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When using a two-bar VASI, the difference between the eye reference path and the gear path results in a low approach with marginal threshold height. Therefore, the two-bar VASI system should not be used to determine proper approach profile. It may provide useful information in alerting the crew to low profile situations.
Some airports have three-bar VASI which provides two visual glide paths. The additional light bar is located upwind from a standard two-bar installation. When the airplane is on the glide path, the pilot sees the two white bars and one red bar. Three-bar VASI may be safely used with respect to threshold height, but may result in landing further down the runway.
For a T-VASI, flying the approach with one additional white fly down light visible provides additional wheel clearance.

Three Bar VASI/T - VASI

 

VASI Landing Geometry
Two-bar VASI installations provide one visual glide path which is normally set at 3°. Three-bar VASI installations provide two visual glide paths. The lower glide path is provided by the near and middle bars and is normally set at 3° while the upper glide path, provided by the middle and far bars, is normally 1/4° higher (3.25°). This higher glide path is intended for use only by high cockpit (long wheelbase) airplanes to provide a sufficient threshold crossing height.
CAUTION: Use of a two bar VASI system results in main landing gear touchdown at, or very near, the end of the runway threshold. Use of a two bar VASI system is not recommended.
Two Bar/Three Bar VASI Landing Geometry
The following diagrams use these conditions:
. 
data is based upon typical landing weight

. 
airplane body attitudes are based on Flaps 30, VREF 30 + 5 and should be
reduced by 1° for each 5 knots above this speed.

. 
eye height is calculated at the moment the main gear is over the threshold.

Two Bar VASI Landing Geometry

Flaps 30  AIM Point at 1,000 Feet 
757  Visual Glide  Airplane  Threshold Height  Main Gear 
Model  Path (degrees)  Body Attitude (degrees)  Touchdown Point - no flare (feet) 
Pilot Eye Height (feet)  Main Gear Height (feet) 
-200  3.0  2.2  48  28  526 
-300  3.0  1.7  48  27  518 

Three Bar (Upper Glide Path) VASI Landing Geometry

Flaps 30  AIM Point at 1,800 Feet 
757  Visual Glide  Airplane  Threshold Height  Main Gear 
Model  Path (degrees)  Body Attitude (degrees)  Touchdown Point - no flare (feet) 
Pilot Eye Height (feet)  Main Gear Height (feet) 
-200  3.3  1.95  98  77  1363 
-300  3.3  1.45  97  77  1356 

 

Precision Approach Path Indicator (PAPI)
The PAPI uses lights which are normally on the left side of the runway. They are
similar to the VASI, but are installed in a single row of light units. When the airplane is on a normal 3° glide path, the pilot sees two white lights on the left and two red lights on the right. The PAPI may be safely used with respect to threshold height, but may result in landing further down the runway. The PAPI is normally aligned to intersect the runway 1,000 to 1,500 feet down the runway.

PAPI Landing Geometry

Landing Geometry Visual Aim Point
During visual approaches many techniques and methods are used to ensure main landing gear touchdown at the desired point on the runway. One of the most common methods used is to aim at the desired gear touchdown point on the runway, then adjust the final approach glide path until the selected point appears stationary in relation to the airplane (the point does not move up or down in the pilot’s field of view during the approach).
In first generation jet transports (e.g. B-707, DC-8), this method is acceptable because of the small difference between landing gear path and eye level path. Flare distance accounts for the small difference in paths. Gear touchdown occurs very near the visual aim point. However, in today’s larger aircraft, the difference in gear path and eye-level path has increased because of the longer wheelbase and the increased flight deck height. Consequently, the main gear do not touchdown on the runway at the selected visual aim point.

Visual aim points versus gear touchdown point differences increase as glide path angle decreases as in a flat approach. For a particular visual approach, the difference between gear path and eye level path must be accounted for by the pilot.

Landing Runway Markings (Typical)
The following runway markings are for runways served by a precision approach.

Threshold Height
Threshold height is a function of glide path angle and landing gear touchdown target. Threshold height for main gear and pilot eye level is shown in the Two Bar/Three Bar VASI Landing Geometry tables on a previous page. Special attention must be given to establishing a final approach that assures safe threshold clearance and gear touchdown at least 1,000 feet down the runway. If automatic callouts are not available, the radio altimeter should be used to assist the pilot in judging terrain clearance, threshold height and flare initiation height.
　
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