Runway Markings

[usp3r]

Here´s a question for you interested in details. I wonder why the aiming point markings of an instrument runway are situated as they are...

is it dependant upon the descent gradient (glidepath) to give the aircraft a clearence of 50 ft over the T/H (a steper gradient would situate the markings closer to T/H according to this theory)

[None]

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."

[usp3r]

Okey!

A very detailed answear indeed, thanks a lot:)

[alf5071h]

“A very detailed answer indeed,” but not necessarily correct ??

Re “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.”
I understanding that the ‘aim point’ discussed is more likely a fixed distance marking from the threshold, which normally coincides with (or is close to) the glide-slope origin. The markings are coded to define the distance from the threshold and are often used as ‘an acceptable landing area’, but this is aircraft dependant and is rarely mandated except in theory for Cat 2/3 operations.

The old style VASI was not a projection system – the red / white lamps interfered and gave an ambiguous pink value at low altitude; however if we assume that the definition used is an abbreviation for the PAPI (VASI), then PAPI is a very accurate projected system. Then as stated, the PAPI visual glide path is harmonized with the electronic glideslope to main the correlation between pilot’s eye position and glidepath aerial – the electronic glidepath.

Again as I understand a two PAPI system (long body aircraft option), the visual glide-slopes are identical and at the same angle as the electronic glidepath (parallel beams); the second set of PAPI lights are positioning further down the runway to enable a long body aircraft to follow a similar flight path to that of standard aircraft although the pilots eye position is much higher. This enables the aircraft to track the same electronic glideslope whilst giving the pilot a ‘normal’ visual cue; it also maintains the wheel height over threshold as explained (plus/minus all the calculations as given).

Re "Visual Aim Point”; I know of very few runway markings or indeed 'techniques' to ensure main landing gear touchdown at the desired point on the runway. Some limiting runways have a yellow landing box marked on the runway (Faeroe Islands?) where the aircraft must either touch down or fly a baulked landing. London City has a non standard fixed distance marking that has been recommended as the last point of touchdown for jet aircraft, but this is not directly associated with the steep approach.

I do not recall that a steep approach specifically requires different runway markings. At LCY the geometry change in the visual glidepath (PAPI) and the ILS is countered by repositioning the PAPI and ILS closer to the threshold whilst maintaining their correlation. The other geometric change accounted for is the 35 ft threshold crossing height, but again all glide-paths are aligned; as yet, there are no complexities of long body aircraft at LCY!

[None]

You are correct...it is the fixed distance marker that is the focus of this question. This marker is a visual aid that is useful for any aircraft. For the 767-300, the previous post shows why many do not "plan" a touchdown prior to this point. Specifically:

1. Boeing does not recommend a TD prior ("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.")

2. Main Gear height above the ground at the threshhold is minimal.

The full touchdown zone is an acceptable position to TD in most cases. For runways in the US (9,000 feet or longer), and for the 767-300, that represents the zone between the 1,000 foot markers and the final TDZ stripes that are located at 3,000 feet down the runway.

If flying a SE Cessna, the 1,000 ft markers are still a meaningful visual aid, be it on a runway with a length of 4,000 ft or 10,000 ft .



>The markings are coded to define the distance from the threshold and are often used as ‘an acceptable landing area’, but this is aircraft dependant and is rarely mandated except in theory for Cat 2/3 operations.

[AirRabbit]

For those of you who are interested in runway markings (for US airports), here is a link to an FAA briefing on Airport Marking. Its actually pretty well done and there are a lot of slides (84) in what was a PowerPoint presentation and presented here in PDF format.
http://www.faa.gov/arp/awp/pub/conf/agenda060305_files/Marking-Chuck.pdf