20130530 - Ab-initio pilots' performance using T-VASIS versus PAPI - 2011

[Data] [<Normal page] [PEREZGONZALEZ Jose D et al [eds] (2013). Ab-initio pilots' performance using T-VASIS versus PAPI. Knowledge (ISSN 2324-1624), 2013, pages 94-96.] [DOI]


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.


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.


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.


  • 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.


  • 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.


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.

1. LEWIS Raymond (2011). T-Visual Approach Slope Indicator System (T-VASIS) versus Precision Approach Path Indicator (PAPI) – the debate revisited. Aviation Education and Research Proceedings (ISSN 1176-0729), volume 2011, pages 20-30.
+++ Notes +++
2. The research was presented as a confirmatory one, including two research hypotheses. However, the results were only descriptive in nature which, together with a small sample size of limited generalizability, makes this research a de facto exploratory study.
3. This is the estimated unstandardized effect size for group differences (Cohen's d and Glass's Δ) given an average standard deviation and following Cohen's d effect size interpretation. It can be used to ascertain the relative importance of descriptive data without the need to perform inferential tests.
4. T-Visual Approach Slope Indicator System.
5. Precision Approach Path Indicator.

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).


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


Steve GEDDES (2013). Massey University, New Zealand. (Steve GeddesSteve Geddes).
Sarah ROSS (2013). Massey University, New Zealand. (Sarah215Sarah215).
David ATKINSON (2013). Massey University, New Zealand. (DAtkinsonDAtkinson).

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