By Staff Writer | November 1, 2012
Regular and thorough helicopter engine maintenance ensures safety, compliance with legal obligations, peace of mind, and enhanced flying pleasure. According to the Helicopter Safety Advisory Conference, engine-related mechanical malfunctions are one of the leading causes of helicopter accidents. Powerplant maintenance is so important because the engine is the main component keeping the helicopter in the air. “An airplane uses wings for much of its lift, but a helicopter relies on the engine to drive the main rotor blades that provide the required lift,” says Jim Freeman, president of Helicopter Specialties in Janesville, Wis. “A neglected engine can’t provide enough horsepower to safely take off or land a helicopter.” A complete engine failure could be catastrophic. “Without a reliable engine, the helicopter is just about useless,” says Jerry McCawley, test pilot and safety engineer at Lockheed Martin in Owego, N.Y. “Proper maintenance is critical to engine long-term reliability and performance. Daily inspections, pilot pre-flights and heavier inspections such as 100 hour/annual inspections and hot-end inspections all play a part in ensuring an engine is both safe and delivering full performance.”
Not all rotorcraft engines receive equal engine maintenance. Smaller engines complete more cycles over the same period of time compared to larger engines. Any maintenance activity prescribed according to engine cycle will be carried out more often on a smaller turbine engine. McCawley notes piston engines require more maintenance than turbine engines due to their more complex makeup, spark plug fowling and cooling issues.
Engine manufacturers establish maintenance standards for their products via an extensive test and qualification process during the design and development phases of the engine-making process. A smart design team includes operators’ input early on to help ensure easy access for maintenance. Additionally, manufacturers often work in cooperation with the helicopter airframe manufacturer. “The airframe manufacturer may impose additional specific maintenance requirements relative to the engine installation, ancillary systems, controls and indicating systems,” say Phil Kemp, vice president of airframe and avionic sales for Vector Aerospace Helicopter Services North America in Langley, Canada.
Engine maintenance procedures are part of the engine certification process (JAR21). “Based on engine architecture, design margin, system redundancies and engine parameters monitored, safety analysis will identify mandatory maintenance tasks and associated periodicities that have to be performed,” says Keith Marshall, vice president at Turbomeca USA. “These maintenance constraints are taken into account along all the engine development phases and design optimization. Most of those tasks are related to control functions which will be activated in emergency and can’t be tested in normal operations. Qualification tests will identify all limitations related to engine operations.” Respecting preventive and scheduled maintenance is the only way to keep engine airworthiness. “It’s important from both a practical standpoint and a legal/regulatory standpoint to maintain a helicopter engine in accordance with the manufacturer’s standards,” stresses Freeman.
Cory Little, senior technical sales manager at Phoenix, Ariz.-based Honeywell, agrees, saying: “Civil certification authorities prohibit continued operations outside of approved maintenance standards. Violating standards can result in fleet grounding by civil authorities until an engine comes back into compliance with maintenance standards.”
From the regulatory standpoint, the FAA plays a big role in U.S.-based civil helicopter fleets in the standards which regulate engine maintenance initially and through issuance of Airworthiness Directives to address issues that arise as an engine matures in service and problems arise. “The Aviation Missile Research Development and Engineering Command (AMDREC) does this for the U.S. Army and NAVAIR for the U.S. Navy/USMC,” says Sam Evans, Colonel (ret.) and U.S. Army Aviator.
|It’s important from both a practical standpoint and a legal/regulatory standpoint to maintain a helicopter engine in accordance with the manufacturer’s standard.|
Operators can also set maintenance standards to better reflect the conditions they face in their operations. The more an operator lowers the hourly operating cost of the engine, the more profitable the operator becomes. “Through coordination with the OEM they can tailor their programs to be both effective in catching any issue as well as cost effective,” says McCawley. “A operator that uses its aircraft in longer distance executive transport has needs that are vastly different than one involved with logging, long lining or offshore applications.”
Engine maintenance includes regular, scheduled maintenance events with routine inspections and overhauls, as well as unscheduled events including repairs, component replacement and troubleshooting.
“The most common maintenance is the daily/pre-flight inspection performed by the mechanic, the pilot, or best (in my opinion) both,” says McCawley. “A thorough visual inspection will spot many issues at an early stage and allow for a quicker fix, though not in every case, of course.” At this time, the engine, systems and components must be examined for cracks, corrosion, general condition, leaks and signs of material distress.
External components and fuel control systems require close inspection of lines, unions and fittings for leaks and cracks. “At specified periods in the engine overhaul cycles, or following specific abnormal events, the engine may require internal inspection of the compressor, combustion or power sections,” says Kemp. “These inspections may be accomplished through either partial disassembly of the engine to access the parts to be inspected, or with a borescope, which allows a detailed inspection of the engine interior through a fiber-optic viewfinder, without extensive disassembly, which may be recorded by photo or video.”
Performing periodic engine maintenance tasks reduces the risk of severe engine damage. Checking oil, fuel filters, magnetic plugs, oil level and air intake are periodic tasks, which help get information to understand engine health to detect and prevent serious engine degradation. For example, the composition of a metallic particle collected on a magnetic plug or oil filter will identify which component it came from and trigger a specific maintenance procedure.
A helicopter’s engine oil is its lifeblood. “Maintenance requires that the engine oil be changed before it becomes acidic,” says Mike Turner, president of Air Technology Engines Inc. in Naples, Fla. Acidic oil results in a drastic reduction in the Ryder gear value of the oil. Under high loads, the oil gets squeezed out between the gear teeth resulting in excessive wear.”
Spectrographic oil analysis programs provide a very detailed analysis of the engine oil, monitoring elevated levels of specific metals and materials. “By reviewing the volumes and type of material in the sample, it is possible to identify the source within the engine and intercept potential failures, or to direct maintenance activities to avoid expensive repair or replacement actions,” says Kemp. “By interpreting trends across individual engines as well as fleets, it is possible to anticipate failures as well as the impacts of operational and environmental conditions.”
Inspection of fuel and ignition systems includes the filtration system to ensue that clean fuel is being delivered to the burner nozzles, ensuring reliable and safe engine operation. Some engine types require periodic inspection of nozzles and igniters, to ensure the nozzles and igniters are operating to specification and are not damaged, which in turn can cause additional damage and problems to the combustion and power sections of the engine. “Air and bleed air systems are an extremely important and essential part of the engine operating and control system,” says Kemp. “In addition to their role in the start process, in many types of fuel-control systems, they play an essential role in the scheduling and governing of the entire fuel system. Chafed line or loose connections can cause air leaks that result in the sudden and uncommanded loss of all engine power.”
The above helicopter engine maintenance procedures along with others are all considered operator-level maintenance. Brian Costello, field service engineer at Lycoming Engines in Williamsport, Pa. believes most required periodic maintenance and inspections can be carried out by a properly trained and equipped in-house staff. Kemp feels much depends on staff training, technician experience, and the ratings, capabilities and access to special tooling.
More intermediate-level activities must be carried out by service centers capable of disassembling and inspecting an engine at a modular level. “These activities might include disassembling the engine into the primary modules (turbine module, compressor module, gearbox module), inspecting those modules per approved inspection procedures, and then carrying out any module-level maintenance,” says Little. Intermediate level maintenance is performed off-wing at certified and properly equipped facilities.
|Advancements in manufacturing technology through improved material properties and/or advanced manufacturing methods allow engine manufacturers to produce engines with increased maintenance intervals that allow operators to safely leave the engines on wing for longer periods of time.|
Depot level engine maintenance is the most advanced and detailed. It includes complete engine disassembly and inspection and/or maintenance activities at the part level. It’s typically completed during major inspections prescribed in intervals often measured in engine hours, cycles or both. “An unusual helicopter operating event, such as a hard landing, may also trigger a depot-level inspection per the approved maintenance standard,” Little adds.
Helicopter engine air intakes are frequently challenged by many airborne contaminants encountered in flight such as: sand, dust, ice, FOD, snow, heavy rain and salt spray. When ingested, these contaminants can seriously affect helicopter safety and availability.
“The air inlet’s cleanliness is vital to prevent damage to the precision high-speed rotating equipment in turbine engines,” says Pierre-Yves Jan, global leader of air intakes at Pall Corp. in Port Washington, N.Y. “Failure to protect the engine air intake can lead to compressor blade erosion, reduced combustion chamber life, turbine blade glazing, blockage of turbine blade air cooling holes, component wear corrosion, decreased MTBUR, power loss or ultimately, engine flameout.”
One of the most important engine maintenance tools to lower these contaminants’ impact is a simple water and chemical wash of the engine’s interior. A clean engine performs better due to increased aerodynamic efficiency across the turbine sections which results in more horsepower, cooler temps and reduced fuel flows. “An engine flush passes solvents or water through the turbines to clean any deposits on the turbine blades after flight in dusty, dirty or salt environments,” says Evans.
Also, there are various engine air protection systems available that eliminate or capture sand and dust particulates such as self-cleaning particle separators, integrated particle separators, dust protection units, oil-wetted inlet barrier filters and debris screens.
“They eliminate or capture airborne sand and dust particulates, thus protecting the engine from premature erosion,” Jan says. “Selecting the best performing, safest, cost effective and—not forgetting—environmentally friendly solution, requires a good understanding of how each works.”
HUMS and condition based maintenance (CBM) are among the most prominent developments in helicopter engine maintenance. CBM describes a set of practices utilized to transform the aircraft maintenance from a reactive environment to a proactive environment. HUMS is one tool for accomplishing the CBM objective.
Self-diagnostic systems such as HUMS or (Digital) Electronic Engine Control Units, or (D)EECs, monitor the engine and will report any faults through a variety of displays or outputs. “They can play a critical role in spotting problems as well as trending an engine’s condition and performance,” McCawley says. These systems allow vast data collection and number crunching of all engine parameters. These produce a highly detailed real-time review of operating equipment’s current condition and may predicate corrective or preventative maintenance actions, as well as single-aircraft and fleet-trend monitoring.
For instance, a Bell 2061 operator can install a Honeywell VXP on-board HUMS system. “The system is comprised of multiple sensors installed throughout the aircraft and connected to a primary data processing unit,” Little says. “This system allows the operator to monitor critical aircraft engine data during a flight that is subsequently analyzed to identify potential maintenance issues before they occur.”
Marshall believes HUMS is just in its infancy in terms of engine maintenance and agrees that it is turning engine maintenance from a corrective to a proactive model. Also, “Computer aided trouble shooting tools, based on statistical case databases will enhance problem isolation and resolution by optimizing resources or access time,” he says. “Real-time data transmission between in-flight helicopters and on-ground maintenance centers using specialized tools will limit helicopters’ on-ground operation.”
Developments like these and others related to advanced manufacturing technology involving improved material properties and/or advanced manufacturing methods continue to surface. “They allow engine manufacturers to produce engines with increased maintenance intervals that allow operators to safely leave the engines on wing for longer periods of time,” says Little.
Ultimately, an operator’s goal should always be the safe operation of their helicopter. Using substandard engine maintenance practices, and unlicensed or substandard parts degrades overall engine performance, which can potentially affect the purpose for which it was designed.