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Why changes in aviation safety are needed - and other benefits that will occur.

Written by Dr L H Brealey (RMCI) presented at EASA Rotorcraft and VTOL Symposium 2022.


The landscape of future aircraft is rapidly evolving. We can no longer view aviation

as either fixed wing or helicopters, as we now need to incorporate the many different

types and variation of aircraft: Fixed wing, rotor craft,integrated unmanned vehicles

and drones. Roles of aircraft are changing and expanding. new technology is

causing disruption on the established aviation sector,with anticipated new roles for

aircraft, such as short distance commuting using electrically powered aircraft,

regional cargo carrying drones and even such things as counter-drone drones!

Alongside this, there has been a renewed drive with space exploration and

supersonic travel, resurrecting the routes of the 1980’s. These changes are pushing

the limits of air transportation safety, potentially leading to the aviation authorities

struggling to keep pace with the transformations occurring daily.

The Challenge

The challenge for the aviation authorities is how to legislate and govern the skies

safely with all these new forms or aircraft, without restricting the ability of traditional

aircraft to perform their roles. In the past, legislation has usually come about because

of thousands of hours of historic data on which to base decisions. The problem for the

aviation authorities today, is that the skies are constantly evolving at a seemingly

increased rate, and whilst change is needed now, there is very little data for the

authorities to base their decisions on. Widespread adoption of Health and Usage

Monitoring systems across all platforms would help in this regard.

A major problem is that we now have a mixed array of old and new aircraft. Many of the older aircraft performing inland roles do not have the safety enhancements of the military or the offshore oil and gas industries, making them higher risk and with lower availability in these busy and important roles.

Are there lessons that we can learn from other forms of transport? – Cars and other

motorized vehicles have for some time been equipped with sophisticated vehicle

monitoring. Many modern vehicles can communicate vehicle status and potential

problems both to the driver and back to the manufacturer/dealer. Communication

is now key to drive down risk and can start a journey knowing the status of your

vehicle. It seems incredible that more information is often available to a car driver

than a pilot or operator! Part of the solution to de-risk using technology has been

addressed by affordable advanced integrated Health and Usage Monitoring

(HUMS). In this paper we will learn not only of the enhanced safety levels

achievable using a HUMS system, but also the quantifiable cost benefits, and

some of the technology advancement which has supported this.

Part of the solution enabled by technology

This has now been addressed by affordable advanced integrated HUMS and FDM (Flight data monitoring) These technologies can solve many of the safety issues faced by the aviation sector today, whilst at the same time providing significant real time data that can be used by both operators and authorities alike.

Barriers to Entry

The main barriers to entry for an operator to incorporate HUMS have always been

lack of awareness, resistance to change and the perceived cost of the technology.

Whilst awareness of such systems is increasing, there still seems to be some reluctance amongst operators to install them. Resistance to change is an obstacle

that needs to be overcome. When the UK first introduced legislation that it was

mandatory to wear seatbelts in cars, unbelievably there was a lot of opposition to

the new law. Now it seems second nature to wear a seatbelt. Both education and

data proving that vehicle casualties were significantly reduced means that seat belts are now regarded as essential, not something to be annoyed about! We

will later detail in this paper both the safety benefits that a good HUMS system

can achieve, and that the cost will almost certainly be outweighed by the savings

the technology can deliver. When presented with these facts, this should help to

overcome the resistance to change and demonstrate that a HUMS system can reduce, not increase costs.

Safety Benefits

The key safety improvements are the accurate identification of potential faults prior to catastrophic failure, informed decision making, risk mitigation and avoidance, minimizing the risk of failure in flight, and decreased need for emergency landings. This enhanced level of safety when installed on U.S. army helicopters has already avoided 4 Class A Mishaps and could avoid 11% (or 40) material-related mishaps of all classes per year. (Source: U.S. Army)

Maintenance Benefits

A good HUMS system results in more efficient maintenance, troubleshooting and diagnosis of potential faults, deferment, or elimination of certain maintenance inspection intervals and diagnosis of problems before they cause collateral damage.This was quantified by the US Army by a reduction of 343,278 maintenance man hours/year with 4,958 maintenance events eliminated. Optimized maintenance practices resulted in over 50 AWRs and 127 improved maintenance procedures.

Readiness Benefits

Demonstrable reduction in downtime for unscheduled maintenance events, proactive maintenance, allowing aircraft downtime to be a scheduled and anticipated event rather than an unexpected inconvenience, resulting in increased platform availability and readiness (up to 11.8% increase) and up to 11.3% non-mission-capable for maintenance reduction. Experienced 1 less mission abort per 100 flights hours.

Operations and Support Cost Benefits

These include increased useful life and efficiency; identification of certain problems that warrant grounding the aircraft immediately, thereby preventing further damage, extension of the life of an aircraft’s avionics and airframe by reducing overall vibration on the aircraft. Proven statistics were TBO extensions on 22 CBM+ monitored parts, a 5.8% reduction in maintenance test flights (MTFs) and a CBM+ CBA projection that Mean Time between Failure cost avoidance was in the region of $25.7M/Year.

A further recent example has been use of data by a customer when a part began to fail within the first 100 hours of operation. This resulted in a successful warranty claim which in turn gave the customer a 200% return on investment.

A Closer Look at the Safety Benefits

Advancement in Technology - Understanding the Data

Advancement in Epicyclic Gear Trains monitoring

RMCI developed methods for the Army to effectively monitor epicyclic gear trains in HUMS- monitored rotorcraft. RMCI supplied formulas to correctly perform the calculations.

For current HUMS gear train diagnostics, STAs (Synchronous Time Analysis) are calculated to diagnose gear faults. Gear faults threatening drive train health and crew member safety, such as worn or cracked teeth, can be tracked using STA signals. If a gear fault exists that experiences contact every rotation, the fault will typically manifest in the gear’s STA through anomalous behaviour including a peak or increase in magnitude. To generate the STAs, tachometer (tach) data are used to divide accelerometer time domain data into sections correlating to gear shaft rotations. Tach data is typically multiplied by gear ratios to obtain the number of tach pulses per rotation, since the tach usually does not reside on the shaft for which the STA is being calculated. The tach divided sections are averaged together to obtain average vibration signals per shaft or gear rotation.

Fixed-axis gear train STA calculation is relatively simple since, despite gear rotation, points of gear mesh contact remain fixed in space relative to the tach’s external frame of reference—the transmission housing. Fixed-axis gear rotation ratios are therefore calculated for the gear for its rotation relative to the transmission housing. However, points of contact in epicyclic gear trains are not fixed relative to the transmission housing or tach orientation, but rotate around the ring, sun, and planet gears at the rate of the planetary carrier’s angular velocity if the ring gear is fixed relative to the transmission housing. Therefore, STA gear ratios must be calculated for the sun and planets as they rotate with respect to the planetary carrier instead of the transmission housing. Otherwise, tach-divided data sections don’t accurately correlate to gear rotations and the desired gear mesh amplitudes average out.

Without correct planetary gear ratios, the Army fleet did not possess epicyclic monitoring capability through the current HUMS. AH-64A, AH-64D, and OH-58D script files did not include sun or planet STAs. Incorrect STAs were incorporated into the MH-47 and CH-47 HUMS script files. The sun and planet gear ratios were incorrect, causing the STAs to track gear rotation relative to the transmission housing, not the planetary carrier.

RMCI supplied the Army with formulas to calculate epicyclic gear monitoring ratios on all HUMS- equipped aircraft. RMCI verified the formulas using AH-64D and UH-60A time-domain data, demonstrating that gear mesh harmonics were dominant within their respective STAs using the new method. Platform specific ratios were provided in a format that could readily be implemented for the new AH-64D Block III main transmission as well as the MH-47G and CH-47D forward and aft transmissions. The supplied formulas also allow for future epicyclic monitoring on platforms monitored by various HUMS vendors.

CBM Metric Development to support improve user perceptions

RMCI was tasked with designing and developing metrics for analyzing the benefits of the Army’s Condition-Based Maintenance (CBM) Program. The CBM Program’s primary objectives include decreasing the maintenance burden on the soldier, increasing platform availability and readiness, enhancing safety, and reducing operations and support costs. To quantitatively measure the progress of these goals, RMCI assisted in the design of both the methodology and processes for calculating operating metrics. These metrics include Readiness, Maintenance Test Flight (MTF) Hours, Mission Abort Rate, Maintenance Man Hours (MMH), Parts Cost per Flying Hour, and Combat Power. Each of the six operating metrics required portions of data from at least two or more databases for performing calculations. Our knowledge of the Army’s logistic databases, including ULLS-A(E), FEDLOG, IMMC, LIW (LOGSA), and OSMIS enabled us to efficiently combine components from multiple databases for the calculation of each metric. These metrics are currently being utilized by Army Material Command G3 Command Analysis Directorate. As a result of these six metrics, the Army can see the benefits of CBM in a quantitative form. RMCI has also helped incorporate analytical results generated from two of the metrics into the Business Case Analysis supporting the effectiveness of CBM. Such continuing innovation and dedication set RMCI apart from the rest.


So far RMCI, Inc. supported the Aerospace Industries Association (AIA) in their efforts to modernize all existing National Aerospace Standards. The NAS series is best known for its state-of-the-art, high strength, precision fasteners. In addition to all types of screws, nuts, and rivets, NAS define high pressure hose, electrical connectors, splices and terminations, rod end bearings, and many other types of hardware and components. RMCI modernized over 670 NAS standards during the project.


Traditionally Health and Usage Monitoring has been mainly seen on helicopters. However, with the extremely small and light weight system from RMCI, there is great potential that the same technology can be incorporated on drone systems in the future, with rotor track and balance problems being a common problem for many drone manufacturers which the RMCI system can solve.


Health and Usage Monitoring must become an essential component of aviation going forwards, no longer being consigned to a luxury product, but a vital product to enhance aviation safety, reduce costs and provide data to help legislate the future of aviation.


International Helicopter Safety Team, Health and Usage Monitoring Systems Toolkit,

Josh Kennedy, Army Aviation

CBM+ and Cost-Wise Readiness, American Helicopter Society Airworthiness, CBM and HUMS Specialists' Meeting, Huntsville, AL, 2013.

• “Investigation of Spiral Bevel Gear Condition Indicator Validation via AC-29-2C Using Fielded Rotorcraft HUMS Data,” NASA/TM-

2014-218406, November 2014.

• Data Fusion Tool for Spiral Bevel Gear Condition Indicator Data,” presented at the Society of Machinery Failure Prevention Technology, May 2014.

• Application of Advanced Vibration Techniques for Enhancing Bearing Diagnostics on a HUMS-Equipped Fleet,” presented at the

American Helicopter Society CBM Specialists Conference, February 2013.

• “Continued Evaluation of Gear Condition Indicator Performance on Rotorcraft Fleets,” presented at the American Helicopter Society

CBM Specialists Conference, February 2013.

• “Health and Usage Monitoring Systems Toolkit,” U.S. Joint Helicopter Safety

Implementation Team, International Helicopter Safety Team.

• “AH64D/E Condition Based Maintenance (CBM) Component Inspection and Maintenance Manual Using the Modernized Signal

Processor Unit (MSPU),” U.S. Army Aviation Engineering Directorate, Apache Systems Division.

• “Evaluation of Gear Condition Indicator Performance on Rotorcraft Fleets,” presented at American Helicopter Society Forum 66, May 2010.

• Acceptance Test Procedure for prototype Aircraft Diagnostic and Vibration Management System (ADVMS) for the United States Coast Guard MH-60T helicopter.

• “Health Management for Marine Power,” Department of Energy Small Business Innovative Research, Principal Investigator,


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