Flight Certification Research Articles

Modeling the unsteady wake of an impulsively started circular cylinder using refined potential flow theory

Cylindrical structures find usage in many engineering applications including tethered oil drums and engine canisters slung beneath helicopters in flight. The motion of air around such circular cylindrical structures and the helicopter presents interesting phenomena including flow separation, wakes and turbulence. The physics of these are enshrined in the continuity equation and the Navier–Stokes equations. Therefore, their studies are not only important in mathematics and physics, but they are also required for efficient helicopter operations. In practice, reduced-order models of these operations that take in aerodynamics models of the tethered loads are utilized for stability analysis, flight certification and pilot training because of the prohibitive cost of experimentation and computational analyses of these configurations. However, there is a dearth of realistic analytical models of finite cylinder flows because of the Navier–Stokes problem. Classical potential flow theory provides an avenue to develop such models, but the extant gaps in its predictions significantly preclude its usage for engineering applications. Attempting to bridge these gaps, this article introduces refined potential flow theory in which the governing equations and boundary conditions are satisfied. Viscous effects, fluctuations of the mean flow and three-dimensional effects are also incorporated. For characterization, refined potential flow theory is employed on an incompressible flow over an impulsively started circular cylinder for Reynolds numbers and non-dimensional times in the range 30<Re<104 and 0.2 ≤ T ≤ 77, 047 respectively. There is an excellent prediction of 0.209 for the Strouhal number at Re=3,900 . At this transitional Reynolds number, the harmonics of the Strouhal frequency are also captured, and the characteristic irregular fluctuations at sub-Strouhal frequencies are discernible in the velocity spectra. As the flow becomes more turbulent, these become more pronounced at Re=9,500 when the predicted Strouhal number is within 10% of experimental result. In the fully developed stage, spectra analyses of the wake velocity components at some downstream locations also display Kolmogorov's Five-Thirds law of homogeneous isotropic turbulence. The present model can thus aid the development of reduced-order models of helicopter operations that feature tethered cylindrical loads.

Practical examples for the flight data compatibility check

Accurate information about aircraft speed, altitude, and aerodynamic flow angles is essential for evaluating aircraft performance and handling qualities. These quantities are determined from air data measurements taken by sensors normally located near the aircraft cockpit. Since these sensors are affected by the distorted flow field around the fuselage, a correction must be applied. Before the first flight, a set of calibration parameters is usually determined from wind tunnel experiments or CFD calculations. However, the Data Compatibility Check (DCC) method allows a more accurate air data sensor calibration during the certification flight test. This method reconstructs air data quantities from inertial acceleration, angular rate measurements and the flight path. By comparing the reconstructed quantities with the measured ones, the structure and parameters of air data sensor models can be identified. In this paper, an introduction to the data compatibility check method and the setup used in a flight test for system identification is given. The DCC is applied on data gathered from a test campaign with the new DLR research aircraft Dassault Falcon 2000LX ISTAR. Use cases for the calibration of the nose boom airflow vanes and the correction of sensors during large sideslip maneuvers will be presented in this paper.

A procedure for obtaining forecast turbulence parameters in the atmospheric boundary layer for acoustic propagation

Propagation through turbulence causes a mean reduction in loudness of sonic booms and in some cases can cause significant fluctuations in loudness about that mean. Understanding these effects is important for accurately planning loudness levels during upcoming community noise tests with the X-59 aircraft as well as mitigating the risk for excessive loudness due to turbulence. Current methods for modeling acoustic propagation through turbulence require several atmospheric parameters including the atmospheric boundary layer (ABL) height, friction velocity, mixed-layer velocity scale, surface-layer temperature scale, as well as ambient pressure, temperature, and humidity within the ABL. Accurate forecasts of these parameters are needed for X-59 flight planning and may also be useful for planning future supersonic aircraft certification flight tests. This presentation showcases one method for obtaining the forecast turbulence and ambient atmospheric parameters from freely available Climate Forecast System Version 2 data. Forecast parameters are compared to measurements to assess accuracy and utility.

Certification-Constrained Vertical Tail Sizing and Power Split Optimization for Distributed Electric Propulsion Aircraft

Distributed electric propulsion (DEP) concepts leverage the synergistic benefits of aeropropulsive coupling and improve the overall flight efficiency by placing propulsors at optimal locations, such as the wingtip. However, placing propulsors far away from the aircraft centerline may bring aircraft difficulties in complying with critical-engine-inoperative flight certification rules. To investigate the impact of certification requirements on the DEP aircraft design, this paper takes NASA’s PEGASUS concept as a use case and presents a study of certification-constrained vertical tail sizing and propulsive power split optimization. For comparison, a vertical tail retrofit study is also conducted on the ATR 42-500 aircraft using the same design space, to gain insights into the different impacts of certification constraints on the design process between conventional and unconventional aircraft. In each study, a design space exploration is performed, including a feasibility test and a multi-objective optimization. The feasibility test shows that the certification constraints yield a much smaller feasible design space for the DEP concept when compared to the conventional aircraft. Despite a much smaller feasible design space, the constrained Pareto optima of the PEGASUS still exhibit lower cruise drag coefficient, fuel burn, and operational energy cost when compared to those of the ATR 42.

Spinal injuries after ejection seat evacuation in fighter aircraft of the German Armed Forces between 1975 and 2021.

The ejection seat is one of the most important rescue tools for military aircrews. However, the high ejection speeds place high loads on the pilots, which is mainly absorbed by the pilot's spine. The differentiated evaluation of spinal injuries is of particular importance because this has a decisive influence on the further personal life and career of the affected aircrew members. Factors influencing the occurrence of a fracture as well as the impact of a spinal injury on military flight certification have not been addressed sufficiently to date. The authors conducted a retrospective evaluation of ejection seat evacuations in German Armed Forces combat aircraft accidents between 1975 and 2021. The total number of aviation accidents with ejection seat initiations and the survived ejections were collected. Accident-specific data and pilot-specific data were collected. In addition, it was evaluated whether aircrew members were recertified for military flight after the accident. The type of spinal injury and the location of the injury were evaluated. In addition to the descriptive statistics, two logistic regression models were developed to assess prediction values of accident- and pilot-specific factors for spine injuries. A total of 103 aircrew members were included in this retrospective analysis. The overall prevalence of spine injuries was 56.3%, and the overall prevalence of spine fractures was 33.0%. In the first regression model analysis, there was no association with pilot flight experience, age, height, weight, BMI, and being diagnosed with a spine fracture. Similarly, aircraft type, altitude, and airspeed were not associated with the occurrence of spine fractures. In the regression model analysis for predictors of rejection in military flight recertification after ejection seat evacuation, lumbar spine fractures were associated with higher odds of being rejected for recertification. Military aircraft crew members have a high risk of suffering from a spine injury after emergency evacuation using an ejection seat. In the subsequent medical workup, special attention should be paid to the spine, and ideally a consultation with a spine surgeon should be performed. Lumbar spine fractures may have major consequences for military flight certification, and therefore special attention should be paid to successful rehabilitation.

Using a run time assurance approach for certifying autonomy within naval aviation

AbstractThe methods and procedures within United States naval aviation to certify an aircraft safe for flight are well established. However, these methods and procedures are based on clearing a system that is operated or monitored by a human. A fully autonomous system will not have a human in or on the loop and will therefore require a new method for certifying it safe for flight. This paper details how to use run time assurance as the framework for a safety of flight certification of autonomous behavior within United States naval aviation. We present an aerial refueling task with run time assurance as use case for the framework for certification. Within the use case we then give more details on the mechanics of using RTA to enable autonomous functionality within naval aviation.

CAMERA: A method for cost-aware, adaptive, multifidelity, efficient reliability analysis

Estimating probability of failure in aerospace systems is a critical requirement for flight certification and qualification. Failure probability estimation involves resolving tails of probability distributions, and Monte Carlo sampling methods are intractable when expensive high-fidelity simulations have to be queried. We propose a method to use models of multiple fidelities that trade accuracy for computational efficiency. Specifically, we propose the use of multifidelity Gaussian process models to efficiently fuse models at multiple fidelity, thereby offering a cheap surrogate model that emulates the original model at all fidelities. Furthermore, we propose a novel sequential acquisition function based experiment design framework that can automatically select samples from appropriate fidelity models to make predictions about quantities of interest at the highest fidelity. We use our proposed approach in an importance sampling setting and demonstrate our method on the failure level set and probability estimation on synthetic test functions and two real-world applications, namely, the reliability analysis of a gas turbine engine blade using a finite element method and a transonic aerodynamic wing test case using Reynolds-averaged Navier-Stokes equations. We show that our method predicts the failure boundary and probability more accurately and at a fraction of the computational cost compared with using just a single expensive high-fidelity model. Finally, we show that our sequential approach is guaranteed to asymptotically converge to the true failure boundary with high probability.

Flight regimes recognition in actual operating conditions: A functional data analysis approach

Helicopters need adequate monitoring to prevent dynamic failures from excessively affecting components’ health status, increase the level of safety, and reduce operative costs. Health and Usage Monitoring Systems have been developed to monitor helicopters during their lifetime in the last few decades. Recent works demonstrated that despite analyzing physical components’ behavior over time, tracking the regimes performed during each flight contributes to estimating the aircraft’s health and usage status, paving the way for designing accurate prognostics algorithms. However, today, most regime recognition systems rely on data recorded during certification flights. It follows that the training regimes differ from the ones proposed in the prediction phase, which are acquired during helicopter actual operating conditions. This affects these recognition system performances. Aiming at overcoming this limitation, in this work, we proposed an unsupervised regimes recognition system capable of better handling the actual helicopter usage spectrum. In detail, we proposed a system based on an unsupervised learning paradigm, which leverages a soft-membership classification technique to account even for mixed regimes and transitions. In addition, the system represents data according to functional data analysis theory, which allows for considering the temporal relationship between samples in the classification process, often neglected in state-of-the-art approaches. The proposed system was tested on experimental data, collected by Leonardo Helicopter Division, assessing outstanding capabilities in recognizing correctly standard and mixed regimes and transients. Also, the presented results demonstrate the approach capabilities in paving the way for the definition of new regimes, more consistent with the actual helicopter usage spectrum.

Research on High-Temperature Cooling Flight Test Technology of a Certain Type of Civil Aircraft Generator

The high-temperature special flight test is one of the important special meteorological flight tests for civil aircraft airworthiness certification flight test, and generator high-temperature cooling flight test is also very important as the key flight test subject of high-temperature special flight test. The purpose of this paper is to summarize and study the airworthiness background, technical research, technical preparation, technical implementation, and data analysis results of a certain type of civil aircraft generator high-temperature cooling flight test, to provide guidance and help for subsequent civil aircraft generator high-temperature cooling flight test.

Modelling Green VTOL Concept Designs for Reliability and Efficiency

All-electric and hybrid-electric aircraft are a future transport goal and a possible ‘green’ solution to increasing climate-related pressures for aviation. Ensuring the safety of passengers is of high importance, informed through appropriate reliability predictions to satisfy emerging flight certification requirements. This paper introduces another important consideration related to redundancy offered by multiplex electric motors, a maturing technology which could help electric aircraft manufacturers meet the high reliability targets being set. A concept design methodology is overviewed involving a symbolic representation of aircraft and block modelling of two important figures of merit, reliability, and efficiency, supported by data. This leads to a comparative study of green aircraft configurations indicating which have the most potential now, and in the future. Two main case studies are then presented: an electric tail rotor retrofitted to an existing turbine powered helicopter (hybrid) and an eVTOL aircraft (all-electric), demonstrating the impact of multiplex level and number of propulsion channels on meeting target reliabilities. The paper closes with a summary of the important contribution to be made by multiplex electric machines, well as the advancements necessary for green VTOL aircraft sub-systems, e.g., power control unit and batteries, to improve reliability predictions and safety further.

Relating Sensor Degradation to Vehicle Situational Awareness for Autonomous Air Vehicle Certification

Pilots use situational awareness (SA) to make appropriate aeronautical decisions. Autonomous vehicles will not have a human pilot (or operator) in the loop when off-nominal conditions present themselves, and will rely on sensors to build SA on their environment to make sound aeronautical decisions. As their sensors degrade, it is hypothesized that a point exists where the SA those decisions are based off will be inadequate for sound aeronautical decisions. It will be shown that this point can be identified through modeling and simulation of a simple sensor network to complete a task currently reserved for qualified pilots. This research highlights the process of determining an objective measure for the subjective end and relates it to a possible safety of flight certification for an autonomous system to perform tasks currently reserved for qualified pilots.

Deep Reinforcement Learning for Power Controlled Channel Allocation in Wireless Avionics Intra-Communications

Wireless avionics intra-communications (WAIC) can play an important role in alleviate the issue of fuel consumption, stability and maintenance costs over traditional wired avionics systems. However, in order for WAIC system to coexist with other systems in aircraft, the transmitted power level of WAIC is strictly limited to 50 mW. Meanwhile, WAIC require extremely low outage probability for the safety-critical avionics applications. Hence, it is urgently needed to effectively allocate channels and utilize the limited power while ensuring the reliability and real-time performance of the WAIC system. In this paper, a deep reinforcement learning (DRL)-based power controlled channel allocation (DRL-PCCA) scheme for WAIC network is proposed, with physical layer of frequency hopping orthogonal frequency division multiplexing (FH-OFDM). First, we formulate a sub-bands allocation and power control optimization problem whose aim is to minimize the overall transmit power provided that all end nodes achieve their requested data rates and desired bit error rate (BER). However, the problem formulated is non-convex and NP-hard. To tackle this problem, we propose a DRL-based scheme, which can effectively solve the optimization problem of sequence decision making in complex environment by using neural networks to extract spatial correlation features of WAIC. In particular, a reliability framework for WAIC is presented to analyse the performance of the proposed scheme against the system requirements of the flight certification. Simulation results demonstrate that the performance of proposed DRL-based scheme is superior to the traditional power control and channel allocation scheme.

Autonomous Flight-Test Data in Support of Safety of Flight Certification

The current safety of flight clearances for unmanned aircraft requires a qualified operator who can make decisions and ultimately bears the responsibility for the safe operations of the vehicle. The future of aviation is unmanned, and ultimately autonomous. Yet, a method for certifying an autonomous vehicle to make decisions currently reserved for qualified pilots does not exist. Before we can field autonomous systems, a process needs to be approved to certify them. This paper analyzes the flight-test data (both developmental and operational) of an autonomous decision engine selecting an appropriate landing site for a large rotorcraft in an unprepared landing zone. In particular, this paper focuses on using legacy test and evaluation methods to determine their suitability for obtaining a safety of flight clearance for a system that possesses autonomous functionality. We will show that the autonomous system under test was able to complete a mission currently reserved for qualified pilots under controlled conditions. However, when confronted with conditions that were not anticipated (or programmed), the software lacked the judgment a pilot uses to complete a mission under off-nominal conditions.

Aeroelastic Vibration Measurement Based on Laser and Computer Vision Technique

The new aeronautical structures have become more flexible and light weight to reduce energy consumption, emissions, and noise, and to operate at high altitudes for long periods in the air, such as those required in the NASA Helios project. The increased structural flexibility of these aircraft has reignited concerns related to aeroelastic instabilities, such as flutter. Improving the techniques and methods used in aircraft certification flights is an important concern of the aeronautical community, because current standards and procedures do not provide recommendations and guidelines for aircraft with a high degree of flexibility. The techniques traditionally used in commercial airplanes cannot be used in this new aircraft concept, because they have a high degree of non-linearity in their flight dynamics. Current research indicates an increasing awareness about the importance of vision in the monitoring of UAV structural health. This work presents a new methodology to measure natural frequencies of aeronautical structures using a computer vision system. We also discusses new approaches to sense and acquire vibration data on aeroelastic certification flights test. These new approaches aim to reduce both the time required to identify the aeroelastic phenomenon and the size of the hardware that must be boarded on the aircraft, thus minimizing the risks and costs of the vibration tests. The advance of computer vision systems enables the use of cameras as a motion tracker sensor with millimeter precision and accuracy. Non-contact sensors are suitable for flutter analysis because they do not interfere with the dynamics of the aircraft. Therefore, this new methodology is able to process the obtained images and provide the user with the data about movements in a ready to use vector, at a reasonable cost. Using the data provided by this methodology the natural frequencies of the first bending modes were identified. This new methodology can be a user-friendly tool to support the Brazilian National Civil Aviation Agency - ANAC program called iBR 2020, which aims to certify small aircraft.

ОСОБЛИВОСТІ ЗАБЕЗПЕЧЕННЯ АЕРОДИНАМІЧНИХ ХАРАКТЕРИСТИК РЕГІОНАЛЬНОГО ПАСАЖИРСЬКОГО ЛІТАКА

Principal results of the computational and research work performed during development of a regional passenger aircraft to ensure its aerodynamic characteristics are given. When creating the An-148-100/An-158 family of aircraft, such level of the aircraft aerodynamic perfection was achieved, which ensured fulfillment of the specified requirements for their flight performance – maximum speed, cruising flight altitude and flight range with different payloads. The developed aerodynamic configuration made it possible to create a family of regional passenger high-wing planes with a flight speed of up to 870 km/h (true speed) (M = 0.8), which have no analogues in the world aviation industry. Developed for the An-148-100 / An- 158 aircraft, supercritical profiles of the new generation with a large maximum relative thickness formed the basis of the aerodynamic configuration of a high-speed wing with moderate sweep. The aircraft lift-to-drag ratio in cruise flight is Kcruise = 15.8, which corresponds to the worldwide values. Developed aerodynamic configuration of the wing high-lift devices provides high bearing properties of the wing during take-off and landing stages, which allows to fully meet the requirements for the runway required length of the base airfields Lrun = 1485...1950 m. Developed algorithms are implemented in the electric remote control system and provide necessary standard characteristics of stability, controllability and flight dynamics in the main control mode. Selected margins of the aircraft’s own static stability and effectiveness of its controls ensure safe completion of the flight in standby control mode. The certification flight tests of the An-148-100/An-158 airplanes confirmed full compliance of their take-off and landing performance, as well as the stability, controllability and flight dynamics characteristics with the requirements of the Certification basis in both standard and in failure situations tested in flight tests. Necessary and sufficient amount of experimental work was conducted in the lowspeed and high-speed wind tunnels of the ANTONOV SC and TsAGI to verify the aerodynamic and spin characteristics of the An-148-100/An-158 airplane models, which improved the aerodynamic configuration of the aircraft and its individual units and allowed to apply the work results in calculation of aircraft strength, as well as for development of their systems.

System Suitability and Pilot Workload Assessments for Instrument Flight Certification of a New Utility Helicopter

System Suitability and Pilot Workload Assessments for Instrument Flight Certification of a New Utility Helicopter

Experimental Research on the Helicopter Sliding Stability after Ditching

As the application of the helicopter increases rapidly, more and more attention is paid to the safety. Thus the survivability and crashworthiness has been an important assessment items for the helicopter design and flight certification. In this paper, a newly designed ditching experiment with a helicopter model scaled 1:8 was conducted to attain the dynamic response of loads and especially the posture. Parameters such as weight, bladder pressure, lateral wind and ditching velocity were set in different groups to investigate the effect on the sliding stability. Based on the result, comparison and analysis was carried out and some conclusions were given. Meanwhile, the method and process of the experiment was presented in detail and proved an effective and available reference for the helicopter flight certification.

Verification of Over-Speed and Burst Margin Limits in Aero Engine Disc along with Low Cycle Fatigue Life

Aero engine rotor burst evaluation is one of the most important problems to be taken care off, whenever it comes to designing a turbo machinery disc. The consequences of a failure can be intense, since the disc fragments into multiple pieces and they are hurled away in all the possible direction at high speeds. Due to high thermo-mechanical loading conditions the disc is subjected to varying degrees of temperature from bore to rim. However, the centrifugal force dominates in the disc which ranges from 80%-90% and the rest can be treated as thermal and gas loads. The challenge lies at designing a disc for off-design conditions with their varying loads and duty cycles. In present work evaluation of safety limits and low-cycle fatigue (LCF) life estimation of an aero engine disc through classical methods and blending the terminologies with simulation engineering to arrive at a probable interpretation of number of duty cycles is carried out. The methodology compares the fatigue parameters involved in evaluation of disc life at off-design condition through sensitivity analysis. The design tool closely connects the flight certification FAA and EASA the regulating agencies for safety in air transportation vehicles. The off-design speed regulations through API and MIL handbook for material specification are considered to carry out design of experiments using finite element analysis approach

Sustainable flight test project tools

One of the most important aspects of the development of a new aviation product is the testing and verification phase. This paper identifies procedures to optimise certification flight test campaigns as one troublesome and unpredictable aspect of aircraft development. Considering a large market historical sample from key aircraft manufacturers for the certification of their products, we conclude that a sustainable approach with flight hour reduction, selected engineering tools and management tools will result in a considerable reduction in fossil fuel expenses and overall product certification time and cost.

Performance Comparison of Industrial Wireless Networks for Wireless Avionics Intra-Communications

Recently, the United Nations voted to grant a frequency band for wireless avionics intra-communications (WAICs) to replace the heavy and expensive cables used in aircraft with wireless systems. However, WAICs require extremely low failure probability for the flight certification of the safety-critical avionics applications. Existing industrial wireless networks are possible candidate technologies, since they are designed to meet the high reliability requirement of industrial automation within a harsh environment surrounded by metals. This letter provides a theoretical framework to compare the performance of representative industrial wireless networks against the system requirements of the flight certification. A mathematical model is proposed to evaluate the considered protocols in terms of the deadline missing probability per flight hour. Furthermore, the model is used to derive the maximum allowable packet loss probability while guaranteeing the flight certification.