Christmas Eve, 2009 in Decatur, Texas, was marked by low visibility with blowing snow. The flight crew of a Bell 407 helicopter hunkered down in the hospital's crew room while waiting for the miserable weather to subside, which it finally did. At that point the dispatcher called with a release for a cross-country positioning flight. The crew climbed aboard, strapped in and the pilot cranked up the Rolls-Royce 250 turbine.

As the helicopter ascended to a height of about 50 to 60 ft., the pilot heard two warning horns sound, followed by the Bell yawing 90 deg. to the left. The pilot immediately lowered the collective in an effort to preserve rotor rpm and maneuvered the helicopter back over the helipad. When about 5 to 8 ft. over the pavement, the pilot increased collective pitch, but the helicopter continued its rapid descent and skids hit the surface so hard that two of the three people aboard were seriously injured.

According to the NTSB's official accident report, the helicopter had been parked outside for approximately 5 hr. in blowing snow conditions without any plugs or covers over the engine inlets or the exhaust. The plugs and covers were later installed and the helicopter remained outside in temperatures ranging from 16F to 34F for the next 19 hr. A video surveillance camera at the scene revealed that at no time did anybody look at the exhaust stack, right-side engine intake, or open any access panels prior to the operation of the helicopter.

An examination of the recorded Engine Control Unit (ECU) data revealed that the helicopter's powerplant experienced a momentary flameout followed by a successful relight. However, due to the helicopter's close proximity to the ground, the pilot was unable to recover main rotor rpm before ground impact. As part of the post-accident examination, the engine was removed from the airframe and an engine run was performed. There were no anomalies found with either the engine or airframe that would have contributed to the loss of engine power.

The operator's procedures manual stated that a pilot must ensure that all ice, snow and frost is removed from the engine inlet area prior to flight. The helicopter manufacturer's flight manual stated that as part of the preflight inspection, the pilot must check that the engine inlet area is cleaned of all debris, accumulated snow and ice. The engine manufacturer's operation and maintenance manual included a precaution that the accumulation of snow or ice may result in the engine experiencing a flameout.

The NTSB determined the probable causes of this accident were the pilot's inadequate preflight inspection and the momentary loss of engine power due to snow or ice ingestion.

It's a given that gas turbine engines operate best when the air is flowing smoothly through the engine. In reality, the airflow within a compressor is quite complex and tends to be especially unsteady near the compressor blade tips. Typical flow phenomena in rotating machinery can include vortices, separations, secondary flows, shock and boundary layer interactions, and turbulent wakes.

The susceptibility to compressor stalls was more common among earlier gas turbine engine designs with manual or mechanical fuel control units. Newer generation turbine engines incorporate a variety of features including hydromechanical and electronic control systems such as Full Authority Digital Engine Controls that greatly reduce the probability of compressor stalls.

To ensure safe engine operation over the entire range of flight conditions, several limitations are included in engine control systems. An N1 speed limiter restricts the maximum gas generator rotational speed to a preset value in order to avoid engine damage from mechanical and pressure forces. The Turbine Outlet Temperature (TOT) indicator is also constantly monitored and fuel flow restricted to avoid exceeding the TOT limit. At low rpm, the engine is naturally not able to pump as much air as during high revs, and so at engine start, the compressor bleed valve opens to boost airflow so that the compressor blades do not stall. Additional control devices such as variable stator vanes extend the stability reserve in the low-power operating region.

When these devices operate as designed, a turbine engine's susceptibility to compressor stall is greatly reduced. Nevertheless, many operational factors remain that result in compressor stalls. These include ingestion of debris during takeoff and landing from unimproved fields; distorted intake flows during abrupt maneuvers or hovering in adverse winds; re-ingestion of hot exhaust gases; degradation of the stall margin over time due to erosion, rubs and normal engine wear of the rotor blades, seals or bleed valves; inlet distortion caused by phenomena such as ice, water or snow; and adverse adjustment of the engine control system. Even dust, wildfire smoke, smog and dirt in the compressor can reduce its efficiency and lead to a stall if the contamination is severe.

A comparatively mild form of compressor stall is a local disruption of airflow within the compressor, but which continues to provide compressed air to the rest of the engine, albeit with reduced effectiveness. This type of compressor stall may be momentary, resulting from an external disturbance such as disrupted unsteady airflow entering the air inlet while hovering in a tailwind condition. This condition will typically produce an odd bang or two.

A steady compressor stall occurs when the compressor finds a working equilibrium between stalled and unstalled areas. This type of stall is more likely to produce a roaring sound and severe vibrations. The Bell 206 L-3 manual lists the engine compressor stall indications to include engine pops, high or erratic TOT, decreasing or erratic gas producer rpm or power turbine rpm, and torque oscillations.

Compressor surge is a complete break–down in compression resulting in a reversal of flow and the violent expulsion of previously compressed air through the engine intake due to the compressor's inability to continue working against the already compressed air behind it. The Boeing CH-47 Chinook's manual warns pilots that a sharp rumble or series of loud reports emanating from the engine normally characterizes the onset of compressor surge or stall. These odd noises and vibrations are accompanied by abnormal engine vibrations, increased TOT, rapid fluctuations in Turbine Inlet Temperature (TIT), torque, and N1 for the affected engine, a noticeable loss of power and rotor rpm, and sudden bolts of flames flashing out of the inlet and/or exhaust.

The severity of the compressor stalls will dictate if the engine should be shut down and treated as failed. The Bell 206 L-3 manual states that with stalls of a less-severe nature (one or two low-intensity pops) continued operation of the engine may be permitted but at a reduced power level and by avoiding the condition that resulted in the compressor stall. If flight is to be continued, it prescribes reducing power, switching engine anti-ice off, maintaining slow cruise flight, checking the TOT and gas producer rpm for normal indications, slowly increasing the collective to achieve a desired power level, rechecking the TOT and gas producer rpm, and landing as soon as practical.

Pilots must avoid conditions of repeated surging or stalling as the transient torsional loads from the engine can damage it, the drive train and associated airframe components. For example, a compressor stall in the Eurocopter AS350 requires an overhaul of the tail-rotor gearbox due to the oscillating loads. In the extreme, compressor surge can potentially lead to total destruction of the engine.

There are many operational environments that make a helicopter more susceptible to compressor stalls and surges. One of these is hovering in a tailwind condition, resulting in distorted flow of air into the engine inlet.

On Sept. 14, 2002, near Rochester, Mass., the pilot of a Bell OH-58AT, converted to civilian use by Garlick Helicopters, made numerous successful lifts into the wind with loads of cranberries before moving on to a new bog. On the first lift there, the pilot hovered the helicopter about 12 ft. above the ground with a left quartering tailwind while the ground crew secured a 900-lb. cargo basket to a 15-ft. sling load line. Once the load was secured, the pilot maneuvered the helicopter forward, dragging the cargo basket. There was a loud pop sound, and a foot-long jet of flame fired from the left exhaust pipe. The pilot jettisoned the load and the helicopter pitched up. At that point, the main rotor blades severed the tail boom, and the main rotor assembly separated from the helicopter. The helicopter crashed to the ground and rolled onto its left side. The pilot, who had 4,000 hr. in the OH-58, was killed. There were no obstructions that would have impeded him from departing into the wind. The NTSB determined the probable cause of the accident included the pilot's improper decision to attempt a takeoff with a quartering tailwind, which resulted in an engine compressor stall.

Helicopters hovering in locations with “squirrely” winds will be especially prone to this phenomenon. Such a location is the downwind side of buildings and obstructions where the direction and velocity of the moving air changes rapidly. Also, airflow interrupted by obstructions will contain strong wind gradients. Airflow studies around buildings at the University of Notre Dame's aeronautical engineering school revealed that windsocks positioned at each of the four corners of the rooftop platform can blow in completely different directions simultaneously, illustrating the complex nature of airflow in the vicinity of these structures. Under such complex airflow conditions it can be impossible for the pilot to determine the “predominant” wind condition in the confines of that location. The probability of distorted inlet airflow during hovering rises significantly in such an environment.

There is an additional hazard created in the hovering environment called Hot Gas Ingestion (HGI) in which the rotor downwash redirects engine exhaust gases into the engine air inlet. This phenomenon is particularly problematic when helicopters hover with a tailwind or during sideways or rearward flight.

HGI causes increased engine inlet gas temperatures, which result in a significant loss of power and potential flow distortion. Assuming an engine is operating at its maximum rated temperature, an inlet temperature rise of 40F due to HGI can cause a power loss of approximately 15%. In addition, temperature distortion (meaning the hot gases) can cause compressor stall.

Helicopter pilots operating in the offshore oil industry are well aware of the effects of flying through hot exhaust plumes around oil platforms whose temperatures can be several hundred degrees above ambient. When these gases disperse into a helicopter's approach, departure and landing paths they potentially create a severe operational risk because any rise in ambient temperature along the flight path will result in a loss of both engine and rotor performance. Additionally, small increases in air temperature in the approach path and over the helideck from the platform hot exhaust gases can result in a dramatic loss of rotor lift.

Pilots not involved in the oil and gas industry can be exposed to a similar phenomenon. Incidents have occurred during missions in which pilots were asked to navigate over power plants so that photographers could take pictures. Smokestacks at energy plants often contain “scrubbers” that help remove visible pollutants from the exhaust, and ingestion of the invisible exhaust gases can lead to a compressor stall.

Aerial firefighting pilots are exposed to hazardous smoke and incredible heat as they move in low and slow in an attempt to snuff the fire with the water or retardant. Beyond producing vast amounts of smoke and intense heat, wildfires consume massive amounts of oxygen and the resulting gaseous mixture above the flames can contain many substances toxic to a gas turbine engine.

When making low-altitude water drops, a firefighting helicopter is under a heavy load and the rotor blades are at an exceptional pitch angle to create sufficient lift. Making the situation even more dire, the helicopter is often within the cross-hatched zone of the Height-Velocity curve, meaning that a successful autorotation is highly unlikely. Should engine failure occur, the pilot immediately needs to release the heavy load as well as reduce rotor blade pitch as much as possible in order to preserve rotor rpm. With the blades at such a high pitch angle, rotor rpm will decay almost instantaneously. Aircraft equipped with auto reignition will automatically try to relight. In single-turbine helicopters without auto reignition, the pilot needs cat-like reflexes to initiate a restart with the starter button, and since there simply is so little time and altitude, momentary torque or temperature exceedance can result.

Flying in heavy rain or hovering close to the water's surface can result in water ingestion. Partial vaporization of ingested water droplets decreases the inlet temperature and compressor outlet temperature and reduces the compressor surge margin. Moreover, increasing the water-to-air ratio leads to an immediate flameout.

Dust, smog, wildfire smoke, salt water and dirt in the compressor — all of them can reduce compressor efficiency and lead to a stall if the contamination is severe. This threat is mitigated by periodic “compressor washes” to remove the contaminants. 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 Hughes (now MD) 369 series of helicopters powered by the Allison (now Rolls-Royce) Model 250 turboshaft engine.

The service letter notes: “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 (Pc) sensing tube. A 5-min. ground run is required to purge and evaporate any residual water after rinsing. It is to be noted that Hughes PN 369H92537 engine compressor water wash kit (installed as optional equipment per referenced Notice No. HN-107.1) is designed specifically for compressor rinsing with a clean water spray only. 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. The Pc tube must therefore be disconnected and the opening capped to prevent introduction of chemical agents into the controls system Pc connection during the wash operation.” Comparable instructions are provided in the maintenance and service manuals for almost all turbine helicopters.

Recent aeronautical engineering studies acknowledge the lack of fundamental understanding of the processes of stall inception with a gas turbine engine's compressor. Since the engines must perform reliably in harsh operating environments while maintaining a high degree of operational availability, advanced aeronautical research continues to refine the “state of the art” knowledge so that future designs will be even more reliable.

In the meantime, it is of keen importance that helicopter pilots remain cognizant and protective of the airflow conditions into the engine inlets, and take the proper preventive actions to ensure the engine is receiving a steady stream of clean air to keep the fire lit and the rotables rotating. BCA