Jet engines are sensitive to the temperature they operate in. The engines typically like cold and sea-level conditions, whereas hot and high environments result in the compromised engine efficiency.
Getting heated
The engines function on the principles of compression (of air), combustion (of compressed air and fuel), and expansion (of hot gasses). As the air is compressed, its temperature increases. As the fuel and air mixture ignites for combustion, the temperature further increases. The temperature in the combustion chamber can get as high as 3,000 degrees F (1,700 degrees C).
The internal components of the engine are designed to withstand such temperatures. However, all engines have a designed redline limit (maximum temperature the material can withstand). The redline limit is measured at the engine’s Exhaust Gas Temperature (EGT).
It reflects the temperature of the hot gasses entering the engine turbines. The engine performance is measured through the EGT Margin, which is the difference between the operating EGT and the maximum redline limit.
New and overhauled engines have a high EGT Margin, whereas older, rundown engines have a higher EGT Margin. Similarly, engines with higher EGT Margins have much greater specific fuel consumption (SFC).
Every little wear, such as a notch or a microscopic crack on the high-pressure turbine (HPT) blades, can add to the SFC of the engine. This is one of the reasons why engine trend monitoring is performed periodically to optimize efficiency through inspections and maintenance.
Photo: Airbus
The operating EGT Margin is influenced by the outside temperature. For example, the engine configured for a specific thrust rating will only generate a certain amount of thrust, say 90%, when operating in hot temperatures. On the other hand, a conditioned engine operating in colder, sea-level conditions generates 100% of the designed thrust.
While 90% is only an example, the output thrust depends on various conditions, including the Maximum Takeoff Weight (MTOW) of the aircraft, outside temperature, and altitude. For every 3 degrees Celsius increase in temperature, the air density decreases by about 1%. Airports at higher elevations also produce less thrust than the configured rating.
So, can flights be suspended due to very hot temperatures?
The answer is yes. An increase in the outside temperature means the engine is running hotter than normal. As a rule of thumb, every 1 degree Celsius increase in the outside temperature corresponds to a roughly 8 degrees C increase in the EGT. It is noteworthy that the change in density due to hotter temperatures only marginally affects the engine performance. Instead, the engines are, by design, limited by the amount of temperature the turbines can withstand.
Photo: Denver International Airport
There have been several occurrences where commercial airliners suspended their operations during the day due to extreme heat, particularly in deserted locations. Airlines usually configure daytime summer flights by having a reduced MTOW to optimize engine efficiency. It is common for airlines in the Middle East to run most of their operations during evening and overnight hours to optimize engine efficiency.
What are your thoughts on the effects of outside temperature on the engine performance and the suspension of flights? Tell us in the comments section.
Source: Rolls-Royce