Coincident with visual GP?
Instructors have always emphasized the transition to land aspect of a low viz instrument approach as being particularly challenging. Most runways have an aimpoint marking associated with the 1,000 ft point (US FAA) or 1,312 ft/400m ICAO, regardless of whether it is an IFR or VFR runway. It seems this marker is not just an aimpoint, but also an excellent reminder of how far the jet has progressed down the runway.
The following are some copy/pastes from a manual (they are not my words).
"The VASI is a system of lights arranged to provide visual descent guidance information during the approach. All VASI systems are visual projections of the approach path normally aligned to intersect the runway at a point 1,000 or 1,800 feet beyond the threshold. Flying the VASI glideslope to touchdown is the same
as selecting a visual aim point on the runway adjacent to the VASI installation
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 airplanes to provide a sufficient threshold crossing height.
CAUTION: B-767-300 use of a two bar VASI system will result 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: [unable to copy diagram]
• 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."
B-767-300
Two Bar [Three Bar]
Flaps 30 AIM Point at 1,000 Feet [1,800]
Visual Glide Path (degrees) 3.0 [3.25]
Airplane Body Attitude(degrees) 2.2 [2.0]
Threshold Height
Pilot Eye Height (feet)
48 [98]
Main Gear Height (feet)
24 [74]
Main Gear Touchdown Point - no flare(feet) 461 [1,300]
"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 wheel base 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."
"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."