Learn what a pulsating VASI means, decode its white and red signals, correct high or low approaches, and understand the system's limits.
A pulsating VASI (PVASI) is a single-unit visual approach slope indicator that shows an aircraft’s position relative to a runway’s prescribed approach path. Pilots see pulsating white when high, steady white on path, steady red slightly low, and pulsating red when well below, then adjust pitch and power smoothly to correct.
For Aviation & Real-World Flying, the key distinction is that PVASI presents one colour-and-pulse signal rather than the pattern of separate lights used by conventional VASI and PAPI systems. PVASI stands for pulsating visual approach slope indicator; the pulsation is deliberate guidance, not normally a fault.
What do PVASI colours and pulses mean?
A PVASI uses four principal indications, with the pulse rate increasing as the aircraft moves farther above or below the intended path.
| Aircraft position | PVASI indication | Pilot interpretation |
|---|---|---|
| Above approach path | Pulsating white | High; the faster the pulse, the greater the deviation |
| On approach path | Steady white | Correct vertical path |
| Slightly below approach path | Steady red | Low; correct before the deviation increases |
| Farther below approach path | Pulsating red | Well low; the pulse quickens as the aircraft descends farther below |
The target indication is steady white. A mistake we see constantly is applying the familiar “red over white” mnemonic from a two-bar VASI to a PVASI. That mnemonic does not apply because a PVASI displays only one signal at a time.
How do pilots use a pulsating VASI on final?
Pilots use PVASI by establishing a stabilised final approach, identifying the signal positively and making small corrections towards steady white.
- Confirm the installation. Check the airport information for the visual slope system, its location and any limitations. A PVASI is commonly positioned beside the runway, but a single light can be mistaken for another airport light.
- Intercept the extended centreline. Read the PVASI from a properly aligned final approach. Its optical guidance and any associated obstacle protection should not be assumed valid from a large angle off the runway centreline.
- Hold the correct approach speed. Use coordinated pitch and power changes appropriate to the aircraft. A pulsating white indication calls for a controlled increase in descent angle; steady or pulsating red calls for reducing the descent angle. The precise pitch-and-power technique depends on the aircraft and operating procedure.
- Allow the indication to settle. Avoid chasing every change with abrupt control movements. Correct the trend, maintain speed and reassess the signal.
- Go around if the approach becomes unstable. Do not dive towards the runway to cure a close-in pulsating white signal, or attempt to salvage a persistent pulsating red indication. If the aircraft cannot regain a safe, stabilised path, go around.
- Transition near the threshold. Shift attention progressively to the runway aiming point, threshold clearance and flare. The objective is not to chase the PVASI indication all the way to touchdown.
How is PVASI different from VASI and PAPI?
PVASI gives its guidance from one visual signal, while conventional VASI and PAPI installations compare several separate light units.
- PVASI: one light unit using steady or pulsating white and red indications.
- VASI: light units arranged in bars, producing combinations such as red over white.
- PAPI: normally a row of four units, with the number of white and red lights showing the aircraft’s vertical position.
PVASI is compact, but its single source makes positive identification especially important. PAPI also provides more discrete steps around the desired path, whereas PVASI communicates increasing deviation through a faster pulse rate.
Can PVASI disagree with instrument approach guidance?
Yes—PVASI and instrument vertical guidance can use different angles or threshold-crossing geometry, so their indications need not coincide exactly.
A 3-degree visual path is common, but it is not universal. The approach chart or airport information may publish the visual slope angle or warn that the visual glide slope indicator and an ILS glidepath are not coincident. Our guide to understanding approach-chart minima, profiles and visual transitions explains where that information fits into the procedure.
During an instrument approach, pilots follow the authorised procedure and its vertical guidance until making the permitted transition to visual references. Once visual, PVASI is a cross-check alongside the runway aiming point, airspeed and descent trend; it does not override a published restriction. Sim pilots can rehearse that scan using our guidance on practising visual approach aiming points and slope-light indications.
When should pilots not rely on PVASI?
Pilots should disregard a PVASI whenever the signal cannot be identified reliably, the aircraft is outside its intended viewing sector or continuing would require an unstable correction.
- Off-axis approach: the indication is intended for the final approach sector, not circling flight or terrain clearance well away from the extended centreline.
- Poor visibility or glare: PVASI is visual guidance, not a substitute for approved instrument guidance.
- Uncertain or unserviceable signal: do not guess whether a distant or intermittent light is the PVASI.
- Large eye-to-wheel height: the indicated path guides the pilot’s eye, while the landing gear is lower. Aircraft with substantial cockpit-to-wheel geometry must use the applicable runway compatibility and threshold-crossing guidance.
- Runway decisions: PVASI says nothing about landing clearance, runway occupancy, crosswind, braking action or available landing distance.