Runway approach lighting is the set of lights that leads you in from instrument flight to a visual landing. In the US the main systems are ALSF-1, ALSF-2, SSALR, MALSR, MALSF, SSALF and ODALS, and they differ in length, brightness and whether they carry sequenced flashing lights. The longer, brighter systems serve precision instrument runways. The shorter ones serve runways with nonprecision approaches.
This guide uses the names and definitions in the FAA's Pilot/Controller Glossary and Aeronautical Information Manual (AIM), so the abbreviations match what you will see on charts and in the Chart Supplement.
What an Approach Light System Does
The AIM says approach light systems provide the basic means to transition from instrument flight to visual flight for landing. A system is a configuration of signal lights starting at the landing threshold and extending into the approach area. It runs 2,400 to 3,000 feet for precision instrument runways and 1,400 to 1,500 feet for nonprecision instrument runways. Some systems include sequenced flashing lights that look like a ball of light travelling toward the runway at high speed, twice a second.
The lights give you a line to the runway centreline to fly along while you look for the runway itself.
The Systems, Named
| System | What the abbreviation stands for | Notes |
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| ALSF-1 | Approach Light System with Sequenced Flashing Lights | ILS Category I configuration |
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| ALSF-2 | Approach Light System with Sequenced Flashing Lights | ILS Category II configuration. May operate as an SSALR when weather conditions permit. |
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| SSALR | Simplified Short Approach Light System with Runway Alignment Indicator Lights | The "R" means runway alignment indicator lights are added. |
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| SSALF | Simplified Short Approach Light System with Sequenced Flashing Lights | Sequenced flashers rather than runway alignment lights. |
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| MALSR | Medium Intensity Approach Light System with Runway Alignment Indicator Lights | Medium intensity, with runway alignment indicator lights. |
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| MALSF | Medium Intensity Approach Light System with Sequenced Flashing Lights | Medium intensity, with sequenced flashers. |
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| ODALS | Omnidirectional Approach Lighting System | Seven omnidirectional flashing lights on a nonprecision runway. |
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| RAIL | Runway Alignment Indicator Lights | Sequenced flashing lights installed only together with another system. |
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| RLLS | Runway Lead-in Light System | Flashing lights that lead you along a curved or straight path where terrain, obstructions or noise abatement make a straight approach hard. |
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Read the last letters of an abbreviation as a code. An "F" at the end means sequenced flashing lights. An "R" means runway alignment indicator lights. "MALS" means medium intensity, and "SSALS" means simplified short. The Chart Supplement and the approach plate tell you which system a runway has.
ODALS: The Simple Nonprecision System
The omnidirectional approach lighting system is the easiest one to picture. It has seven flashing lights. Five sit on the extended runway centreline, the first 300 feet from the threshold, then at equal intervals out to 1,500 feet. The other two sit one on each side of the threshold, 40 feet from the runway edge, or 75 feet when the runway has a VASI.
Lights That Show Your Glidepath
Approach lights tell you where the runway is. A separate set of lights tells you whether you are on the right descent angle.
VASI
The Visual Approach Slope Indicator uses bars of light units that project a white segment above and a red segment below. Most installations have a near and a far bar. The basic idea is colour: you see the combination of red and white lights that tells you whether you are high, low or on the path. A two-bar VASI gives one glidepath, normally set at 3 degrees. A three-bar system adds a second, higher path for tall-cockpit aircraft. VASI lights are visible from 3 to 5 miles by day and up to 20 miles or more at night, and the visual path provides safe obstruction clearance within 10 degrees either side of the extended centreline and out to 4 nautical miles from the threshold.
PAPI
The Precision Approach Path Indicator uses light units installed in a single row of two or four, normally on the left side of the runway. They are visible from about 5 miles by day and up to 20 miles at night, and the path typically gives safe obstruction clearance within 10 degrees either side of the centreline and to 3.4 nautical miles from the threshold. With a four-light PAPI, all white means well above the path, three white and one red means slightly high, two white and two red means on the path, one white and three red means slightly low, and all red means well below.
Tri-colour indicators
These use a single light unit that shows red below the glidepath, amber above it and green on it.
Do not start your descent using a VASI or PAPI until you are visually aligned with the runway. They give you descent guidance only, and the runway lights provide the lateral guide.
When You See the Lights
On an instrument approach, seeing the approach lights can let you descend below the decision altitude or minimum descent altitude, within the limits of the regulations in 14 CFR 91.175. Which lights count, and how far down you may go, depends on the approach and the lighting installed, so read the approach plate notes rather than relying on memory.
Frequently Asked Questions
What does ALSF-2 stand for?
ALSF-2 is the Approach Light System with Sequenced Flashing Lights in ILS Category II configuration. The FAA glossary notes it may operate as an SSALR when weather conditions permit.
How long is an approach light system?
The AIM says systems extend into the approach area 2,400 to 3,000 feet for precision instrument runways and 1,400 to 1,500 feet for nonprecision instrument runways.
What is the difference between VASI and PAPI?
A VASI uses bars of red and white lights, often a near and a far bar. A PAPI uses a single row of two or four light units. Both show whether you are above, below or on the glidepath, and the PAPI is visible from about 5 miles by day.
What is a sequenced flashing light?
It is a flashing light installed in the approach area that fires in order toward the runway, so it looks like a ball of light moving to the threshold twice a second.
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