What should you know about helicopter engine maintenance? How do helicopter turboshafts work, how often is maintenance required, and what does it cost? Chris Kjelgaard provides answers on the key questions…
How do helicopter turboshaft engines work?
One of the most important characteristics of any helicopter turboshaft engine is that the air-and-fuel vapor mixture is not designed to produce thrust. Instead, via the power spool turned by the turbine stages and the reduction gearbox linking the power spool to the drive shaft, the exhaust gas makes the drive shaft rotate.
Helicopter turboshaft gas turbine engines use fast-rotating compressor and turbine stages to drive the power spool. This spool, which turns at an extremely high rotation per minute (rpm) rate, transfers its rotational energy to a drive shaft through a reduction gearbox containing an arrangement of planetary gears in order to turn the aircraft’s propeller blades.
The drive shaft usually turns at a greatly lower rpm than the power spool. Either directly or indirectly (through additional gearboxes which form part of the helicopter airframe, not its engine), the drive shaft turns the helicopter’s large rotors and the anti-torque device which counteracts the torque the rotors produce.
In most helicopters that device is a rear-mounted tail rotor embedded in the helicopter’s vertical tail surface. The rotation of the drive shaft ultimately rotates the helicopter’s rotors and its anti-rotor-torque device.
Why do helicopter engines need maintenance?
If the working of a helicopter turboshaft sounds like a complex arrangement, it is, and — as we’ve discussed – it requires the engine to contain fast-moving rotating stages and parts. All this rotation produces vibration.
The level of that vibration can be very pronounced in some older helicopters but is much less so in modern designs.
Regular scheduled maintenance which follows the maintenance plan the engine manufacturer has developed, or a different plan developed by the operator or maintenance facility (but in every case requires approval from the operator’s national airworthiness regulator) is vital.
This will ensure the helicopter remains safe to operate and that the helicopter/engine combination continues to operate at peak performance levels.
When is helicopter engine maintenance required?
For scheduled maintenance, most turboshaft engines offer times between overhaul (TBOs) ranging from 3,500hrs to 5,000hrs. While the TBO is a recommended interval for FAR Part 91 private flying, for Part 135 passenger-carrying and cargo charter operations it is mandatory.
Various turboshaft engines, particularly older ones such as Pratt & Whitney Canada’s PT6T family (powering as the Bell 212 and Bell 412), also require mid-TBO hot section inspections (HSIs).
A turboshaft’s TBO interval could also depend on the mission for which it’s being used and the environment in which it is being operated. As an example, the maintenance plans developed for the turboshaft engines of helicopters engaged primarily in heavy-lifting operations, fighting wildfires or flying over saltwater may require much lower HSI and TBO intervals, and in some cases the TBO could be as low as 1,000hrs.
For modern turboshaft designs, the degree to which the engine is electronically controlled (via dual-channel FADEC), and the amount of digital condition data the engine’s internal sensors can provide the helicopter’s computerized health monitoring system will also influence the turboshaft helicopter engine’s maintenance intervals.
And as helicopter engine manufacturers increasingly offer their customers engine trend monitoring services based on engine sensor-derived data, various components and even the entire engine can be placed on an on-condition maintenance schedule which calls for preventative maintenance.
Because of the likelihood that the fast-rotating parts inside a turboshaft engine will wear, those parts, known as life limited parts (LLPs) are subject to strict life limits determined either in hours or flight cycles. (While replacing a set of turbine blades is expensive, the good news is that LLP life intervals are usually at least twice as long as the full-engine TBO determined by the maintenance plan.)
What does helicopter engine maintenance cost?
Helicopter turboshaft engines regularly used in Business Aviation are generally similar in size and power output to turboprop engines utilized in BizAv. Turboshaft maintenance costs are also generally similar turboprop engine costs. Helicopter operators can expect to pay $100k to $300k per engine, per shop visit.
However, maintenance for helicopter turboshaft engines may be required more often than for turboprops; especially when the turboshaft is primarily used for heavy-lifting operations or where it accumulates high numbers of flight cycles.
Turboshaft operators should give careful consideration to one of the hourly maintenance cost plans the engine’s manufacturer or third party provider, such as JSSI, offer. Here, the operator makes multiple payments of equal amounts over time, rather than pay a lump sum to settle the invoice for an overhaul or inspection.
These plans usually cover far more than the basic cost of the inspection involved, and might include parts, labor, transportation of the engine to the facility performing the work, and the loan or short-term lease of a replacement engine (either for free or for a modest amount).
Manufacturers offer other, optional services which set caps to any additional costs for scheduled and unscheduled maintenance.
Helicopter turboshaft engine maintenance costs can be mitigated by the operator by paying careful attention to using high-quality lubricating oil, performing oil inspections and changes, per the manufacturer’s recommended schedule, and performing engine washes frequently – particularly when operated in a saline or polluted environment.
Who makes helicopter engines today?
Although turboshaft designs have more in common with turboprops than jet engines, the variety of helicopter turboshaft families in existence for Business Aviation use is about as wide as the variety of jet engine families available for business jets.
Like turboprop engines, helicopter turboshaft engines are available for a wide range of shaft-horsepower (shp) requirements, but the overall power range helicopter turboshafts offer isn’t as great as the wide thrust range offered by jet engines.
GE Aviation, Pratt & Whitney Canada (with the PT6B, PT6C and PT6T sub-families of the huge PT6 engine family), Rolls-Royce and Honeywell all have substantial turboshaft helicopter engine manufacture and support businesses.
However, the dominant force in turboshaft manufacturing currently is Safran Helicopter Engines, which under its original name Turbomeca pioneered the use of gas turbine helicopter engines.
Where can I find helicopter engine maintenance?
Because all the major engine manufacturers are heavily invested in manufacturing and supporting turboshafts, each has a global network of maintenance and parts support centers, which combine in-house and licensed third-party providers.
There are many MRO facilities around the world who offer maintenance and repairs for turboshaft engines – particularly the smaller engines such as the Rolls-Royce’s M250 and RR300.
To help ensure operators select the right one for their helicopter engine maintenance needs, industry insiders suggest a five-step process is adhered to…
- Learn if a maintenance provider belongs to the engine manufacturer’s licensed network. If they do, they will perform each repair and overhaul task according to the process the manufacturer approves and use spare parts made by the manufacturer.
- Ask the engine manufacturer to advise on which maintenance provider might be best for your requirements—in terms of location, the turnaround time the provider will need to perform the overhaul or repair, and the pricing structure and flexibility offered.
- Establish which technical capabilities a prospective maintenance provider offers (i.e. can it perform maintenance for a given component in-house rather than having to outsource repair and overhaul of the component to a third-party provider? Outsourcing maintenance components can add weeks—and considerable cost—to your engine’s overhaul).
- Insist on seeing the contract for the planned work up-front and study it carefully with a technical consultant. Avoid contracts which exclude some of the services, parts or labor required for completion of the work, or fail to specify that such items are included in the contract. Open-ended contracts can result in operators facing much higher invoiced amounts for overhauls than quoted.
- Follow the manufacturer’s prescribed maintenance plan for the turboshaft engine. This plan is based on what it learned during type certification of the engine and on the engineering data on which that helicopter turboshaft engine design is based. Failure to follow the plan closely could result in additional costs for repairs.