18th annual conference on manual control.pdf - Acgsc.org

constant speed and constant altitude. Under these conditions the pilots can
achieve very consistent spacing with standard deviations of less than 0.I n. mi.
(refs. 5 and 6), which equate to an interarrival time standard deviation of
less than 3 seconds.
As the lead aircraft begins to maneuver (decelerate), even mildly, the
self-spacing performance as reflected by the standard deviations is adversely
affected. For example, reference 2 presents spacing data for two constant
distance spacing segments during which the lead aircraft is performing a
very docile approach. The standard deviations ranged between 0.13 and 0.28 n. mi.,
depending on the display size employed. These deviations are equivalent to
interarrival times between about 3 to 7 seconds. Another study using a similar
task also produced standard deviations in the same 3 to 7 second range.
The most recent studies conducted at Langley have involved relatively
active lead aircraft. In the study of time-based self-spacing techniques,
the simulated lead aircraft was being vectored by ATC onto the final approach.
In the groundspeed resolution study, the lead aircraft was performing an idle
thrust profile descent with a close in turn to final. In both of these studies
the standard deviation of the interarrival times was found to be 7 to 8 seconds.
Figure 18 summarizes our experience to date in terms of the lead aircraft's
effects on self spacing performance. Keep in mind that these are only "ball park"
numbers; the test results from which they are derived have invariably involved
lead-followers of the same type aircraft. This probably leads to better
spacing performance than could be obtained with pairs of different types, which
is to say different performance characteristics. On the other hand, the data
used in deriving the "ball park"numbers have not been massaged to eliminate
particularly poor performance producing constraints such as not having target
groundspeed information. As such, the data base is not necessarily biased,
since there are offsetting features incorporated in it.
Potential
Payoff
Assuming it is possible to achieve a standard deviation of 8 seconds for
interarrival time at the threshold using CDTI in the real world, then we can
estimate the potential payoff with the aid of figure 19.
This figure illustrates the probability density curves for interarrival
times using two different spacing techniques. The standard deviation value
of 18 seconds used for the curve labeled "ground controlled" was taken from
reference 7 and represents the accuracy of today's ATC system. The mean for
the curve in figure 19 has been established'by applying a ground rule that
only 5 percent of the arrivals can have an IAT less than 50 seconds. (This
actually results in a mean of about 79.6 seconds.) The basis for selecting
a specific IAT and buffer size are explained in reference 7. In this paper,
the somewhat arbitrary choice of 50 seconds and 5 percent will serve as an
anchor point for comparing CDTI to present day techniques.
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