ARTICLES RESEARCH & TECHNOLOGY

Bleed System Failure Analysis of Airbus A320: The measures mainly taken in line maintenance

Maintenance-LineWork-SRTechnics

CHAPTER 3


Line maintenance is concerned with the planning and execution of different maintenance activities performed in order to keep an aircraft in an airworthy condition in accordance with company and National Authority procedure while the aircraft is assigned to the revenue schedule.

The operational environment requires visibility of all new, deferred and cleared work items against an aircraft, enabling a longer reaction time window for planning and assignment of the work items to the executing line station.

Proper knowledge, sense of utilization and management of the line station resources (i.e. technicians, parts, tools and documents) is crucial to ensure the on time dispatch of an aircraft. To a manager of line maintenance department, the best utilization of own physical resource, human resource and improvement in line maintenance department procedure is a very important task.

Three aspects on which improvements have been made are;

(1) Improvement dispatch procedure in engine bleed fault,

(2) Strengthen the monitoring of engine bleed and

(3) Perfect the communication with the crew.

Every aspect has been discussed separately.

Improvement in dispatch procedure

As we know, the airline will hold permission for the pilot to operate with some equipment unserviceable for a limited time, just like one engine bleed fault, subject to the Minimum Equipment List (MEL), which is based on the Master MEL produced by the aircraft manufacturer. A Master MEL will not necessarily apply to everyone as circumstances differ from time to time. So operators must prepare their own which may not be less restrictive than the master.

MEL are lists of systems and equipment installed on an aircraft showing how many of them can be defective and for how long. In other words, there are changes to the type desigthatan do not require recertification. In some cases, additional restrictions are applied. For example- you may have to troubleshoot, inspect or secure items as conditions to be met before takeoff. As the MEL is a detailed list, it follows that any item not on it must be working at the time of dispatch.

The deferred defects are those which will not prejudice the safety of a flight but should be rectified as soon as practicable after it. However, it can fly when one of the above is unserviceable if it is not reasonably practical to effect repairs or replacements before taking off especially as outside maintenance should not be used without approval.

If the crews as commanders are satisfied with the forecasted weather conditions, regulations, etc. The crew is allowed to complete one flight to a place where repairs may be carried out. Notes: the idea is to get the crew to a place where the defect can be fixed or to fly while awaiting spares; it’s not for skimping on maintenance.

As a general rule, a defect will only be allowed for a return to base. Only under exceptional circumstance should the crew depart (see MEL). The aircraft should not then fly until they are either cleared or defected, which means that it can be scheduled for a more convenient time such as the next service when the machine is in the shop anyway.

Next, the MEL related to the Engine bleed temperature fault has been shown. If an engine over-temperature fault is reported in line maintenance, it is very difficult to judge which part actually has the fault; TCT, FAV or others but engine bleed system can be considered affected and is inoperative and then referring to MEL 36-11-01, the aircraft can be dispatched.

the_mmel

In the case of a single bleed fault, the aircraft can still be dispatched with the associated bleed valve switch selected to OFF and the cross bleed valve selected open in accordance with the MMEL 36-11-01, provided that icing conditions are not predicted and the altitude is limited to 31,500 feet.

But attention must be paid to the fact that if one bleed system is disabled, the other system becomes overloaded which can cause both systems being inoperative. Like the typical example mentioned above in this very paper. So when the deferred defect in engine bleed system is made, the other engine must be checked to ensure that there is no class 3 fault in the BMC.

A class 3 fault is shown: FAN AIR-V 9HA1 (2) OR TCT (2) OR SENSE LINE, further action must be taken to avoid both engines’ bleed fault. If this report contains a fault message relating to the engine bleed system, it is recommended, as a preventative action, to check the TCT FAV sensor line for leakage and if no leak is found to replace the TCT then both engines running is a must, which can be done like procedure mentioned in chapter 3.2.

Strengthen the monitoring of engine bleed status:

Further Over-temperature troubleshooting, it is recommended to perform an engine run in order to check the correct levels of bleed parameters before dispatching the aircraft:

Improved engine run procedure is available in aircraft documentation since February 2008 revision, supplied by Airbus, Operational test of the Bleed Air System of the Engine 1 (2) can be founded in the AMM (Aircraft Maintenance Manual).

When engines have an idle speed and each engine bleed is used to supply the air, the bleed temperature should be between 150ºC and 200ºC. When the N1 of engine 1(2) is between 35% and 40 %( the time HPV will open), the bleed temperature should be between 170ºC and 210ºC. When the N1 of engine 1(2) is between 65% (the time HPV will close), the bleed temperature should be between 160ºC and 210ºC.

According to the AMM (Aircraft Maintenance Manual), if the engine bleed temperature is in the temperature range mentioned above, no maintenance action is needed. Something must be mentioned that if the temperature of engine bleed is near the high limitation, it means that the performance of TCT is decreasing.

So the test result of the operational test of engine bleed must be mentioned if the engine bleed temperature is 5ºC lower than the highest temperature limit. The engine bleed temperature should then be monitored in line maintenance.

The monitoring work to be done in line maintenance is:

Ask for the crew‘s assistance: The crew can record the engine bleed temperature in every flight phrase (discussed below in segment 3.3), although some data offered by the crew may be not be reliable, up-to-date, it is still an effective way to monitor the engine bleed parameter in current technical condition.

The parts which may be used in the troubleshooting and the tools were prepared to take precautions against the sudden occurrence of engine bleed fault. In the line maintenance, if over-temperature was not reported by the crew, but in the PFR (Post Flight Report), there was associated fault messages, the troubleshooting work should be finished as soon as possible.

To further improve the reliability of engine bleed temperature regulating the system, and lessen the workload to line maintenance department, a suggestion was given to the MCC (maintenance control center) and Project Engineering department that being: a pneumatic system‘s performance check can be done in a shorter time interval than the regular check.

The maintenance personal have to write down the real temperature of the engine bleed when the engines start and then have to compare the real temperature with the document required. If fault found, do the troubleshooting. The MCC (maintenance control center) and Project Engineering department accepted the suggestion, and the CMP (Customized Maintenance Plan) of A320 was revised in May 2008.

A pneumatic system‘s performance check will be done in 2A check, which was previously done in the 1C check. (1A-checks are done: from 500 to 600 flight hours; 1C-checks are done: every 15 months to every 20 months or 6,000 flight hours.)

All above are preventive maintenance work, the monitoring work in engine bleed temperature status can in time found the decreasing performance of parts, to minimize the operational interruptions.

To monitor the engine bleed temperature, it is very necessary to optimize the part supply system, develop the aircraft condition system monitoring system and to raise the maintenance personnel’s troubleshooting ability which needs the combined efforts of the all department in the company beside the line maintenance department.

Perfect the communication with the crew

For every aircraft fault, the pilot needs to exercise some professional judgment. But there are circumstances where this operation is definitely not permitted and although they are given the authority to operate with specified equipment unserviceable, they do not have to if they do not think it is safe (which is a point they may have to argue sometimes with the company) When in doubt, consult an engineer but engineer’s signature in the log book guarantees their work. So the responsibility is still of the pilot. Once a MEL is approved, compliance is mandatory.

If the crew has a problem before taxiing out, they would refer to the MEL, once they start taxiing, they would consult the operation manual. To prevent unnecessary delay or returning to base, the crew must get the right action. As we know: with the rapid development of china eastern, there has been a lack of pilots. In the meantime, there are many young pilots who are not that experienced. So the good communication between the maintenance personal and the crew is very necessary.

EXAMPALE: On SEP 15, 2006, in an airplane B-2400, the crew reported engine bleed fault in the city of Yan-Tai in transit and FAV fault was shown in the PFR (post flight report). Because the time limitation and lack of part and also the fact all the conditions fulfill (there is no icy condition in the line) were according to MEL, DDF (deferred defect form) was made.

But the crew was worried about that the climate changing and then rejected to take this flight. So an A320 with nobody onboard was arranged from Shanghai to Yan-Tai. It was a big loss to the airline. In the condition where the safety of flight is ensured, a fine communication with the crew should be made.

The department of Pudong line maintenance had a technical meeting every week, attended by the engineer and the technical service behalf of Airbus. In the meeting the technical condition of the fleet was analyzed. The maintenance personals investigated the operational interruption that occurred in that week and discussed how to avoid operational interruption or how to shorten troubleshooting time.

After the meeting, meeting topic material was sent together with the suggestion to flight operation and the pilot. They could also get the relevant information in China Eastern‘s website .In addition, a conference with the department of pilots was held twice a month. A face to face communication was very necessary as well as it was a good time for the line maintenance department to make sure whether their operation suggestions have good effects or not, and to know if the crew had new questions.

Here is something which was found in china eastern airlines’ website:

Bleed Overheat fault mostly happened in the climb phase, when the fault happened in one engine, the crew could cut off the bleed and 3 min later get the engine bleed on again. They can try this several times to avoid unnecessary delays and returning to base.

Several A320 operators reported cases of dual bleed failure. Most reported cases occurred during the climb, at phase 6. The rate of dual bleed loss events across A320 Family fleet had always been below certification requirements.

However, Airbus had introduced additional support measures to further reduce dual bleed loss events. Due to both engine bleed systems being inoperative, cabin pressurization was lost and the aircraft emergency descent was then required unless one or both engine bleeds had been reset successfully.

Some operators had reported cases of simultaneous failure of both engines bleed systems due to over temperature conditions. The failures were identified by “air eng 1(2) bleed fault” ECAM warnings associated to “FAV 9HA1 (2) or TCT or sense line” class 1 CFDS fault messages. The temperature control thermostats (TCT) removed after troubleshooting was confirmed faulty during shop examination.

In a case of single “air engine bleed fault” ECAM warning, aircraft could be dispatched with associated bleed P/B switch selected off as per MMEL 36-11-01, provided icing conditions were not predicted and the altitude is limited to 31,000 feet.

In order for troubleshooting of pneumatic system failure in summer days, the crew was given a sheet before pre-flight, to help record the engine bleed parameters in different flight phrase and then it was more convenient to deal with the engine bleed fault. A sample of the sheet is shown below:

sample_sheet

Through the maintenance personal‘s hard work and the cooperation of the pilot, many operational interruptions were avoided successfully, just as the example given below.

EXAMPLE: On MAY 15, 2008, airplane B-2415 crew reported Bleed Overheat fault in the climb phase in the city of Beijing. At first, the crew decided to return to the base. Through the communication with the engineer on the ground and their advice, they cut off the bleed. A few minutes later, got the engine bleed on again. They tried this operation 3 times, then the parameter of engine bleed was OK and the fault message in the upper ECAM was cleared.

 

Read More: CHAPTER 4