An introduction to Hot-Section Inspections, Overhauls & Downtime
How can you manage the cost and downtime of a business aircraft engine overhaul or inspection, asks Dave Higdon? Discover the key tips to minimize inconvenience and surprises…
Some basic maintenance occurs on virtually every flight – when the flight crew visually checks the engine condition, lubricant levels and various points designed to highlight an anomaly – such as a dysfunctional low-oil level indicator or any other component indicating something out of specification.
Other powerplant maintenance items arrive largely on an hours- or cycles-based schedule, and include the two major events in the life of any turbine engine: The hot-section inspection (HSI) and engine overhaul.
While some overhauls are proscribed as ‘on-condition’ events, the HSI generally arrives at a set time period.
For engines with a proscribed TBO (Time Between Overhaul), HSI events tend to arrive at the mid-point between zero hours and the TBO limit. For engines with an on-condition TBO cycle, however, HSI times are typically set on a firm schedule, whether 1,500 hours, 1,800 hours, 2,000 hours, or more.
The downtimes vary and costs vary accordingly, but anticipate a low six-figure fee to be a starting point for engines that cost in the high six-figures new. Anticipate seven-figure fees for powerplants with new-production prices well into eight-figure sums. One engine-shop operator uses a rule of thumb that overhauls will cost one-third to one-half of the engine's cost when new.
Regardless of the renewal costs, the necessity remains to help engines reach their limits, remain cost-effective and, most importantly, reliable and safe. Let's first look at the two different cyclical events.
Hot-Section Inspections (HSI)
Here's a relative rarity among aviation acronyms – one that means exactly what it says: An inspection of the components subjected to the pressures and heat of fuel combustion – the hot section. HSIs help verify that all hot-section elements of an engine remain capable of producing rated power, and they assure efficient and safe performance until the time comes due for the next HSI or overhaul.
Turbine engines, whether jet or propjet, share several components: air compressors; combustion chambers and liners; and the turbine wheels turned by the blast of hot air exiting nozzles on the combustion chamber. Inspections generally cover the blades of power and compressor turbines, stationary vane rings, turbine-shroud segments, temperature sensors and connections, and compressor inlet.
Components found to be short of standard may need to be replaced or repaired – a step that may entail further disassembly of the engine.
Conversely, other items in the hot section may only be flagged for future attention, but will be left in the engine as long as they won't impact the powerplant's integrity.
Take, for example, tiny cracks in stationary parts that may be kept in operation; as can rotating components showing minimal erosion and/or corrosion, or wear of ceramic coatings.
An HSI is generally the lowest-cost major maintenance event in an engine's life and the shortest, with some engines down for only a couple of days for an uneventful HSI.
The flip side comes when an inspection shows more-significant deterioration of those hot-section components (in which case partial or full overhaul may be required). And we can't forget about life-limited parts (LLPs), as set by the engine OEM. Whether cycles or hours define the life-limits, LLPs typically determine the outside limit for how long an engine can stay on the aircraft. The good news is that an overhaul can renew that powerplant's useful life and send it back to work.
The Full Overhaul
Whether an engine achieves TBO, exceeds TBO, or comes up short as discovered by a hot-section inspection; an overhaul significantly raises the bar on complexity, cost and downtime. Some engines can be overhauled ‘on-wing’ – as most HSIs occur. But many overhauls of jets and propjets entail removal of the powerplant from the aircraft.
The first step after removal is the disassembly and item-by-item inspection of components to determine their level of wear, and whether they remain within specified limits and can thus continue working or whether the wear exceeds limits and requires replacement.
Modern turbine engines consist of several modules, each of which serves a specific purpose – whether the inlet section with the large fan, the compressor section that follows, the fuel-control components, the hot-section components and others. As each module comes off the engine its components undergo detailed cleaning prior to inspection to compare their condition to the manufacturer's specifications and limits.
In general, rotating parts undergo inspections to monitor wear, as well as detailed non-destructive testing in search of tiny cracks that can propagate and ultimately lead to a failure of that component.
Among the non-destructive testing available for components are dye-penetration tests, eddy current electronic inspections, or x-ray inspections that reveal details about the insides of metal parts.
Inspecting all the components of a high-bypass jet turbine engine can take several days before work begins on replacing and reassembling the engine for a test run and installation back on the aircraft. The time can vary widely depending on the size of the engine and the number of components that require replacement to bring the engine back to factory specification.
But at the end of the process the engine should be in like-new condition and ready to make another run for TBO. Of course, whether down for an HSI or a complete overhaul, the work will interfere with the aircraft's availability for its primary mission: moving you and your people to meet the needs of your business.
Continuing Business During HSI/Overhaul
Depending on the engine, the job and other elements of aircraft maintenance, an engine overhaul or HSI may be scheduled in proximity with other major maintenance, as determined by the airframe OEM, and the type of operation.
Remember, the maintenance requirements vary significantly between private operations flying Part 91 missions, and on-demand, charter or leaseback flying Part 135. To keep the business flying, operators often arrange for temporary lift through a leased aircraft, or, depending on demand, charter solutions.
For some airframes temporary engine replacements are an option that keeps the owned aircraft working on the thrust of loaned engines. This option, where available, still incurs some downtime.
While newer model engines may allow for easier removal and replacement processes requiring fewer hours, such a swap with its post-work checks and return-to-service flights can still take a week or more. But the operator retains access to the familiar aircraft and its cabin accouterments.
Scheduling an overhaul to coincide with another major maintenance job helps minimize downtime – not to mention provide an opportunity to accomplish more (such as an ADS-B Out upgrade).
Some operators choose to schedule an overhaul for a time when the airframe itself faces a lengthy inspection process, say a C-Check or D-Check period. HSIs can often be accomplished simultaneously with an annual or 100-hour inspection.
Making the timing work may enable performance of some work to become a little earlier than might otherwise happen, or conversely a little later (where regulations and conditions allow). The rule of thumb, however, is that the greater the job – or the greater the need – the longer the process takes.
Consequently, an overhaul for which everything checks out in spec and the removal, tear down, reconditioning and replacement processes for a pair of powerplants proceeds predictably and smoothly, can see the aircraft out of action for several weeks. But the end product is an aircraft with a renewed residual value, as well as a restored level of owner confidence in the safety and efficiency of the engines.
Preparing for the Day...
Savvy operators and flight departments begin preparing for the dual eventualities of HSIs and full overhauls even before taking delivery of the aircraft. Some handle the advance preparations in-house by regularly making deposits into an engine or powerplant escrow account.
Regardless of the deposit cycles (weekly, monthly, quarterly), the amounts should reflect an overhaul-cost estimate divided by the hours between overhaul. If overhaul costs are $200,000 and the TBO is 4,000 hours, the deposits should at a minimum reflect $50 for every hour the engines ran since the previous deposit.
Some operators suggest upping that formula by 25 percent to cover any unplanned maintenance and/or to bank extra funds for the hot-section inspection costs. As one operator told AvBuyer, there’s no sense in robbing the overhaul account for the HSI when you can also prepare a cushion for both needs.
The other option used by many savvy operators and owners takes the form of a pre-paid hourly-based program like the ones offered by Rolls-Royce CorporateCare and independent providers such as Jet Support Services Inc. (JSSI).
The advantage of a pre-paid approach is the delivery through renewal management of an operator's engine maintenance and powerplant overhaul needs, charged at a convenient per-hour basis. Some tip-to-tail programs may even include alternative lift for the downtime when an overhaul or hot-section inspection comes due.
A Healthy Aircraft Heart Means Flying...
Just like those of their passengers, the heart of every aircraft stays healthiest when regularly, vigorously exercised. Every aircraft and powerplant technician broached, every planemaker and powerplant producer, offers the same message.
Powerplants – the systems on which we depend to power our flights and drive the support systems integral to aircraft use – respond best when flown well and flown often. And like the human heart, confirming the results of that exercise regimen with regular scheduled hot-section exams can help assure that the need to overhaul doesn't arrive prematurely.
Hourly Maintenance Programs
Just as foot soldiers are advised to take great care of their feet, aircraft operators should pay particular attention to the wellbeing of their aircraft’s engines. Careful attention to powerplant maintenance protects an aircraft’s value and contributes measurably to safety.
An hourly cost maintenance program offers the added advantage by removing much of the unpredictability of the costs associated with aircraft engine upkeep.
JSSI, the world’s largest independent provider of hourly cost maintenance programs for aircraft engines and airframes, for example, offers programs tailored to a highly extensive range of airframes and powerplants. An aircraft owner pays an amount of money per hour flown, which provides them with coverage for maintenance needs along with the technical expertise from a company with over 25 years’ experience in the market. Discover more at www.jetsupport.com.
Some operators prefer a program provided by the powerplant’s OEM, and Rolls-Royce (among others), provides just such a solution through its CorporateCare engine management program. The concept is the same – pay a per-flight-hour amount, and receive complete engine management, from line-maintenance parts to full engine overhauls. Find out more at www.rolls-royce.com/civil/services/corporatecare/
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