HUD With a Velocity (Flight-Path)
Vector Reduces Lateral Error During
Landing
in Restricted Visibility
Örjan Goteman
Scandinavian Airlines
Stockholm, Sweden
Kip Smith and Sidney Dekker
Department of Industrial Ergonomics
Linköping Institute of Technology
The majority of approaches (the segment of flight immediately before
touchdown) performed in U.S. and European civil air transport operation are
conducted as Category I instrument landing system (ILS) approaches. For a
Category I approach the two critical factors when making the decision of
whether to
land are the decision height and the RVR. Decision height refers to the
aircraft’s vertical distance above the runway threshold where the pilot must
make the decision to land or make a go-around. The pilot must be able to see at
least some of the approach or runway lights at the decision height. The
approach lights will then guide the pilot to the runway.
The RVR is a measure of horizontal visibility defined by the length of
visible approach and runway lights in the ambient atmospheric conditions. If
the RVR is too low the pilot will not be able to see any of the approach lights
at the decision height and must make a go-around.
This RVR is expected to allow the pilot to see a visual segment of the
ground that contains enough of the runway approach lights to judge the
aircraft’s lateral position, cross-track velocity, and position in roll when
the aircraft is at decision height.
EXPERIMENT
1
Method
Forty-eight pilots from a major European airline volunteered to
participate. All were qualified to fly the B–737–700 aircraft and had completed
their HUD training for the operator. The HUD training sessions consisted of 1
day of theory and two simulator sessions of 4 hr duration each. Experience on
the B–737–700 varied from 50 hr to more than 1,000 hr. There is every reason to
believe that these participants are representative of the population of
commercial pilots to whom regulators need the data to generalize. Apparatus. We used a CAE
B–737–700 training simulator with aircraft aerodynamics and visual angles valid
for B–737–700 to collect data. This six-axis full-motion simulator is approved
for low-visibility operations down to an RVR of 200 m. The simulator’s visual
system had a field of vision of 180°/40° with a focal distance greater than 10
m.
The head-down instrumentation was a B–737–700 instrument panel in
primary flight display (PFD) configuration with flight director guidance. This
instrumentation was available in both the with-HUD and the no-HUD conditions.
The HUD installed in the simulator was a Rockwell-Collins Flight
Dynamics HGS–4000® (Head-up Guidance System), certified for low-visibility
operations down to and including an RVR of 200 m. The HUD symbology and
functionality used in the experiment met the production-line standard
specification for the instrument meterological conditions (IMC) mode used when
conducting Category I ILS and non-precision approaches (see Figure 6). This
mode was deliberately chosen to improve the external validity on the form of
operational usability of the study. In normal operations, the vast majority of
ILSs are only approved for Category I operations. The HUD provided conformal
display of flight path (velocity) and flight-path guidance. The flight path was
displayed as a circle with slanted wings. Flight-path guidance was displayed in
the form of a ring inside the flight-path symbol. Flight-path guidance was not
available in the conventional head-down instrumentation.
The radio navigation facilities simulated in the study conformed to the
ICAO (1996) standard for ILS radio navigation aids for Category I approaches
transmitting a radio beam for both vertical and lateral reference. The
integrity of the transmitted beam is guaranteed to keep the aircraft within
allowable airspace, safe from
Gulfstream
GVII-G500 in level flight at 40,000 feet, Mach 0.90.
obstacles
down to 200 ft height, corresponding to lowest allowable decision height. Approaches in Experiment 1 were flown to a
simulated runway with a system of approach lights 900 m in length. This
length falls within the full facilities system category of
European aviation regulation (JAA, 2004b).
Procedure.
Each pilot manually flew two approaches
using standard operating procedures of the airline. The scenarios started as a
6 nm final to the runway in lower than standard or standard RVR in a simulated
10 kt left crosswind. In the with-HUD condition, the pilots kept the aircraft
on lateral and vertical by following the flight-path guidance ring with the
flight-path symbol on the HUD. At 50 ft above the runway threshold, the
guidance cue was automatically removed and the pilots performed the landing
flare using external visual cues in conjunction with the HUD flight-path
symbol. The HGS–4000® IMC mode incorporating automatic removal of the guidance
cue was deliberately chosen to ensure that the pilots could not attend solely
to the HUD symbology in the with-HUD conditions.
In the no-HUD condition the pilots kept the aircraft on lateral and
vertical track by following the flight director bar guidance. At decision
height they continued the approach and landing using the external cues only.
All approaches were recorded to determine approach success. Approach and
landing plots for approaches ending with a landing were printed using the
aircraft’s center of gravity as the reference to determine the size of the
touchdown footprint.
Results
Approach
success rate. Each of the 48 pilots
attempted two approaches.
Thirty-seven made two successful landings, one with HUD and one without.
Three made go-arounds in both conditions. Two pilots landed with the HUD and
made go-arounds without the HUD; six landed without the HUD and made go-arounds
with the HUD.
The McNemar change test is the appropriate statistical procedure for
testing the null hypothesis that pilots were equally likely to (a) land with
the HUD and go-around without it, and (b) land without the HUD and go-around
with it (Siegel & Castellan, 1988). Because the observed test statistic,
calculated from the values given earlier (2 and 6), is 1.125 and is less than
the criterion, x2 (.05, 1)
= 3.84, we cannot reject the null hypothesis. Accordingly we infer that HUD use
had no impact on approach success rate.
Touchdown
performance. As noted previously,
11 of the 48 pilots conducted one or two go-arounds. The simulator failed to
capture the location of the landing footprint for another 7 pilots. As a result
the data set for comparing the touchdown footprints across the HUD and no-HUD
conditions consists of 30 pairs of approaches. Of these 30, 15 were flown using
the HUD in the first approach and 15 using the conventional head-down
instruments (no-HUD) in the first approach; 15 were flown in standard RVR (550
m) conditions and 15 in lower than standard RVR conditions (450 m).
Lateral touchdown performance was measured as the absolute lateral
deviation from the runway centerline at touchdown. The data for the main effect
of HUD use are shown in Figure 7. Landings were closer to the centerline when
pilots used the HUD. The two-factors repeated-measures analysis of variance
(ANOVA) revealed a strong effect for HUD use on the lateral component of the
touchdown footprint, F(1, 28) = 9.05, p < .006, n2 = .12 indicating a power of .80 at α = .05.
The main effects for RVR and order of HUD condition were not
significant. There was, however, a marginally significant interaction between
HUD use and the order
EXPERIMENT 2
Method
The method, procedure, and design used in the second experiment were
identical to those used in the first experiment with the few exceptions
discussed here. The different criteria for standard RVR across facility types
preclude collapsing and analyzing the data as a single experiment.
Participants. Forty-five pilots
from the same major European airline volunteered to participate. None of the
volunteers had participated in Experiment 1. All were qualified to fly the
B–737–700 aircraft and had completed theirHUDtraining for the operator.
Experience on the B–737–700 varied from more than 50 hr to more than 1,000 hr.
Simulated ground
facilities. Approaches
in Experiment 2 were flown to simulations of a runway with 420mof approach
lights. This length falls within the intermediate facilities category of
approach lights as defined by European aviation regulations (JAA, 2004b).
Results
Approach success
rate. Each of
the 45 pilots attempted two approaches.
Thirty-two made two successful landings, one with HUD and one without.
Four pilots made go-arounds in both conditions. Three pilots landed with the
HUD and made go-arounds without the HUD; six pilots landed without the HUD and
made go-arounds with the HUD. Once again we used the McNemar change test to
test the null hypothesis that pilots were equally likely to (a) land with the
HUD and go-around without it, and (b) land without the HUD and go-around with
it. Because the observed test statistic, calculated from the values given
previously (3 and 6) is 0.44 and is less than the criterion, x2 (.05, 1) = 3.84, we infer that HUD use had no impact
on approach success rate.
Touchdown
performance. As noted
earlier, 13 of the 45 pilots conducted one or two go-arounds. The simulator
failed to capture the location of the landing footprint for another 4 pilots.
As a result, the data set for comparing the touchdown footprints across the HUD
and no-HUD conditions consists of 28 pairs of approaches.
Of these 28 pairs of approaches, 9 were flown using the HUD in the first
approach and 19 using the conventional head-down instruments (no-HUD) in the first
approach; 14 were flown in standard RVR (700 m) conditions and 14 in lower than
standard RVR conditions. The opportunistic nature of data collection precluded balancing
the order of HUD use.
The main effect for HUD use, shown in Figure 9, is the only factor in
the two-factor repeated-measures ANOVA to achieve statistical significance. As
in Experiment 1, the effect for HUD use on the lateral component of the
touchdown footprint is strong, F(1, 26) = 14.9, p <
.001, n2 = .10, indicating a
power greater than .70. Once again, landings were closer to the centerline when
pilots used the HUD. Because participant order was not fully counterbalanced,
it is not possible to assess the potential for asymmetric transfer effects. As
in Experiment 1, there was no significant effect for HUD use or RVR on the
longitudinal component of the touchdown footprint. Once again, the difference
in the observed variances across conditions of HUD use would be significant at α = .10 if the data sets were independent.
There are three findings. First, HUD use per se did not influence the
pilots’ decision to land or go-around at the decision height. The lack of an
effect of HUD suggests that the additional information in the HUD did not
distract the pilots’ attention or interfere with their decision making during
the most critical portion of the approach and landing sequence. Second, HUD use
significantly reduced the size of the lateral component of the touchdown
footprint for all RVR conditions.
Arguably it can be said that the difference between the HUD and the no-HUD
conditions lay in the presence of a conformal flight-path vector in the pilots’
primary field of view during the landing. Third, in contrast to its effect on the
lateral component of the touchdown footprint, it appears that the HUD did not
influence the size of the longitudinal footprint. The first two findings
conform to our hypotheses. Here we reexamine our hypotheses about the impact of
HUD use on the touchdown footprint and offer an explanation for its
differential impact on the lateral and longitudinal components.
The HUD largely eliminates uncertainty about drift. The addition of a conformal
flight-path vector projected over the runway provides instantaneous feedback
about aircraft drift and actual flight path. The additional information enables
precise control of the flight path during approach and landing and reduces the variance
in lateral displacement practically to nil.
Control of the longitudinal component of the touchdown footprint is
largely an effect of how pilots handle the aircraft’s energy (operationally
manifested as sink rate) in the final seconds before landing. The pilot uses
information provided by the optic flow from the looming runway to control the
aircraft’s energy (Lee, 1974). It is important to note that pilots of large
commercial air transport aircraft are also aided by radio altimeter callouts
that count down from 50 ft to 0 ft (runway contact) in 10-ft decrements. The
initiation of the landing flare has been shown to be a function of time to
contact (Mulder, Pleijsant, van der Vaart, & van Wieringen, 2000). A small change in
the timing of a landing flare at the nominal glide slope of 3° results in large
longitudinal differences. The HUD mode
used in the experiments provided no flare guidance and no additional
information that could be used to guide the pilots when to initiate the landing
flare. So, if the pilots are relying more on timing of a flare maneuver than on
velocity cues from the HUD to initiate the flare, it is easy to understand why
we failed to detect any effect of the tested HUD with a flight-path symbol on
longitudinal touchdown performance. It remains to be seen whether similar
results are found for HUD modes with flare guidance, and which visual
representations are actually most effective in prompting pilots to reduce sink
rate at the optimal height above the runway.
CONCLUSIONS
Data from the experiments reported here show that HUD with a conformal
velocity symbol (flight path) improved lateral touchdown performance, likely
because the conformal flight-path vector in conjunction with the visual ground segment
makes it easier for pilots to determine and correct for aircraft drift. We did
not find an effect of HUD use on longitudinal touchdown performance, probably
because the pilots flared the aircraft using a time-to-contact strategy, rather
than using the flight-path vector available in the HUD modes studied here. The
beneficial effects of HUD use on landing performance were seen in both in
standard and lower than standard RVR conditions in both experiments, which
implies that the minimum RVR for approaches using an HUD could be set lower
than for approaches without an HUD.
BACKGROUND
“Where
is the nose of the aircraft?”
It
is shown as a flat vee with wings and is called a boresight.
“Where
is the horizon?”
It
is below you, as shown by the synthetically drawn terrain.
The
pilot’s primary focus is no longer where the aircraft is pointed, but where it
is going. That is represented by the Flight Path Vector (FPV) and is drawn as a
circle with small wings and a tail. The white horizontal line can be thought of
as a horizon that has been adjusted to consider the altitude of the aircraft,
as if the earth’s diameter has been increased. In some HUDs it is called a Zero
Pitch Reference Line. You maintain level flight by placing the FPV on the Zero
Pitch Reference Line.
The
U.S. rules are given in 14 CFR 91.176 and are further explained in Advisory
Circular 90-106A. If you meet the requirements of section (b) of that FAR, you
can operate using EFVS to 100 feet above the touchdown zone elevation, at which
point you must take over what the regulations call “natural vision” to complete
the approach and landing. If you are operating under part 91K, 121, 129, or
135, you will need a management or operations specification. If you meet the
requirements of section (a) of that FAR, you can operate using EFVS to
touchdown and rollout. You will need a letter of authorization, management
specification, or operations specification. AC 90-106A lists 1000 RVR as an
adequate flight visibility and authorizations are normally written with this as
a minimum.
Recent flight experience: EFVS. Except as provided in paragraphs
(f) and (h) of this section, no person may manipulate the controls of an
aircraft during an EFVS operation or act as pilot in command of an aircraft
during an EFVS operation unless, within 6 calendar months preceding the month
of the flight, that person performs and logs six instrument approaches as the
sole manipulator of the controls using an EFVS under any weather conditions in
the category of aircraft for which the person seeks the EFVS privilege. The
instrument approaches may be performed in day or night conditions; and
(1) One approach must terminate in a full stop landing; and
(2) For persons authorized to exercise the privileges of
§ 91.176(a), the full stop landing must be conducted using the EFVS.
Source: 14
CFR 61, ¶61.66 (d)
comparison of flight
director symbology
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