GTEquiz3mod15.21 Engine Monitoring & Ground Operations PDF

Engine Monitoring and Ground Operations (15.21) Learning Objectives 15.21.1 Provide a detailed description of procedures for starting and ground run-up (Level 3). 15.21.2 Interpret engine and ambient parameters to calculate engine power output (Level 3). 15.21.3 Provide a detail...

Engine Monitoring and Ground Operations (15.21) Learning Objectives 15.21.1 Provide a detailed description of procedures for starting and ground run-up (Level 3). 15.21.2 Interpret engine and ambient parameters to calculate engine power output (Level 3). 15.21.3 Provide a detailed explanation of trend monitoring (including oil analysis, vibration and borescope) (Level 3). 15.21.4 Provide a detailed description of the inspection of a gas turbine engine and components to criteria, tolerances and data speci ed by the engine manufacturer (Level 3). 15.21.5 Describe compressor washing and cleaning (Level 3). 15.21.6 Describe foreign object damage (Level 3). 2022-08-24 B1-15c Gas Turbine Engine Page 62 of 104 CASA Part 66 - Training Materials Only  Maintenance Engine Runs Purpose of Maintenance Engine Runs Maintenance engine runs are carried out for a variety of reasons, including: Testing following repair or overhaul – carried out in a test cell Fault detection and correction – con rms a reported fault, diagnose the fault, con rm the recti cation and remedy the fault Performance monitoring – carried out at various intervals to con rm the power output or serviceability of an engine, with results closely monitored and graphed Engine removal and installation – con rms serviceability pre-removal or post installation and checks for the presence of any leaks Aircraft system power supply – carried out to enable other systems on the aircraft to be tested or operated. B737 - RR Trent 1000 full power run 2022-08-24 B1-15c Gas Turbine Engine Page 63 of 104 CASA Part 66 - Training Materials Only  Preparation for Engine Running Before starting an engine, a good practice is to carry out a walk-around check of the entire aircraft. This establishes whether anything is out of place that might cause problems or damage. Check all the main engines – you may need to run more than one engine during the run-up. Engine-related items that should be checked include opening access panels to check uid levels and visually examining the engine inlet, fan, tailpipe and LP turbine. Maintenance ground runs Aircraft come in various sizes and power ratings. While two aircraft may be roughly the same size, there may be a large difference in their weights and the thrust available from their engines. When preparing an aircraft for an engine run, the many considerations come under the following two main headings: Position Conditions. 2022-08-24 B1-15c Gas Turbine Engine Page 64 of 104 CASA Part 66 - Training Materials Only  Position The position or location of the aircraft has a large effect on what power settings can be used when starting the engines. Considerations that may apply to running aircraft engines are: Aircraft size and weight – dimensions and limits must be considered if an aircraft must be moved to allow for engine running Engine power – safety distances to be observed, tarmac capable of handling jet blast, enough weight on aircraft to maintain brakes at high power Tarmac and surrounding area material – suitable for engine run, weight rated Boeing 737 with damage due to an engine run-up on loose bricks Distance to buildings/roads or air eld facilities – possibility of damage to buildings, structures or vehicles from jet blast Blast de ectors – devices used to change the direction of the jet thrust, which is normally directed upwards. 2022-08-24 B1-15c Gas Turbine Engine Page 65 of 104 CASA Part 66 - Training Materials Only  Blast de ectors 2022-08-24 B1-15c Gas Turbine Engine Page 66 of 104 CASA Part 66 - Training Materials Only  Danger Areas Danger areas around aircraft dramatically increase as soon as the engines are started. The suction from an engine intake is capable of drawing personnel into the engine even at low power settings. Small material such as gravel or sand is easily picked up and erodes compressor and turbine components. At the same time, the jet exhaust may pick up and blow loose dirt, rocks, sand, bits of paving and other debris a large distance from the aircraft. For this reason, the aircraft must be parked in a location where no damage can result from ying particles which are driven from the rear. As can be seen in the diagram below, the areas in front of and behind an aircraft engine are extremely dangerous. The areas shown, for a Boeing 737-600 to 900 aircraft with a CFM56 engine, are only one example. Speci c aircraft restrictions as listed in the applicable Aircraft Maintenance Manual (AMM) must be observed when any engine run is attempted. Engine danger areas at idle thrust 2022-08-24 B1-15c Gas Turbine Engine Page 67 of 104 CASA Part 66 - Training Materials Only  Engine danger areas at full power Engine inlet danger areas at full power Other danger areas around the engine may include the engine and starter turbine discs. Danger also exists from the propeller of turbo propeller aircraft. 2022-08-24 B1-15c Gas Turbine Engine Page 68 of 104 CASA Part 66 - Training Materials Only  Warning - propeller danger 2022-08-24 B1-15c Gas Turbine Engine Page 69 of 104 CASA Part 66 - Training Materials Only  Conditions The conditions for an engine run require serious consideration of the following points: Fuel requirements – Suf cient fuel must be present to complete the engine run, plus a safety factor. Also, some aircraft have a minimum fuel state that must be in speci c fuel tanks prior to start. Noise – Gas turbine engines produce noise levels considered dangerous to humans if no protective devices are worn. Also, running an engine at full power just outside a maintenance facility may curtail work inside. Weather – Wind direction and velocity can change the stability of the engine. Where possible, the engine must be pointed into the wind. Excessive wind in any direction affects the performance gures of the engine. A typical wind direction and velocity restriction graph is shown below. Allowed windspeeds Trim running of engines at high ambient temperatures is often aborted because engine operating limits are exceeded before the trim graph minimum limits are reached. 2022-08-24 B1-15c Gas Turbine Engine Page 70 of 104 CASA Part 66 - Training Materials Only  Support equipment – Equipment requirements vary with aircraft type and the reason for which the engine is being run. They may include tow motors, tool kits, a re extinguisher, personnel communication equipment, aircraft steps, wheel chocks and AMMs. Personnel – The number of personnel required for the run should be kept to a minimum, but includes ight deck runners, an outside ground observer and a re attendant. Communication – Aircraft engines are extremely noisy (up to 160 dB) when operating, so normal means of communication (speaking) are not always effective. Specialised headset communication equipment may be available to make communication easier; these systems use long leads that can present a potential danger around engines which are operating. Maintenance crew during an engine ground run Protective equipment – The correct protective equipment must be worn at all times when engines are running. The equipment required depends on the aircraft and the type of engine run contemplated, but normally includes kidney belts, earmuffs and earplugs, and appropriate clothing. 2022-08-24 B1-15c Gas Turbine Engine Page 71 of 104 CASA Part 66 - Training Materials Only  Engine Running Local instructions offer guidance regarding the restrictions and authorisations required to carry out engine run procedures. Pre-Start Pre-start is the step of an engine run which involves everything from the noti cation of a requirement for the engine run to climbing into the aircraft to carry out the engine run. Once you have been tasked to carry out an engine run, the aircraft maintenance documentation should be inspected to determine the requirement for the run and to ensure nothing in the documentation would restrict or prohibit the operation of the engines or aircraft, such as: Fuel status A disconnected VHF radio antenna, preventing communication with the tower Fire bottle squibs that have been removed, stopping use of the re extinguishers, etc. After con rming the engine run requirements and aircraft serviceability, organise the location for the run, considering: Power settings required Wind Local ying program Positions that are available Other local restrictions. Determine the personnel and equipment required to complete the engine run, taking into account what specialised checks may be required during the run. On arrival at the engine run location, position the aircraft into the wind with appropriate wheel chocks or tie-downs installed. Position the power cart and any unneeded equipment off to the side of the aircraft in the event the aircraft jumps the chocks or holdbacks. The aircraft pre-run walk-around determines that the aircraft, support equipment and surrounding area are in a safe condition (no foreign object damage, or FOD) for the engine run to commence. 2022-08-24 B1-15c Gas Turbine Engine Page 72 of 104 CASA Part 66 - Training Materials Only  Aircraft walk around Engine Starting and Run As run procedures vary with engine and aircraft types, the exact procedure for running is laid down per type; however, this is a guide to how an engine run would be carried out. Brief all personnel on the required procedures, including responsibilities, safety and emergency procedures. Perform the relevant ight deck entry checks. Perform the relevant pre-start checks and actions. Contact the control tower for start and/or run clearance. The control tower normally requires the aircraft type, tail number and location, number of personnel on board (POB) the aircraft, and duration of the run to be carried out. Obtain clearance from the external safety observer. Carry out the start procedure, taking care to remain within the limits of all parameters, including Exhaust Gas Temperature (EGT), rpm, oil pressure and starter limitations. Carry out the requirements of the engine run. Perform relevant shutdown checks and actions. 2022-08-24 B1-15c Gas Turbine Engine Page 73 of 104 CASA Part 66 - Training Materials Only  Engine Limitations All engines have a variety of starting and operating limits that must not be exceeded. These may include: Maximum turbine starting temperature Minimum oil pressure for start and operation EGT limits during starting and continuous operation as depicted in the example below. EGT limits Area of Maintenance Required Operation Starts in Area A must be recorded and immediate corrective action must be taken A prior to further start attempts. A borescope inspection must be performed prior to further start attempts. Starts in Area B must be recorded. Persistent starts in this area are cause for B corrective action. C Any operation in Area C requires removal of the engine for overhaul. 2022-08-24 B1-15c Gas Turbine Engine Page 74 of 104 CASA Part 66 - Training Materials Only  Emergency Procedures Emergency procedures are normally carried out upon the detection of a dangerous situation which, if not acted upon immediately, could damage the engine or aircraft and injure personnel, either inside or outside the aircraft. Emergency situations require that relevant checklist items are performed immediately. Emergency situations could be: Engine re Engine runaway Engine seizure Engine overheat Aircraft re Aircraft system failure. Although some of the above may not initially appear to be emergency situations during engine running, they may rapidly accelerate, within seconds, to life-threatening events. In all emergency situations, quick reaction times and crew co‑operation are essential to prevent/minimise damage to the aircraft and prevent crew injury. Note: If at any time during an engine run an emergency situation appears imminent, the best action to take is to follow emergency shutdown procedures to abort the engine run. 2022-08-24 B1-15c Gas Turbine Engine Page 75 of 104 CASA Part 66 - Training Materials Only  Post-Engine-Run Checks After completion of the engine run, the aircraft should be returned to its standard con guration: If tted, remove any specialist engine recording equipment and perform engine run leak checks as required. Remove any specialist tie-down equipment. Re t all blanks. Ensure engine and gearbox oil levels are within prescribed limits. Carry out a post-engine-run aircraft walk-around inspection: Look for evidence of air, fuel or oil leaks. Look for FOD damage to the engine intakes and exhausts, propellers and airframe. Re t any panels or doors removed for access or engine running. Refuel the aircraft as necessary. Return all equipment used for the engine run. Complete all necessary documentation. 2022-08-24 B1-15c Gas Turbine Engine Page 76 of 104 CASA Part 66 - Training Materials Only  Factors Affecting Engine Performance Introduction to Factors Affecting Engine Performance Power output of a gas turbine engine is affected by: Design factors, such as compressor and turbine ef ciency and engine pressure ratio (EPR) Operational factors, such as rpm and EGT External factors, such as airspeed, altitude and Outside Ambient Temperature. Design Factors Although design factors cannot be altered, the greater the engine ef ciencies and EPR, the greater the resultant engine power output. Engine RPM Gas turbine engines are generally most ef cient when operating at maximum allowable rpm. Normal operating range is usually 90%–100% of max allowable. At high engine speeds, thrust increases rapidly with small increases in rpm. Maximum compressor and turbine ef ciency are only possible at one speci c rpm, therefore engines are designed for best performance at max power. Operation at other power settings degrades speci c fuel consumption. RPM vs thrust 2022-08-24 B1-15c Gas Turbine Engine Page 77 of 104 CASA Part 66 - Training Materials Only  Turbine Temperature Maximum engine thermal ef ciency, fuel ef ciency and power output occur at maximum allowable turbine inlet temperature, although severe restrictions are laid down with regard to exceeding maximum EGT to protect the turbine from heat stresses. Power Limiting Low altitude and low ambient temperature both produce high air density, which gives high mass ow for a given engine rpm. High mass ow allows the engine to produce maximum power before the maximum allowable turbine temperature is reached. When this condition occurs, the engine is restricted from increasing burner pressure any further and is said to be power (EPR) limited, thrust limited, N₁ limited or torque limited. High density altitude and temperature conditions reduce air density and mass air ow. Maximum allowable turbine temperature is therefore reached before the engine produces maximum power output. Under these conditions, the engine is said to be temperature limited. If not monitored, these conditions may lead to engine overheat, leading to turbine fatigue through heat stress. Some engines are de-rated so that operational limits are less than design limits. This reduces engine wear and extends engine life. These engines are different from at rated. Adjustments In service adjustments to non-FADEC gas turbine engines, fuel control units (FCUs) are usually limited to: Throttle rigging Speci c gravity Idle rpm Maximum power. An FCU suspected of being faulty should be removed and replaced with a serviceable item. 2022-08-24 B1-15c Gas Turbine Engine Page 78 of 104 CASA Part 66 - Training Materials Only  Performance Checks Trimming Adjustment of idle rpm and maximum thrust settings must be set to match the manufacturer’s speci cations. Idle adjustment sets engine idling rpm to that speci ed by the manufacturer, allowing for variations in ambient conditions. Depending on the particular engine, maximum power (or thrust) adjustment ensures that the engine is producing rated thrust or shaft horsepower as speci ed by the manufacturer, and again allowing for variations in ambient conditions. Trimming is carried out whenever engine power output is suspect and after certain maintenance tasks as prescribed by the manufacturer (e.g. engine, FCU or module change). In a turbojet engine, EPR is used as a measure of engine power output. In a turbofan engine, EPR or fan rpm may be used; if the engine is not producing full power, mass ow through the turbine assembly is reduced, and fan rpm is reduced. Turboprop or turboshaft engines use output shaft torque as a measure of engine power output. Part Power Trimming Some engines may be trimmed at less than take-off power. A stop (called a part power trim stop) is placed so that the power lever is blocked before reaching the full power position. Engine trimming is carried out with the power lever against the stop. Acceleration Check When technicians perform an acceleration check, the power level is moved from the idle to full power setting, and engine acceleration time to full power is compared to the manufacturer’s acceleration time. Data Plate Speed Check After manufacture, the engine is test-run and a data plate attached. The data plate records conditions under which required thrust was obtained during part power trimming (e.g. 87.25% N2 at 1.61 EPR and 59 °F). The data plate speed check is carried out by running the engine at 1.61 EPR, noting N2 tachometer indication, correcting the observed gure for any temperature variation from 59 °F, and comparing it with the original rpm gure. Any variation outside manufacturer speci ed tolerances is an indication that either the FCU needs adjustment or the engine is worn, and maintenance or repair may be necessary. 2022-08-24 B1-15c Gas Turbine Engine Page 79 of 104 CASA Part 66 - Training Materials Only  Trend Monitoring Purpose of Trend Monitoring Accurate forecasting of impending component failure allows forward planning of a unit’s maintenance commitment by taking advantage of manpower and spares availability and reducing aircraft downtime. Conventional trend monitoring has taken two paths: Mechanical parameters – oil consumption, oil analysis, chip detection and vibration measurements Performance parameters – fuel ow, thrust, rpm, EGT and pressure readings. The more recent advances in microprocessor-aided monitoring, displaying and recording of engine parameters have expanded the information available to both the aircrew and maintenance crew to determine engine condition. Engine trend monitoring is based on the consistency with which a gas turbine engine follows its corrected performance gures at steady-state operating conditions. An engine does not vary from its new or post-overhaul performance gures unless some internal or external effect forces it to do so. If the cause of the deviation can be determined, maintenance action can be taken to correct the fault. Most modern engine manufacturers are making engines which no longer have scheduled overhaul intervals. The engines are manufactured to be modular. This means each major engine sub-assembly, such as a compressor assembly or turbine assembly, is a self-contained module. The engine as a whole is not overhauled, only the engine modules, and then only when a fault occurs. This type of engine maintenance is known as ‘on-condition’ maintenance. With engines now being maintained on an on-condition basis, monitoring their performance trends is crucial. It allows aircraft operators to accurately determine when maintenance on an engine is required and takes maximum advantage of available spares and manpower. Accurate trend monitoring also saves aircraft operators money and aircraft downtime. 2022-08-24 B1-15c Gas Turbine Engine Page 80 of 104 CASA Part 66 - Training Materials Only  Performance Parameters The following engine and ight indications are required to operate an effective trend monitoring program: Altitude EPR or N₁ rpm N₂ rpm EGT Fuel ow. The instrument indications obtained during ight or during an engine ground run are corrected to a set of standard conditions. The corrected readings are then compared to the average readings previously obtained or the gures from the post-overhaul engine test. Graphical Presentation The data obtained from corrected engine performance gures are plotted on a graph as shown below. This method displays an easily readable record of the engine’s performance trend over a signi cant time. Any malfunction or deterioration in performance is quickly recognised and maintenance may be scheduled at the most convenient time. Engine performance gures - normal 2022-08-24 B1-15c Gas Turbine Engine Page 81 of 104 CASA Part 66 - Training Materials Only  Effective trend monitoring is wholly dependent on accurate observations of engine instruments and the operator’s ability to detect small deviations in the instrument indications. The deviation of two or more parameters generally results from a change in engine performance. If there are minor uctuations (within the tolerances), the engine should continue to operate normally until its next overhaul. Engine performance gures - abnormal The chart above shows the same parameter data for the same model of engine as the previous chart. This engine was removed for excessive oil consumption. On disassembly, the No. 4 and No. 5 main bearings and carbon oil seals were found to be leaking bleed air into the bearing compartments, causing the problem. Because of the bleed air leak, the engine required more fuel to produce the required thrust. With the addition of more fuel, the compressor rpm of both the low- and high-pressure compressors also increased. If you study the chart, you will see that not only are the oil consumption data high, but so are the fuel ow, N₁ rpm and N₂ rpm data. If these gures were plotted for an engine whose oil consumption was normal, you would suspect that the engine compressor bleed valves were leaking. 2022-08-24 B1-15c Gas Turbine Engine Page 82 of 104 CASA Part 66 - Training Materials Only  Spectrometric Oil Analysis Program A spectrometric oil analysis program, or SOAP, is another tool available to aircraft operators to help detect developing problems in an engine. Spectrometric analysis of metal particles suspended in oil is possible because metallic ions emit characteristic light spectra when vaporised by an electric arc. Each metal produces a unique spectrum, allowing easy identi cation of the metals present in the oil samples. The wavelength of spectral lines identi es each metal, and the intensity of a line is used to measure the quantity of that metal in a sample. Engine trend monitoring (SOAP) Alloyed metals in turbine engines may contain amounts of aluminium, iron, chromium, silver, copper, tin, magnesium, lead, nickel or titanium. Silver is accurately measured in concentrations down to one part silver in 2,000,000 parts oil. The wavelength of the spectral line identi es the metal. The intensity of the line measures the quantity of the metal. 2022-08-24 B1-15c Gas Turbine Engine Page 83 of 104 CASA Part 66 - Training Materials Only  SOAP analysis Vibration Monitoring Engine vibration during ight is also subject to trend monitoring and analysis. Increasing vibration can indicate a deteriorating engine condition. An increasing vibration trend could be caused by pending bearing failure or rotating assembly deterioration or damage. Usually two vibration sensors are tted: one to the fan case and the other on the turbine case. Vibration intensity at these points can point to the source. A warning lamp on the ight deck warns the pilot when an unacceptable level of vibration is reached so the engine can be shut down before more damage occurs. Vibration monitoring 2022-08-24 B1-15c Gas Turbine Engine Page 84 of 104 CASA Part 66 - Training Materials Only  Borescope Inspection A borescope is a viewing device which allows you to visually inspect areas inside a turbine engine without disassembling the engine. A borescope may be compared to a small periscope with an eyepiece at one end and a strong light, mirror and lens at the other end. A conducting cord connects the probe to a control panel for adjusting light intensity and magni cation. To aid the inspection process, many gas turbine engines are equipped with openings, or ports, that allow inspection tools to enter. Some of the common tools used for such inspection are the borescope, brescope and electronic imaging. Borescope Borescope inspections are carried out following suspected engine damage or as part of a trend monitoring process. Borescope ports are located in the engine gas path, in both the cold and hot sections. To provide a view of each N₂ compressor or turbine blade, the engine can be manually rotated through a manual cranking pad on the accessory gearbox or via a special motor attached to the starter pad. The N₁ can be rotated by hand-turning the fan. 2022-08-24 B1-15c Gas Turbine Engine Page 85 of 104 CASA Part 66 - Training Materials Only  Engine borescope inspection ports Borescope inspection is often carried out following trend analysis that identi es the following: Deteriorating performance Increasing vibrating Over-temperature Overspeed. Borescope inspection 2022-08-24 B1-15c Gas Turbine Engine Page 86 of 104 CASA Part 66 - Training Materials Only  Borescope inspections are also part of routine and special maintenance inspections, such as: FOD Bird strike Lightning strike. Mechanic using a borescope 2022-08-24 B1-15c Gas Turbine Engine Page 87 of 104 CASA Part 66 - Training Materials Only  Damage Inspections Damage Inspection Precautions Fan compressor and turbine components are often damaged by loose items left in engine intakes or exhausts. Before performing any work inside intake or exhaust ducts, ensure all loose items are removed from your pockets. Before closing up, thoroughly inspect your work area for loose items, including nuts, bolts, screws, rags, lockwire, tools and personal items. Fan Blades CAUTION: Before entering the inlet, place a soft rubber mat in the inlet duct to protect the inlet lining. Like propellers, fan blades often suffer minor erosion and stone damage. Blade damage is normally dressed by blending out the erosion or nick using a ne le and abrasive cloth. Damage and repair limits are speci ed in the manufacturer’s maintenance and repair manuals. Any damage to fan blade roots is critical due to the high centrifugal stress loads. Debris, used abrasives, tooling and the protective mat must be removed before the job is nished. Following blade dressing, an engine ground run and vibration survey may be necessary to check for fan imbalance. 2022-08-24 B1-15c Gas Turbine Engine Page 88 of 104 CASA Part 66 - Training Materials Only  Blade Shingling A speci c type of damage that may be caused by FOD is fan blade shingling. Shingling is the overlapping of midspan shrouds on fan blades. Anytime rotating fan blades encounter a force that pushes a blade sideways, shingling can occur. Common causes of shingling include FOD, compressor overspeed and compressor stall. Whenever shingling occurs, you must inspect the top and bottom surfaces of the shrouds involved for scoring or galling. In addition, any blade that encounters shingling typically must be removed and inspected in accordance with the manufacturer’s maintenance manual. If any cracks are found during the inspection, the fan blade must be replaced. Blade shingling 2022-08-24 B1-15c Gas Turbine Engine Page 89 of 104 CASA Part 66 - Training Materials Only  Inlet Guide Vanes Inspect inlet guide vanes for FOD damage, and blend out damage in accordance with the AMM; blending should be nished with emery cloth to polish the area and restore the original scratch-free surface. Blade blending 2022-08-24 B1-15c Gas Turbine Engine Page 90 of 104 CASA Part 66 - Training Materials Only  Compressor Blades Inspect the compressor for FOD damage, cracking and evidence of blade rub. If damage falls within predetermined limits, record the extent and location of the damage for further reference and to aid trend monitoring. Depending on engine type, it may be possible to carry out in- eld repairs and blend out damage in accordance with the AMM; blending should be nished with emery cloth to polish the area and restore the original scratch-free surface. The purpose of blending is to minimise stresses that concentrate at dents, scratches or cracks. The illustration shows some examples of blade damage to an axial ow engine. Blade damage 2022-08-24 B1-15c Gas Turbine Engine Page 91 of 104 CASA Part 66 - Training Materials Only  Compressor Washing Compressor Washing and Cleaning During normal operation, dirt particles, industrial fallout, salt and volcanic ash accumulate on the compressor stators and blades. These build up over time and reduce compressor ef ciency. Reduced EPR, unsatisfactory acceleration and high fuel ow and EGT might indicate a dirty compressor. This contamination can be removed by one of the following recommended processes: Water rinsing Water washing. Compressor washing Before washing and/or rinsing, take precautions to ensure water does not enter the engine’s sensors and fuel control. The AMM describes which sensors and sensing lines need to be removed or blanked off. For a rinse, de-mineralised water is used. For a water wash (performance recovery wash), a detergent and de-mineralised water mixture is used, followed by a de-mineralised water rinse. Most turbofan, turboprop and turboshaft engines are dry motored for the rinse or wash process. After the process is completed, the sensors are reconnected and an idle ground run is carried out to thoroughly dry out the engine. 2022-08-24 B1-15c Gas Turbine Engine Page 93 of 104 CASA Part 66 - Training Materials Only  Turbine Nozzle Guide Vanes and Blades Inspect nozzle guide vanes for burnt spots, erosion and cracks. Depending on engine type, it may be possible to carry out in- eld replacement. NGV damage Exhaust Section The exhaust section of the engine is susceptible to heat cracking. Inspect the exhaust section and rear of the LP turbine for cracks, warping, buckling, hot spots or carbon deposits. Hot spots or carbon deposits on the exhaust cone are a good indication of faulty fuel nozzle operation. 2022-08-24 B1-15c Gas Turbine Engine Page 92 of 104 CASA Part 66 - Training Materials Only  A compressor grit wash (comprising of ground walnut shells or apricot pits) was used on older turbojet engines, but with the advancement of cooling technology for turbines where the burnt grit could clog the cooling holes, this process is not used today. Video – Compressor Wash – Relevant Youtube link: Compressor Wash (Video) Relevant Youtube link: Compressor Wash Fuel-Saving Figures (Video) Foreign Object Damage Foreign object damage (FOD) is an ongoing problem for the safe operation of aircraft engines, especially gas turbine turbofan engines. The engine inlet acts as a huge vacuum cleaner and ingests small debris, such as concrete chips or rocks from the ramp and tools that have been carelessly left where they can be sucked up. In ight, birds and ice are the main causes of FOD. Any FOD damage, regardless of how small it is, results in a loss of performance by the engine and may be indicated by an increase in EGT and a decrease in EPR or N₁ fan speed. All technicians must remain extremely vigilant for any FOD lying around the aircraft and remove it before the engines are started. FOD damage Relevant Youtube link: Compressor Damage (Video) 2022-08-24 B1-15c Gas Turbine Engine Page 94 of 104 CASA Part 66 - Training Materials Only