Flying qualities: subjective assessment and other topics

If test manoeuvres are too dangerous for a skilled test pilot to perform in a tightly controlled environment, it is unreasonable to expect the user to fly such manoeuvres in an unfamiliar, unfriendly environment in the fog of war.

(Key, 1993)

7.1 Introduction and Scope

While objective measurements and assessment are necessary for demonstrating com­pliance with quality standards, they are still not sufficient to ensure that a new he­licopter will be safe in achieving its operational goals. Gaps in the criteria due to limited test data, and the drive to extend operations to new areas, continue to make it vital that additional piloted tests, with a subjective orientation, are conducted prior to certification. A further issue relates to the robustness of the criteria and an aircraft’s flying qualities at higher levels of performance. The point has been made on several occasions that criteria in standards like ADS-33 represent the minimum levels to en­sure Level 1 in normal operation. A good design will do better than just meet the objective Level 1 requirements, and the absence of upper limits on most of the han­dling parameters means that there is practically no guidance as to when or whether handling might degrade again. An aircraft will need to be flight tested to assess its flying qualities in a range of mission task elements MTEs, throughout its intended operational flight envelope (OFE), and including operations at the performance lim­its to expose any potential handling cliff edges. During such testing, measurements will be made of aircraft task performance and control activity, but there is, as yet, no practical substitute for pilot subjective opinion. The measurement and interpreta­tion of pilot opinion is a continuing theme throughout this chapter but is exclusively the subject of Section 7.2, where a range of topics are covered, including handling qualities ratings (HQRs), MTEs and the design and conduct of a handling qualities experiment.

Section 7.3 deals with a selection of what we have described as special flying qualities, including agility, carefree handling and flight in poor visual conditions.

One of the areas omitted from the comprehensive treatment of objective assess­ment in Chapter 6 was the requirements for pilot’s inceptors or controllers. The issues surrounding the assessment of quality for inceptors, particularly sidesticks, are so pilot centred that coverage in this chapter was considered more appropriate; Section 7.4 deals with this topic.

For both military and civil helicopters, the potential improvements in flying qual­ities offered by active control technology (ACT) through the almost infinite variety

of response shaping, where computers take on the ‘compensation’, have prompted a more serious examination of the benefits of improved flying qualities to safety and performance. Helicopters’ accidents and incidents due to so-called pilot error are still far too high and many can be attributed to poor flying qualities in a broad sense. Even those accidents caused by system failures can ultimately be attributed to degraded flying qualities, as the pilot struggles to fly a disabled aircraft to a safe landing. With these ideas in mind, Section 7.5 examines the contribution of flying qualities to per­formance and safety by viewing the pilot as a system element, with the potential of failing when under ‘stress’, and outlines a new approach to quantifying the risk of failure.

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