T-VASIS versus PAPI

Lewis (2011¹) compared the accuracy with which a group of ab-initio pilots tracked the correct approach-to-landing glide path when using two different approach lighting systems (T-VASIS⁴ and PAPI⁵). Results (see illustration 1) show that pilots were better at tracking the correct glide slope in 8 scenarios out of 10 when using T-VASIS and in 2 scenarios out of 10 when using PAPI. On average, ab-initio pilots performed better with T-VASIS (mean difference = 6), a sensible difference of moderate magnitude (when attending to the unstandardized effect size).

Illustration 1. Average deviation from glide slope*
Scenario	I	II	III	IV	V	VI	VII	VIII	IX	X	Mean	StDev
PAPI	51	47	59	79	71	82	57	62	46	47	60	13
T-VASIS	43	46	61	63	59	72	47	52	53	45	54	9
Difference	8	1	-2	16	12	10	10	10	-7	2	6
Unstandardized effect size³				small		2	medium		6	large		9
( Angular deviation from 0, multiplied by 100)*

When questioned about their perception of fit-for-purpose and preference for one lighting system or the other, most pilots perceived T-VASIS as being a better visual system when flying on the glide slope or when above it, but perceived PAPI as being a better visual system when flying below the glide slope. Irrespective of perception, though, most preferred PAPI over T-VASIS.

Illustration 2. Assessment of best-fit-for-purpose and preference for approach lighting system
Number (%) of pilots who think best displays when				Number (%) of pilots
	On glide slope	Above glide slope	Below glide slope	Who prefer
PAPI	6 (42%)	3 (21%)	8 (57%)	9 (55%)
T-VASIS	8 (57%)	11 (79%)	6 (43%)	5 (35%)

Research approach

Exploratory research² done under laboratory conditions for assessing whether ab-initio pilots would maintain the correct glide path when using two different visual approach slope indicator lighting systems (PAPI and T-VASIS) under similar conditions. It also measured personal assessment and preference for each system.

Sample

A convenient sample of 14 third-year military aviation students from the Australian Defence Force Academy (UNSW, Canberra, Australia), between 20 and 23 years of age.

The participants had a minimum of 21 flying hours (assumed to be as cadet pilots), while some had accumulated up to 50 hours when counting prior flying experience (although none of these had flown an aircraft in the past two-and-a-half years other than for the 21 flying hours as cadets, supposedly). None had experience with visual approach slope indicator lighting systems.

Design

A repeated measures design with 10 simulated approach-to-land maneuvers per slope indicator lighting system, and with random allocation of each participant to the first slope indicator lighting system (which started the sequence, followed by the second slope indicator lighting system, and so on). The PAPI system was linked to simulated runway 05 at Adelaide airport, while the T-VASIS system was linked to simulated runway 19 at Brisbane airport. Both runways were selected for their contextual similarity, thus helping control potential confounding variables.

Variables

Dependent variable: angular deviation from correct approach flight slope.
Independent variable: two different visual approach slope indicator lighting systems (T-VASIS and PAPI), and ten pairs of scenarios simulating different flight conditions.
Confounding variables:
- Two simulated runways (at different airports), which were controlled by choosing runways that were similar in their contextual information
- Flight experience, which was controlled by the official requirement of pilots not having flown for the past two-and-a-half years, other than as part of their military training.

Materials

Simulated flights using Microsoft Flight Simulator 2004. Ten pairs of flight scenarios were put to the pilots, varying in visibility, weather condition and altitude.
The flight simulator function was used to record deviation from the correct glide slope per second of flight.
A four-item questionnaire for measuring perception of fit-for-purpose and preference for one or the other slope indicator lighting system.

Procedure

After selection, participants sat in a simulator and piloted a Beechcraft King Air 350 using Microsoft Flight Simulator 2004. Each participant flew 20 approaches-to-land, 10 approaches using T-VASIS (at the simulated Brisbane airport) and another 10 approaches using PAPI (at the simulated Adelaide airport) under varied conditions of visibility and altitude. The approaches were made in sequence, with the starting condition (T-VASIS or PAPI) being selected at random for each participant. After the experiment, participants filled in a questionnaire about their assessment of perceived fit-for-purpose of, and personal preference for, each lighting system.

Generalization potential

Limited, if any, to ab-initio pilots with about 21 flying hours. The main limitation of the study is that the low experience with flying may account for the variability in tracking the flight path (with T-VASIS offering more precise feedback than PAPI, thus the better performance). The results seem of little relevance to any other population.

Editors

Jose D PEREZGONZALEZ (2013). Massey University, Turitea Campus, Private Bag 11-222, Palmerston North 4442, New Zealand. (JDPerezgonzalez).
dotwikifan (2012). (dotwikifan).

Peer-reviewers

Steve GEDDES (2013). Massey University, New Zealand. (Steve Geddes).
Sarah ROSS (2013). Massey University, New Zealand. (Sarah215).
David ATKINSON (2013). Massey University, New Zealand. (DAtkinson).

Want to know more?

LEWIS (2011) original article: The original article provides full detail about the research.
Skybrary - VASIS: This Skybrary page offers a small introduction to different visual approach slope indicators (such as T-VASIS and PAPI).

Other interesting sites