Anti-Skid & Auto-Brakes - Aircraft Brakes - Airframes & Aircraft Systems #23
Summary
TLDRThis 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
- π« **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.
- π **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.
- π **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.
- π§ **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.
- π« **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.
- π **Adaptive Pressure Bias Modulation**: Electronic anti-skid units adapt brake pressure to prevent immediate return to skid conditions, maintaining optimal braking efficiency.
- π **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.
- π **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.
- βοΈ **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.
- π **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
π« 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.
π¬ 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.
π 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.
Mindmap
Keywords
π‘Brake Modulating Systems
π‘Optimum Braking
π‘Anti-Skid Systems
π‘Deceleration
π‘Touchdown Protection
π‘Locked Wheel Protection
π‘Electronic Anti-Skid Unit (ASU)
π‘Auto Brake System
π‘Rejected Takeoff (RTO)
π‘Brake Torque Sensor
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
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