Airbus A320 CFM LEAP-1A Thrust Reverser Explained: How Reverse Thrust Really Works
- Sunil Rawat
- 7 days ago
- 8 min read
Thrust Reverser
The Hidden Engineering Behind Every Landing
When an Airbus A320/321 touches down, the aircraft is still moving at a very high speed. Even after the wheels are on the runway, thousands of kilograms of aircraft mass continue moving forward with a large amount of energy.
To safely reduce this energy, pilots use multiple systems:
Wheel brakes
Ground spoilers
Engine thrust reversers
Among these systems, the thrust reverser is unique because it uses the engine’s own airflow to help slow the aircraft.
From the passenger window, it looks simple—the engine changes sound and panels move open. But inside the engine, a carefully coordinated system is working.
This hidden system is called the: Thrust Reverser Actuation System (TRAS)
Scope of This Article
This article explains the Thrust Reverser Actuation System (TRAS) installed on Airbus A320 aircraft powered by the CFM International LEAP-1A engine. The system design, components, monitoring methods, and safety features explained here are specific to the A320 LEAP-1A application.
Other aircraft and engine combinations may use different thrust reverser designs.
What Does a Thrust Reverser Do?
During normal flight, the CFM LEAP-1A engine produces forward thrust by accelerating air through the engine. After landing, the aircraft needs additional assistance to slow down.
The thrust reverser helps by changing the direction of the fan bypass airflow.
The main moving component is called the:
Translating Cowl (Transcowl)
The translating cowl moves between two positions.
Stowed Position — Normal Engine Operation
In the stowed position:
The translating cowl remains closed.
Normal forward thrust is produced.
The thrust reverser is locked safely.
This is the condition used during:
Takeoff
Climb
Cruise
Approach
Deployed Position — Reverse Thrust Operation
When reverse thrust is selected:
The translating cowl moves aft.
The bypass airflow path changes.
Fan air is redirected.
Reverse thrust helps slow the aircraft.
Although the movement appears simple, many systems must work together before the cowl can safely move.
What Is the Thrust Reverser Actuation System (TRAS)?
The Thrust Reverser Actuation System (TRAS)Â is the system responsible for moving, controlling, locking, and monitoring the translating cowl.
A simple way to understand TRAS is to imagine a team where every member has a specific role.
The Thrust Reverser Actuation System (TRAS) Team
When the Pilot Says "Reverse"
When the pilot selects reverse thrust, a command from the cockpit starts a carefully controlled sequence inside the engine which is designed for safe operation.
The operation happens in several steps.
TRAS Safety Systems
A thrust reverser is a powerful system designed to operate only when commanded.
An accidental deployment during flight could create a serious safety risk, so the TRAS uses multiple protection layers to make sure the translating cowl moves only when the correct conditions are met.
The system uses three main safety barriers:
Primary locks
Hydraulic isolation
Electrical tertiary lock system
Together, these systems protect the aircraft from unwanted thrust reverser movement.
Understanding the TRAS Components in Detail
The main operation gave us the big picture.
Now lets look deeper into the individual members of the TRAS team and understand how each component performs its role.
Isolation Control Unit (ICU) — Detailed Operation
The ICU controls hydraulic pressure supplied to the TRAS.
The main component inside the ICU is the: Isolation Control Valve
Normally:
The valve is held in the closed position by a mechanical spring.
Hydraulic pressure is isolated from the thrust reverser system.
When the ICU receives an aircraft command:
The solenoid operates.
The closed solenoid valve is energized.
Pressure is sent to the pilot area of the spool.
The isolation valve moves to the deploy position.
Hydraulic fluid is supplied to the DCU.
The ICU monitors two important conditions:
Output Pressure Status
The pressure switch detects hydraulic pressure and sends status information:
Pressurized.
Not pressurized.
Inhibition Status
The limit switch provides:
Inhibited status.
Active status.
The inhibition signal is controlled by the inhibition lever.
Both the pressure switch and limit switch contain two channels, and each channel is continuously monitored.
The ICU supplies EEC channels A and B with:
Output pressure information.
Inhibition status information.
Directional Control Unit (DCU) — Detailed Operation
The DCU controls hydraulic flow direction to the actuators.
The main component inside the DCU is the: Directional Valve
The valve has two positions:
Stow position.
Deploy position.
Normally:
The valve is held in the stow position by a mechanical spring.
When the DCU solenoid is energized:
Hydraulic pressure moves the directional spool valve.
Hydraulic fluid is directed to the deploy side of the actuators.
The translating cowl moves toward reverse thrust position.
When solenoid power is removed:
The DCU returns to its normal condition.
High-pressure hydraulic fluid is removed from the deploy side.
Pressure is supplied to the stow side.
The translating cowl returns to the forward position.
Actuators — Detailed Overview
The TRAS uses four actuators to move the translating cowl.
Two Synchronized Locking Feedback Actuators (SFLA).
One Synchronized Manual Locking Actuator (SMLA).
One synchronized non-locking actuator (SNLA).
The actuators are connected through the synchronization system to ensure:
Equal movement.
Correct alignment.
Smooth operation.
Synchronized Locking Feedback Actuators — Locking + Feedback
The two upper actuators are the synchronized locking feedback actuators.
They are the most advanced actuators in the system because they provide:
Mechanical movement.
Locking capability.
Position feedback.
Each actuator contains:
Primary lock.
Single-channel LVDT.
Two single-channel proximity sensors.
The LVDTs provide position information to:
EEC channel A.
EEC channel B.
The proximity sensors confirm:
Fully stowed position.
Lock condition.
The LVDTs are part of the actuator assembly and are not Line Replaceable Units (LRUs).
The proximity sensors are LRUs and do not require adjustment.
Lower Actuators — Supporting the Main Movement
The two lower actuators support the movement of the translating cowl and work together with the upper synchronized locking feedback actuators.
They are:
Synchronized manual locking actuator (SMLA).
Synchronized non-locking actuator (SNLA).
Both lower actuators are connected to the upper actuators through the synchronization system, ensuring all four actuators move together.
Synchronized Manual Locking Actuator (SMLA) — Maintenance Safety Feature
The synchronized manual locking actuator is installed as the: Right lower actuator
Its main purpose is to support normal thrust reverser movement.
It also contains an important maintenance safety feature:
Manual Locking Lever (Bumper Lever)
The bumper lever allows maintenance personnel to mechanically lock the translating cowl during ground maintenance.
This prevents unwanted movement while technicians are working on the aircraft.
The actuator also contains a fire seal where the actuator body passes through the thrust reverser forward frame.
Synchronized Non-Locking Actuator (SNLA) — Movement Support
The synchronized non-locking actuator is installed as the: Left lower actuator
Its function is to:
Support translating cowl movement.
Maintain synchronization with the other actuators.
Unlike the synchronized locking feedback actuators:
It does not provide LVDT feedback.
Its position status is not monitored by the EEC system.
The actuator also contains a fire seal where it passes through the thrust reverser forward frame.
Synchronization Shafts — Keeping Four Actuators Working as One
Moving four actuators together requires accurate synchronization.
The TRAS uses three synchronization shafts installed inside tube assemblies.
Their purpose is to make sure:
All actuators move at the same time.
All actuators move at the same rate.
The translating cowl remains correctly aligned.
Manual Drive Units (MDUs) — Maintenance Operation Without Power
The TRAS includes a manual operating feature for maintenance activities.
This feature uses: Manual Drive Units (MDUs)
An MDU is installed on each of the two lower actuators. The MDU allows maintenance personnel to move the translating cowl without:
Hydraulic power.
Electrical power.
Before using the MDU:
The ICU must be in the deactivated position.
The synchronized locking feedback actuators must be unlocked.
The tertiary lock system must be unlocked.
Maintenance personnel apply torque to the MDU using a wrench to manually move the translating cowl. Only one MDU is required to deploy or stow the cowl.
Electronic Engine Controls (EECs) — Supervising the Entire System
The Electronic Engine Controls are the supervisors of the TRAS operation.
They continuously monitor information received from:
LVDTs.
Proximity sensors.
ICU pressure monitoring.
Lock systems.
The TRAS uses two monitoring channels:
EEC Channel A.
EEC Channel B.
This provides independent system monitoring and improves reliability.
The EECs monitor:
Actuator Position
Provided by:
LVDT feedback from synchronized locking feedback actuators.
Lock Status
Provided by:
Primary lock proximity sensors.
Tertiary lock proximity sensors.
How TRAS Prevents Accidental Thrust Reverser Deployment ?
1. Primary Locks — The First Safety Barrier
The primary locks are installed inside the: Synchronized Locking Feedback Actuators
Their main purpose is to keep the translating cowl locked in the stowed position.
When the thrust reverser is not being used:
The primary locks remain engaged.
The translating cowl is held securely.
Accidental movement toward the deployed position is prevented.
When reverse thrust is commanded:
Hydraulic pressure is supplied from the ICU and DCU.
The primary locks release.
The actuators are allowed to move toward deployment.
The primary locks are continuously monitored by the Electronic Engine Controls (EECs) using proximity sensors.
2. Hydraulic Isolation — Controlling Access to Power
Hydraulic pressure provides the force needed to move the thrust reverser.
However, hydraulic power must only reach the system when commanded.
This protection is provided by the: Isolation Control Unit (ICU)
The ICU controls the hydraulic connection between the aircraft hydraulic system and the TRAS.
Without the correct command:
Hydraulic pressure is isolated.
Actuators cannot move.
The translating cowl remains locked.
This makes the ICU an important safety barrier as well as a hydraulic control component.
3. Electrical Tertiary Lock — The Additional Safety Guard
The electrical tertiary lock system provides the third protection layer.
It acts as an additional mechanical lock to prevent accidental movement of the translating cowl.
The tertiary lock system is installed:
On the left latch beam of the thrust reverser.
At the 6 o’clock position.
It is controlled directly by the aircraft systems and continuously monitored by the EECs.
How the Tertiary Lock Works
When electrical power is applied:
The tertiary lock unlocks.
The translating cowl is allowed to move toward the deployed position.
When electrical power is removed:
The tertiary lock automatically returns to the active condition.
The translating cowl is locked in the stowed position.
The roller is attached to the translating cowl. When the cowl reaches the stowed position:
The roller engages with the tertiary lock hook.
The translating cowl is secured.
The lock condition is monitored through two single-channel proximity sensors:
One monitored through EEC channel A.
One monitored through EEC channel B.
Summary of the TRAS Operation.
Step 1: Pilot Command
The pilot moves the reverse throttle lever.
The aircraft systems send a command signal to the TRAS.
The system checks that the required conditions are available before allowing movement.
Step 2: ICU - Allows Hydraulic Power
The first component to act is the: Isolation Control Unit (ICU). The ICU controls hydraulic pressure supplied to the TRAS.
The ICU does not create hydraulic pressure. Instead, it controls when hydraulic pressure is allowed to enter the system.
When reverse thrust is selected:
The ICU solenoid operates.
The isolation control valve opens.
Hydraulic pressure becomes available for the DCU.
The ICU also provides system information to the Electronic Engine Controls (EECs), including:
Hydraulic pressure status.
System inhibition status.
Step 3: DCU - Directs Hydraulic Flow
The next component is the: Directional Control Unit (DCU). The DCU decides where hydraulic pressure should go.
During deployment:
The DCU sends hydraulic pressure to the deploy side of the actuators.
The actuators begin moving the translating cowl.
During stowing:
Hydraulic flow changes direction.
The actuators return the cowl to the forward position.
Step 4: Actuators - Move the Translating Cowl
The hydraulic pressure reaches the: Actuators. They convert hydraulic energy into mechanical movement.
The four main actuators are:
Two synchronized locking feedback actuators.
One synchronized manual locking actuator.
One synchronized non-locking actuator.
All four actuators must move together at the same rate to keep the translating cowl aligned.
Step 5: Sensors - Confirm Operation
While the translating cowl moves, sensors continuously monitor the system.
They provide information about:
Actuator position.
Lock condition.
System status.
This information is sent to the EECs.
The Result: A Controlled Reverse Thrust Operation
A simple command from the pilot becomes a coordinated action between:
Hydraulic systems.
Mechanical components.
Electrical sensors.
Engine computers.
A Simple Action Powered by Complex Engineering
From the outside, reverse thrust looks like a simple action—the engine changes sound and the thrust reverser opens.
Inside the engine, however, a highly coordinated system is operating.
The Airbus A320 CFM LEAP-1A TRAS combines:
Hydraulic control.
Mechanical movement.
Electrical monitoring.
Computer supervision.
Multiple safety systems.
The pilot gives one command: Select reverse thrust.
Behind that command, the TRAS team performs hundreds of coordinated actions to safely move, monitor, and protect the thrust reverser system.
This is what makes modern aircraft systems reliable: many individual components working together as one.