Hot Section Inspections For Turbofans

A hot section inspection (HSI) examines the condition of several key engine components when the engine reaches the halfway point of its time between overhauls. The hot section is the part of the engine where combustion temperatures and pressures are highest. During an HSI, a trained technician will visually inspect the engine's hot section components for signs of wear or damage. They will also measure critical tolerances to ensure that they haven't degraded beyond what is considered safe.

Hot section inspections play an important role in ensuring the continued airworthiness of your engine. By catching potential issues early, HSIs can help prevent more serious and costly problems down the road. In some cases, an HSI may reveal that an engine component has reached the end of its service life and needs to be replaced.

In the case of a turbofan, the hot section module is removed from the engine and taken to an approved maintenance facility. Per the manufacturer's manuals and recommendations, typically the turbine blades, combustion chambers, stators, vane rings, compressors, and more are all examined and diagnosed.

An HSI is typically carried out when an engine reaches half of its time between overhauls (TBO). The TBO of an engine is specified in its maintenance manual. As an example, a Pratt & Whitney JT15D Series engine has a recommended overhaul interval or time between overhaul (TBO) of 3,500 hours, this would mean that the HSI should be scheduled at 1,750 hours after overhaul or time since new.

According to Valdemar Porto, Customer Manager, Regional Turboprops at P&WC, explains that “we [P&WC] monitor parameters like the inter-turbine temperature [ITT], the speed of the compressor and fuel consumption. When we see a trend of deterioration in performance, that tells us exactly when to do the HSI.” Thanks to upgraded technologies, such as P&WC FAST program, they may be able to determine when a HSI is needed instead of relying on hours or timeframes.

A hot section inspection can take one to five days, depending on the engine type and size.

For larger engines, such as JT15D engine can be done relatively quickly but must be removed from the engine. A technician opens the hot section and inspects the components with basic equipment, along with any specialized instruments recommended in the technical handbook. Once the inspection is complete, the engine is reassembled and reinstalled on the aircraft.

Hot section inspections are usually performed as a preventative measure during an aircraft's regularly scheduled maintenance usually at the half way point of the engine's TBO (time between overhaul).

By performing an HSI, you can ensure optimal engine performance and power. Additionally, it increases the engine's durability and lowers maintenance costs by giving operators more control over their upkeep schedules. 

There are times that an HSI is unscheduled. For example, a hot start, FOD, or when selling an aircraft. A recent HSI is an asset to the sale. An engine that has recently been inspected (“fresh since HSI" or “fresh since overhaul") is more valuable than one which hasn't.  

The short answer is that it's not a good idea to skip a hot-section inspection. Doing so could put your engine at risk of serious damage or failure. In addition, you may find yourself in violation of regulatory requirements, particularly for Part 135 Operations.

Skipping an HSI can also negatively impact the resale value of your aircraft. As we mentioned earlier, a recent HSI is an asset when selling an airplane.

An HSI can help you save money by finding little issues before they develop into larger problems, requiring the replacement of parts. As mentioned above, there are scheduled and unscheduled HSI’s. With scheduled HSI’s, this is routine maintenance, that in the long run helps keep your aircraft running properly and catches things before they become a larger issue down the road.

For example, an HSI can reveal a minor crack. Thanks to the HSI, the technician can repair it with welding or grinding right away. If the crack is overlooked at an early stage, it can cause the entire component to need to be replaced later, therefore costing more money and even a delay in repair if you can't get the part.

The actual cost of the HSI depends on what information is uncovered. If the engine is in good shape, all you have to do is pay for the examination itself. However, if the HSI reveals that important components must be replaced, this can result in significant expenses.

An HSI is a perfect opportunity to inspect the engine's health since it is already in the shop. There are some easy maintenance activities you can do during an HSI that will bring additional benefits. External fuel and oil leaks can be checked for, as well as gearbox oil seals. Cases can also be inspected for corrosion, which is particularly vital for aircraft that operate near salt water.

Our TBO Extension offers two packages, one with an HSI and one without. Within 300 hours of an overhaul, by including the HSI, TBO Extension increases your overhaul interval by 2000 hours +300 for a P&WC JT15D Series engine. The benefits of doing an HSI with TBO Extension STC means an increased overhaul interval (compared to not doing an HSI in conjunction with TBO Extension), you don't have to do a hot section down the road (it's already done), and if the plan is to sell the aircraft in the near future this package adds incremental value to the aircraft.