uses a rotary selector switch to control

the level of braking required

the rejected takeoff or RTO selection

provides maximum braking

on an Airbus push buttons are used with

max being selected on takeoff to give

rejected takeoff protection

the D cell lights illuminate during the

landing role to indicate that the

selected level of deceleration is being

achieved

anti-skid protection is provided during

auto break operation if the anti-skid

system is unserviceable then the auto

brake system is disabled

on systems with a rotary selector the

landing auto brake system is armed by

selecting one of the deceleration rates

on the auto brake selector

the system is now armed provided the

anti-skid system is serviceable and the

normal brake system is being used

if these parameters are not met then the

autobrake disarm light will illuminate

on touchdown with ground mode and wheels

spin up sensed the brakes will be

automatically applied and will slow the

aircraft at the selected rate of

deceleration to a complete stop or until

the auto brakes are disarmed

the deceleration rate may be changed

during Auto break operation without

disarming by rotating the selector

with our teo selected maximum break

pressure modulated by the anti-skid unit

will be applied automatically when all

thrust levers are closed at ground

speeds above a specific speed depending

on aircraft type normally around 85

knots

this brings the aircraft to a stop in

the shortest possible distance

if a rejected takeoff is initiated below

this specific speed the auto brake

system will not be activated and manual

braking will be required

the pilot can disarm the auto break at

any time and revert to manual braking

simply by applying pressure to the brake

pedals

as we have already seen the auto brake

system disarms immediately and the

disarm light will illuminate if an auto

brake or normal anti-skid system fault

occurs or if manual braking is applied

during autobrake operation disarming

will also occur if after landing any

thrust lever is advanced

or after landing if the speed brake

lever is returned to the down detent

after the speed brakes have been

deployed this is because advancing the

thrust levers or retracting the speed

brakes is an indication of a rejected

landing

the auto breaks can also be disarmed by

moving the auto break selector to disarm

or to off in this case the disarm light

will not illuminate

however the auto brake system is

normally disarmed by the handling pilot

tapping the brake pedals when he is

happy that the speed of the aircraft is

under control

that is the end of the lesson on

anti-skid and auto brake systems

remember that a modern anti-skid system

as well as producing optimum

deceleration also has the additional

features of touchdown protection

preventing the brakes being applied

before landing and locked wheel

protection releasing the pressure to any

wheel that locks because of hitting a

very slippery patch on the runway

the auto brake system if armed before

landing will automatically apply the

brakes after touchdown and decelerate

the aircraft at a predetermined rate

if the rejected takeoff mode is armed in

the event of a rejected takeoff it will

stop the aircraft in the shortest

possible distance