The starter valve cannot be open
When it is cold, the high-pressure air that enters the actuation mechanism of the low-temperature starter valve will condense and freeze as the high-pressure air of the aircraft bleed air system contains steams. This will increase the effective resistance. As the power provided by the high-pressure bleed air cannot overcome the resistance of valve, the valve is stuck in the closing position. Regarding the procedures, please refer to the checklist.
Engine Surge and Stall
Engine surge simply refers to collisions created by normal and reverse airflows inside the engine. To be more specific, surge is an abnormal work status of engine and is triggered by the airflow in the compressor and off-speed design of compressor. Compressor surge refers to the low-frequency and high-amplitude oscillation created by the airflow along the compressor axis. It can result in strong vibrations of or even serious damage to the engine components; over-temperature of the engine hot-end components; worsen the engine performance within a short period of time; and other defects such as Flameout. Airflow separation is the root cause of compressor surge, and such separation is caused by serious failure of the compressor.
In this QRH checklist, engine surge and stall are on the same list. What is stall and what is the difference between these two terms?
Stall refers to the rotation stall of compressor. That is, at least one low-speed airflow zone rotates the direction of compressor rotor with a speed that is lower than the rotor. When the rotation of compressor reaches certain level, the airflow of compressor starts to reduce for some reasons. It results in a decrease of the axial component of the absolute speed at the inlet of the moving blade, and then the airflow starts to separate on the back of the blade. This phenomenon that happens on the back of the blade is known as stall. If it happens to too many blades, it will result in compressor surge.
The stall phenomenon and indications can be summarized as: high-frequency mechanical vibration, increase of EGA and increase of amplitude indicator
What is the difference between surge and stall? When surge takes place, the airflow pulsation happens along the compressor axis and the flow field is symmetric, whereas stall occurs along the circumferential direction and is not symmetric.
The surge phenomenon and indications during the flight can be summarized as: canon sounds; a rapid decrease of thrust; the total pressure at the outlet of compressor decreases; air breathes in and out obviously at the inlet of the engine; an obvious increase of the sound of running engine with a low pitch and heavy sound; the high-temperature and high-pressure gas inside the combustion chamber flow back and results in "fire spitting"; severe mechanical amplitude; increase of EGT with unstable N1 and N2 rotation speed; increase of vibrator exponent; flying away the flight path.
What conditions can result in surge? An decrease in the performance of engine, such as the cracking of compressor blade and degraded performance of compressor; external environment, such as change of the summer wind; when a bird or foreign object is sucked into the engine and results in a blade damage and airflow separation (ex. volcanic ashes and ice); flight envelope that is out of the aircraft’s design (compressor condensation); corroded compressor and damaged engine components (turbine); defective engine control, gas control or surge protection devices; when the turbulence or hot air is sucked into the compressor (used reverse thrust at low speed); the generator design is out of the standards and resulted in the a disproportion of the engine’s acceleration power and turbine power (pushed the throttle forward quickly).
It is not complicated to implement the checklist. However, we shall think about engine surge carefully. Surge is defined as the over-limits caused by thrust shock, intake disturbance or aerodynamic disturbance inside the engine. Something that is worth for us to pay attention to is that some of the surges can be heard and come cannot. It is not true that all surges can create big noises.
In the flight simulator training, students can make the following mistakes when engine surge occurs: not being able to control the aircraft; attempting to stop the take-off process after V1 or rising the head; shut off an engine that operates normally; other procedural procedures. These improper measures often occur when the aircraft takes off, climbs up, approaches or overshoot. Common mistakes include choosing the wrong checklist; failing to accomplish all steps on the checklist; and mistakenly determine the affected engine.
Improvement Measures for Engine Surge
Three methods are often adopted in the engine design to solve compressor stall: air bleed valves; variable angle stator blades, or known as variable stator blades; and multiple-spool compressor. The descriptions thereof are as follows:
- Air bleed valves
As compressor stall is more or less caused by the large amount of airflow accumulated in the post-compression components, front compressor blades can, if the compressor functions abnormality and if the aircraft is in a high-altitude environment, have difficulty in compression if the airflow is insufficient. This can result in insufficient compression ratio of the compressor and blockage in the post-compression section. Therefore, when designing the compressor, the designer often seeks for a balance point between the compression ratio and engine efficiency to prevent the said conditions and to enhance the stability of the aircraft by reducing or losing little mechanical efficiency. In this way, when the airflow reaches a specific level, the decrease of compression ratio can be used to reduce the loading of engine in order to minimize the occurrence of compressor impulse. This is the reason that aircrafts are often designed with a number of air bleed valves in the middle of compressor to release some of the airflow in order to adjust the compression ratio and maintain appropriate airflow relations to prevent stall. These air bleed valves can be automatically turned on or off according to the rotation speed; temperature and pressure at the compression inlet; and the size of compression ratio to prevent the air being accumulated in the post-compression section, which often has a high pressure, and prevent the compressor from being blocked.
In general, air bleed valves are often used in the engine starting phase and prevention of the high-altitude compressor stall. When the thrust and rotation speed are low, the air bleed valves will be open to release some of the compressed air in order to reach expected functions. However, when the thrust and rotation speed are high, the air bleed valves will be automatically closed to maintain the normal operations of the aircraft engine.
- Variable angle stator blades
Where a single-shaft compressor with high-pressure compression ratio is adopted in relatively bigger turbojet and turbofan engines, the airflow control will be one of the key point that shall be taken into consideration. Normally, in the first-level compression, variable intake guide vanes are adopted together with some of the first few variable angle stator blades to prevent compression stall. When the rotation of compressor slows down, these angle stator blades will gradually close, enabling the back blades to maintain at an acceptable airflow angle and prevent a stall. In the meantime, these variable blades can have their angles automatically adjusted using the oil control system. The main reasons for the adjustment are the intake air temperature and engine rotation.
Basically, the main functions of variable angle stator blades are:
- roviding a flowing direction or angle that can have the first few compressor blades be maintained at the most appropriate level (close to the design);
- Minimizing the possibility for the first few compression blades becoming stall, and the later blades being locked due to the windmill effect;
- Preventing or making an improvement on the influence of compression effect to all levels of compression.
- Multiple-spool compressor
In theory, general single-shaft compressor may have the compression level increased or decreased according to the actual demand in order to produce a needed compression ratio. If it is so, the last level of compressor blade can, when the rotation speed reaches specific level, become rather inefficient, whereas the first-level blades may be overloaded and results in stall or impulse. As described in previous paragraphs, although this condition can be improved by opening air bleed valves, it can also reduce the compression efficiency of aircraft engine if too much compressed air is released. Therefore, to enhance some of the throttle operations; increase the adaptability and stability of compression in start-up; and maintain that compression efficiency at a normal range, the compression body is often divided into two independent systems. Basically, the high-pressure compressor has shorter blades compare with those of the low-pressure compressors, and is therefore lighter. As the air temperature will gradually increase during the compression process, the temperature of high-pressure compressor is higher than that of the low-pressure compressor, allowing the high-pressure compression to have a relatively higher blade tip speed. Therefore, when the temperature of compressed air increases, the velocity of sounds also increases.
When the rotation of the back high-pressure compressor is controlled by the oil control system, the front low-pressure compressor is powered by the low-pressure turbine and rotates automatically and freely to ensure the best airflow pass through the compressor. Besides, whenever necessary, the low- and high-pressure compressors can be manipulated through the throttle to adjust their respective speed. This can, while satisfying the minimal air separation demand, prevent the happening of compressor impulse. As the freely rotated low-pressure compressor is responsible for matching the compressors and preventing stall triggered by the blockage of high-pressure compressor, the high- and low-pressure compressors actually complement each other. Where the airflow between the high- and low-pressure turbines are matched, it is possible to enhance the compression ratio without reducing the engine efficiency. Besides, this dual-shaft compressor only requires the back-end high-pressure segment in the start-up phase. Therefore, among the high-pressure parts, only those that are relatively lighter will participate in the bounce off, resulting in a decrease of torque.
Conclusions
The NASC helicopters often need to implement mission is dangerous weather conditions and environment. Challenging by the strict operating environment, it is a must for us to understand potential symptoms that can cause the malfunction of aircraft engine. This will help us to, when an incident occurs, effectively and accurately handle it and exclude all defects to prevent flight safety hazards.
Sources: Read One – Do you know about these confusing engine defects?
Commercial Aircraft Corporation of China Ltd.