MOC VTOL.2135 Minimum Acceptable Handling Qualities Rating

1.      Background and Introduction

The aircraft needs to be controllable and manoeuvrable to cope with adverse weather conditions and to avoid late detected obstacles or traffic appropriate to the type. The control and manoeuvring of the aircraft requires a certain amount of physical and/or mental workload from the crew. Satisfactory Handling Qualities (HQ) give the opportunity for the crew to better manage high workload situations, and allow them to operate safely for longer periods, and to be able to deal with aircraft system failures and contingencies. Degraded HQ lead to an increased crew attentional demand for aircraft control, hence reduced high workload capacity for other tasks and for Situational Awareness.

The following is a method of determining and evaluating, for compliance demonstration, the HQ for VTOLs in the Category Enhanced in normal and abnormal/emergency conditions. The Category Basic VTOLs may also elect to use this method; however, the Minimum Acceptable Handling Qualities Rating section 4 will need to be adapted. The focus is on the crew task of flight path/trajectory control. All the other characteristics of the flight controls such as number of inceptors, size and mechanical forces (friction, breakout etc.) are out of scope of this MOC. These other characteristics however will influence the achievable HQ, so they will be indirectly assessed.

This method is different from CS-23 and CS-27, since in those certification specifications, the HQ of an aircraft are suitably assessed on the addition of the compliance to static or dynamic stability requirements along with other requirements for controllability and average piloting skills. HQ are evaluated without any specific generally recognised method, and are mainly evaluated to measure the workload to determine the minimum crew in respect to the kind of operations. Usually the Cooper Harper Handling Qualities Rating Scale (CHR) is used to measure the Handling Qualities, while the Bedford rating scale (or NASA Task Load Index as alternative) is used to measure the workload. However, each applicant can choose the methodology to determine the HQ and/or workload.

This Modified Handling Qualities Rating Method (MHQRM) is an accepted means of compliance with  VTOL.2135, and can also be used to assess compliance, fully or in part, with the following SC VTOL requirements that require a determination of HQ: VTOL.2110 Flight Envelopes, VTOL.2115 Take-off performance, VTOL.2130 Landing, VTOL.2135 Controllability, VTOL.2140 Control forces, VTOL.2145 Flying qualities, VTOL.2150 Stall characteristics and stall warning, VTOL.2160 Vibration, VTOL.2300 Flight control systems and VTOL.2305 Landing gear systems.

This method should not be considered to be the only method. Applicants may propose alternative methods or deviations based on the characteristics of their design, or on their compliance determination strategy. Unless otherwise specified in a special condition, the HQRM does not replace or override any of the systems and equipment requirements of §§ VTOL.2500, VTOL.2505 and VTOL.2510.

2.      List of Acronyms

AD              Atmospheric Disturbance

ADQ            Adequate

AFM            Aircraft Flight Manual

CHR            Cooper Harper Rating Scale

CON            Controllable

CONOPS      Concept of Operations

EFCS           Electronic Flight Control System

FC               Failure Conditions

FE               Flight Envelope

FEP             Flight Envelope Protection

FHA            Functional Hazard Assessment

FltC             Flight Condition

GNSS          Global Navigation Satellite System

HQ              Handling Qualities

HQR            Handling Qualities Rating

HQRM         Handling Qualities Rating Method

IMC             Instrument Meteorological Conditions

LFE              Limit Flight Envelope

MHQRM      Modified Handling Qualities Rating Method

MTE            Mission Task Elements

NFE             Normal Flight Envelope

NVIS           Night Vision Imaging System

OFE             Operational Flight Envelope

SAT             Satisfactory

SC               Special Condition

TBD             To be determined

VFR             Visual Flight Rules

VisC            Visual cues

VTOL           Vertical Take Off and Landing

3.      MHQRM Process

The overall process is derived from the FAA Advisory Circular 25-7D Appendix E, which was intended mainly to define a method for evaluating Failure Conditions (FCs). In particular, the principle of determining the minimum HQR based on the probability of being in a given Flight Condition (FltC) was adopted. The “tool” to evaluate and show compliance with the minimum acceptable HQR will be derived from ADS-33E. The Mission Task Elements (MTE) manoeuvres of this military standard will be adapted to the SC VTOL based on the Concept of Operations (CONOPS) for VTOL that is being produced by industry. There will be also provisions on the competences of the test pilots (fixed wing or rotary wing background) and on the minimum number of evaluators. This tool is being developed together with industry and research centres, and will be published at a second stage.

This MHQRM starts by determining the minimum acceptable HQR for each phase of the flight and for a given FltC, defined as a combination of the Flight Envelope (FE) and the level of Atmospheric Disturbance (AD), relative to the Nominal State of the aircraft systems, or the probability of the FC being evaluated. A pre-requisite to start the MHQRM process is thus to have Functional Hazard Assessment (FHA) available and have preliminary quantitative assessments for the FCs to be analysed in the MHQRM. If this MHQRM process is intended for validating FC classification in the Aircraft FHA, early coordination with the Agency is advised.

The methodology developed in this MOC is aimed at identifying which FCs need to be considered in the handling quality assessment. One possible outcome of the HQ assessment is that the failure condition classification of a given FC needs to be increased.

To limit the risk of iterations of the FHA content and the subsequent side effects on the MHQRM, early coordination with the Agency on the safety assessment outputs (FHAs, preliminary quantitative analysis) is also advised.

The visual environment, or the quality of the Visual Cues (VisC), is not defined, and the assumption is that the VisC, in terms of external visual environment and displays/sensors feedback, are sufficient to allow the crew to perform their tasks and be able to achieve and assess Desired and Adequate HQ performance criteria. The most conservative external visual environment (Day, Night, IMC, NVIS) should be used for each phase of flight for which certification is requested. For example, if the aircraft is intended to be certified for flight in Night VFR, the climb, cruise, descent and approach phases of flight should be evaluated by using an appropriate external visual environment, while the take-off and landing phase may use a better external visual environment. The VisC will be defined in the evaluation document and should be agreed with the Agency on a case by case basis.

To apply this method it is first necessary to divide the profile of the aircraft into different phases of flight, e.g. taxi (if applicable), take-off (including rejected take-off), climb, cruise, descent, approach and landing (including landing following a failure condition and balked landing). The classification for each phase of flight is done because there could be failure conditions at aircraft level that affect HQs only in one particular phase of flight, as for example the loss of Global Navigation Satellite System (GNSS) could result in a reduced accuracy in the Translational Rate Command FCS mode in low airspeed, or a failure condition (i.e. multiple electric engine failures) could result in less precise turn coordination in cruise.

For each phase of flight, the different FltCs that have a probability of being encountered of greater than 10-9 per hour are then identified. Special care should be given also to the transition between different phases of flight and aircraft configuration changes (if any). The FltCs probability is given by combining (multiplying) the probability of the aircraft being in a specific FE, the probability of the aircraft having a FC that affects HQ (not only flight control system failures, but any other, including lift/thrust system failures) and the probability of an AD being experienced.

When there is an interrelationship between the different probabilities, the FE probability will be adjusted to take this into account. For each FltC, the minimum HQR level is assigned based on the requirements derived from SC VTOL. The applicant should then show compliance by using an approved rating tool in actual flight test, or in a simulator that has been validated and shown to be representative for the test.

4.      Minimum ACCEPTABLE HQR

Table 1 describes the different Handling Qualities Rating (HQR) levels and compares them to the System Failure Classification that is contained in MOC VTOL.2510, and to the Cooper Harper Rating Scale.

Exceptional piloting skills should not be required for the achievement of any HQ performance criteria. The evaluation should assess whether Desired or Adequate criteria are met, and the associated workload in terms of physical and/or mental compensation required by the crew.

Table 1: Handling Qualities Ratings definition (Example for Cruise)

Table 2 is an example for the Cruise phase of flight, and shows the minimum HQR for each FltC, defined as a combination of the FE and the level of AD, relative to the probability of the FC being evaluated. Similar tables could be created for the other phases of flight, as the type of FC, most critical from a HQs point of view, could vary depending on the phase of flight. The minimum HQR for each table will not vary across the different tables, but, since the FC, FE and AD levels may vary depending on the phase of flight, including the probabilities of occurrence, it might be beneficial to have different tables or groups of tables depending on the phase of flight.

The different FE are (Table 3): Normal Flight Envelope (NFE), Operational Flight Envelope (OFE) and Limit Flight Envelope (LFE).

The AD level (Table 4) can be Light, Moderate or Severe.

The FC probabilities (Table 5) are in accordance with the aircraft level MOC VTOL.2510 quantitative probability values. Probability values for Probable up to Remote Failure Conditions have been grouped together for table readability reasons, as the minimum HQR would be the same.

It is important to highlight that NOT every combination of AD, FC and FE should be tested.

Table 2 Minimum Acceptable Handling Qualities Rating

The probabilities in Tables 3, 4 and 5 apply when they are considered separately. When obvious interrelationships exist due to the design or the intended or expected operation of the aircraft, the way to address this within MHQRM is to modify the FE probability value. For example, for FltCs with Moderate and Severe AD levels in CRUISE and APPROACH, an atmospheric (windshear) event may require an escape operational procedure that results into entry in the LFE, resulting in a LFE probability of 10⁰ (^{i.e. 1 or certain}). Similarly, an aircraft flying at the boundaries of the NFE, may experience overspeed due to a gust and fall into the OFE, hence the modified FE would be 10⁰ (^{i.e. 1 or certain}). This probability adjustment concept would also apply to FCs where, for example, a loss of warnings or a loss of envelope protection might contribute to excursions outside the NFE or OFE, in which case the flight envelope probability should be increased appropriately. In this latter case, the change of probability will be evaluated case by case and should be agreed with the Agency.

5.      Probability definitions and determination

(f)      Flight Envelope (FE)

The flight envelope probabilities will depend on the aircraft architecture. The automatic envelope protection provisions (if available) and the cues to the crew will be the determining factors.

The flight crew should operate the aircraft by definition in the NFE. Excursions into the OFE and LFE are determined by AD, by transient conditions due to failures (that can have different probabilities based on the design), or by expected human errors.

These probabilities should be adjusted to account for the interrelationship between AD and FC events (section 4).

Applicants should provide probabilities based on the evidence that they have available to substantiate them, and based on their aircraft characteristics.

Visual and aural warnings, or specific aircraft characteristics at the boundaries of the envelopes (vibrations, noise) could grant credits for increasing the probability of remaining within a given FE.

The probability values proposed by the applicant should be substantiated with actual flight test data.

Table 3: Probability of Occurrence of the Flight Envelope (FE)

(g)      Atmospheric Disturbance (AD)

The atmospheric disturbance level ranges from the complete absence of any disturbance up to the atmospheric disturbance level (gusts) that are considered for the structural limits of the aircraft. The AD considered could be different depending on which phase of flight is being evaluated.

Additional steady state relative winds values, for the most critical azimuth, are established to show compliance with the applicable requirements when the aircraft is operating based on ground references (e.g. Take-off, Hover, Landing).

The amplitude of the gusts to be considered for the structural design will be defined in MOC VTOL.2215 “Flight Load Conditions”. Non-symmetric gust cases should be considered when evaluating HQ. The shapes of the gusts may also be a critical factor for HQ and should be evaluated.

The steady state relative wind values are derived from the experience from CS-27, and have been identified as being 17 kt. This value is the minimum to be used for airworthiness approval; applicants may choose higher steady wind values based on market requirements.

The steady wind value should be evaluated only in the phases of flight that are close to the ground. The controllability in steady winds should be demonstrated for all FC in Light AD level (without gusts and turbulence).

The exact values of the gusts are currently not defined for each AD level. Even the related probabilities (X_AD), which are modified in respect to Appendix E to AC25-7D to account for the Urban Environment, will need to be verified by recorded data which are currently not available.

Table 4: Probability of Occurrence Guidelines of Atmospheric Disturbance (AD)

(h)     Aircraft or System Failure Condition affecting HQ (FC)

The Failure Condition probabilities (X_FC) relate to the probability of encountering a Failure Condition which affects HQs. This may include, but is not limited to, the FCS or lift/thrust system. The MHQRM should be linked to the Safety Assessment Process at aircraft level. Feedback should be provided into the different Safety Assessment Elements, such as the Functional Hazard Assessment (FHA), Preliminary System Safety Assessment (PSSA), Fault Tree Analysis (FTA), System Safety Assessment (SSA) or Failure Mode and Effect Analysis (FMEA), and vice versa to check if the assumptions of these Safety Assessment Elements in terms of effect (when the driving factor are HQ) are confirmed by the MHQRM evaluation.

Table 5: Probability of Occurrence Guidelines of Failure Condition affecting HQ (FC)