Introduction to Repair of Aero Engine Components
Organizations that are holding a large inventory of various types of aircraft and related equipment where most of the aircraft might have been imported from foreign countries. So, to keep the fleet serviceable, it is necessary to maintain high serviceability of all components of aircraft including aero engine parts by repairing/replacing them.
When an aircraft is dismantled for overhaul, all the components are checked and tested. If any component is found worn out or damaged, either it is to be replaced with new one or is to be repaired. This depends on, whether the component repair is permitted as per Overhaul Manual or not. If permitted then the component is sent to concerned agency for repair, if not then the component is replaced with new one. An assembled aero engine is illustrated in image below.
Those workshops who are involved in repair and manufacture of various components of aircraft, AGS parts, rotables (aero engine spares which can be repaired and reused) and aero engine components may get some guidance from this article about the repair of few major aero engine components which are discussed below.
Repair of Compressor Rotor (CR)
Compressor rotor is divided in two parts, Low Pressure Compressor Rotor (LPCR) and High-Pressure Compressor Rotor (HPCR). These rotors are fitted with blades in stages. Number of stages depends on type of aircraft.
When the engine is dismantled, as discussed earlier, it has to undergo various types of checks. In these checks, blades are checked for damage and wear. At times blades may also need replacement due to completion of total technical life. New blades may require reduction in length. The rotor has to be loaded to machine shop for reducing length of blades to the specified size by grinding.
Procedure for Grinding
For reclamation of CR by grinding blade tip to the specified diameters, following steps need to be considered:
(a) Lift the rotor with the help of sling provided for lifting CR.
(b) Use overhead crane for this purpose.
(c) Ensure availability of adequate man power for loading of CR on the machine.
(d) Use cylindrical grinding machine or a heavy-duty lathe with grinding attachment.
(e) Hold the job in the machine with one end held in chuck and the other end in fixture, which is supported by tail stock.
(f) True the rotor from both ends. The trueness should be with in specified limit, e.g. 0.01 mm in case of R-29 Engine of Mig 23 & Mig 27 aircraft.
(g) Mount the grinding attachment in place of tool post.
(h) Mount the grinding wheel of correct specification.
(i) Dress the grinding wheel.
(j) Take the grinding wheel close to 1st stage blades and very slowly give the feed till the wheel touches the blade
Note. It is always to be kept in mind that the length of all the blades may not be equal, hence while giving the feed for first touch operator must take care and rotor should be rotating. Never give the feed when rotor is stationary.
(k) Grind the blade to the specified dimensions. Take the measurement with the standard provided for measuring 1st stage. Record the dimension on the job card.
(l) After the first stage is over, the wheel is to be brought to IInd stage and the same operation to be repeated i.e. as mentioned in sub Para (j) and (k). Subsequently the other stages are also to be ground to the specified dimensions. All the dimensions should be recorded stage wise and operator, supervisor and QAS must sign. There after the rotor is to be off loaded from the machine using sling and crane by taking help of fellow workers.
Precaution to be observed during the Repair of CR
Compressor rotors are the vital component of an aero engine therefore maximum care must be taken while handling these components. Following precautions must be observed during transition and repair of the low-pressure compressor rotor:
(a) There is specific trolley available for CR. Always keep rotor on its trolley only.
(b) While transporting the rotor from one shop to another, make sure that blade is not touched with anything so that the damage can be avoided.
(c) While loading and offloading of rotor proper slings are to be used so as to avoid damage of blades.
(d) Grinding wheel guard should be on.
(e) Feed should not be more so that the burning of blades tip can be avoided.
(f) Proper measurement to be taken and supervisor must check the final dimensions.
Repair of Compressor Casing
Compressor casing is also a component of aero engine, which is fitted over the compressor rotor. It is in two halves joined together with nuts and bolts. It has the same number of stator blade stages as in compressor rotor, fitting in between the rotor stages. There is abradable coating on the inside side wall of casing like 20b in case of R-29 engine of MIG-23 and MIG-27 aircraft. The abradable coating ensuring minimum gap to the rotor blades and avoids metal to metal contact. It is worn out due to thermal expansion in rotor after prolonged use due to slight rubbing action. During the overhaul, this casing is loaded for fresh coating. Before coating, the old abradable coating has to be removed by machining. The following procedure is adopted to carry out the repair of compressor casing.
(a) Mount the compressor casing fixture on the chuck of heavy duty lathe machine.
(b) Casing is mounted on the fixture with the help of sling and overhead crane.
(c) Hold the casing on the face of the fixture with the help of suitable clamps.
(d) Concentricity at specified reference points should be adjusted within the accuracy of 0.02 mm.
(e) Mount the long and robust boring tool on the tool post and set it to the required angle. Always keep in mind that each stage is having different angle, therefore, the tool must be set at required angle for each stage.
(f) Remove coating of each stage by boring. Do not remove the material from base metal.
(g) Clean each stage thoroughly with the help of emery and with scriber from the serrations.
(h) The compressor casing is then loaded to coating bay for fresh coating. After the coating is fully cured and dried it is again loaded to machine shop for final machining to the specified diameter.
(i) Repeat the process stated in sub Para (a) to (e) above for mounting and setting of the casing.
(j) Bore all the stages to the specified dimensions. Keep all the parameters as specified in the Overhaul Manual.
(k) The dimensions of each stage are counter checked by supervisor and QAS and to be recorded on the job card.
Repair of Turbine Rotor (TR)
Blades of TR gets worn out after long use. These blades are replaced with new blades. These blades are to be finished to the specified dimensions by grinding as per the Overhaul Manual to ensure proper clearance. Procedure to be adopted the same as that of compressor rotor.
Repair of Turbine Rotor Blade
TR blades of R-29 engine of MIG-23 and MIG-27 aircraft gets worn out from sides and the clearance between two blades increases beyond the permissible limit. These blades cannot be rejected or thrown away owing to their high cost. Instead these blades are repaired by building up the dimensions by welding small plates known as BZL plates in Russian terminology. BZL plates are made out of same material that of turbine blades. For repair of these blades following steps are to be followed:
(a) Pre-Weld Grinding. Before the welding of BZL plates turbine blades have to be ground on the portion where the welding will take place. This will ensure that the seating of the plate is proper and firm homogeneous weld joint can be made. The blades are held in the fixture and ground to the specified dimensions as per the Overhaul Manual. Then it is sent for BZL plate welding and subsequently crack detection is carried out to check the welded joints for any cracks which might have developed due to heat during welding operation.
(b) Post-Weld Grinding. After the blades are welded with BZL plates, these are to be ground by using the appropriate fixture leaving 0.10 to 0.15 mm material for final grinding. Then the blades are subjected to heat treatment to relieve stresses set up by repeated grinding and heating. After the heat treatment is over, the blades are again loaded to machine shop for final grinding.
Now these blades are ground to the final dimensions which has been left in previous grinding. Care must be taken that during the final grinding over heating of the blades should not occur due to heavy feed. Also, it is always kept in mind that very little dimension is left for grinding therefore very less feed to be given and the final dimensions to be made as specified in the overhaul manual. Finally, blades are ready for use after the supervisor and QAS have checked the dimensions for its accuracy.
Repair of Stator Blades
Stator blades of stator casing unit of R-29 engine were fitted using 7mm bolts. In most of the cases it was found that these bolts were not able to withstand the load as these bolts were breaking frequently during operation. A modification was suggested to replace 7mm bolt with 8mm bolt. The stator casings were loaded to machine shop to carry out the modification. The following steps are to be followed for carrying out the modification:
(a) Mount the fixture on the drilling machine table. True the fixture using precision spirit level.
(b) Mount the stator with the help of clamps of fixture and check the perpendicularity of the hole with help of Dial Test Indicator.
(c) Drill out the existing 07 mm bolt.
(d) Clean the hole thoroughly.
(e) Using 7mm mandrel or accurate drill, adjust the concentricity of the hole.
(f) Using 7.8 mm drill enlarge the hole.
(g) Take 8 mm reamer and ream the hole to accurate dimension within specified tolerance limit, i.e. 8 +0.02 mm
(h) All the blade holes are to be enlarged following the same procedure as stated above.
(j) All the stages of stator holes are to be modified using appropriate fixture and following the same procedure as stated above in sub Para (a) to (h).
(k) The job has to be checked by supervisor and Quality Assurance Service (QAS) Personnel.
Repair of Combustion Chamber
After Prolonged use combustion chamber bearing housing bore is worn out and the outer race of bearing becomes loose in the housing. Chamber’s bearing housing bore has to be reclaimed by building up the bore with chrome plating and final grinding to the required diameter. The following procedure is adopted for reclamation of combustion chamber.
Procedure of Grinding. Following procedure should be adopted for grinding the combustion chamber bearing housing:
(a) Pre- Chrome Grinding.
(i) Combustion chamber is mounted on the machine.
(ii) Concentricity and perpendicularity is adjusted with in the permissible limit.
(iii) Using appropriate grinding wheel the bore dia is ground to the specified dimension.
(iv) Check the dimension and record it.
(b) Chrome Plating. Combustion chamber is to be loaded to electroplating shop for hard chrome plating. Procedure as stated in overhaul manual chrome plating is to be done.
(c) Post Chrome Grinding. Combustion chamber is again loaded to machine shop for post chrome grinding. Chamber is mounted on the machine, concentricity and perpendicularity is adjusted as per the specified limit. Using appropriate grinding wheel the bearing housing bore is ground to the final dimensions. The measurement is checked by supervisor and QAS and recorded in the work order. Combustion chamber is handed over to assembly bay for fitment in the engine.