Commercial, Military, Products

An Eye on Operations

By James T. McKenna | September 1, 2005

As health and usage monitoring systems prove their worth in the North Sea and elsewhere, more civil and military fliers want the safety and operational benefits they offer.

As operational experience proves the scope and value of benefits from health and usage monitoring systems in helicopters, more and more users are pushing for the application of such systems to their fleets.

Known as HUMS, such systems have demonstrated their worth in North Sea offshore support helicopter operations from the United Kingdom and Norway, where they have been in use since the early 1990s. Now a number of groups, including major offshore energy exploration companies led by Royal Dutch/Shell and the United Kingdom's Civil Aviation Authority, are pushing for worldwide adoption of HUMS to improve helicopter safety. Others, like the military services of the United States, are pursuing HUMS applications to help them manage and lower the cost of operating, maintaining and supporting their rotorcraft.


HUMS "is an area that's growing in the level of interest around the world, both in military and civil operations," said Piet Ephraim of Smiths Aerospace's Electronics Systems unit in Grand Rapids, Iowa. Smiths is a HUMS pioneer, having worked with the U.K. CAA and operators on the development and testing of the first systems fielded in the North Sea.

A HUMS installation uses a central computer, called a data processing (or acquisition) unit to gather in-flight data from accelerometers and tachometers installed on critical dynamic and structural components on the aircraft. The system may also draw data from the flight data recorder if the aircraft is equipped with one. Key components that are monitored include the main and tail rotors and their gear boxes and drive trains. The installation can weigh 10-100 lb. depending on the aircraft type on which it is fitted.

Typically, the data is captured by a quick-access recording device that facilitates downloading by a mechanic once the aircraft has returned from a flight. A critical set of data is one that matches vibration levels and other information to particularly flight regimes for the aircraft.

A primary function of such a system is to gather enough data to enable engineers to identify normal and abnormal deviations from vibration levels on an aircraft's critical components, then derive deviations that are likely precursors of incipient failures in those units. In developing the S-92 HUMS, for instance, Sikorsky Aircraft engineers worked from an extensive database gathered during flight and certification testing of that aircraft to identify the boundaries of "normal" performance of its critical components.

HUMS data generally is not available to pilots in flight; it must be downloaded by telemetry or a ground connection and analyzed at a central facility run either by an operator, if its fleet is large enough to justify the cost of that center, or by a vendor such as Smiths, Goodrich or Thales.

HUMS was born of safety problems in North Sea helicopter operations. While an advisory panel for the U.K. CAA's Airworthiness Requirements Board identified the need to develop systems to monitor the health of critical helicopter components in the early 1980s, their development did not begin in earnest until after the 1986 crash of a Boeing Vertol 234 Chinook off the Shetland Islands. Investigators found that the crash, which killed 45, was caused by disintegration of the forward main gear box and the subsequent de-synchronization of the main rotors that forced a ditching in frigid waters. The crash prompted North Sea trade unions to agitate for tougher safety standards for flights there. As a result, the CAA launched a project to develop an effective HUMS. By the early 1990s, under labor and public pressure, North Sea operators had fitted their aircraft with such systems before the CAA had mandated their use.

The positive results from those installations led the CAA in 1999 to require HUMS on certain transport-category aircraft capable of carrying more than nine passengers. In addition, Europe's Joint Aviation Authorities in 1993 required that "technically feasible" and "economically justifiable" health monitoring systems be considered part of the design assessment of new aircraft. Neither the JAA nor the U.S. FAA yet requires HUMS on helicopters.

CAA studies point to numerous cases in which routine analysis of HUMS data provided the first indications of faults that were subsequently confirmed as requiring maintenance action. "These faults, if undetected, would have led to failures of, amongst others, an engine, the tail rotor, tail rotor shaft, gearbox and engine to gearbox coupling," states the 2005 Safety Plan of the CAA's Safety Regulation Group. "Such failures may be hazardous or even catastrophic."

According to the CAA, such HUMS "successes" are found at a frequency as high as 22 per 100,000 flight hours.

One early CAA study determined that HUMS had detected data identifying about 70 percent of helicopter problems requiring significant maintenance action. Of the cases that HUMS identified, the agency found, six were classified as potentially catastrophic or hazardous problems. The CAA analysts judged that one or two of those cases likely would have led to an accident if HUMS had not detected them.

An independent study in Norway concluded that development and use of HUMS is "probably the most significant isolated safety improvement of the last decade." It said HUMS is "expected to mature over the next decade and will probably contribute to a further risk improvement."

HUMS suffered a number of problems early on. One was the reliability of sensors on which the system (and operators) relied for data. In many cases, sensor failures led to groundings and maintenance checks for problems that did not exist. With subsequent generations of HUMS, the reliability and robustness of sensors has improved.

Another problem was a lack of understanding and acceptance of the indications that HUMS was providing. In a recent article for the journal of the Australian Petroleum Production & Exploration Assn., Jed Hart, managing director of the Melbourne, Australia risk management consultant Hart Aviation, cited the 1995 case of a Super Puma whose tail-rotor blade-flapping hinge retainer fractured, nearly resulting in loss of the blade. The U.K Air Accident Investigation Branch found the aircraft's HUMS identified unusual vibrations starting 50 hr. before the failure and triggered an alert 5 hr. before that vibrations were exceeding limits. But mechanics failed to detect the crack, which propagated undetected until the near-catastrophic failure.

"To make effective use of HUMS, the standard of the supporting training and procedures are just as important as the aircraft hardware and ground stations," Hart wrote. A veteran of the Royal Australian Navy, Hart also was an aviation adviser for Shell International, an aviation manager for BHP and corporate manager of safety for BHP Billiton. He is a former chairman of the Aviation Safety Foundation of Australia.

Given the success of HUMS initiatives, the CAA's Design and Production Standards Div. is undertaking this year to promote the use of HUMS globally. (The division also plans to investigate the potential use of HUMS in other propulsion applications, such as fixed-wing turbofan and turboprop engines.)

For HUMS analysis to be effective, it is critical that data be collected and compared from as many operational aircraft of a particular type and model as is practical. If a particular type has a total fleet of 60 aircraft and Shell Aircraft--the flight department of Royal Dutch/Shell has data on only the three or four that it operates, "guess how effective HUMS is," said Bob Sheffield, that department's managing director. "Buying and installing the systems and downloading and analyzing the data is very expensive for use and very much less effective. It's essential that the systems be installed on and collect data from as many aircraft as possible."

Required use of HUMS in aircraft flying its employees to offshore platforms is one of the seven key elements of Shell's 7/7=1 program to reduce the offshore sector's fatal accident rate from seven per one million flight hours to one--the same as good commercial airlines in the world.

"HUMS ain't perfect, but it's damn well worth the investment, in my mind," Sheffield said. "We clearly do have to have HUMS in helicopters."

Hart estimated that of the roughly 1,100 helicopters flying offshore support around the world, less than half are equipped with HUMS. Newer generation helicopters have HUMS integrated in their systems, which is one reason why Shell is pushing for replacement of older aircraft with the latest models. That's No. 1 on its 7/7=1 program of initiatives.

Shell has been briefing energy and aviation industry officials on that program. It presented its analysis and findings to manufacturers and some operators at the HAI Heli-Expo show in Anaheim, Calif. last February and briefed top FAA officials in December. The company has won the backing for the initiatves from the aviation subcommittee of the International Assn. of Oil and Gas Producers, Sheffield said, as well as the overarching safety committee of that group. The group represents the world's major energy exploration and production companies--key customers of offshore helicopter companies--so their unity behind higher safety standards that include HUMS will likely spur greater use of the systems.

Manufacturers also are acting to fit their older aircraft with HUMS-like devices. Bell Helicopter, for instance, has worked with Aeronautical Accessories, Inc. and Intelligent Automation Corp. to develop what it calls an advanced helicopter vibration monitoring system for the Bell 412.

The U.S. military, in the meantime, is looking to HUMS to help it control maintenance and operations costs for both old and new helicopters. The U.S. Army has contracted with Goodrich to incorporate HUMS on the 1,200 UH-60Ms that Sikorsky is developing as the next-generation Black Hawk as well as special-forces MH-60Ms. It has tapped Smiths to demonstrate the feasibility of retrofitting a HUMS on UH-60Ls. The U.S. Marine Corps has teamed with Goodrich to install that company's Integrated Mechanical Diagnostic System on the service's CH-53Es, one of the most expensive rotorcraft to operate in the U.S. military fleet.

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