Helicopter Mechanical Diagnostics

Since their introduction into the aviation world, health and usage monitoring systems (HUMS) have gained traction and expanded from the offshore oil and gas industry to the military, unmanned aerial systems, and commercial and business operations. HUMS are designed to automatically monitor the health of mechanical components in a helicopter, as well as usage of the airframe and its dynamic components. HUMS enable aircraft to record structural and transmission usage, transmission vibrations, rotor track and balance information, and engine power assurance data. HUMS not only monitor the health of rotating components such as gearboxes, bearings, shafts, engines, and rotors through vibration, they can also record parametric data from the aircraft’s bus for usage and event analysis.

Subtle changes in vibration are recorded in flight, visualized on the HUMS ground station computer, and evaluated by technicians. The intelligence gained from the use of HUMS allows aircraft maintainers and fleet operators to make informed decisions about flying and maintaining their aircraft. As a result, HUMS have been shown to enhance safety, decrease maintenance burden, increase availability and readiness, and
reduce operating and support costs.

One of the key elements in identifying mechanical problems is to take note of changes in the vibration patterns. These changes can identify the beginning of the issue and also offer further clues as to why the event occurred.

Vibration is an indicator of condition. It is natural for even the smoothest of machines to vibrate. Therefore, each will have an acceptable range. In the event that the vibration levels increase or vary considerably, a mechanical issue is often the cause. Causes of vibration can be attributed to anything from the original part design to unfavorable operating conditions or even poor maintenance practices. Vibration indications often appear before defects can be detected by traditional means. This can serve as a guard against “substandard” components, possibly prompting investigations into particular production batch numbers.

Vibration indications are a tool that can detect flaws outside of the normal scheduled inspection intervals. This detection ability enhances mechanical integrity by complimenting the numerous prescribed inspections, maintenance actions, and service limits in the aircraft’s maintenance manuals. Vibration indications help guard against a failure of maintenance procedures, continually monitoring for
patterns that would justify procedural alterations.

Typical helicopter vibrations include:
• Rotors (main, tail): Low-frequency vibrations travel throughout the airframe
• Power train (driveshaft, gearbox): Medium-frequency vibrations must be closer to detect
• Bearings: High-frequency vibrations travel very small distances

Sources of vibration can include:
• Mass unbalance
• Misalignment
• Eccentricity
• Distortion
• Looseness
• Wear
• Interference
• Friction
• Gear contact
• Resonance
• Aerodynamic forces
• Operational circumstances

Effects of sustained levels of excessive vibration can include:
• Fatigue cracks
• Component or structural failures
• Accelerated component wear
• Increased unscheduled removals
• Fretting and chaffing
• Corrosion and looseness at riveted joints
• Electrical component failures
• Passenger and crew discomfort and fatigue
• Rough ride, excessive noise in cabin