Every runway at a controlled or uncontrolled airport is surrounded by a carefully designed system of lights, each serving a distinct purpose. Some lights tell you where the runway begins and ends. Others guide you down a safe glidepath so you cross the threshold at the right altitude — not so high that you float past your intended touchdown zone, and not so low that you clip terrain or obstacles on the way in. For student pilots, learning to read these lights fluently is as important as learning to read the airspeed indicator.
This article covers the three most commonly tested lighting systems: the Visual Approach Slope Indicator (VASI), the Precision Approach Path Indicator (PAPI), and runway edge lights. Together they form a visual picture that lets pilots conduct safe, stabilized approaches even in marginal visibility or at unfamiliar airports.
Runway Edge Lights
Runway edge lights define the lateral boundaries of the usable runway surface. They are installed in rows along both sides of the pavement and give pilots a clear outline of where the runway is — especially critical at night or in reduced visibility. There are three intensity classifications:
- High-Intensity Runway Lights (HIRL) — Used at larger airports and those with instrument approaches. Brightness can be adjusted by the tower or, at uncontrolled airports, by pilot-controlled lighting (PCL) via the CTAF frequency.
- Medium-Intensity Runway Lights (MIRL) — Common at general aviation airports with moderate traffic.
- Low-Intensity Runway Lights (LIRL) — Found at smaller, lower-activity airports.
The color coding of edge lights carries important information. Along the majority of the runway, edge lights are white. However, on instrument runways, the lights transition to yellow in the last 2,000 feet of runway or the last half of the runway length — whichever is less. This yellow section is known as the caution zone and signals that the available runway ahead is diminishing. At the far end — the departure end as you roll out — the edge lights are red, marking the end of usable pavement.
In addition to edge lights, pilots should be aware of runway end identifier lights (REILs), which are pairs of synchronized, flashing white lights placed at the threshold of a runway to help identify it quickly in a cluttered visual environment. REILs are not glidepath indicators, but they are often tested alongside edge light color coding.
The Visual Approach Slope Indicator (VASI)
The VASI is one of the most widely installed visual glidepath systems in the United States and remains heavily tested on FAA knowledge exams. Its purpose is simple: it tells you, at a glance, whether your approach angle is too high, too low, or just right. The standard glidepath provided by a VASI is approximately 3 degrees, though the exact angle at a given airport may vary slightly based on terrain and obstacle clearance requirements.
The most common VASI configuration is the 2-bar VASI, which consists of two rows of light units: a near bar (closer to the runway threshold) and a far bar (farther from the threshold). Each bar can display either red or white light, depending on the angle from which it is viewed. Reading the combination of colors tells you your position on the glidepath:
- White over white — You are above the glidepath. A useful memory aid: "White over white, you're high as a kite."
- Red over white — You are on the glidepath. This is the correct, stabilized approach picture.
- Red over red — You are below the glidepath. Memory aid: "Red over red, you're dead" — below glidepath is dangerous.
The 3-bar VASI adds a third (upper) bar and is primarily installed for high-cockpit aircraft like wide-body jets, where the pilots' eyes sit well above the main gear. For these aircraft, the upper and middle bars provide the glidepath signal. Private pilot students flying light aircraft will encounter the 2-bar configuration almost exclusively, but should know the 3-bar variant exists.
The VASI provides obstacle clearance within 10 degrees either side of the extended runway centerline and up to 4 nautical miles from the runway threshold. This means if you are established on the VASI glidepath within that protected corridor, you have obstacle clearance assurance. Flying above the VASI is always safe from an obstacle standpoint; flying below it is where things get dangerous.
The Precision Approach Path Indicator (PAPI)
The PAPI is the modern, more precise cousin of the VASI. Rather than two bars of lights, it uses a single row of four light units installed to the left of the runway (in the United States). Each unit projects a beam that appears either red or white depending on the viewer's vertical angle. The combination of reds and whites across the four lights gives a finer resolution of glidepath information than a standard VASI.
Reading the PAPI is straightforward once you memorize the pattern:
- Four whites — Significantly above the glidepath.
- Three whites, one red — Slightly above the glidepath.
- Two whites, two reds — On the glidepath. This is the correct picture.
- One white, three reds — Slightly below the glidepath.
- Four reds — Significantly below the glidepath. Immediate correction required.
The standard PAPI glidepath is also approximately 3 degrees, providing the same basic angle as a VASI. Like the VASI, the PAPI offers obstacle clearance within a protected corridor — specifically within 10 degrees either side of the extended centerline and up to 3.4 nautical miles from the threshold for a standard installation. Because the PAPI uses four lights rather than two bars, it gives a more nuanced indication, which is why it has largely replaced the VASI in new installations.
One practical difference to note: the PAPI's single row is mounted closer to the runway threshold than the VASI's near bar, and pilots transitioning from VASI-equipped fields to PAPI-equipped fields sometimes initially feel they are reading the glidepath differently. The key is to internalize that two whites and two reds always means on-path, regardless of airport.
Why These Systems Matter
The FAA-published accident data consistently shows that controlled flight into terrain (CFIT) and landing short of the runway are among the most preventable accident categories. A pilot who ignores or misreads the PAPI or VASI eliminates the primary visual safety net during the most dynamic phase of flight. Flying a stabilized approach — at the right speed, configuration, and glidepath — dramatically reduces the risk of hard landings, runway overruns, and threshold strikes.
Additionally, these systems are especially valuable at night and in haze, when a pilot's unaided depth perception degrades significantly. The human eye is poorly suited to judging distance and angle in featureless darkness. The PAPI and VASI remove the guesswork by presenting a binary color signal that is almost impossible to misinterpret once a pilot knows the code.
At airports with an instrument approach procedure (such as an ILS), the PAPI or VASI glidepath generally aligns with the electronic glideslope angle, providing a visual confirmation that the instrument approach is proceeding correctly. Students learning their first instrument approaches will find PAPI lights a useful cross-check.
Key Numbers and Rules
- Standard VASI and PAPI glidepath: approximately 3 degrees (may vary by airport).
- VASI obstacle clearance corridor: 10 degrees either side of extended centerline, up to 4 nautical miles.
- PAPI obstacle clearance corridor: 10 degrees either side of extended centerline, up to approximately 3.4 nautical miles.
- VASI reading: red over white = on glidepath; white over white = high; red over red = low.
- PAPI reading: two whites, two reds = on glidepath; more white = high; more red = low.
- Runway edge lights: white along most of runway; yellow in the last 2,000 feet or last half (whichever is less); red at the departure end.
- Pilot-controlled lighting (PCL): activated by clicking the microphone on the CTAF frequency — typically 7 clicks for high intensity, 5 for medium, 3 for low.
Memory Aid
For the VASI, use the classic rhyme: "Red over white, you're all right. White over white, you're high as a kite. Red over red, you're dead." This captures all three states: on glidepath, above, and below.
For the PAPI, remember: "More red, more dread" — as the number of red lights increases, you are getting lower and closer to danger. Conversely, more white means you are riding high. The balanced two-and-two picture is your target.
Common Test Traps
- Confusing VASI bar order: Students mix up which bar is near and which is far, then misread the color combination. Remember the near bar is upwind (seen first on approach) and its color dominates the top of the reading — red over white puts red on top, meaning on glidepath.
- Yellow edge lights mean stop: Some students think yellow edge lights mean the runway is closed or caution due to a hazard ahead. Yellow means the end of the usable runway is within 2,000 feet — adjust your rollout plan, but you are still on usable pavement.
- PAPI vs. VASI obstacle protection distance: Tests may ask which system offers protection farther from the threshold. The VASI (4 nm) provides a slightly longer protected range than the standard PAPI (3.4 nm).
- Assuming the glidepath is always exactly 3 degrees: FAA materials say "approximately" 3 degrees. At some airports, terrain or obstacles require a steeper path. Always check the Chart Supplement (formerly Airport/Facility Directory) for the published angle.
- Forgetting REILs are not glidepath indicators: Runway end identifier lights flash to help identify the threshold, not to indicate whether you are on, above, or below glidepath. Do not confuse their flashing white light with PAPI or VASI signals.