The experiment presented in the Chapter was undertaken to test the hypothesis that pilots use a single, invariant value of to initiate the flare manoeuvre when approaching from a variety of angles. A secondary hypothesis was that performance would be improved when the pilots were directed to initiate the flare at a constant value of compared to at a
constant value of or . Following analysis of the results, the conclusions can be summarised as follows:
Both and ̇ were shown to exhibit an approximately linear relationship to for all four pilots. As such, it is concluded that a single, invariant value of either of these parameters is not used as the basis of a flare initiation strategy. Conversely, the value of was shown to be relatively consistent across the range of approach angles. This finding supports the primary hypothesis of the experiment.
was shown to provide an appropriate flare cue for cases in which the aircraft was positioned accurately (longitudinally) relative to the touchdown marker on the runway. However, for cases in which the aircraft overflew the touchdown marker, the
based cue did not provide appropriate guidance. The pilot with the most recent and relevant experience of the real-world equivalent task was shown to have the most consistent performance in terms of longitudinal position at flare initiation. As such, the value of was relatively consistent for this pilot (though not to the same extent as ). A further potential limitation of a flare initation strategy based on a constant value of is this parameter’s dependence on the width of the runway in question. Indeed, this was highlighted by Ref. 55 as a potential source of accidents, as pilots incorrectly apply an initiation strategy learned at one airfield whilst approaching an unfamiliar runway.
Flares initiated at a constant value of (4 sec) were shown to be more successful than those at constant values of or for 3 of the 4 pilots tested. This was measured in terms of against
the performance criteria defined by Ref. 71. Such a finding supports the previously stated secondary hypothesis.
When flying the manoeuvre without flare initiation guidance, the four pilots were not always able to achieve desired performance in terms of vertical velocity at touchdown, . This demonstrates the high level of task difficulty of this task, supporting the findings of Ref. 55.
The recommendations for further work arising from this Chapter are summarised as follows:
In terms of the development of pilot aids, the constant based initiation strategy was shown to be appropriate, and as such should be integrated into the design of any future tau-based pilot aid for the flare manoeuvre.
The experiment focussed on the timing of the flare initiation, and its effect on the subsequent manoeuvre. However, the actions of the pilot between initiation and touchdown also clearly have an effect on flare performance. For this reason, the piloting strategy during the flare itself should be investigated further.
Although this experiment sought to address the limitations of some of the previous studies outlined in Chapter 2, it should be noted that a number of simplifications were still incorporated into the design of the experiment. Most notably, the lateral dynamics of the aircraft and wind / turbulence effects were not modelled. It is therefore recommended that a further iteration of this experiment could investigate the effects of these factors on the selection of flare initiation strategy. Additionally, a larger sample of pilots or
an increased number of test point repetitions would allow conclusions to be made as to the statistical significance of the findings.
Pilots of varying levels of experience were used during the course of this investigation, and some differences in performance and strategy implementation were evident. However, it would be necessary to test additional pilots with a broader range of experience levels to form any firm conclusions on the effects of experience on flare initiation strategy. It may also be of interest to test non-pilots, to enable a direct comparison with the results of Refs 50, 51, 53 and 55.
The tests used a single runway of dimensions representative of a large civilian airfield. Two of the previously proposed flare
initiation cues, and ̇ were shown to be defined as functions of the runway geometry. Ref. 8 suggested that pilots attempting to execute a -based flare initiation strategy (learned at their home airfield) at an unfamiliar airfield with different runway geometry is a potential cause of accidents. This could be investigated by repeating the experiment described in Chapter 4 with a variety of runway geometries. If is truly runway- independent, then a -based flare initiation strategy would be valid for any runway (or indeed any surface). Such a finding would further support the appropriateness of the tau parameter for use in this area of flight.
C h a p t e r 5
PILOT AID DEVELOPMENT
The previous Chapter presented an investigation into the factors used to determine the appropriate flare initiation point for a large transport aircraft, and one of the key conclusions was that selection of an appropriate initiation point is required to achieve a adequate performance. However, appropriate flare initiation alone does not necessarily guarantee a successful touchdown, implying that the actions of the pilot during the flare itself can influence the outcome of the manoeuvre. For this reason, Chapter 5 details an investigation into the piloting strategies used in the period between flare initiation and touchdown. A related investigation was previously conducted as part of Ref. 3, which included the initial stages of development and evaluation of a pilot aid for the flare manoeuvre. This demonstrated that a tau-based strategy provided appropriate guidance for this phase of flight, and provided recommendations for further development. The work presented in this Chapter, therefore, builds upon these recommendations to enable the development of a new pilot aid, featuring both a novel tau-based guidance strategy and a novel method of implementation.
5.1. Background
5.1.1. Previous Findings
The high proportion of fatal airliner accidents in the approach and landing phase of flight, reported in Chapter 2, indicates that there is scope for improvement of safety in this area. As such, Ref. 3 reported on the development and evaluation of a tau-based guidance algorithm for the flare manoeuvre. The findings of this previous study are briefly summarised in this Section.
The motion gap selected as the control parameter for the guidance strategy in Ref. 3 was the distance between the aircraft c.g. and the runway. It had previously been demonstrated that pilots made use of the constant ̇ strategy for the flare, and so this was to be commanded by the guidance algorithm. Specifically, this was achieved by commanding a constant value of rate of change of time to contact with runway ( ̇ ) for each of the two phases of the type 1 flare (Figure 67).
Figure 67. Schematic of the type 1 flare strategy.
Figure 67 shows that a constant vertical approach velocity ( ̇ =1) was reduced to the touchdown velocity by selecting some value of ̇ <1, which results in a continuous but not necessarily constant deceleration to touchdown. For the algorithm developed in Ref. 3, the value of ̇ for the flare phase was set to 0.75 based upon subjective pilot opinion. In order to evaluate this concept, the guidance strategy was used to drive the set of HUD symbology shown in Figure 68. This was achieved by converting the constant values of ̇ into a flight path angle command as shown in Equation 12:
⌊ √
̈
̇ ⌋
Equation 26
where is the vertical flight path angle, is the pitch angle, is the roll angle and is the forward velocity of the aircraft. The value of was thus derived from parameters relating to the current position / orientation of the aircraft and the predefined guidance strategy. It was then used to determine the onscreen position of the guidance cue symbol, which is shown in Figure 68 (symbol 2).
Figure 68. HUD symbology for piloted evaluation in Ref. 3.
Figure 68 shows both the guidance cue (2) and the Flight Path Vector (FPV)(5) symbols. The FPV displayed the current flight path angle of the aircraft, which enabled the pilot to determine whether the current trajectory of the aircraft matched that commanded by the guidance cue. This was achieved by application of longitudinal stick inputs, which caused the FPV symbol to move up (aft input) or down (forward input) the screen. The
other symbology shown in Figure 68 was used to provide additional primary flight information to the pilot. The purpose of this was to emulate some of the secondary functions of the in-service example HUD (Section 5.3.2) so as to enable a fair comparison between the respective guidance cues.
The results of Ref. 3 demonstrated that, in good visual conditions, the novel tau-based algorithm produced results comparable to the in-service example HUD. There was also an improvement in performance in a degraded visual environment compared with a baseline case using only a Head Down Display (HDD); however this was not as consistent as for the in-service example. This suggests that tau-based guidance can be appropriate for use in the flare manoeuvre. However, two key limitations of this novel tau-based flare algorithm were identified by Ref. 3:
The type 1 strategy was selected as the basis of the flare guidance algorithm as it was shown to produce more predictable performance when flown manually. It was also deemed to be the simpler of the two strategies to implement as it features two distinct phases rather than three. However, the type 2 flare strategy was shown to result in consistently lower, albeit less predictable, touchdown velocities.
The display symbology was not initially intuitive, resulting in some confusion as to which symbol the pilot should be following. This was reflected in the pilots’ assessment of the display, resulting in poorer controllability and workload ratings due to the ergonomics of the symbology rather than the behaviour of the guidance cue.
5.1.2. Hypothesis
The objectives of this study were divided into two elements, the first of which was to implement a tau-based flare guidance algorithm to command the type 2 strategy. It was hypothesized that, based on the findings of Ref. 3, such an approach could also provide appropriate guidance for the flare manoeuvre. It was proposed that the use of the type 2 strategy could give a performance benefit compared to the type 1, as flares of this type were shown to result in consistently lower touchdown velocities when performed manually. Secondly, the display symbology used for piloted evaluation of this flare algorithm was to emulate the “look and feel” of an in-service example as closely as possible, in order to allow a more objective comparison between the guidance of the novel and in-service algorithms.
5.2. Display Development