APPROACH OPERATIONS IN REDUCED VISIBILITY

 

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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