MAINTENANCE MATTERS
two years of inflation has increased these dollar amounts, but the three- to-one ratio has not changed. This illustrates the importance of compressor maintenance. Every mechanic knows that only a small amount of ice or snow accumulation on an aircraft’s wing will degrade the lift that the wing can produce significantly. The rotating blades and the stationary vanes in a compressor are also affected adversely by anything that degrades blade and vane surface finish and airflow. Regular motoring washes with plain water only helps remove loose “dirt” and contaminants that cause airfoil corrosion. Periodic but less frequent performance recovery washes with a cleaning compound and water help remove stuck on “dirt” and contaminants. Both the compressors and turbines must be rinsed with clean water after any wash procedure. Then run the engine to dry it out. Obviously you must follow the engine manufacturer’s specific wash instructions.
A clean compressor is a more efficient compressor. A more efficient compressor requires less fuel to produce the same power. A lower fuel flow rate at the same power level reduces hot section temperature, prolongs hot section life and provides a modest fuel savings. At today’s high jet fuel prices, modest fuel savings can pay for the time and cost of the wash procedures. Many engine manufacturers recommend the use of de-ionized water. A machine to produce de-ionized water is expensive! As an alternative, many mechanics go to Wal-Mart and purchase distilled water in one- gallon plastic bottles. Distilled water at Wal-Mart is less than $2.00 per gallon and it only takes four gallons of distilled water to wash a PT6 engine. It is also necessary to examine the compressor periodically, looking
20 |
DOMmagazine.com | march 2016
for dirt, corrosion, etc. If dirt or the like is observed, changes to the wash procedure or wash schedule are necessary.
SILICON DIOXIDE The chemical name for ordinary beach sand is Silicon Dioxide. Plain old-fashioned dirt contains a large percentage of Silicon Dioxide. Turbo prop aircraft can stir up sand and dirt during takeoff and landing, especially when propeller reversing is utilized. Helicopters can stir up even larger quantities of dirt during takeoff and landing. This stirred-up sand and dirt can be ingested into the engine. When the sand and dirt particles are struck by the compressor blades, they are shattered into a very fine dust. Air from the compressor that is contaminated with this fine dust is then extracted to pressurize the engine labyrinth seals that help keep the oil in the bearing compartments. This process allows the dirt/dust to enter and contaminate the engine oil system. Grit mixed with moving air is called sand blast. Grit mixed with a moving liquid is called slurry blast. Once a mechanic realizes an engine oil system that is contaminated with dirt/dust is a flying slurry blast machine, and that engine oil is slurry blasting the ball and roller bearings, gears, etc., in the oil system, the mechanic understands the need for careful monitoring. Periodic spectrometric oil analysis is an effective means to monitor the quantity of silicon in the engine oil. Obviously when elevated silicon levels are detected, oil filter cleaning and an engine oil change are necessary. Aviation Laboratories in Kenner, LA, can perform spectrometric oil analysis on an oil sample to measure the amount of silicon (as well as other metals) in an engine oil sample. At the same time, Aviation Laboratories can analyze the debris from your oil
filter to determine which metallic alloys, etc., are present in your oil filter debris. The cost of this analysis is reasonable and provides a major benefit. Additionally, the regular utilization of inlet covers and exhaust covers helps minimize the entry of dirt and sand into the engine(s) while the aircraft is parked.
FUEL NOZZLES The purpose of the fuel nozzles is to atomize the fuel into very tiny droplets (e.g., a fine mist). It is important to keep the fuel nozzles in proper working condition to atomize the fuel properly and continually. As the fuel nozzles’ condition deteriorates, the size of the fuel droplets begin to increase. The fuel droplets are only able to burn on the surface. As a result, slightly larger fuel droplets will not burn completely and very fine carbon particles can be produced. The air flowing through the turbine
vane ring (i.e., turbine airflow nozzle) approaches the speed of sound. These very fine carbon particles can then erode the coating off turbine nozzles and turbine blades. Leading edge tip erosion on the first stage turbine blades is frequently caused by faulty fuel nozzles. As the condition of the fuel nozzles continues to deteriorate, the size of the fuel droplets continues to increase. When the fuel droplets are sufficiently large they will not burn completely, and unburned fuel droplets will impact the turbine nozzle airfoils (vanes) where they will splatter. If you have seen napalm attacks in movies or on television, you can easily visualize the damage that fuel droplet splatter can do to turbine vane rings. The Pratt & Whitney Canada PT6A maintenance manual recommends an initial fuel nozzle inspection and cleaning interval of 400 hours, and allows this to increase to 600 hours, provided that the fuel nozzle condition is satisfactory.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68
