Like any high-speed machine, jet engine temperatures significantly increase during operation. Jet engines are composed of thousands of parts manufactured with specialized materials. A typical Boeing 747 engine (a General Electric CF6-80C2) comprises upwards of 40,000 parts. Critical parts are manufactured with unique metals and alloys to withstand extreme temperatures.

The combustion chamber is one of the hottest sections of the engine. This is where the homogenized mixture of pressurized air and fuel is ignited. A healthy jet engine running near maximum thrust levels experiences approximately 3,000 degrees F (1,700 degrees C) in the hot section.

The combustion chamber comprises highly effective thermal coatings on the inner liner. The thermal coatings serve as a heat barrier, protecting the parent material. Without the thermal barrier, the maximum allowable temperature would be significantly limited due to the melting point threshold of superalloys.

Ground technician checking the engine
Photo: Fasttailwind/Shutterstock

Notably, the jet engine’s hottest component may not be the combustion chamber. For example, the hottest component on the GE CF6-80 engine is the high-pressure turbine (HPT) stage 1 nozzle. The HPT stage 1 nozzle is installed downstream of the combustor at the onset of the turbines. It is a passageway (for hot gasses) to travel from the combustion chamber to the HPT stage 1 turbine.

Besides the Boeing 747, current commercial aircraft types that use the GE CF6-80 engines are Boeing 767 (CF6-80C2), Airbus A310 (CF6-80C2), and Airbus A330 (CF6-80E1).

Engine temperature measurement

Thermocouple probes are installed at various locations (sections) on the engine to obtain temperature readings. The Exhaust Gas Temperature (EGT) determines the engine’s performance by measuring the temperature leaving the turbine exhaust. The EGT Margin provides the difference between the incurred takeoff EGT and the Redline (maximum limit) EGT. The EGT Margin determines the on-wing time and the engine’s health.

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Operational performance

The operational performance of the engine is monitored to detect deterioration, accelerated wear, and other damage. The health and stability of the engine are diagnosed through the collection of on-wing data. A narrowbody engine operating at maximum takeoff thrust in a deserted region would reveal different trends than the identical engine running (at similar thrust levels) in a tropical region. Engine Trend Monitoring (ETM) tools are designed to provide real-time visibility into the status of the engines.

N310DU Delta Air Lines Airbus A220-300 (1)
Photo: Vincenzo Pace | Simple Flying

The EGT margin allowable limit varies depending on the engine type and thrust rating. Typically, the maximum allowed EGT margins for most new and overhauled lower-rated engines are 75-100 °C. Similarly, the maximum permitted EGT margins for typical high-rated engines in a series are 50-70 °C.

The EGT margin is consumed over time depending on the aircraft usage and operating conditions. For example, the CFM56-7B27, a high-rated engine fitted on a Boeing 737-900ER, can consume the 50°C EGT Margin in less than 10,000 engine cycles. This means the engine must come off the wing for a performance restoration. A complete overhaul may be required if the Life Limited Parts (LLPs) have reached their limit.

On the other hand, a lower-rated engine, such as the CFM56-7B24 fitted on a Boeing 737-900, can remain on the wing for over 15,000 cycles. While there are always trade-offs between required performance and time between overhauls, a higher thrust rating generally equals lower wing time before a maintenance visit.

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