quinta-feira, 28 de julho de 2022

LATERAL ERROR DURING LANDING

 


Conventional head-down display X Head-up display (HUD)

Source:

Örjan Goteman

Scandinavian Airlines Stockholm, Sweden

 

Kip Smith and Sidney Dekker

Department of Industrial Ergonomics Linköping Institute of Technology

References

Refer to FAA Advisory Circular 90-106A, issued 3/2/17

Nichol, Ryan J., “Airline Head-up Display Systems: Human Factors Considerations”. International Journal of Economics and Management Sciences, 4:248, May 3, 2015. https://www.omicsonline.org/open-access/airline-headup-display-systems-human-factors-considerations-2162-6359-1000248.php?aid=54170

 

AC No: 90-106A 2017

AC No: 20-167A 2016

AC No: 25-118 2014

AC No: 90-106A 2017





Third-generation aviation HUDs use optical waveguides that produce images directly in the combiner, without the need for a projection system. Some of the latest HUD systems use a scanning laser, which can display images and video on a clear transparent medium, such as a windshield.

It is possible that, during approach and landing, the HUD might affect the pilot’s ability to assimilate outside cues at the decision height, thereby reducing the success ratio for landings using an HUD.

HUD use reduced the width of the touchdown footprint in all tested visibility and lighting conditions, including visibility below the minimum allowed.

HUD use had no effect on the length of the touchdown footprint.

How ambient RVR affects approach and landing operations.

HUD USE IN COMMERCIAL FLIGHT OPERATIONS

A computer-generated aircraft flight-path and energy symbols presented onto a transparent screen in the pilot’s primary view.

HUDs replicate the information on the pilot’s conventional flight instruments, showing aircraft attitude, speed, altitude, and heading, and containing a flight-path symbol showing the aircraft velocity.

Conformal HUDs

A conformal HUD with a flight-path symbol can explicitly show the pilot where the aircraft is going relative to the surrounding world.

A pilot flying with conventional flight instruments must infer the aircraft’s flight path from a synthesis of the H-angle (Lintern & Liu, 1991), optical flow (Gibson, 1986), and possibly also the relative perspective gradient (Lintern, 2000).

Comparisons between HUDs and head-down displays in manual flight have found that conformal HUDs use improved track, speed, and altitude maintenance (Lauber, Bray, Harrison, Hemingway, & Scott, 1982; Martin-Emerson & Wickens, 1997).

The civil aviation community assumed that these HUD performance advantages over conventional head-down instrumentation could reduce the number of approach and landing incidents and accidents (Flight Safety Foundation, 1991).

Two well-documented problems associated with approach and landing: visual approaches to runways without radio navigation aids or with unreliable navigation aids, and the transition from instrument to external visual cues for landing in low visibility (e.g., Newman, 1995).

Pilot performance during landing in low-RVR conditions where transitioning from instrument to external cues for maneuvering is an issue.

The presumed sources for the advantage in flight-tracking performance for the HUD are that it eliminates the need for the pilot to move his or her gaze from head-down instruments to the outside world to look for maneuvering cues (Stuart, McAnally, & Meehan, 2003) and it minimizes scanning requirements (Mar[1]tin-Emerson & Wickens, 1997). The transition from head down to the outside world requires a change in visual accommodation (e.g., the visual depth of field changes from less than 1 m to infinity). Because conformal HUD symbology is focused at infinity, HUD use eliminates the need for and time demand of visual accommodation, simplifying the pilot’s task.

Cognitive tunneling

One possible negative effect of HUD in the landing situation is that inserting a glass plate with symbols in front of a pilot may affect his or her ability to visually acquire the approach lights, which is necessary to continue the approach below the decision height.

The light transmission through the HUD is not 100%. A commercial HUD will let about 85% to 90% of the incoming light pass through the glass plate. A detrimental effect of HUD use during the landing would then show up as a lower success ratio for HUD than for a conventional flight deck without HUD.

The segment of flight immediately before touch[1]down performed in U.S. and European civil air transport operation are conducted as Category I instrument landing system (ILS) approaches.

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 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.

Missed approach

It  is part of normal operations (and formal procedures), it adds an undesired additional risk(International Civil Aviation Organization [ICAO], 1993).

The approach light lengths differ from runway to runway. Geographical constraints sometimes make the standard full length of 720 m (Full Facilities) impossible to achieve. Fewer approach lights means less guidance and later contact with the approach lights during the approach.

For example, to commence a Category I ILS approach to a runway equipped with a 720 m length of approach lights, the RVR measured at the runway must not be less than 550 m (Federal Aviation Administration [FAA], 2002; Joint Aviation Authorities [JAA], 2004b). 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.

Apart from the obvious effect that the approach lights will come in view later with lower RVR, other effects of shorter approach lights lengths could also come into play. If runway length has been shown to influence the perceived descent path (Lintern & Walker, 1991) it is also possible that a reduced length of approach lights can have similar effects, adding a source of uncertainty to the vertical control of the aircraft.

During landing the pilots have to concurrently process both outside cues and HUD cues to get any benefit from the HUD. The operational benefit from a reduction in touchdown variability could be that regulations would allow approach operations using HUD in lower than standard RVR conditions. The current operating minima were not set bearing HUD operations in mind and may be too restrictive for operations using HUD, a fact that the existing legislative text acknowledges (JAA, 2004b).

Setting RVR for approach too low will ultimately reduce approach success rate. In low RVR with very few external cues available at decision height, there is a risk that the pilots will focus their attention on the HUD symbology to the extent that they might not perceive the few visible approach lights at decision height. Pilots who do not pick up the out[1]side cues may thereby initiate a go-around when the approach actually could have been continued, an outcome that is not desirable from an operational stand[1]point.

The effect of HUD on touchdown performance for two different approach lights conditions as defined in the European regulations:

F     Full Approach Light facilities (≥ 720 m) and

b     Intermediate Approach Light facilities (420 to 719 m; JAA, 2004b).


EXPERIMENT 1

Experience on the B–737–700 varied from 50 hr to more than 1,000 hr.

It was 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 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 meteorological conditions (IMC) mode used when conducting Cate[1]gory I ILS and non-precision approaches.


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.

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.

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[1]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.

Touchdown performance

11 of the 48 pilots con[1]ducted 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.





EXPERIMENT 2

Approaches in Experiment 2 were flown to simulations of a runway with 420 m of approach lights. This length falls within the intermediate facilities category of approach lights as defined by European aviation regulations (JAA, 2004b).

Each participant flew one approach with HUD and one approach without HUD to a runway with a system of approach lights of 420 m length. The between-subject variable was RVR at two levels, a standard minimum RVR (700 m) for intermediate facilities and a lower than standard minimum RVR (550 m to 600 m).

Forty-five [45] 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 their HUD training for the operator. Experience on the B–737–700 varied from more than 50 hr to more than 1,000 hr.

Each of the 45 pilots attempted two approaches.

Thirty-two made two successful landings, one with HUD and one without. Four pi[1]lots 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.

Touchdown performance

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.

As in Experiment 1, the effect for HUD use on the lateral component of the touch[1]down footprint is strong, F(1, 26) = 14.9, p < .001, η2 = .10, indicating a power greater than .70. Once again, landings were closer to the centerline when pilots used the HUD.

Three findings:

111. 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.

2.2.HUD use significantly reduced the size of the lateral component of the touchdown footprint for all RVR conditions.

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.

3.3.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 ubiquitous and ever-varying direction and velocity of wind is likely to preclude the development of true automaticity at touch[1]down. Crosswinds introduce an element of uncertainty regarding drift (the shift in lateral location of the aircraft relative to the runway’s centerline). For the pilot to detect drift the visual ground segment needs to be long enough to determine the aircraft’s movement over the ground. That means that to detect drift at all, a notice able lateral displacement must take place and not all of this displacement can be corrected before touchdown.

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.




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