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

Human Factors / Research

Dear All,

For 10 years and until date, it becomes very important for Companies Managements, Employees, Decision Makers, and we can say even everyone to study the HUMAN FACTORS.
Human Factors are divided into TWO TOPICS. They are: 1) Physiology Factors. 2) Psychology Factors.

These two topics are important to understand because if someone think that they don't affect his life then he must be sure that they are going to affect others life.

How? When? Why?

I am going to explain this by giving examples on How? When? and Why?

A) Let us start with HOW? WHEN? WHY? from the physiological angle.

We must know about the human body and his senses like hearing, sight sense, smelling, touching, Tasting. These senses are very important to understand that they have functions which will affect taking decisions. Examples on these affected senses in aviation are:

1) When your eyes are affected, then the pilots, maintenance personal,..etc will take improper decisions because they are not able to see things correctly like when an engineer do his walk around for visual inspection or like pilot whe he observes the aircraft parameters during flight or while he reads the check lists,..etc.

2) When ears are affected, then the people working in aviation may make mistakes such as can't evaluate strange noise occurs during flight, or improper evaluation for a noise coming out from the engines while running, ..etc.

3) When the touch sense is affected, then and according to my experience sometimes you can evaluate internal leak for a component by feeling how hot it is after running. So, if I dont have the full ability to feel how hot a component is then I will take a decision to release back a component with internal leak existing and this will make troubleshooting harder.

4) When your smell sense is affected then I can for example see a leak below the wings and I will take a sample from it using my finger and smell it. If the smell sense is affected maybe I will think it is a water condensation and not a fuel leak.

5) If my tasting sense is affected, then maybe hydraulic will come on my lips and I lick it thinking that it is a water and not able to distinguish that it is hydraulic or poisonous material.

In Human Factor You study the human body parts and systems, how it works and what is it consists of. So, when you study it you will know how to care about it and keep it strong senses.

B) Let us start with HOW? WHEN? WHY? from the psychological angle.

All of us are humans, and we know how psycological factors affect us such as stress, environmental atmosphere, pressure, living costs, ...etc these factors will affect the way we take decisions like when you have financial problems and you are on duty flying an aircraft or repairing a component then most of your thinkings will be in your personal problem and this will reduce concentration of the brain on the procedure, safety, and how to troubleshoot.

There are something in the human factors study called DIRTY DOZEN. They are:

1) Lack of communication: like when somebody write a note for his colleague regarding an issue in doing a task. This note can be understood wrongle. So always it must be written in full details or at least without using alot of abbreviations.

2) Complacency: When you do something many times and you didn't find anything changed. then oneday you will say I have always done this check and I have never found anything wrong then I will not do it this time. VERY VERY COMMON AND DANGEROUS THINKING.

3) Lack of Knowledge: When you do something and you reach an area while doing it and you dont know how to do it. You may feel shame or afraid to say I DON'T KNOW. Then the shame will be bigger if you did it and caused a disaster.

4) Distraction: Very common factor. While you are working your mobile phone rings. You answer it and then after finishing the call you complete the job forgetting some points you must do. ALWAYS GO BACK 3 STEPS BEFOREE THE CALL.

5) Lack of Team Work: When you hate to work with someone or when you forget that all of people in the company are team then you will fail. Imagine that there are two pilots one is the captain another is the first officer. If they are not working in the cockpit as a team, be sure that in case of emergency the aircraft will crash.

6) Fatigue: Like working for many hours, not sleeping properly, all what you are thinking about is to make extra money with overtime. You think you are able and you are aware but believe me if you are honest with yourself then think how many times you have done mistakes because you didn't sleep well for days ago.

7) Lack of resources: Like having a problem on an aircraft and you need a part which is not available in your store. You may decide to disregard the problem and say "I will dispatch it". No, Ground the aircraft for sure is better.

8) Pressure: Liek when you get a call from your boss to inform you that you have to finish your job within an hour or less and you know that it takes more. Feeling afraid to lose the job. Believe me LOSE YOUR JOB IS MUCH BETTER THAN LOSING THE PEOPLE LIVES.

9) Assertiveness: A factor like when somebody brought you something to fix and he fill on you orders what to do according to his financial situation or needs of time. He orders you things just because he wants it like this and you know that it is wrong. DON'T DAMAGE YOUR STANDARDS for others to be happy.

10) Stress: Like political news. A war may happen. I will lose my job because of this war and such factors and thinkings. LIVE YOUR DAY, ALWAYS THINK THAT TOMORROW IS BETTER.

11) Lack of Awareness: When you do something always think of the consequences. Build a scenarios for what may happen if I install this here or there.

12) Norm: Like using things as equipment to reduce the time and taking short cuts that you think you are smarter than the manual or the manufacturer instructions. Sorry, you will be stupid and silly because even you succeed one or two times at the end you will fail. DONT DEPEND ON LUCK, THERE IS NO PLACE FOR LUCK IN OUR BUISINESS.

I hope you like such study and research I have done, and I hope we all are going to follow such instructions and make sure to stop doing any of the DIRTY DOZEN.

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

Turbine Inlet Temperature - TIT SENSOR

Turbine Inlet Temperature Sensor (Engine):

The turbine engine is basically composed of different stages of compressors, combustion chamber, turbine stages. Now, when the engine is running, the air will ingested into the engine and will be compressed through the compressor stages. After that the air will enter the combustion chamber as an element for combustion to take place with the fuel. then, the discharged hot exhausted air will leave the combustion chamber under very high in both (Pressure and Temperature). This exhausted air is used to hit the turbine (High Pressure Turbine) causing them to rotate at which in turn will rotate the compressor and so on.

It is used to measure the temperature of the discharged air coming from the combustion chamber and hitting the turbine. the sensing of this temperature is very useful to control the engine operation by the computer that is used this information and according analysis the engine combustion effeciency and at the same time control the power output of the engine automatically through a computer called ECU - Engine Control Unit -. the ECU is a computer that receives different signals from different sensors and systems of the aircraft. One of these inputs is the Turbine Inlet Temperature.

Turbine Inlet Temperature Sensor (Aircondition System):

There is also a sensor called TIT sensor installed on the airconditioning pack. The aircondition system pack. This pack composed of
1- ACM - Air Cycle Machine - (FAN, Compressor, and Turbine)
2- Heat exchangers
3- Condenser
4- Reheater.

Now, the aircondition system is controlled by computer called Pack Controller and another computer called Zone Controller. Now the engine bleed air is delivered to the airconditioning packs to be conditioned both (Temperature and pressure) in the following sequence:

1- the bled air enters the pack via a pack flow valve.
2- the hot bled air enters the primary heat exchanger to be cooled little bit before entering the ACM compressor at which the air is going to be compressed so its pressure will increase.
3- the air will leave the compressor with high pressure and very high temperature due to the compression.
4- then the air will enter the main heat exchanger to be cooled. Then the air will enter the reheater to increase its temperature so we can eliminate any water particle suspended in the air before entering the turbine - The elimination of any water particle is useful to protect the turbine blades from being daamaged or corroded-. Then the turbine will cause the compressed air to extract due to giving force for the turbine to rotate. At this point the air entering the turbine will be so hot and the air leaving is so cold.

Now the computer controlling the system operation needs input to control the flow rate of air entering the packs and also to control the air entering the heat exchangers. Also the computer needs these inputs for maintenance personal analysis and to monitor the efficiency of packs.

Turbine inlet temperature sensed will be signaled to the pack controller which in turn with another sensors signals will control the aircondition system operation.

Ayman Shak'ah
Licensed Aircraft 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