Particulates Challenge Turbine Engines, Part 1

An aircraft engine is like a big vacuum cleaner that sucks everything in front inside. The impact of airborne particulates on aircraft engines is a huge challenge for today’s aviation industry.

When hail, ice, heavy rain, dust and sand, volcanic ash, lightning, or corrosive gases enter a turbine engine, internal components are exposed to stresses that can lead to premature wear, degraded engine performance, unanticipated component failures, and sometimes even complete failure of the power.  

Imagine yourself descending into your destination when every engine on your aircraft loses power, turning you into a heavy inefficient glider with a mediocre glide ratio. This occurred on Dec. 14, 1989, when KLM flight 867, a Boeing 747-400 enroute from Amsterdam to Tokyo, was cleared to descend from FL390 at the pilot’s discretion and a vector given to avoid the last known area of ash cloud.  

During the descent through FL260, the aircraft encountered the ash cloud. An ash/smoke haze entered the cockpit, which prompted the crew to don their oxygen masks, then use max power to climb. One minute later all four engines lost power and were stuck at roughly 28-30%. The main generators dropped off line, causing a power interruption, loss of airspeed indications and a fire warning alarm for the forward cargo area.  

During momentary power interruptions the flight instruments transfer to standby power, which is provided by two batteries and inverters. Multiple attempts to restart the engines were only momentarily successful, but the switching between main generator and standby power caused repeated power transfer interruptions to the flight instruments, sometimes causing a temporary blanking.  

After eight or nine attempts and a descent to 13,300 ft., all engines restarted and the flight landed without further incident at Anchorage. The 245 aircraft occupants were uninjured. The post-incident inspection found significant damage on external surfaces of the aircraft and in the high-pressure turbines of all engines. More than $80 million in damage was done to the 747, requiring that all four engines be replaced.

Volcanic ash particles are predominantly composed of silicates with a melting point of 1100 deg. C. When heated by the engine combustion section, these particles become molten glass that can coat fuel nozzles, the combustor and the turbine blades. This reduces the efficiency of fuel mixing and restricts air from passing through the engine.

The abrasive ash particles also cause erosion along the leading edges of compressor and turbine blades. The roughening of the surfaces over those airfoils causes extra drag and early boundary layer separation. This will lead to higher engine temperatures and reduced fuel economy. Engine surges, total loss of thrust and flameout have occurred in many previous incidents.    <\/p>

Even brief encounters with the fringes of a volcanic ash cloud can produce long-lasting damage that will significantly shorten an engine’s life. On Feb. 26, 2000, the Mount Hekla volcano in Iceland erupted, producing an ash and steam cloud rising to 45,000 ft. Two days later, while enroute from Edwards AFB, California to Kiruna, Sweden, scientists onboard the NASA Douglas DC-8 noted a sudden rise in measurements indicating the presence of a volcanic ash cloud. The flight crew noted no change in cockpit readings, no St. Elmo’s fire, no odor or smoke, and no change in engine instrument indications.<\/p>

After landing in Kiruna, the engine oil, oil filters and heat exchanger filters were removed for analysis. Visual inspection of the DC-8 and first-stage engine fan blades showed no apparent damage or erosion to any parts of the aircraft. The research flights continued in Sweden, after which the aircraft was ferried back to Edwards AFB, accumulating 68 flight hours since the ash encounter.  

Engine borescope inspections revealed clogged cooling passages and some heat distress in the high temperature section of the engines. All engines exhibited a fine white powder coating throughout. There was leading edge erosion on the high-pressure turbine vanes and blades, blocked cooling air holes which reduced or eliminated cooling airflow, blistered coatings and a buildup of fine ash inside passages.  

The uncooled blades still performed aerodynamically but required expensive overhauls. Even though engine trending data had not yet revealed a problem, many hot section parts had begun to fail. Total cost of refurbishment for all four engines was $3.2 million.

Dust, Dirt, Sand, Wildfire Smoke

Those who live in the U.S. Southwest Desert region have been exposed to “haboobs.” These are massive, wall-like dust storms that lift sand and dust into the air, coating everything with vast amounts of sediment.

Many new-generation engines have encountered some problems with dust ingestion in regions such as the Middle East. This has led to increased wear on engine components and premature removals from wings. Recently, Embraer encouraged customers of its E2 regional jet that operate in hot and dusty environments to defer deliveries of the aircraft until a new combustor is signed off for its Pratt & Whitney PW1900G engines.

Tiny particles of sand, dirt, dust, and wildfire smoke have the abrasive equivalent of sandpaper. These particles can cause rapid erosion of the compressor airfoil surfaces, resulting in loss of compressor efficiency, loss of Exhaust Gas Temperature margin, and compressor stall if the contamination is severe enough.

Sand and dirt particles that flow through the engine into the hot section will also destroy the protective coatings on hot-section components, leading to accelerated erosion. They also block small-diameter pressure sensing and control lines, leading to erratic fuel metering to the combustion chamber, erratic engine operation, and malfunction of accessories, as well as blocking cooling holes within turbine blades.

The silica in sand, similar to volcanic ash, melts at high temperatures, forming a glass-like substance that clogs fuel nozzles and blocks cooling holes. This results in the loss of cooling air to the turbine blades, exposing blades to temperatures beyond their design parameters. Continuous exposure to high temperatures may result in blade failure, blade creep, blade rub, and extensive damage to the turbine leading to an in-flight shutdown.

As turbine blades experience high temperatures they can begin to elongate. If the blades elongate too much, they will begin to “rub” against the boundary walls.  Research at The Ohio State University’s Gas Turbine Laboratory found that blade-to-case rub can degrade the performance of jet engines through the introduction of high amplitude shaft vibrations and severe blade/seal wear. This can lead to catastrophic failure of the entire engine if left undetected.

Airborne particles will also cause deposition of contaminants inside heat exchanges (e.g. oil cooler) that may block free air flow and result in decreased heat transfer that will then elevate fluid temperatures. Higher than permitted fluid temperatures result in rapid deterioration of internal engine components.

Blowing snow, freezing precipitation, or even airport snow removal operations can cause contamination on a wide spectrum of engine components, we explain in Part 2 of this article.