ARTICLES RESEARCH & TECHNOLOGY

Bleed System Failure Analysis of Airbus A320: Introduction to aero-engine (CFM56-5B)

CFM56-5B-engine

CHAPTER 2


Introduction to aero – engine (CFM56-5B)

To make an aircraft move forward there requires a pushing force i.e. thrust; which is obtained by the acceleration of air passing through an engine. The engine most widely used in the Airbus-320 fleet is the CFM56-5B engine. China eastern airlines’ A320 also mainly uses engine CFM56-5B, manufactured by GE.

CFM56-5B_engine

The CFM56-5B is a high-bypass turbofan engine with several variants having bypass ratios ranging from 5:1 to 6:1, which in turn generates 80KN to 150KN of thrust. It is a two spool engine i.e. it has two rotating shafts, one high pressure and other low pressure powered by its respective turbine. The components of the CFM56-5B include a fan, compressor, combustion chamber, Turbines and the exhaust assembly.

The fan is the large diameter propeller in front of the engine. It’s the airflow inlet of the engine. The airflow is categorized into the primary and secondary flow. The primary flow passes (20%) through the fan to the compressor whereas the secondary flow (80%) passes through the fan to the bypass duct and provides 80% of the engine’s thrust.

movement-of-air-in-CFM56-5B

The compressor classified as low pressure and high pressure are the receivers of the primary airflow from the fan. The airflow gets compressed i.e. the pressure of the airflow increases as it moves along the 13 stages of the compressor. High-pressure bleed air is extracted from the stage 5 and 9 of the compressor which is located in low and high-pressure compressor respectively. The air passing through the compressor reaches a temperature of 450 ºC.

The high-pressure air then moves to the combustion chamber where it is mixed with jet fuel and burnt. The temperature of the air then rises to 1700ºC. The air after being burnt in the combustion chamber immediately moves to the turbines where the accumulated energy is extracted in 5 turbine stages; 1 high pressure, 4 low pressure.

The pressure of the air decreases as it moves along the turbine and makes them spin. The turbines are connected to concentric shafts which are connected to the fan and the two compressors. Thus as the turbine rotates it drives the fan and the compressors. The air from the turbines is then expelled through the primary duct joining the air from the bypass duct.

Thus the CFM56- 5B turbofan engine is a flow cycle engine. Air is compressed and then heated by burning fuel after which it passes through the turbines which in turn drive the compressors and the fan.

Engine bleed system principle

Bleed air is the high-pressure air extracted from the compressor in an engine and then distributed to various systems with the help of different ducts, valves, and other components. These various components helping in the transportation of engine bleed air comprise the engine bleed system. The engine bleed air system is a significant system for an aircraft flight operation and stands to be the heart of the engine pneumatic system.

China eastern A320 mainly uses the CFM56- 5B engine manufactured by GE. Other A320 aircrafts which use the V2500 manufactured by IAE have the same bleed system with only a little difference. Our main focus shall remain on the bleed system of the CFM56-5B engine.

CFM56-5B-engine-bleed-system

Engine air is generally bled from an Intermediate Pressure (IP) stage of the engine High Pressure (HP). The IP stage is the 5th HP compressor stage. At low engine speeds, when the pressure from the IP stage is insufficient, air is automatically bled from a higher compressor stage (9th HP compressor stage).

The HP bleed valve working with PRV pneumatically limits the downstream static pressure: 36 to 44 psi. Downstream of the PRV an overpressure valve (OPV) (5HA) is installed to protect the pneumatic system against damage if overpressure occurs. When the upstream pressure increases and reaches 75 psi, the OPV starts to close as the pressure on the piston overcomes the spring force.

This decreases the air flow and thus reduces the downstream pressure. At 85 psi upstream pressure the OPV is fully closed, it opens again when the upstream pressure has decreased to less than or equal to 35 psi. To keep the temperature within the limits, a fan air valve (FAV) (9HA) is installed in the cooling air duct which supplies fan air to the pre-cooler exchanger. The FAV operates pneumatically and is connected by a sense line to the FAV control thermostat (7170HM). The thermostat is installed downstream of the pre-cooler exchanger.

For analysis the engine problem we encounter it is better that we first get the number of parts which cause bleed problem in 2006, 2007, 2008 from the reliability system of The engineering and technology Company of China Eastern Airlines which is as shown below.

parts-changed-in-deal-with-ebs

table-2.1

The chart is the parts (including TCT, FAV, and TCT air filter) changed to deal with Engine bleed problem in (2006, 2007, and 2008). Through these charts, we concluded that TCT,FAV,TCT air filter, HPV,TLT were components with high rate of failure. The TCT air filter is a part of TCT. From the report it is clear that two of HPV are NFF. In line maintenance, the Engine bleed problem caused by HPV and TLT mostly can be dispatched by MEL.

Engine bleed problem caused by TCT and FAV are the ones that cause maximum interruption in operation. So we have reached a conclusion that the TCT and FAV are reasons for more Engine bleed failure. The two parts mentioned are used to control the Engine bleed temperature.

It is necessary to know the design and operation of TCT and FAV. Only by deeply understanding the engine bleed temperature regulating system, can the troubleshooting in our daily work be finished, and then the maintenance program is optimized.

Description and operation of TCT

The Temperature Control Thermostat (TCT) is part of temperature control/limitation section of the engine air bleed system. Installed on the pre-cooler outlet duct, it limits the pre-cooler outlet temperature by modulating the servo-pressure of the fan air valve.

Characteristics of TCT

Supply Pressure: 3 bar rel. (43.5 psi).
Maximum Supply Temperature: In the event of failure: 260 °C (500 °F).
Normal value: + 200 ± 15 °C (365 to 419 °F) within the pressure and temperature ranges.

Structure of TCT

The Temperature Control Thermostat has two main parts: Temperature-sensing part and pressure-regulation part. The sensing assembly has a tube made of stainless steel, a rod made of INVAR, a metal which has a low coefficient of thermal expansion. The lower tip of the INVAR rod is soldered to the bottom of the stainless steel tube. The rod is attached to the body of the sensor.

The pressure regulation valve has a body made of light alloy incorporating a clapper controlled by a diaphragm. The diaphragm delimits two chambers: The opening chamber (A), and The closing chamber (B) (which is air vented). Clapper is held in contact with the diaphragm centering guide by a return spring. A union secured into the body of the pressure reducing valve interconnects pressure reducer with the servo chamber of the fan air valve.

temperature-control-function

Temperature Control Function of TCT

The thermostat pneumatically supplies and controls the servo chamber of the FAV in relation with the temperature measured at the pre-cooler outlet. Two functions adjust the servo pressure in the chamber (A) of the pressure reducer.

When the pre-cooled air temperature is more than or equal to a set value, the regulating probe controls the FAV to adjust the fan air flow rate. This function is related to the differential thermal expansion between the stainless steel tube and the Invar rod, which opens the rod valve and releases the pressure in chamber A.

This supplies a pressure into the servo chamber of the Fan Air Valve that changes frequently between clapper and its seat. Because of this, the Fan Air Valve adjusts a cooling airflow in relation to the temperature conditions measured at the pre-cooled air outlet.

operation-temperature-contr0l-function-of-TCT

TCT air filter clogging: The pressure regulating valve failure are the main causes of the TCT failure, especially TCT air filter clogging. The air filter in the TCT is always clogged for dirtiness especially in the summer days during the thunderstorm. TCT air filter can be changed in line maintenance but the pressure regulating valve is in the body of TCT and so cannot be changed separately.

The picture of The TCT air filter clogging is shown below.

the-air-filter-location

example-of-air-filter-clogging

Description and operation of FAV

The Fan Air Valve is part of the temperature limitation system in the engine bleed air circuit on the aircraft.

Fan Air Valve (FAV) structure

The Fan Air Valve comprises of a number of structures including a Valve Body S/A, which has a body with a butterfly which is installed on a shaft. A simple effect piston/cylinder with a return spring operates the Valve Body S/A. A Pneumatic Actuator S/A, which has a piston seal assembly and a return spring which operates the butterfly.

The pressure that is applied to the control piston comes from a temperature control thermostat. Two end-of-travel micro-switches that the butterfly shaft operates and which indicate the position of the valve as “FULLY CLOSED/NOT FULLY CLOSED” and “FULLY OPEN/NOT FULLY OPEN”.

the-fav

FAV operation

The function of the FAV is to supply airflow to the pre-cooler. This airflow decreases the temperature of the charge air at the outlet of the pre-cooler. A temperature control thermostat senses the charge air outlet of the pre-cooler. The pressure in the (A) chamber of the pneumatic actuator changes the position of the butterfly and the butterfly position changes the rate of flow through the FAV.

With zero pressure, the butterfly is fully closed (with the load of the spring). With maximum pressure, the butterfly is fully open. With a controlled pressure, the butterfly is in a controlled position.

principle-of-operation-of-fav

The FAV is often found to have an inner leak in maintenance work, which can be found by the test tool.

One typical engine bleed fault

A typical engine bleed fault encountered in the reliability system of the engineering and technology Company of China Eastern Airlines has been put forward below.

9 JULY 2007, the aircraft (Airbus 320-214, B-2362) departed from Nanjing at 0750 hrs, at approximately 0845 hrs the No 2 engine HP bleed valve started to cycle between the open and closed positions but appeared to stop cycling after about five minutes. At 0852 hrs, however, an ECAM warning AIR ENG 2 BLEED FAULT was annunciated to the crew.

Some information must be mentioned: engine 1 supply was always off in this flight, because the engine 1 bleed fault had been reported by the crew before this flight , the aircraft was dispatched ,according to MEL, the affected engine bleed must be switched off. The crew declared an emergency and began an emergency descent to FL100 as advised and during the descent the APU was started. A diversion to Beijing was initiated but after discussion with the company the aircraft diverted to Shanghai Pudong airport.

Through the company’s subsequent investigation: On the previous day the aircraft had operated from Guangzhou to Nanjing, the same fault on the No 1 engine bleed system was reported during the climb from Guangzhou. However, the fault cleared after resetting the system which then operated satisfactorily during the cruise and descent.

The maintenance personnel dispatched the aircraft according to the MEL, but they did not check the status of engine 2’s bleed system, which can be seen in the CFDS (Centralized Fault Display System).

Maintenance action

Both Bleed Monitoring Computers were tested via the Built-In Test Equipment (BITE) and no faults were found. The initial No1 engine bleed fault was traced to a loose union within the sensor line within the sensor line for FAV from the TCT.

The aircraft returned to Nanjing on a non-revenue flight for further investigation. No faults were identified with the No 2 bleed system. As a precaution, the No 2 FAV TCT, No 2 PACK flow control valve, and No 1 pressure regulating valve (PRV) control solenoid were removed.

All sensor lines were inspected and No. 1 bleed control sensor line was replaced. Strip examination of the No. 2 TCT revealed that the valve within the regulator assembly was sticking within its guide and a leak test of the unit was out of tolerance.

The Airbus’ reply

The loss of both bleed systems has occurred on several occasions in the past. This led to the issue of manufacturers Technical Follow-Up Document (TFU reference 36.11.43.005) first issued in 1998. This noted that several operators had reported cases of simultaneous failures of both air engines’ bleed system due to over-temperature conditions.

The investigation revealed that a simultaneous failure of both systems could be due to a dormant failure within one TCT, which only became apparent after the first system failure. The remaining TCT could not compensate for the normal increase of the bleed air temperature resulting from the increase in air bleed demand on the remaining system.

Strip examination of TCTs removed from these aircraft revealed that there was movement within the regulator valve guide assembly which could result in a permanent or temporary limitation in the orifice section (through which operating pressure is sent to the FAV opening chamber.)

As a case of increased bleed temperature, such as the one that occurs in single bleed operation. As an interim solution the size of the guide bore was slightly increased in production from unit serial number 2706 and was also applied on units returned for repair through revision of Vendor Service Bulletin (VSB).

In order to prevent any potential movement of the regulator valve guide a special ring together with a change in assembly process has been defined and introduced through VSB with a new Part Number.

As mentioned above, the bad performance of TCT could cause dual bleed loss and thus it is a big threat to aviation safety. There are other operational interruptions including flight delay, flight canceled and returning to base caused by over-temperature, which can be extracted from the reliability system of the engineering and technology Company of China Eastern Airlines.

The engine bleed system’s bad performance causes abnormal operation, big waste of money, and also diminishes the company’s reputation. Through Close analysis of an abnormal operation, it can be observed that the reasons for above abnormal operations are complicated, some are human reasons like the mechanic’s experience, the crew‘s operation, the communication between the mechanic and the crew.

Others are incidental and uncontrollable like the performance of parts, the bad weather condition. These reasons are not always independent and are integrated sometimes. The Company can optimize the maintenance program in some aspects such as; Engineering, Maintenance plan, Material support, Communication with the crew, which will be shown in detail in the following words.

The maintenance program’s aim is to reduce failure rate of bleed system ,as well as decrease the airline delay rate, and reduce AOG time, without increasing more human resource and material cost.

Introduction to bleed system leakage

Bleed air leakage can be defined as the process in which there is an uncontrolled loss of bleed air from any part of the aircraft pneumatic system or from the services that utilize bleed air. It takes account for all the tubes, pipes, ducts, etc that help in bleed air transportation or which make up the bleed air system. This loss of bleed air from the pneumatic system or any other system pneumatically powered can cause damages to aircraft wiring, overheat components and even cause fire.

It also has serious consequences on other systems such as anti- icing system, environment control system, etc. The leakage of hot air from the bleed air duct can pass onto other sensitive components nearby and cause serious damage. Since the aircraft is constantly in motion and is subjected to stress and strains from landing,

The leakage of hot air from the bleed air duct can pass onto other sensitive components nearby and cause serious damage. Since the aircraft is constantly in motion and is subjected to stress and strains from landing, take-offs, turbulences etc it is very important to monitor the condition of bleed air ducts for any leakage occurrence.

Different types of leak detection systems are inbuilt in the aircraft to monitor the bleed system for any type of leakage and send the information to the cockpit if found any. An example of bleed leakage detection system is the ODS (Overheat Detection System). In such system, a cable externally installed in parallel with the bleed duct includes two resistive wires that are immersed in a special salt solution.

When leakage occurs, the temperature outside the duct rises and the heating causes the equivalent conductance between the two wires to decrease significantly. This change in conductance is detected by electronics (e.g., a microprocessor) which closes the bleed valve and provides a “duct leakage” message to the pilot’s control panel.

Because the equivalent conductance depends on the length of the cable, this Overheat Detection System is able to detect the exact place where the leakage is occurring. Unfortunately, the Overheat Detection System is a relatively expensive and complex system.

One another arrangement uses a thermal switch connected in series with the shutoff valve and also comprises of a monitoring control device. The thermal switch interrupts the shutoff circuit whenever a predetermined limit temperature within the bleed air duct is exceeded. The thermal switch prevents possible overheating relating to the temperature within the bleed duct. Thus, a variety of different thermostat designs

The thermal switch prevents possible overheating relating to the temperature within the bleed duct. Thus, a variety of different thermostat designs have been developed and optimized in an attempt to improve bleed leakage detection reliability and low cost. A technical challenge is to attain an acceptable level of reliability.

Although the commonly available thermostatic thermal switch is considered to be highly reliable, the ratio between the thermostat failure rate and duct leakage failure rate may still remain significant. Therefore, the probability is not negligible that a thermostat used to detect bleed air duct failure may have latently failed by the time duct leakage occurs. A thermostat latently fails when it has failed but its failure has not yet been detected.

The exemplary illustrative non-limiting technology herein provides a bleed leakage detection system including an arrangement of series-connected, disparately placed thermostats. The bleed leakage detection system is capable of detecting the exact place where the bleed air leakage is occurring (e.g., the precise failed junction in bleed air duct work) by detecting which thermostat has opened.

Such detection can be accomplished by monitoring voltage or current on a line that connects the thermostats in series with respective resistances. The exemplary illustrative non-limiting implementation also provides a sequential bleed leakage detection system thermostat self-test function (Initiated Built In Test-“IBIT”) which allows continuous monitoring of thermostat sensor wiring during flight. The pilot is alerted when the bleed leakage detection system has failed so that appropriate countermeasures and maintenance may be performed.

Read More: CHAPTER 3