A little review of avialable VASI systems, their operations and their limitations for anyone so interested...................
Visual Approach Slope Indicator (VASI): The same one we're all familiar with. Color-coded visual glidepath indication using a system of lights positioned along the runway along the touchdown point used for IFR and VFR. Standard indications of white over white- too high; red over white- on GP, red over red, too low. For final approach use, the system is normally coincident with the ILS and PAR glideslopes servicing the same runway. If the glideslope angles differ, such deviations will be noted on the IAP plate. System consists of anywhere from 2-box on one side of runway to 4/6/12 and 16 box units mounted on both sides of the runway and up to three "rows" of bars, the normal being two rows. VASI is especially effective in providing GP guidance during approaches over water or featureless terrain where other sources of visual reference are lacking or misleading. USAF and US Navy VASI systems operate continuously on the active runway unless noted otherwise in IFR/VFR supps. Civil VASIs may be full-time or part time and this is noted in the Airport Facilities Directory. The intensity of VASI lights at civil fields can be adjusted by the tower controller at the pilot's request, or by Pilot Controlled Lighting at an uncontrolled field or after ATC hours, if available. Three bar VASI systems are used for jumbo aircraft. Pilots of other aircraft should use the lower two rows of bars like a standard VASI. When on glidepath, you're essentially overshooting the near bars (white) and undershooting the far bars (red). Departure from glidepath is indicated by color changes from red to pink to white, or vice versa; giving the white/white for high and red/red for low.
Note that although VASI indicates a deviation from the glideslope, full white/white and red/red indications show only that your position is above or below the GP. VASI has no ability to tell you how far you are above/below the GP. Pilots must therefore employ other references to ensure a positive position. Additionally, caution should be exercised when using VASI systems that are associated with runway threshold bar lights due to color similarities. Also, some erroneous glideslope indications could occur during low viz periods fro reflection of the light beams. As VASI is merely an approach aid, it shoulc be crosschecked against other available aides. On the plus side, VASI blends well with instrument approaches. For precision approaches, easy transition can be made from the instrument glidepath and VASI glidepath without much pitch/power changes due to their being coincident. For non-precision approaches, upon leveling at MDA, capture of the VASI glidepath can be made the same as VFR.
Precision Approach Path Indicator (PAPI): One of the most widely seen glidepath aids in use today, PAPI makes up for some of the limitations that VASI has. As we all know, PAPI consists of 4 light boxes, similiar to VASI, but situated in a horizontal arrangement on one side of the runway, normally the left side and used for IFR and VFR. When on glidepath, the pilot will see two red lights and two white lights. Deviations from glidepath cause deviations in the red/white light indications. When the aircraft is 0.2 degrees below glidepath, the system will indicate 3 red lights and 1 white light; when 0.4 degrees below GP, the system will indicate 4 red lights; and vice versa for above glidepath indications. At some fields, light boxes will be installed on both sides of the runway, and will be synchronised.
Pulsating Visual Approach Slope Indicator (PVASI): For those of you that fly our of DVTs north runway, you've seen these. PVASI is a visual aid designed for VMC use only. System consists of single-light unit projecting a two-color visual approach path into the final approach area of the runway. On glidepath is indicated by either a steady white light, or a alternating white/red light. Above glidepath is indicated by a pulsing white light; further deviations cause the pulsing to become faster; as the pilot re-intercepts the glidepath, the pulsing becomes slower until a steady white on-glidepath indication. Below glidepath is indicated by a steady red light, further to a pulsing red light. The on glidepath light is an angular wedge of 1/3 of a degree, corresponding to approximately 1/2 dot on an ILS glideslope indicator. The system, while relatively simple, has some inherent limitations and hazards.
Limitations:
1. Limited first-look reliability: Based on the specific installation, PVASI can confused with other lights in the vicinity of the runway during night ops (runway lights, aircraft landing lights/anti collision lights, emergency vehicle lights, other vehicle lights, etc). During lower viz/lower ceiling conditions, the ability to pick-out PVASI, especially with runway lights at high-intensity, can be difficult with a solid-white (on glidepath) indication. When breaking out of IMC, first look reliability can be difficult.
2. Dual installations: Some fields have PVASIs installed with one boxon each side of the runway. Unlike VASI and PAPI, PVASI was not designed for dual installation. The left and right units are independant systems and cannot be synchronized. Often, there are differences in signals between between the left and right side units (one steady white, one pulsing white).
Caution when viewing the PVASI in the pulsing white or red indications not to mistake these as lights of another aircraft.
Fresnel Lens Optical Landing System (FLOLS, or OLS): This system is an electro-optical landing aid designed by the Navy for use aboard ship. However, all Naval/Marine Corps Air Stations, including joint-use fields such as MCAS Yuma/Yuma Intl (KNYL/KYUM) have this system installed on their primary runways. OLS appears as two sets of green datum (reference) lights arranged horizontally with a gap in the middle. Inside the gap is a large yellow light known as the meatball, or ball. When above glidepath, the "ball" will appear to be above the horizontal green lights formed by the datum lights; vice versa for below. At the lowest limit of the ball, the ball will turn red. The object is to line up the ball horizonatlly with the green datum lights. The 5 ball lights (one yellow on glidepath, two yellow above, one yellow and one red below glidepath) are framed by red "wave off" (go around) lights that are not normally visible. When activated by tower, they flash bright red and command a go-around if go-around commands aren't possible due to frequency congestion, etc. These lights will also be activated when the go-around instruction is given by tower. Additional red wave-off lights are installed on each side of the runway extending to 2500' from the threshold, and will flash when the OLS wave-off lights are activated. Note that on this system, when more than 3/4 of a degree above or below the glideslope, the ball will disappear as it transitions to the next light unit. If in doubt, assume below the glidepath.
Tri-Color VASI: Not found in too many places, but still around, the Tri-Color VASI consists of a singular light unit located on the left side of the runway, and displaying a different color (one of three) correlating to the aircraft's position on the glidepath. On glidepath is indicated by a green light, above glidepath is indicated by an amber light, below glidepath is indicated by a red light. Caution should be noted that during the transition from on glidepath (green) to below glidepath (red), the pilot may see a dark amber color if in-between these lights that could be confused for above glidepath indications.
T-VASI: T-VASI consists of an upright T on both sides of the runway that is lighted white. On the correct glideslope, only the crossbar is visible. If the "T" appears upright like the letter T, that is a fly-up or below glidepath indication, inverted "T" is vice versa. Gross undershoot is indicated by all lights becoming a red upright "T" indication (severe below glidepath). Simple system requiring pilot to merely follow the crossbar up or down.
Alignment of Elements and Low-Cost VASI (APAP and LCVASI): These systems consist of three orange or black/orange painted panels that may be lighted at night. The panels are situated horizonatlly and the glideslope is determined by the position of the center panel in relation to the outer panels. With the panels aligned, the pilot in on glidepath. Center panel above the outer panels is a "fly-up" indication, or below glideslope, while center panel below the outer panels is a "fly down" indication. LCVASI is the same operating principle, but uses lighted a singular box of three horizontal lights. Useful range of these systems is approximately 3/4 of a mile.
MD
Visual Approach Slope Indicator (VASI): The same one we're all familiar with. Color-coded visual glidepath indication using a system of lights positioned along the runway along the touchdown point used for IFR and VFR. Standard indications of white over white- too high; red over white- on GP, red over red, too low. For final approach use, the system is normally coincident with the ILS and PAR glideslopes servicing the same runway. If the glideslope angles differ, such deviations will be noted on the IAP plate. System consists of anywhere from 2-box on one side of runway to 4/6/12 and 16 box units mounted on both sides of the runway and up to three "rows" of bars, the normal being two rows. VASI is especially effective in providing GP guidance during approaches over water or featureless terrain where other sources of visual reference are lacking or misleading. USAF and US Navy VASI systems operate continuously on the active runway unless noted otherwise in IFR/VFR supps. Civil VASIs may be full-time or part time and this is noted in the Airport Facilities Directory. The intensity of VASI lights at civil fields can be adjusted by the tower controller at the pilot's request, or by Pilot Controlled Lighting at an uncontrolled field or after ATC hours, if available. Three bar VASI systems are used for jumbo aircraft. Pilots of other aircraft should use the lower two rows of bars like a standard VASI. When on glidepath, you're essentially overshooting the near bars (white) and undershooting the far bars (red). Departure from glidepath is indicated by color changes from red to pink to white, or vice versa; giving the white/white for high and red/red for low.
Note that although VASI indicates a deviation from the glideslope, full white/white and red/red indications show only that your position is above or below the GP. VASI has no ability to tell you how far you are above/below the GP. Pilots must therefore employ other references to ensure a positive position. Additionally, caution should be exercised when using VASI systems that are associated with runway threshold bar lights due to color similarities. Also, some erroneous glideslope indications could occur during low viz periods fro reflection of the light beams. As VASI is merely an approach aid, it shoulc be crosschecked against other available aides. On the plus side, VASI blends well with instrument approaches. For precision approaches, easy transition can be made from the instrument glidepath and VASI glidepath without much pitch/power changes due to their being coincident. For non-precision approaches, upon leveling at MDA, capture of the VASI glidepath can be made the same as VFR.
Precision Approach Path Indicator (PAPI): One of the most widely seen glidepath aids in use today, PAPI makes up for some of the limitations that VASI has. As we all know, PAPI consists of 4 light boxes, similiar to VASI, but situated in a horizontal arrangement on one side of the runway, normally the left side and used for IFR and VFR. When on glidepath, the pilot will see two red lights and two white lights. Deviations from glidepath cause deviations in the red/white light indications. When the aircraft is 0.2 degrees below glidepath, the system will indicate 3 red lights and 1 white light; when 0.4 degrees below GP, the system will indicate 4 red lights; and vice versa for above glidepath indications. At some fields, light boxes will be installed on both sides of the runway, and will be synchronised.
Pulsating Visual Approach Slope Indicator (PVASI): For those of you that fly our of DVTs north runway, you've seen these. PVASI is a visual aid designed for VMC use only. System consists of single-light unit projecting a two-color visual approach path into the final approach area of the runway. On glidepath is indicated by either a steady white light, or a alternating white/red light. Above glidepath is indicated by a pulsing white light; further deviations cause the pulsing to become faster; as the pilot re-intercepts the glidepath, the pulsing becomes slower until a steady white on-glidepath indication. Below glidepath is indicated by a steady red light, further to a pulsing red light. The on glidepath light is an angular wedge of 1/3 of a degree, corresponding to approximately 1/2 dot on an ILS glideslope indicator. The system, while relatively simple, has some inherent limitations and hazards.
Limitations:
1. Limited first-look reliability: Based on the specific installation, PVASI can confused with other lights in the vicinity of the runway during night ops (runway lights, aircraft landing lights/anti collision lights, emergency vehicle lights, other vehicle lights, etc). During lower viz/lower ceiling conditions, the ability to pick-out PVASI, especially with runway lights at high-intensity, can be difficult with a solid-white (on glidepath) indication. When breaking out of IMC, first look reliability can be difficult.
2. Dual installations: Some fields have PVASIs installed with one boxon each side of the runway. Unlike VASI and PAPI, PVASI was not designed for dual installation. The left and right units are independant systems and cannot be synchronized. Often, there are differences in signals between between the left and right side units (one steady white, one pulsing white).
Caution when viewing the PVASI in the pulsing white or red indications not to mistake these as lights of another aircraft.
Fresnel Lens Optical Landing System (FLOLS, or OLS): This system is an electro-optical landing aid designed by the Navy for use aboard ship. However, all Naval/Marine Corps Air Stations, including joint-use fields such as MCAS Yuma/Yuma Intl (KNYL/KYUM) have this system installed on their primary runways. OLS appears as two sets of green datum (reference) lights arranged horizontally with a gap in the middle. Inside the gap is a large yellow light known as the meatball, or ball. When above glidepath, the "ball" will appear to be above the horizontal green lights formed by the datum lights; vice versa for below. At the lowest limit of the ball, the ball will turn red. The object is to line up the ball horizonatlly with the green datum lights. The 5 ball lights (one yellow on glidepath, two yellow above, one yellow and one red below glidepath) are framed by red "wave off" (go around) lights that are not normally visible. When activated by tower, they flash bright red and command a go-around if go-around commands aren't possible due to frequency congestion, etc. These lights will also be activated when the go-around instruction is given by tower. Additional red wave-off lights are installed on each side of the runway extending to 2500' from the threshold, and will flash when the OLS wave-off lights are activated. Note that on this system, when more than 3/4 of a degree above or below the glideslope, the ball will disappear as it transitions to the next light unit. If in doubt, assume below the glidepath.
Tri-Color VASI: Not found in too many places, but still around, the Tri-Color VASI consists of a singular light unit located on the left side of the runway, and displaying a different color (one of three) correlating to the aircraft's position on the glidepath. On glidepath is indicated by a green light, above glidepath is indicated by an amber light, below glidepath is indicated by a red light. Caution should be noted that during the transition from on glidepath (green) to below glidepath (red), the pilot may see a dark amber color if in-between these lights that could be confused for above glidepath indications.
T-VASI: T-VASI consists of an upright T on both sides of the runway that is lighted white. On the correct glideslope, only the crossbar is visible. If the "T" appears upright like the letter T, that is a fly-up or below glidepath indication, inverted "T" is vice versa. Gross undershoot is indicated by all lights becoming a red upright "T" indication (severe below glidepath). Simple system requiring pilot to merely follow the crossbar up or down.
Alignment of Elements and Low-Cost VASI (APAP and LCVASI): These systems consist of three orange or black/orange painted panels that may be lighted at night. The panels are situated horizonatlly and the glideslope is determined by the position of the center panel in relation to the outer panels. With the panels aligned, the pilot in on glidepath. Center panel above the outer panels is a "fly-up" indication, or below glideslope, while center panel below the outer panels is a "fly down" indication. LCVASI is the same operating principle, but uses lighted a singular box of three horizontal lights. Useful range of these systems is approximately 3/4 of a mile.
MD