Two whites and two reds — and you are exactly where you should be. Drift high and the reds turn white; sink low and the whites turn red. The PAPI is the most honest instrument on final approach, and EASA loves to test whether you truly understand it.
Most students memorise the rhyme and stop there. The EASA ATPL exams reward the candidates who go further — who know the ICAO geometry behind the lights, why a low cockpit can lie to you, and where the PAPI sits in the syllabus. Here is what you need.
What a PAPI actually is
A Precision Approach Path Indicator (PAPI) is a bar of four light units that gives visual glidepath guidance on final approach. Under ICAO Annex 14 and the Aerodrome Design Manual (Doc 9157), the standard installation sits on the left-hand side of the runway, roughly 300 m beyond the threshold, with the four units spaced about 9 m apart and the inner unit set back around 15 m from the runway edge.
Each unit throws a sharp beam that is white in its upper segment and red in its lower segment. The colour transition is engineered to happen over an angle no greater than three minutes of arc, which is why the switch from white to red looks so crisp from the flight deck. Set each of the four units to a slightly different angle and you get a four-step scale of your position on the slope.
Reading the signal
The rule is simple once you see the geometry. More white means too high; more red means too low. On a correct approach the two units farthest from the runway show white and the two nearest show red — two white, two red.
- Four white — well above the slope. "Check your height."
- Three white, one red — slightly high.
- Two white, two red — on slope. "You're all right."
- One white, three red — slightly low.
- Four red — well below the slope. "You're too low" — correct or go around.
The classic mnemonic — red over white, you're all right; white on white, check your height; red on red, you're too low — is a memory aid, not an exam answer. Examiners want the underlying logic.
The numbers EASA can ask you
For a nominal 3° approach slope, the four units are set at 3°30′, 3°10′, 2°50′ and 2°30′ (that is 3.50°, 3.17°, 2.83° and 2.50°). The designated glidepath sits midway between the second and third units — so "on slope" is the band between 2°50′ and 3°10′. Fly above 3°30′ and all four read white; sink below 2°30′ and all four read red.
Two more facts worth carrying into the exam hall. In good visibility the PAPI is usable from about 5 NM by day and night, and the light bars can be picked up at ranges of 20 NM or more at night. An abbreviated version, A-PAPI, uses only two units for certain lower-category operations.
PAPI versus VASI — know the difference
EASA questions like to contrast the two systems. The older VASI (Visual Approach Slope Indicator) uses bars of lights and typically gives guidance only down to about 60 m (200 ft). The PAPI is a single row of four units and provides guidance all the way down to flare initiation, around 15 m (50 ft). If a question hinges on which system guides you deeper into the approach, the answer is PAPI.
The trap: eye height and the MEHT
Here is where good candidates lose marks. A PAPI is calibrated to a Minimum Eye Height over Threshold (MEHT) — the geometry assumes a particular pilot eye position. Fly an aircraft with a low cockpit and you may see a "slightly low" indication even while you are on the ILS glideslope, because your eyes sit below the reference. The famous opposite case was Concorde: its main gear sat so far behind the flight deck that pilots deliberately flew a "slightly high" PAPI to keep the wheels safely over the threshold.
This is also where the PAPI connects to Human Performance. On a dark night or over featureless terrain — the black-hole approach — or when a runway slopes, your eyes will happily talk you into a dangerous profile. The PAPI is the objective cross-check that defeats those visual illusions.
Where PAPI shows up in your ATPL exams
PAPI is a genuinely cross-subject topic. It lives mainly in Operational Procedures (070), appears in Air Law (010) through the ICAO Annex 14 aerodrome visual aids, and ties into Human Performance (040) via approach illusions. Expect it in your theory papers and, later, in your skill-test briefings. Learn it once, properly, and it pays off three times.
Frequently asked questions
What do PAPI lights mean?
Two white and two red means you are on the correct approach slope. More white lights mean you are too high (four white is well above the slope); more red lights mean you are too low (four red is well below).
What is the difference between PAPI and VASI?
Both are visual glidepath aids. VASI is the older system and generally guides down to about 60 m (200 ft), while PAPI uses a four-unit bar and guides all the way to flare initiation at roughly 15 m (50 ft).
What angle is a PAPI set to?
For a standard 3° slope, the four units are set at 3°30′, 3°10′, 2°50′ and 2°30′. The on-slope indication of two white and two red corresponds to the band between the second and third units, centred on 3°.
Which EASA ATPL subject covers PAPI?
Primarily Operational Procedures (070), with links to Air Law (010) through ICAO Annex 14 and to Human Performance (040) through approach visual illusions.
Ready to turn theory into recall? Drill PAPI and hundreds more Operational Procedures questions in the exam-style bank at atpltraining.io.
Photo by John McArthur on Unsplash
