Helicopter Operations Research Articles

A Robust Fault-Tolerant Control for Quadrotor Helicopters against Sensor Faults and External Disturbances

This paper presents an active fault-tolerant control strategy for quadrotor helicopters to simultaneously accommodate sensor faults and external disturbances. Unlike most of the existing fault diagnosis and fault-tolerant control schemes for quadrotor helicopters, the proposed fault diagnosis scheme is able to estimate sensor faults while eliminating the effect of external disturbances. Moreover, the proposed fault-tolerant control scheme is capable to eliminate the adverse effect of external disturbances as well by designing a disturbance observer to effectively estimate the unknown external disturbances and integrating with the designed integral sliding-mode controller. In this case, the continuous operation of the quadrotor helicopter is ensured while avoiding the unexpected control chattering. In addition, the stability of the closed-loop system is theoretically proved. Finally, the effectiveness and advantages of the proposed scheme are validated and demonstrated through comparative numerical simulations of the quadrotor helicopter under different faulty and uncertain scenarios.

Safety of twin-engine helicopters: Risks and operational specificity

ObjectivesTo examine the difference between twin-engine and single-engine helicopter operations and accidents, and to understand the main factors that are associated with fatal twin-engine helicopter accidents. MethodsData concerning a ten-year period taken from the US National Transportation Safety Board online database on twin-engine helicopter accidents were analyzed and compared to a dataset of single- and twin-engine helicopter accidents in Part 135 Air Taxi and Commuter operations. Results and recommendationsTwin-engine helicopters show a lower risk of accidents compared to single-engine helicopters but with a similar proportion of fatal accidents. Twin-engine helicopters show a frequent involvement in air medical operations, which expose pilots more often to night and IMC flight conditions that are significantly more often associated with fatal accidents. The differences in accident rate cannot be explained by the profile of the pilots, the kind of accident causes or their type of operations but may possibly be associated with differences in geographical location or type of organization.

Prediction of the aerodynamic behaviour of a full-scale naval ship in head waves using Detached Eddy Simulation

The airwake behaviour around a ship provides useful information for the safe operation of helicopters on naval ships as well as in helicopter pilot training. This study investigates the impact of ship motions on the airwake behind the superstructure of a naval ship using Detached Eddy Simulation. A full-scale simplified frigate geometry is analysed stationary and in head waves at three different wavelengths under a uniform wind field and in the presence of an atmospheric boundary layer. The results reveal that an atmospheric boundary layer impacts significantly the airwake, as well as the vertical wave-induced motions of the ship, which reduce in amplitude by between 20.9% and 22.39% in heave, and up to approximately 38% in pitch. Moreover, the results show that the presence of an atmospheric boundary layer impacts the ship's heave and pitch motion periods. The flow field is also significantly altered depending on the ambient wavelength and period of motion, particularly in the case where an atmospheric boundary layer is modelled.

Blood Product Supply for a Helicopter Emergency Medical Service

Background: Long patient transport times to trauma centers are a well-known problem in sparsely populated regions with a low hospital density. Transfusion of red blood cell concentrates (RBC) and plasma improves outcome of trauma patients with severe bleeding. Helicopter emergency services (HEMS) are frequently employed to provide early advanced medical care and to reduce time to hospital admission. Supplying HEMS with blood products allows prehospital transfusion and may help to prevent exsanguination or prolonged hemorrhagic shock. We have investigated the maintenance of blood product quality under air transport conditions and the logistical steps to introduce a HEMS blood depot into routine practice. Methods: A risk analysis was performed and a validation plan developed. A special, commercially available transport container for blood products was identified. Maintenance of temperature conditions between 2 and 6°C in the box were monitored at ambient temperatures up to 35°C over 48 h. Quality of blood products before and after helicopter air transport were evaluated including (1) for RBCs: hemoglobin, hematocrit, hemolysis rate; (2) for thawed plasma: aPTT, INR, single clotting factor activities. The logistics for blood supply of the regional HEMS were developed by the transfusion service of the Greifswald University Hospital in collaboration with the in-hospital transport team, the HEMS team, and the HEMS operator. Results: The transport container maintained a temperature below 6°C up to 36 h at 35°C ambient temperature. Vibration during helicopter operation did not impair quality of RBC and thawed plasma. To provide blood products for HEMS at least two transport containers and an additional set of cooling tiles is needed as the cooling tiles need a special temperature priming over 20 h. The two boxes were used at alternate days. To reduce wastage, RBCs and thawed plasmas were exchanged every fourth day and reintegrated into the blood bank inventory for further in-hospital use. Conclusions: Supplying HEMS with RBCs and plasma is feasible. Helicopter transport has no negative impact on blood product quality. The logistic challenges require close collaboration between the HEMS team and the blood transfusion service.

Recommendations for emerging air taxi network operations based on online review analysis of helicopter services

The effects of traffic congestion are adverse, primarily including air pollution, commuter stress, an increase in vehicle operating costs, and accidents on road. In efforts to alleviate these problems in metropolitan cities, logistics companies plan to introduce a new Urban Air Mobility (UAM) service called air taxis. These are electric-powered vehicles that would be tested and operated in the forthcoming years by international transportation companies like Airbus, Uber, and Kitty Hawk. Since these flying taxis are an emerging mode of transportation, it is necessary to provide recommendations for initial design, implementation, and operation. This study proposes managerial insights for these upcoming UAM services by analyzing online customer reviews and conducting an internal assessment of helicopter operations. Helicopters are similar to air taxis in regards to their vertical takeoff and landing (VTOL) operations, and therefore, customer reviews pertaining to the former can enable us to obtain insights into the strengths and weaknesses of the short-distance aviation service, in general. A four-stage sequential approach is used in this research, wherein online reviews are mined in Stage 1, analyzed using bigram and trigram models in Stage 2, 7S internal assessment is conducted for helicopter services in Stage 3, and managerial recommendations for air taxis are proposed in Stage 4. The insights obtained in this paper could assist any air taxi company in providing better customer service when they venture into the market.

Aerodynamic Optimization over Frigate Helicopter Flight Deck by Hangar Shape Modifications

Helicopter operations on frigates imply complex maneuvers for pilots. During the aircraft approach to the frigate, the helicopter rotor is immersed in a changing aerodynamic flow, with high speed and turbulence intensity gradients. This occurs as a consequence of the nonaerodynamic surfaces that compose the frigates and generate detached flow on their decks. Typically, the helicopter operations on frigates are located at the stern. That is, after the superstructure of the ship and just behind the hangar where the helicopter can be hosted. Using a simplified frigate shape model, tests have been carried out at the National Institute of Aerospace Technology’s low-speed wind tunnel by modifying the hangar geometry in a bubble-shaped way to optimize it aerodynamically and improve the flow over the frigate aftdeck. Roof and side walls have been modified by linear , circular , and elliptical geometries. A particle image velocimetry (PIV) technique has been used for obtaining the flow velocity field above and behind the hangars proposed to evaluate the effectiveness of each geometry modification proposed. Finally, a numerical comparison with the PIV results and parameters such as the storage capacity reduction of the hangar due to its geometry change has been carried out.

Evaluation of Aerodynamic Characteristics of Mega-Yacht Superstructures by CFD Simulations

Air wake distribution around the superstructure of a mega-yacht is a key concern for the designer because of various reasons such as comfort expectations in recreational deck areas, self-noise generation, air pollution and temperature gradients due to exhaust interactions, and safety of helicopter operations such as landing/take off and hovering. The Reynolds-averaged Navier-Stokes (RANS) technique in computational fluid dynamics (CFD) is frequently used in studies on mega-yacht hydrodynamics and aerodynamics with satisfactory results. In this article, a case study is presented for the utilization of CFD in a mega-yacht's superstructure design. The flow field in recreational open areas has been analyzed for the increase in velocity due to the existence of the superstructure. A reduction in self-noise of the mast structure has been aimed by reducing flow separation and vorticity. Time-dependent velocity data obtained with scale-resolving simulations are presented for the evaluation of helicopter landings. The capabilities and limitations of the RANS technique are discussed along with recent developments in modeling approaches.

Aerodynamic Characteristics of Helicopter with Ducted Fan Tail Rotor in Hover under Low-Speed Crosswind

The tail rotor of a helicopter operating under low-speed crosswind undergoes highly complex flow due to the interaction between the main rotor, fuselage, and tail rotor system. In this study, numerical simulations have been conducted on the complete configuration of a helicopter with a ducted fan tail rotor system (comprising a main rotor, ducted fan tail rotor, fuselage, and empennage) to analyze the wake interaction in hovering flight under various crosswind directions. The flow characteristics around the tail rotor, the tail rotor thrust, and the yawing moment of the helicopter are investigated and evaluated. The aerodynamic forces are compared with those of a helicopter with an open-type tail rotor. The results indicate that the aerodynamic performance of the ducted fan tail rotor is highly affected by the wakes of both the main rotor and port wing. Nevertheless, the helicopter with a ducted fan tail rotor is observed to be much more directionally stable under various crosswind directions, than that with an open-type tail rotor. This is because the rotor is protected by the fixed part of the tail rotor system in the former case.

Data-Driven Technique-Based Fault-Tolerant Control for Pitch and Yaw Motion in Unmanned Helicopters

This article proposes a fault-tolerant control (FTC) algorithm to monitor the anomalies and control them for unmanned helicopters. This overcomes the drawbacks due to heteroscedasticity and restrictions on the input variables in conventional FTCs and improves the training and testing efficiencies with enhanced control. This is achieved by adopting the support vector data descriptor (SVDD) to learn the operating states of the unmanned helicopter during normal and motor fault operations. Furthermore, a neural integrated fuzzy (NiF) controller is trained to cope up with the states classified by the developed classifier. To realize the development of the proposed control algorithm, the motor faults in the unmanned helicopter are developed by observing the pitch and yaw motor operation of unmanned helicopters. A two-class classification is developed using SVDD for identifying the motor faults and the NiF is trained for controlling the plant during the anomaly. The results depicted 98.6% training accuracy and 98.96% prediction accuracy along with efficient control when tested for a faulty condition on a helicopter test rig.

DAMAGING AND DESTRUCTION OF REDUCTION TRAINS OF MI-8 HELICPTER AND ITS MODIFICATIONS

The results of quantitative factographic study of the Mi-8 helicopter (its modifications) reduction trains with different types of gear coupling, the destruction of which occurred while in flight due to the presence of preliminary material damage at the stage of production of the gear wheels and during their operational employment are presented in a generalized form. Contact teeth destruction as the main cause of failures of high-speed gear wheels in well-lubricated enclosed gears are considered. The causes of the wearing and sticking of the teeth along their lateral surfaces as well as scoring and destruction depending on the origin of the cracks are revealed.
 The main causes of damaging and destruction of helicopter reduction trains, locations and origination of fatigue cracks spots are identified. Different types of contact and fatigue destructions due to various reasons are considered.
 Dynamic loading of gear wheels disks as a result of their rotation damages the material of the wheels in the area of multi-cycle fatigue due to the repeatability of flight load cycles.
 Lamination or deep contact destruction of the material, which sometimes take place in gears with surface hardened (cemented, tempered, nitrided, etc.) teeth, are much more dangerous than surface destruction of the material since sufficiently large fractions of the hardened material layer are split away from the teeth that can quickly break down an entire gear train.
 The main causes of contact teeth destruction are load concentration along the length and profile of the teeth, the presence of residual tensile stresses on the working surfaces of the teeth, adverse operational damage to the tooth profiles and negative effect of lubricating fluid on the teeth.
 For a repaired gear train the cause of the uneven load distribution along the length of the teeth may be the poor assembly of the gear wheels and other transmission parts which leads to unsatisfactory contact of the gear teeth in the assembled unit.
 The revealed peculiarities of the destruction process indicate regular crack proliferation from flight to flight. According to the operation conditions of the gear weel, its regular extensive loading refers only to the full cycle of starting and stopping the reducer train or unloading the helicopter in flight, which in this case corresponds to the cycle of helicopter operation in one flight. Therefore, the number of detected fatigue lines corresponds to the number of helicopter flights.
 To increase the smoothness of gear train running which affects positioning accuracy, reduce wear during running-in, decrease noise, eliminate early jamming, and ultimately, to increase the resource, it is necessary to eliminate the edge interaction of the teeth, which causes their excessive wear.

Balance measurements on a frigate type ship model

Balance measurements performed by testing sub-scaled ship models determine the global forces and moments acting on the ship, which allows knowing the power required for the ship's movement and provides insight to be applied in the design of the control systems used to steer the ship and to avoid instabilities while sailing.The ship superstructure may produce large separated regions and high air wake turbulence levels resulting in a set of fluctuations of the flow parameters usually determined by measuring velocity or pressure. This paper presents the balance measurement of the aerodynamic forces acting on the ship hull. Aerodynamic forces and moments produced on the ship can be interpreted as an integration of the flow parameters (velocity and pressure distributions) over the ship surface wetted by the air. Balance method provides averaged values and fluctuations of forces coefficients.Aerodynamic environment in the vicinity of a ship is influenced by a large number of factors (atmospheric wind, sea state, ship superstructure, masts, stacks, antennas …) affecting helicopter operations on board ships and their safety during the take-off and landing manoeuvres.

Comparison of the HRV of Emergency Physicians in the HEMS During Helicopter Operations: Analysis of Differences as a Function of Number of Operations and Workload.

Chronic stress can lead to physiological illness. Emergency physicians in danger of developing such illnesses due to their demanding working environment. The aim of this study was to investigate the physiological stress level of emergency physicians (EPs) of the Helicopter Emergency Medical Service (HEMS) between emergency operations in one shift. Furthermore, the phases of the operations were compared with respect to the activity of the autonomic nervous system. The physiological and self-perceived stress levels of 20 EPs were recorded on a HEMS air-rescue-day (age: M = 44.95, SD = 4.80). The measurement of the heart rate variability (HRV) was performed during a complete air-ambulance day and examined using analyses of variance. The heart rate rises significantly at the beginning of the emergency operationto its highest valueafter landing at the operation site. The HRV parameter standard deviation of all NN intervals shows a significant decrease between the alarm phase and the end of the operation. Furthermore, high values were reached regarding chronic stress. On the Symptom Checklist-90 the EPs show lower values than the norm sample. In conclusion, over the course of three emergency operations, no physiological fatigue indicators were found. In contrast, the subjective stress load was assessed as high among EPs and on average higher than the norm sample. Compared to standard values, the EPs showed lower HRV values, which indicates a strong activation of the autonomic nervous system. These lower HRV findings might be caused by a high psychological stress load.

Evaluation of an advanced slung load control system for piloted cargo operations

Helicopter operations with externally slung loads are highly demanding for the flight crew. As the pilot does not have a direct view of the load, assistance is required from an additional crew member to achieve operational requirements (e.g., for load handling to achieve precise load positioning). An automatic load stabilization and positioning system for cargo operations has been designed with the aim to reduce pilot workload, damp load pendulum motion and to improve the load positioning performance. This system uses the concept of load-motion feedback to the rotor control. To avoid degradation of Handling Qualities (HQs), as found in previous investigations, a function has been developed that monitors pilot control inputs. Dependent on the amplitude and duration of pilot control stick deflection, the feedback signal for slung load damping is blended between two different gain sets. One set provides improved HQs during piloted control and one set provides good load damping when the pilot is passive. A further novel aspect is the evaluation of an automatic load control system using a Translational Rate Command as a method of helicopter control. A piloted simulation study has been conducted using this advanced load control system with automatic load stabilization and positioning. Three test pilots evaluated the system in different control law configurations using a Mission Task Element simulating an external load cargo operation. HQs and pilot workload were evaluated using the Cooper-Harper Rating Scale and NASA Task Load Index, respectively. The results of the study show that improved HQs and reduced pilot workload in combination with improved task performance can be achieved with the advanced slung load control system.

Helicopter Cockpit Audio Data Analysis to Infer Flight State Information

In recent years, the National Transportation Safety Board has highlighted the importance of analyzing flight data as one of the effective methods to improve the safety and efficiency of helicopter operations. Since cockpit audio data contain various sounds from engines, alarms, crew conversations, and other sources within a cockpit, analyzing cockpit audio data can help identify the causes of incidents and accidents. Among the various types of the sounds in cockpit audio data, this paper focuses on cockpit alarm and engine sounds as an object of analysis. This paper proposes cockpit audio analysis algorithms, which can detect types and occurrence times of alarm sounds for an abnormal flight and estimate engine-related flight parameters such as an engine torque. This is achieved by the following: for alarm sound analysis, finding the highest correlation with the short time Fourier transform, and the Cumulative Sum Control Chart (CUSUM) using a database of the characteristic features of the alarm; and for engine sound analysis, using data mining and statistical modeling techniques to identify specific frequencies associated with engine operations. The proposed algorithm is successfully applied to a set of simulated audio data, which were generated by the X-plane flight simulator, and real audio data, which were recorded by GoPro cameras in Sikorsky S-76 helicopters to demonstrate its desired performance.

Prehospital Stressors: A Cross-sectional Study of Norwegian Helicopter Emergency Medical Physicians

ObjectivePersonnel working in helicopter emergency medical services (HEMS) and search and rescue (SAR) are exposed to environmental stressors, which may impair performance. The aim of this survey was to study the extent HEMS and SAR physicians report the influence of specific danger-based and non–danger-based stressors. MethodsThe study was performed as a cross-sectional, anonymous, Web-based (Questback AS, Bogstadveien 54, 0366 Oslo, Norway) survey of Norwegian HEMS and SAR physicians between December 2, 2019, and February 25, 2020. ResultsOf the recipients, 119 (79.3%) responded. In helicopter operations, 33.6% (n = 40) reported involvement in a minor accident and 44.5% (n = 53) a near accident. In the rapid response car, 26.1% (n = 31) reported near accidents, whereas 26.9% (32) reported this in an ambulance. Of physicians, 20.2% (n = 24) received verbal abuse or threats during the last 12 months. When on call, 50.4% (n = 60) of physicians reported sometimes or often being influenced by fatigue. ConclusionThis study shows that Norwegian HEMS and SAR physicians are exposed to several stressors of both a danger-based and non–danger-based nature, especially regarding accidents, threatening patient behavior, and fatigue. Very serious incidents appear to be seldom, and job satisfaction is high.

A Novel Condition Indicator for Bearing Fault Detection Within Helicopter Transmission

Helicopter usage and monitoring system (HUMS) is one of the critical systems for helicopter’s safety and reliability. Whilst HUMS has proven to be effective in detecting gears’ defects, bearing failures are not adequately detected using current monitoring indicators. Detection of bearing faults in helicopter gearboxes is made challenging by the presence of the complicated signal transmission path attenuating the monitored signal to the receiving sensor. To ensure safe operation of helicopters, this research proposes a novel condition indicator to detect bearing faults in helicopter gearboxes. For this purpose, vibration measurements captured from a CS-29 category ‘A’ helicopters test rig were utilized for detecting a bearing defect that has occurred in the epicyclic module of the main gearbox. Signals of rolling element bearings under various fault conditions were collected, and an adaptive filter algorithm was utilized to separate the random component of the signal. The resultant signatures were then further processed using wavelet analysis to extract the bearing signal of interest. Results showed that this new indicator successfully detect bearing faults. Besides, the impulse energy indicator responds consistently to the fault severity compared to the traditional indicators such as RMS and kurtosis. A technique to extract frequency band corresponding to the bearing fault impulses has been developed and tested. The technique employs the adaptive filter signal separation, wavelet packet decomposition and the combination of RMS and kurtosis to select the optimum filter band.

Robust Continuous Finite-Time Control of a Helicopter in Turbulence

This letter synthesizes a C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> smooth, continuous, output-tracking sliding mode controller for a full-scale helicopter operating in high-intensity turbulence. The closed-loop dynamics are required to satisfy ideal on-axis attitude and rate response characteristics. Additionally, robust command tracking and suppression of off-axis responses are required in the presence of matched and unmatched uncertainties. Lyapunov based theoretical proofs are utilized for the tracking problem via application of a continuous second-order sliding mode (SOSM) controller. A second order sliding mode observer is employed to estimate the higher derivatives of the sliding variable for the output feedback controller. Simulations in hover illustrate the continuity, smoothness, and robustness properties of the proposed controller as compared to a conventional sliding mode controller and a linear quadratic controller.

Ship Airwakes in Waves and Motions and Effects on Helicopter Operation

Insight into the effects of motions and waves on a ship airwake can be used to provide information regarding safe conditions for at sea flight operations and pilot training. We present a study on the effects of waves and motions on a ship's airwake and a helicopter operating above the flight deck using full scale computational fluid dynamics simulations. The ONR Tumblehome Ship (ONRT) geometry is analyzed in wind and regular head waves corresponding to nominal sea states 3 and 6. In order to separate the effects of waves and motions, simulations are conducted for both sea states with combinations of: waves and motions, waves and no motions, no waves with motions, and no motions or waves. A triple velocity decomposition is conducted in order to quantify changes in the airwake due to motions and waves. The aerodynamic loads on a helicopter hovering in the airwake are studied with one-way and two-way coupling approaches. The one-way coupled analysis uses the velocity field data from the full scale airwake simulations along with disk actuator theory to calculate thrust fluctuations for three different rotor sizes operating above the flight deck. In the two-way coupled study a helicopter loosely based on the Sikorsky SH-60 Seahawk hovering above the flight deck of the ONRT is simulated, including dynamic overset grids of the main rotor, tail rotor and fuselage. This approach captures the interaction between the rotor downwash and the ONRT airwake. The study shows that for the lower sea state 3 the motions and waves have little effect on the airwake behavior, exhibiting little change with respect to the baseline case without motions and waves. At sea state 6 the flow field is modified significantly, which also affects the forces on the helicopter. Force fluctuations show main peaks at the wave encounter frequency and the blade passage frequency, as well as higher harmonics. The one-way coupling approach shows that larger rotors effectively filter out small scale fluctuations while smaller diameter rotors are affected by smaller flow structures.

DAMAGING AND DESTRUCTION OF REDUCTION TRAINS OF MI-8 HELICPTER AND ITS MODIFICATIONS

The results of quantitative factographic study of the Mi-8 helicopter (its modifications) reduction trains with different types of gear coupling, the destruction of which occurred while in flight due to the presence of preliminary material damage at the stage of production of the gear wheels and during their operational employment are presented in a generalized form. Contact teeth destruction as the main cause of failures of high-speed gear wheels in well-lubricated enclosed gears are considered. The causes of the wearing and sticking of the teeth along their lateral surfaces as well as scoring and destruction depending on the origin of the cracks are revealed.&#x0D; The main causes of damaging and destruction of helicopter reduction trains, locations and origination of fatigue cracks spots are identified. Different types of contact and fatigue destructions due to various reasons are considered.&#x0D; Dynamic loading of gear wheels disks as a result of their rotation damages the material of the wheels in the area of multi-cycle fatigue due to the repeatability of flight load cycles.&#x0D; Lamination or deep contact destruction of the material, which sometimes take place in gears with surface hardened (cemented, tempered, nitrided, etc.) teeth, are much more dangerous than surface destruction of the material since sufficiently large fractions of the hardened material layer are split away from the teeth that can quickly break down an entire gear train.&#x0D; The main causes of contact teeth destruction are load concentration along the length and profile of the teeth, the presence of residual tensile stresses on the working surfaces of the teeth, adverse operational damage to the tooth profiles and negative effect of lubricating fluid on the teeth.&#x0D; For a repaired gear train the cause of the uneven load distribution along the length of the teeth may be the poor assembly of the gear wheels and other transmission parts which leads to unsatisfactory contact of the gear teeth in the assembled unit.&#x0D; The revealed peculiarities of the destruction process indicate regular crack proliferation from flight to flight. According to the operation conditions of the gear wheel, its regular extensive loading refers only to the full cycle of starting and stopping the reducer train or unloading the helicopter in flight, which in this case corresponds to the cycle of helicopter operation in one flight. Therefore, the number of detected fatigue lines corresponds to the number of helicopter flights.&#x0D; To increase the smoothness of gear train running which affects positioning accuracy, reduce wear during running-in, decrease noise, eliminate early jamming, and ultimately, to increase the resource, it is necessary to eliminate the edge interaction of the teeth, which causes their excessive wear.

CAA research programme – Helicopter Operations to Moving Offshore Helidecks

ABSTRACTThe background and rationale for the research reported in this paper is detailed in the companion paper subtitled Part A: Modelling the on-deck helicopter Reserve of Stability (ROS).This paper (Part B) derives the fundamental principles underpinning the safe operational envelope and the definition of Motion Severity and Wind Severity Indices (MSI/WSI), and then presents the probabilistic modelling methodology developed for calculating safe limits for these parameters (MSI/WSI limits curves). The introduction of the Relative Wind Direction (RWD) as an additional limiting parameter is also discussed, together with an explanation of how RWD limits have been derived and how they are linked with the MSI/WSI limits calculation.The MSI/WSI limits modelling methodology presented in this paper builds on the analytical helicopter stability modelling discussed in Part A. It uses a probabilistic approach to manage the variability in helicopter operations across the UK and to deal with the uncertainty of predicting helideck motion and wind conditions for the period the helicopter is landed on the helideck based on measurements taken prior to landing. This has been developed with the assistance of industry experts and has been subjected to consultation with stakeholders in the UK with the aim of identifying the right balance between enhancing safety and preserving operability.A new Helideck Monitoring System (HMS) standard has been developed which incorporates the MSI/WSI/RWD functionality. The standard was published by the Helideck Certification Agency in April 2018 with a compliance date of 31 March 2021. Operations to moving decks not equipped with HMS meeting the new standard will be restricted to stable helideck conditions.