FUNDAMENTACIÓN DE LA ORIENTACIÓN EN LENGUAS EXTRANJERAS

4.1.1. Limitations of Previous Studies

A number of previous studies have sought to identify the parameters that pilots use to determine the flare initiation point. Whilst it is generally accepted that initiation is triggered by arrival at a threshold value of a certain parameter, there is no clear consensus as to the nature of this parameter. The body of evidence from previous studies has focused on the following as potential initiation cues:

 Height above runway : The distance between the main landing gear and the surface of the runway (Figure 53a)[10, 49, 50].

 Time-to-contact with runway : The predicted time to main gear touchdown at the current descent rate (Figure 53b) [3, 53, 55].

 Runway width angle : The angle formed by the apparent expansion of the edges of the runway at the aiming point as the aircraft approaches it (Figure 53c). This parameter can be expressed as a function of the line-of-sight range between the pilot and the runway aiming point, , as shown in Equation 22 [50, 56].

(

) Equation 22

It should be noted that is also a function of runway width, . This parameter varies from one airfield to another, from approximately 23m for a small (general aviation) aerodrome 43m for a regional airport in the UK, to 50m for an international hub [58]. As such, the value of at which the flare is initiated would not necessarily be expected to remain constant over approaches to different runways, potentially limiting the value of

as a universal cue for flare initiation.

 Runway side angle rate ̇ : As the aircraft approaches the runway, the angle described by its sides increases. It has

previously been proposed that the rate of change of this angle, ̇ , could be used as a cue for flare initiation (Figure 53d). ̇ can be defined as a function of the height of the height of the pilot’s eye above the runway, , and (Equation 23) [57].

̇ (

) Equation 23

Equation 23 shows that, as was the case for , ̇ is a function of the geometry of the runway being approached. As such, it is

subject to the same potential limitations as in terms of being the basis of a universal flare initiation cue.

Figure 53. Potential cues for flare initiation.

Studies investigating the parameters shown in Figure 53 are discussed in more detail in Section 2.3.3. It was found that each of these studies featured limitations which reduced the similarity of the experiment to the real-world scenario. Firstly, Refs 50, 53 and 55 did not require the pilot to actually perform the flare manoeuvre, but rather to initiate a predefined flare by pressing a button. This method was used in order to simplify the experiment by removing the requirement for piloting ability, but is clearly not representative of the real task. Additionally, the predefined manoeuvre used by Ref. 53 was a simple exponential flight path, whereas Ref. 3 previously demonstrated that piloted flares do not usually conform to such a trajectory. A related limitation in Refs. 50, 53 and 55 was that the participants involved in the experiment were non-pilots. Although it is not always practical for large numbers of participants, when conducting an

a) b)

investigation into a piloting task, clearly it is preferable to use qualified pilots as test subjects.

Whereas Refs. 50, 53 and 55 investigated the effect of varying approach angle on flare initiation strategy, Refs. 56 and 57 used only a single approach angle; a “normal” 3° scenario. Although it is true that standard operating procedures [10] define a standard approach as being approximately 3°, the dynamics of the aircraft and the atmospheric conditions can often result in an unintentional variation in approach angle (as reported in Ref. 51). Additionally, the geography of certain airfields necessitates non-standard approaches. For example London City Airport mandates a nominal approach angle of 5.5° due to noise abatement requirements related to its urban location [58]. For this reason, it is necessary to include variation in approach angle when investigating flare initiation strategy, rather than assuming the nominal 3° scenario.

The visual scene presented to the pilots in Refs. 50, 53 and 55 were simplified or sparse. A comparison between such a simplified scene and one which is typically used in the Heliflight simulator at UoL is shown in Figure 54.

Figure 54. Comparison of visual scene used by Ref. 53 (a) and a typical scene from the

Heliflight simulator (b).

One of the fundamental principles of the ecological approach to visual perception is that the observer makes use of the information received from the optical flow. If the content of the flow were to become in some way reduced (as in Figure 54a), it seems reasonable to conclude that the perceptual mechanisms of the pilot would be affected. Indeed, Ref. 50 demonstrated that the flare initiation strategy used by the pilot changed when the scene content was varied. For this reason, a visual scene which is closely representative of the real-life situation is clearly desirable. In addition, a number of previous studies [50, 55] restrained the pilots’ heads through the use of a chin rest to restrict their fields of view. Although this is a common feature of psychological experiments, it is clearly not representative of the real-life cockpit environment, in which the pilots are able to move their heads freely.

A key objective of this experiment, therefore, was to address the limitations of previous studies whilst investigating flare initiation. In order to achieve this, the following features were included in the experiment reported in this Chapter:

 Four qualified pilots of varying experience levels were used for the simulated flight tests (Section 4.2.2).

 A detailed visual scene including a representative runway with appropriate markings was used (Figure 54b).

 The field of view and cockpit environment were representative of the real-life scenario, in that the pilot’s head was not restrained and the visual scene was presented through a wrap-around visual system.

 The pilots were required to control the aircraft throughout each flight, including manually performing the flare manoeuvre.

4.1.2. Hypothesis

A number of previous studies at UoL have provided compelling evidence for the use of tau-based control strategies for a variety of piloting tasks. For this reason, the hypothesis of this experiment was that pilot make use of a constant value of time-to-contact with the runway ( ) to initiate the flare. In addition, it was hypothesised that this strategy would remain unchanged for a range of approach angles (and hence vertical velocities), which would demonstrate the universal nature of such a strategy. Finally, it was hypothesised that directing pilots to initiate the flare at a threshold value of would result in superior flare performance than directing flare initiation based on other constant cues. The method used to command the flare initiation point is described in Section 4.2.2.

4.2. Experimental Set-up