Aircraft Engine Blade Damages & Contribution to the Engine Health
Engine Blade Damages, Causes & Mitigation
Introduction
Aircraft engines that provide thrust are obviously important to the aircraft but significant to engines are the blades (fan, rotor, or stator) that play with the ingested air & make a thrust out of that. At different stages of aircraft [read as Fan stage, Low-pressure compressor, high-pressure compressor, high-pressure turbine, low-pressure turbine], the integrity and efficiency of blades determine the overall health of the engine.
Generally overlooked, blade defects or a damaged or bad blade can lead the engine to fail in its intended performance & result in In-flight shutdowns (IFSD).
I personally advise identification of the blade damages are one of the most critical & contributory factors towards engine health. Let’s learn today, how these blades get damaged or become counter-productive in the service life of an engine.
Types of Blade Damage
Range of the types of engine Blade damages that Pilots, Engineers or inspectors do not differentiate & get away with, proper identification of which is key to the analysis of a chain of events that can grow uncontrolled to result in an ‘IFSD’ or Uncontained Engine Failure are :
1. Surface Damage (Material loss &/or separation): Particle Deposits, Corrosion, Oxidation, Pitting, Sulfidation, Guttering, Burns, Blistering, Erosion.
2. Contour Deviation or Material Deformation: Dent, Bent, Scratch, Creep, Bow, Bulge Burr, Curl, Peening, Waviness.
3. Wear: Abrasion, Score, Gouge, Groove.
4. Material separation: Nick, Crack, Chipping, Breaking, Tear.
Contributory Factors
Once we could identify the types of damages, let’s see who caused them or what were the contributory factors!
The range of contributory factors [7 types in Broad] is again quite diverse & tricky.
1) Impact Damages: This type of damage can be caused to individual aircraft & dealt with case by case.
• Foreign Object Damage (FOD) Like Grit, Dust, Fine sand, Ground debris, Left-behind items entering the engine through the inlet.
• Known Object Damage (KOD) like Abrasion from worn parts, Broken off pieces from upstream engine parts which are otherwise parts of the engine.
• Organic contributions like Birds or any other Wildlife getting ingested in the engine.
• Weather contributions by Ice ingestion or Hail storm impacts.
2) Environmental Damages: My personal favorite type, these types of damages basically affect a fleet of aircraft operating at the same time or region. Contributions are from :
• Polluted air, Intake air contaminants, Volcanic ash, Salt deposits from sea or runway de-icing, Moisture, Agricultural chemicals, Acids.
Note: Aircraft have had run-ins with volcanic ash about 253 times between 1953 and 2016, according to a report from the US Geological Survey.
3) Engine Operational damages also contribute to more than 40% of cases of blade damage or malfunctions & common to all engines equally over a period of time. Various factors are as below.
• Overheating, Turbulences, Abnormal flame pattern, Overload, Incorrect burning process, Lack of lubrication, Improper clearance, Complex thermal & mechanical loads, Heavy landings, Compressor surge.
4) Poor Manufacturing process or flaw in the material of blades can also lead to failure. This generally happens to a batch of engines with sub-standard subparts.
• Defective material (material defect, e.g. wrong alloy, composition, microstructure or inclusions, or a faulty process, such as casting or forging or heat treatment), Internal stresses, Improper bonding, Incomplete bonding, Defective coating, improper deburring after machining.
Note: Similar Cases of A320 NEO-PW engines or Rolls-Royce Intermediate Pressure (IP) turbine blade of Trent 1000 engines are recent year highlights.
5) Poor Maintenance practices can also initiate blade damage & failure as well.
• Careless handling of engine parts or tools, Improper assembly or disassembly, Left behind hand tools or parts in the engine, Disregard of maintenance procedures, Use of prohibited pencils markings, Missed tip clearance check, improper repair or Non performed repair, Fire extinguisher agents, or Use of corrosive agents.
6) Fatigue Damages will come up with aging of the engine or after certain engine hours, hence early detection is necessary.
• Thermal fatigue, Random stress fluctuations, Stress concentrations, Surface finish, Residual stresses, High cycle fatigue.
7) Aggressive environment is another parameter that has an effect on the life of a blade & its deteriorated conditions.
• Aggressive gases, Excessive heat, Loss of cooling, Blocked cooling passages, Blockage or malfunction of the cooling airflow, Sulphur deposits, Sulphurous jet fuel, Sulphur oxides from combustion or airborne salts, pressure.
Now, we are conversant with the types of damages & contributing factors behind the damages. All of the damages inside the engine won’t be revealed by a visual inspection [GVI], some of the damages need further ‘Intra engine station inspections’ which can be performed by Borescope inspection of the engine [BSI] at different stages and is not feasible during daily checks or transit checks. But, there are many factors or signs that will give the operator/engineer/pilot an early warning of damaged or failed blades affecting engine operation.
Early warning
Vibration, high EGT (Exhaust gas temperature), engine blow-ups, engine shutdowns, increased fuel consumption, resonant airflow, Reduced fatigue life etc are among the warnings to the operator for a quick correction course.
Importance of FOD
The most common type of damages to blades is by FOD (Foreign object damage), which can be identified by a general visual inspection [GVI ] & FOD happens as below.
Foreign-Object-Damage (FOD) occurs due to ingestion of hard particles like small pebbles and sand particles or Birds [Organic]while the engine is dynamic, taxing, or running on the runway with ideal speed. These events usually take place during takeoff and landing when loose debris (size: mm to cm) from the airfield gets ingested into the engine. The velocity of the ingested object varies from 100–350 m/s, depending on the engine specification.
The damage caused due to the impact of a foreign object on the leading edge of the rotating blade results in premature crack initiation and thereafter failure of the blade due to the time-varying tensile loads in the running engine. Residual stresses which arise due to impact of foreign object play an important role in controlling the rate of crack initiation and propagation. Acceleration and retardation of crack growth depend on the characteristics of stress whether it is tensile or compressive in nature ahead of the incipient crack.
Example of engine blade failure & effect on airworthiness
Though there are many examples available of hailing blade failure resulting in Inflight Engine Shut Down [IFSD], let's see this one. On the evening of 1st March 2002, a Boeing 747–436 aircraft sustained the failure of the number#3 engine during a scheduled passenger transport flight from Sydney to Bangkok.
The investigation revealed that a fan blade had failed & punctured the engine cowl & aircraft, which caused the engine shutdown & succedent aircraft backout. Actually, these kinds of events could be avoided by early detection, inspection, repair & technical improvement of the engine substructures including fan blades.
Summary
Blade Damages are significant, Specifically, Low-pressure Fan blades which give the operator first-hand information on the last cycle events that happened in the engine & also, what to expect in the subsequent stages of the engine.
Called out Borescope inspections (BSI) should be done meticulously to record each substance & substrate for an effective Engine Health Trend Monitoring.
Do not overlook any such damages during the periodical inspection phases (may it be a transit daily or an ‘A’ check). Report it, repair it, do the trim balancing, do the transient acoustic.
