The operational environment

A typical operational requirement will include a definition of the environmental con­ditions in which the helicopter needs to work in terms of temperature, density altitude, wind strength and visibility. These will then be reflected in an aircraft’s flight manual. The requirements wording may take the form: ‘this helicopter must be able to operate (i. e., conduct its intended mission, including start-up and shut-down) in the following conditions – 5000 ft altitude, 15°C, wind speeds of 40 knots gusting to 50 knots, from any direction, in day or night’. This description defines the limits to the operational capability in the form of a multidimensional envelope.

Throughout the history of aviation, the need to extend operations into poor weather and at night has been a dominant driver for both economic and military effectiveness. Fifty years ago, helicopters were fair weather machines with marginal performance; now they regularly operate in conditions from hot and dry to cold, wet and windy, and in low visibility. One of the unique operational capabilities of the helicopter is its ability to operate in the NoE or, more generally, in near-earth conditions defined in Ref. 2.1 as ‘operations sufficiently close to the ground or fixed objects on the ground, or near water and in the vicinity of ships, oil derricks, etc., that flying is primarily accomplished with reference to outside objects’. In near-earth operations, avoiding the ground and obstacles clearly dominates the pilot’s attention and, in poor visibil­ity, the pilot is forced to fly more slowly to maintain the same workload. During the formative years of ADS-33, it was recognized that the classification of the quality of the visual cues in terms of instrument or visual flight conditions was inadequate to describe the conditions in the NoE. To quote from Hoh (Ref. 2.4), ‘The most crit­ical contributor to the total pilot workload appears to be the quality of the out-of – the-window cues for detecting aircraft attitudes, and, to a lesser extent, position and velocity. Currently, these cues are categorized in a very gross way by designating the environment as either VMC (visual meteorological conditions) or IMC (instrument meteorological conditions). A more discriminating approach is to classify visibility in terms of the detailed attitude and position cues available during the experiment or proposed mission and to associate handling qualities requirements with these finer grained classifications.’ The concept of the outside visual cues (OVC) was introduced, along with an OVC pilot rating that provided a subjective measure of the visual cue quality. The stimulus for the development of this concept was the recognition that handling qualities are particularly affected by the visual cues in the NoE, yet there was no process or methodology to quantify this contribution. One problem is that the cue is a dynamic variable and can be judged only when used in its intended role. Eventually, out of the confusion surrounding this subject emerged the usable cue en­vironment (UCE), which was to become established as one of the key innovations of ADS-33. In its developed form, the UCE embraces not only the OVC, but also any artificial vision aids provided to the pilot, and is determined from an aggregate of pilot visual cue ratings (VCR) relating to the pilot’s ability to perceive changes in, and make adjustments to, aircraft attitude and velocity. Handling qualities in de­graded visual conditions, the OVC and the UCE will be discussed in more detail in Chapter 7.

The MTE and the UCE are two important building blocks in the new parlance of flying qualities; a third relates to the aircraft’s response characteristics and provides a vital link between the MTE and UCE.