Preamble:

It is no secret that the geographical location of the airline bases from Central Asia is characterized by sharply continental climatic conditions with the presence in this zone of a predominant number of sandy deserts, one of which is the Karakum Desert, which literally means "Black Sands". Its name, which directly speaks of the presence of carbon in the chemical composition, and in addition to the main composition of quartz sand, which is predominantly silicon dioxide, includes certain inclusions of aluminum in the form of corundum and other impurities. Thus, this entire "explosive mixture", absolutely resistant to acid-base effects, is present in the atmosphere and especially the ground air (1000 m and above) for almost 300 days a year, including periods of flowering plants and migration of flying insects, carrying elements of sand, dust, etc. fractions.

Risks and critical conditions of exposure to sand during engine operation:

Common risks associated with the air flow in the engine include:

- abrasive wear, or developing deposits/sticking, of the fan blades and low/high pressure compressor, including the blade mounting positions, lead to increased vibration levels, up to cases exceeding 4.0 units in take-off mode;

- reduction of the flow area or complete blocking of the pressure take-off elements/filters of the FADEC system, engine cooling system, turbine blade clearance control system (LPTACC/HPTAC), as well as the EEC cooling system, lead to incorrect operation of the ATA 73, 75, 76 and 77 systems;

- a decrease in the flow area or complete blocking of the elements of the fuel injectors of the combustion chamber leads to incomplete formation of the fuel-air mixture, which in turn does not ensure complete combustion of the fuel and subsequent drop in thrust, with a corresponding increased fuel consumption, which in the form of a directed stream burns out on the walls of the combustion chamber, with the subsequent formation of cracks and burnouts;

- as noted above, incomplete formation of the fuel-air cloud along with the formation of thermally formed deposits/sintering of quartz sand on the walls of the combustion chamber lead to insufficient cooling of the combustion chamber surface, with the subsequent formation of cracks and burnouts;

- also, a violation of the heat exchange balance leads to the occurrence of cracks and chips in the thermal barrier coating (TBC), which also, in the process of separation from the surface, represent a solid foreign body (FOD / FDI) that subsequently attacks both the guide vanes (HPT Nozzle Guide Vanes) and the high-pressure turbine blades themselves (HPT Blades) and their piping (Shrouds);

- reduction of the flow area or complete blocking of the cooling holes of the guide vanes (HPT Nozzle Guide Vanes) and high-pressure turbine blades (HPT Blades) leads to partial burnout or complete burnout / destruction of these elements, as well as chipping / spalling of the trim (Shrouds), subsequently leading to exceeding the TGT in takeoff mode, especially in the case of elevated ambient temperatures (+30° C and above), which subsequently leads to a forced change in the takeoff mode by the pilots, which also poses a certain risk;

- combustion products with inclusions of the combined sintered chemical and biological elements specified in the Preamble, in the process of impact on the blades of the low-pressure turbine (LPT) also affect the surfaces of the blades in the form of residual abrasive, which in practice did not record a particular excess of the vibration level.

Airline and Organisational Procedures for Continuing Airworthiness:

Despite all the innovations, modifications and improvements in technologies in the field of filtration of sampled air, as well as the use of wear-resistant coatings, introduced by developers and manufacturers of the said systems, in turn, the most effective preventive measures applied in practice by operators of this zone are:

- full use of the SAGE system from CFM;

¾ gentle engine operating policy on the part of operators;

- increased frequency of engine washing procedures using certified materials and detergents;

- full interaction between pilots and engineering and technical personnel in the matter of operational monitoring of engine operating parameters;

- prompt delivery of decrypted information by Readout Shop for subsequent proactive actions by the operator/SAMO;

- increased frequency of checks and inspections, including borescope inspections and NDT inspections (if necessary or suspected), on the engine;

- full interaction between operators and the meteorological service regarding correct information determining the threshold/coefficient of visibility (transparency) of the ground layer of air, characterizing the concentration of foreign fractions.

Note: This aspect may affect the adjustment of the flight schedule operated by the airline, optimally applicable to the state of air/atmosphere purity.

Conclusion:

Be that as it may, airlines face the said problem as an inevitable reality, and additional investments in the use of new materials and technologies by developers/manufacturers of systems and engines will naturally entail an increase in the cost of supplied components/engines, as well as their technical/service maintenance. It is important in this case to find a balance in the development of the AMR (technical maintenance program) in the area of ​​harmonization of maintenance work, both on the aircraft and on related systems and engines, minimizing the risks of unscheduled technical maintenance.

The following are the results of adverse climatic conditions and the lack of sound engine operating policies on the part of the operator.

Author: Myrat Koturov, who held key positions in the Civil Aviation Administration of Turkmenistan.