Particulates Challenge Turbine Engines, Part 2

Aerial firefighting helicopters ingest significant volumes of airborne particles, and operators are advised to conduct periodic “compressor washes” to remove the contaminants from the compressor.

An example of recommended cleaning instructions is included in the service letter entitled “Engine compressor cleaning– daily water rinse and periodic water wash” for the MD 369 series of helicopters powered by the Allison Model 250 gas turbine engine. 

The service letter instructs: “First, a daily water rinse is recommended for helicopters operating in a corrosive environment, to remove any contaminants and corrosive air particles from the compressor. This daily water rinse uses no soap or cleaning solvents and is accomplished without disconnecting the compressor discharge pressure sensing tube. A five-minute ground run is required to purge and evaporate any residual water after rinsing.”  

For helicopters operating in smoggy areas, a periodic (200-to-300 hr.) compressor water wash is also recommended. The periodic water wash uses cleaning solvents to remove dirt buildup in the compressor. Comparable instructions are provided in the maintenance and service manuals for almost all turbine helicopters.

Icing During Ground Operations

Blowing snow, freezing precipitation, freezing fog, slush, ground contaminants or even airport snow removal operations can cause contamination on a wide spectrum of components within a jet engine.  

Even after an engine is started it is possible for snow and slush to accumulate within the engine intake ducting as well as on the rear surfaces of engine compressor/fan blades during ground operations in conditions of moderate to heavy freezing precipitation. Ice accumulation on the surfaces of engine compressor/fan blades may severely affect the aerodynamic performance of the blades and cause compressor stall, engine surging, and engine malfunctioning and/or reduced thrust.

Intake ice occurs when accumulations of snow and/or slush in the engine air intake during low-power engine operations, such as taxiing after landing or prior to takeoff. Relatively long/curved intake ducts are particularly prone to this phenomenon. It is most likely to occur during heavy snow or rain at temperatures close to 0 deg. C before and after engine start. The consequences will only become evident when applying power for takeoff.

These accumulations may not be prevented by using engine anti-icing, especially when engines are operated at or close to ground idle rpm. Intake duct deposits and engine blade deposits may detach and be ingested by the engine during the subsequent application of high-power settings for takeoff, resulting in adverse effects on engine operation and possible flameout.

According to GE Aviation technical pilot Andy Mihalchik, if flight crews or maintenance personnel note ice or snow on the engine’s spinner with a thin layer or no ice/snow visible on the fan blades, the flight crew should accomplish the ground ice shed procedure. 

If your preflight inspections notes ice/snow on the spinner and a heavy accumulation on fan blades, discuss with maintenance the necessity to de-ice the engine prior to engine start. Do not confuse this with an engine’s anti-icing system.  Engine de-icing is a procedure in which engine-specific coverings are placed on the engine, then hot air is piped into the engine.

If de-icing is not an option, attempt to procure a heated hangar as another option.  Spraying aircraft deicing fluid into the engine is NOT approved because it reduces engine efficiency and causes corrosion on the hardware. In addition, de-icing fluids can cause contamination of the bleed air system.

Engine anti-ice must be selected ON immediately after both engines are started and must remain on during all ground operations when icing conditions exist or are anticipated. Most manufacturers recommend the use of engine anti-ice during ground operations when the Outside Air Temperature is 10 deg. C or less and visible moisture is present.  

When air is sucked into the engines there is a venturi effect that will decrease temperature. It is possible to get ice accumulation within the engine’s inlet at +10º C. Since engine anti-ice extracts a performance penalty from the engine’s thrust, don’t forget to apply the necessary corrections stipulated in the aircraft’s flight manual to the aircraft’s takeoff and climb performance.

Jet engines are susceptible to accumulation of ice on fan blades in freezing fog or freezing precipitation conditions when the engine is at or near its ground idle speed. Isn’t engine anti-ice capable of preventing this ice accumulation? No, engine anti-ice systems are designed primarily to deter the accumulation of ice on the intake of the engine nacelle. Ice that has accumulated on the fan blades while the engine is at idle speed  must be removed by engine run-ups prior to takeoff.

Ground ice shed procedures usually contain an acceleration of the engine RPM to a minimum thrust setting and then a dwell time at that thrust setting. The acceleration increases centrifugal forces and slightly flexes the fan blades resulting in mechanical shedding of ice. The dwell time contributes to the thermomechanical ice shedding of rotating and static hardware resulting from increased fan airflow temperatures and pressures. Asymmetric fan ice shedding may cause momentary increases in perceived and indicated engine vibration. Fan vibration levels should return to normal levels as fan ice sheds.

During severe icing conditions, freezing fog, rain or drizzle or heavy snow, repeating the ice shed procedure at 10-min. intervals may reduce fan ice accumulation. Avoid doing the ice shed procedure in areas with loose ice and snow to minimize the FOD potential.  

Be aware of the additional jet blast caused by your engines during an ice shedding procedure, so avoid doing the procedure where the jet blast may damage other aircraft or nearby structures.  Also don’t forget that ice contamination of engine probes can cause erroneous flight deck instrument indications.

Post-incident inspection of an engine whose performance deteriorated during a ground-run discovered a greenish, lacquer-like coating on the compressor blades.  The investigation subsequently determined that paint mist being conducted 50 m upwind during the acceptance run had entered the engine, causing several hundred thousand Deutsche Marks of damage.

Artificial fertilizers and dust from industrial operations can be sucked into engines, even over apparently safe distances. Lacquer mists can bond on the outside of compressor blades which degrade the aerodynamic efficiency of the blades.  

Heavy metal pigments in the paint mist can corrosively react with the hot parts.  The sulfur and phosphorus compounds in fertilizer can damage hot parts. Air impurities must always be taken seriously, even when they are far away.

Extreme rain and hail can cause a decrease in engine RPM, compressor stalls or even complete loss of power, we explain in Part 3 of this article.

Click here for Particulates Challenge Turbine Engines, Part 1.