The Importance of Hot Section Inspections
A critical aspect of regular turbofan or turboprop engine maintenance is the hot engine inspection. A hot section inspection (HSI) refers to the examination of the condition of many key engine parts, such as the turbine blades, combustion chamber, stators, vane rings, compressor turbine disk, and shroud segments. In this blog, we will further discuss what HSIs are, and why they are so important.
An HSI is a significantly invasive form of inspection. As such, most aircraft engine manufacturers recommend a minimum number of flight hours before carrying out an HSI and any associated MRO (maintenance, repair, overhaul). This number of flight hours is known as the time between overhaul (TBO). As the aircraft’s TBO nears, a maintenance contractor should conduct an HSI. Generally speaking, HSIs are to be conducted around the halfway point of the TBO. For example, if the TBO is 4,000 flight hours, an HSI should be carried out after 2,000 flight hours. The TBO varies greatly depending on aircraft type and model, sometimes differing by 1,500-2,000 flight hours or even more.
During an HSI, the maintenance crew will inspect a segment of the aircraft engine known as the hot section, a segment that contains components such as the air compressors, combustion chambers, compression turbines, turbine-shroud segments, and temperature sensors. The main purpose of an HSI is to ensure that these critical components are capable of producing the required power to run the engine. Any of these components that do not meet certain criteria will be considered failing and must be replaced immediately. Minor problems such as small cracks on less significant or stationary parts can be ignored, as long as they do not negatively alter the performance of the engine. These issues will still be noted and checked again during future maintenance. Though HSIs are invasive, they can generally be completed in a matter of a few days.
Hot section inspections are required for many reasons. For one, they help ensure the aircraft engine will continue providing the necessary power and performance. It will also increase the engine’s overall durability. Finally, HSIs reduce maintenance costs by providing operators with greater autonomy over their maintenance activities. A recent HSI is also beneficial when selling a used aircraft. Engines that have been recently inspected/overhauled are more attractive than those that have not.
Regular HSIs can also reduce operating costs by identifying small problems before they cause more serious damage, resulting in a need to replace important parts. For instance, an HSI might help an operator discover a small crack in a non-rotating part. Although this part is still within the acceptable airworthiness standards, inspection allows the operator to note the problem and take immediate action if they so choose. The cost of a hot section inspection will depend on what is revealed. If the engine is in good shape, the sole cost will be the inspection itself. However, should it reveal major problems, there will be significant expenses.
Lastly, an HSI is a good opportunity to examine the overall health of the engine, simply because it is already in the shop. There are many simple maintenance activities operators can do during the HSI that will provide extra benefits. These include looking for fuel and oil leaks, inspecting the oil seals, and checking for corrosion.
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