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ENGINE INSPECTION FAA orders fan blade inspections

A directive has been issued requiring the ultrasonic inspection of fan blades in older CFM56-7B engines to detect signs of cracking.

❱ ❱ On-wing inspection of all the engine fan blades takes approximately 4 hours


ollowing a fatal engine explosion on Southwest Airlines flight 1380 on April 17th, the US Federal Aerospace Authority (FAA) swiftly issued

a directive requiring all Boeing 737 aircraft with CFM56-7B aero engines with over 30,000 take-off and landings recorded to be ultrasonically inspected for signs of fan blade cracks within 20 days. The FAA also requires those with over 20,000 cycles to be inspected by the middle of August with regular inspections to take place after that.

METAL FATIGUE The fan blade failure that resulted in the directive took place in the air on a flight between New York and Dallas. According to the USA’s National Transportation Safety Board (NTSB), it is likely that the destruction of the fan blade which caused the engine to disintegrate and penetrate the cabin was due to metal fatigue but the organisation is continuing with its investigation into the Southwest Airlines incident. According to the airline, it has now

accelerated testing of its remaining fleet of Boeing 737s equipped with the CFM engine with help from General Electric (GE), which performs maintenance for the airline, and Safran Aircraft Engines,

20 /// Aerospace Test & Validation 2018

whose subsidiary CFM supply the engines. CFM and GE have supplied around 40 technicians to support the accelerated inspection programme. According to CFM, approximately

14,000 CFM56-7B engines are in operation. The fan-blade inspections recommended within the next 20 days for engines with more than 30,000 cycles will impact about 680 engines and the further inspections recommended by the end of August for fan blades with 20,000 cycles will impact an additional 2,500 engines. The CFM service bulletin suggests

that the inspection is conducted on-wing using an ultrasonic probe along the surface of the fan blade. This process takes about four hours per engine.

ULTRASONIC INSPECTION The use of ultrasonic inspection is a very effective way of detecting fatigue related defects, such as cracking, in turbine blades. The disadvantage is that the whole of the blade needs to be accessible for positioning the ultrasound transducer. Aero engines have many such blades, hence the significant amount of time needed to perform the inspection. The ultrasound transducer emits a beam of ultrasonic energy, which is

❱ ❱ Non-Destructive Testing using ultrasound provides fast, visual feedback of any defects found within the fan blade

partially reflected by changes within the material, such as inclusions and cracks. Such defects can be very quickly detected, characterised and even measured, thus giving a complete picture to the inspector. Any fan blades found with defects using this technique, will need to be replaced, according to the FAA directive. According to CFM, the FAA has also added eddy current inspection to the directive for carrying out inspections on engines that have reached 20,000 cycles. Eddy current inspection performs a similar role to ultrasonic inspection and involves the use of an electromagnetic pulse probe that detects defect related disturbances to the generated magnetic field.

FUTURE INSPECTIONS Once all of the inspections have been carried out on the initial aircraft by mid May and the end of August, the directive caters for ongoing inspections of engine fan blades every 3,000 cycles after reaching 20,000. What happens after that will largely depend upon the results of these tests and the conclusions reached by the NTSB after it has completed its investigations into the engine failure on flight 1380. n

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