CFM56-5C Thrust Reverser Operation / HCU

Dear All,

Here, we are going to brief the sequence of thrust reverser deployment on engine CFM 56-5C installed on the A340-200/300.

Note: The schematic photo is taken from AIRBUS CBT CD / Its here for studying purposes only. It is not allowed to copy it for trading purposes without prior notice from AIRBUS.


The ECU controls the deployment of the thrust level as follow:

1) Taking direct signal from the TLA -Thrust Lever Angle- while aircraft on ground and engines are running. Or taking a signal from EIVMU through the inhibitation logic when the reverser requisition confirmed by the TCU – Throttle control unit-.

The HCU has a Deploy Solenoid Valve and Isolation Solenoid Valve. Also it has Pressure switch and inhibit switch controlled by ECU for reverser control and monitoring.

2) The hydraulic is supplied to the HCU through the Hydraulic Shut Off valve which opens according to the TLA signal computed in the FCPC – Flight Control Primary Controller-.

3) The ECU will energize or de-energize deployment or stowing solenoids to open or close valves for hydraulic to pass through the HCU – Hydraulic Control Unit- which is responsible for the sequence unlocking, deploying, stowing and then locking the reverser doors.

4) When reverser deployment required both solenoids energized, the hydraulic is supplied to the locking mechanism to unlatch the doors then hydraulic supplied to the stow side, the door and actuator will be unlocked the stow switches will send this indication of unlatched reverser on ECAM (REV In AMBER) then the hydraulic will be supplied to the extend side of the pivoting door actuator. When the door is fully open the deploy switches will close and the deploy signal is sent to ECU and indication appears on ECAM (REV In GREEN). The ECU will de-energize the isolation solenoid.

5) When the TLA set back to normal (STOW) the ECU will de-energizes the deploy solenoid and energizes the Isolation solenoid. The hydraulic is then supplied to the retract side of the actuator piston. The deploy switch will open and unstowed will be sent again through ECU. The hydraulic shut off valve will close.

The HCU -Hydraulic Control Unit- consists of:



1) Isolation Control Valve and its solenoid responsible to supply hydraulic to the HCU. Its spring loaded closed, its two positions valve.

2) Pressure switch signals the HCU of hydraulic pressure availability.

3) Directional control valve and its solenoid supplies the actuator.

4) Flow control valve controls the stowing speed of the doors.

5) Deploy solenoid valve supplies hydraulic to the latches.

The hydraulic supplied, the isolation solenoid is energized the hydraulic will enter the HCU, and then the hydraulic pass through the energized deploy valve to release the latches of the four doors actuators one by one. Then the return hydraulic will return to the HCU to pass through the directional control valve which allows fluid flow to the actuators. Both sides of the piston in the actuator have now hydraulic but due to the differential of the area on the piston sides, the actuator will move in the deploy position. When at least one door start to deploy actuator more than .7% of its travel the stow switch will signal the ECU. This will give REV indication in amber On the E/WD. At 94 % of the travel, the door deployment start to decrease in motion and the deploy switch will be activated. REV in green will be indicated when all doors are deployed. Then the ECU will de-energize all the solenoids and the blocker doors remain open by the aerodynamic forces of the FAN AIR FLOW.

When set to stow, the ECU will energizes the isolation valve while the deploy solenoid isn’t energized. The hydraulic flow in the stow direction on the actuator and the flow control valve will control the stow speed. At 94% of the travel the REV on indication returns amber until it disappears at .7% of the travel.


There are four modes for the thrust reverser. They are: 1) Pre-deployment. 2) Deployment. 3) Stowing 4) Locking.

For maintenance purposes, the Thrust Reverser can be tested on ground through the CMC and be deployed without engines run. The CMC will simulate engine N2 conditions to allow the deployment during test.

The thrust reverser operation logic (AND GATE LOGIC) are: TLA position, Aircraft on Ground, and N2 (Engines are running).

B.Regards
Ayman Shak'ah
Licensed Aircraft Maint. Engineer

Inlet Engine Fairings / Shapes / Swirl Mark

Dear All,

Alot of people in the field are always asking why the engine inlet fairings differ from engine to another?? Some of them are cones another are half ball??

What does the Swirl White Solid Line Means??

I would like to clarify this issue in a very simple way giving an example of some engine types.

On CFM56-5A/5B/5C, the engine inlet fairing looks like a cone. This cone has a specific sliding angle which is called (Anti Ice Algebric Angle). According to the studies done by the designers, they have noticed that at this specific angle of cone slide the ice will not accumulate or build up on the engine inlet cone which in some cases it may become dangerous on engine operation.

On V2500, although it is a cone, but the cone slide doesn't have Anti Ice Algebric Angle as it is more acute angle. Instead of that angle, the cone head is made of small piece made from rubber. This peice will keep vibrating while engine is running preventing engine inlet ice accumulation or build up mechanically. (REALLY SMART DESIGNERS).

On CF6-8, the engine inlet fairing isn't completely a cone. It is half ball shape and as we know on this shape, it is almost impossible to be the perfect shape for ice accumulation or build up. Now most of the Engine Inlet Fairings have swirl solid white line. This line will become like EAGLE EYE shape when the engine starts to run. This shape can be distiguished clearly by the birds. The EAGLE EYE shape will make birds frightened from coming near the engine while running and this will prevent bird ingestion or what we call (ENGINE BIRD STRIKE).

Please note that in the near future, you will find alot of photos and schematics explaining most of common questions generated by different people in the field and from passengers.

B.Regards
Ayman Shak'ah
Licensed Aircraft Maint. Engineer

A320 Electrical System / Sources

Dear All,

The A320 has alot of electrical sources. They are as follow:

1- Two AC main buses. Each AC bus is energized from an IDG - Integrated Driven Generator - installed on each engine. The output values are: 115VAC / 90 KVA / Frequency 400 Hz.

2- One main AC bus is feeding something called essential bus and essential shed bus. The shed bus is connected to the equipment at which is important for the safety of the aircraft when buses are lost. When the main AC bus failed, then the another AC bus can be connected to the essential bus to feed it instead of the faulty one either automatic transfer or manual by push button when the auto logic fails.

3- The AC essential bus also can be fed by the emergency generator. This generator is hydraulically powered to give output electrical AC power to feed the AC essential bus. The generator is hydraulically powered by hydraulic pressurized either from the related hydraulic system if the blue hydraulic system pump is operative or by the RAT - Ram Air Turbine -. Note that the RAT can only turn at specific minimum speed on some A320 its 150 KNOTS on another A320 it is 100 knots or until the flaps slats extended.

4- When the RAT stop working, the AC essential bus will be fed from the batteries through a static inverter that will convert the 28 VDC into 115 VAC. So as you can see, the major equipment are connected with essential buses (AC and DC through essential transformer). This essential buses are always fed either from IDG, APU generator, CSM/G - Emergency Generator - using the RAT, and or static inverter. This will give the electrical system a very very high redundancy.

Thanks for AIRBUS.

B.Regards
Ayman Shak'ah
Licensed Aircraft Maint. Engineer

Rules For Dispatching Aircraft For Next Flight

Dear All,

I would like to brief some information regarding the basics in dispatching an aircraft for the next flight. These information are as follow:

1- When the aircraft reached the destination, the pilot will register any comments, faults, warnings, observations,that occured during his flight in the ATL - Aircraft Technical Log Book-. If he has no comment then he will write down "NIL DEFFECTS". At the same time, when the aircraft reaches to the gate, the ground engineer, technician, and or mechanic will carry out walk around, and transit check. Accordingly if he observe any finding such as leak, damage, ...etc he will also report it in the ATL.

2- The Aircraft Log Book is usually has two columns. One for the Finding where to be reported another for the engineer to write down the ACTION TAKEN.

3- There is a book called MMEL "Master Minimum Equipment List" in each aircraft. The MMEL is issued by the manufacturer and approved by the Civil Authorities like FAA, EASA, CAA, ...etc. Note that any airline can make more restrictions to what is mentioned in the MMEL but can't reduce what is already in it. In this case the MMEL will become MEL - Minimum Equipment List-.

4- The MMEL or MEL is the book that has most of the faults mentioned in. It will give the minimum requirment needed for the fault to be dispatched. Like it will mention if this fault is "GO", "NO GO", or "GO IF...". In the MEL also there will be for some faults the code "M" or "O" and or BOTH. The "O" is stated for OPERATIONAL PROCEDURE while the "M" is stated for MAINTENANCE PROCEDURE. That means if this fault occurs then the Pilots have specific procedure to carry out during aircraft operation regarding that system or flight and in "M" case that means the maintenance has procedure to do before dispatching the aircraft. Please note that MEL items are categorized in dispatchable time limitation such as maybe some faults are GO item for maximum limited number of flights another are dispatcheable for maximum number of days and some them are dispatcheable in some atmospheric conditions.

5- The captain can REJECT what the engineer is saying if he felt not convinced in such dispatchable conditions mentioned in the MEL because in some cases he believe in this atmospheric climate condition he may face the next flight as he can see in the meteorology report some troubles. Anyway he can REJECT the action taken and usually in this case this issue will be reported to the MANAGEMENT so they will advice the flight crew what to do or advice the Maintenance personal of what to do. LAST DECISION WILL BE FOR THE PILOTS as they are going to fly this aircraft.

6- there are some faults that you will not find in the MEL. like LEAK. We can find the dispatchable limitation for any leak in the AMM - Aircraft Maintenance Manual - that will state the dispatchable leak limitation and according also to the leak limitation.

7- Some findings maybe like structore damage, in this case the reference will be something called SRM "Structural Repair Manual" that will give the dispatchable measurements of that damage and always we put into consideration if this damage location in pressurized or unpressurized zones on the aircraft.

So far, I hope that this briefing clarify the way of dispatching any aircraft for the next flight. At the end of each aircraft log book page, the ground engineer, or technician, or mechanic will sign certification that the aircraft is safe for the next flight and then the CAPTAIN will sign final acceptance for that aircraft to take.

Best Regards
Ayman Shak'ah
Licensed Aircraft Maint. Engineer