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I have a question about the PAPI system. One of the first things student pilots learn during their ground school is the PAPI. You get two whites and two reds on a visual approach, you're on the 3 degree glide path. If you get three reds and one white, you're a little low. If you get four reds, then you may consider going around since you're way too low.

My question is: Can we always trust the PAPI lights? Is there no chance the PAPI lights might be wrong, so for example, when you get three whites and one red on the 5 mile final, but you are actually below the 3 degree glide path?

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Yes, you can trust the PAPI lights. As usual in aviation, safety is priority #1, this also applies to airport lighting, which is checked and monitored continuously.

PAPI lights are calibrated regularily to ensure their indications are correct. As with all technical equipment, there is a non-zero risk PAPI lights may fail, however, given how simple they are (literally just a light bulb, a colour filter and a lens - no moving parts) a failure is very unlikely.

You should note that not all PAPI lights are calibrated for a 3° glidepath. While 3° is most common, there are runways where the normal approach angle is different and the PAPI lights will be calibrated to that. So make sure to check the airport charts before using the PAPI system, or you may risk coming in steeper or shallower than you are used to.

Also note that, as pointed out by @wbeard52, approaching with the PAPI showing two whites and two reds is not a guarantee of obstacle clearance if you are far from the runway - it is only a guarentee that you are following the nominal glideslope of the approach procedure with which the PAPI is aligned.

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    $\begingroup$ For a student pilot, PAPI gives an easy reference, but one really should learn to judge glidepath without PAPI, VASI or any technical aid... I know this sounds like oldskool bs, but you prolly are not going to always fly to airfields with tech aids... $\endgroup$
    – Jpe61
    Commented Sep 13, 2020 at 20:21
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    $\begingroup$ If you capture the PAPI too far out, it may not keep you safe. See my answer below. $\endgroup$
    – wbeard52
    Commented Sep 13, 2020 at 21:00
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    $\begingroup$ @Jpe61 that's not oldskool bs, judging an approach by sight alone is very important and very accurate. Humans are pretty good at discerning small angular differences. More than once I've been on final and it looked right, but the papi was showing all red/all white, and when reported it was discovered the box had been knocked out of alignment somehow. $\endgroup$
    – nexus_2006
    Commented Sep 14, 2020 at 17:15
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    $\begingroup$ No, you should not ALWAYS trust any one thing, you cross-check all the available information. As @nexus_2006 says, accidents happen, and especially at smaller airports, may not get reported & repaired immediately. Say for instance the person mowing the grass around the runways happens to hit the box and doesn't bother to report it. $\endgroup$
    – jamesqf
    Commented Sep 14, 2020 at 18:12
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    $\begingroup$ Before I experienced it myself, I agreed with your opinion, that a PAPI is very unlikely to fail. Then I came out of the clouds on an IFR approach at about 500ft and saw the PAPI indicating: white/red/white/red. I don't know what was wrong with it, but a NOTAM was published afterwards that the PAPI is now out of service until repaired. $\endgroup$
    – TobiBS
    Commented Sep 15, 2020 at 8:50
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As with all things in aviation, there is a service volume or limit to the area PAPI and other visual approach lights ensure obstacle clearance. It is important to know what those limits are. I have heard of multiple accidents where the pilots were on the PAPI but still hit terrain because they intercepted the PAPI lights too far out without much thought about the terrain around them.

Please reference AIM 2-1-2

Precision Approach Path Indicator (PAPI). The precision approach path indicator (PAPI) uses light units similar to the VASI but are installed in a single row of either two or four light units. These lights are visible from about 5 miles during the day and up to 20 miles at night. The visual glide path of the PAPI typically provides safe obstruction clearance within plus or minus 10 degrees of the extended runway centerline and to 3.4 NM from the runway threshold. Descent, using the PAPI, should not be initiated until the aircraft is visually aligned with the runway. The row of light units is normally installed on the left side of the runway and the glide path indications are as depicted. Lateral course guidance is provided by the runway or runway lights. In certain circumstances, the safe obstruction clearance area may be reduced by narrowing the beam width or shortening the usable distance due to local limitations, or the PAPI may be offset from the extended runway centerline. This will be noted in the Chart Supplement U.S. and/or applicable NOTAMs.

Visual Approach Slope Indicator (VASI). The VASI is a system of lights so arranged to provide visual descent guidance information during the approach to a runway. These lights are visible from 3-5 miles during the day and up to 20 miles or more at night. The visual glide path of the VASI provides safe obstruction clearance within plus or minus 10 degrees of the extended runway centerline and to 4 NM from the runway threshold. Descent, using the VASI, should not be initiated until the aircraft is visually aligned with the runway. Lateral course guidance is provided by the runway or runway lights. In certain circumstances, the safe obstruction clearance area may be reduced by narrowing the beam width or shortening the usable distance due to local limitations, or the VASI may be offset from the extended runway centerline. This will be noted in the Chart Supplement U.S. and/or applicable notices to airmen (NOTAM).

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  • $\begingroup$ This is an interesting point, but, it doesn't mean per se that you can't trust the PAPI outside 3.4NM - the indication should still be correct. It just means that obstacle clearance might not be guaranteed beyond that distance. OP is asking specifically if the indication may be wrong: "for example, when you get three whites and one red on the 5 mile final, but you are actually below the 3 degree glide path?" (still upvoted btw) $\endgroup$
    – 60levelchange
    Commented Sep 14, 2020 at 11:05
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    $\begingroup$ @Bianfable I would tend to agree with you. If you can see the PAPI's then by definition there is nothing between your eyesight and the runway. Unfortunately, there is the underside of the airplane that could be subjected to terrain even though you can see the PAPI's. During the day, you will most likely see that terrain and fly higher or around the mountain. At night, you will only notice the lights surrounding the terrain leading to the runway start to disappear as you get close to the runway. It is those dark spots at night (in a city environment) that concern me. $\endgroup$
    – wbeard52
    Commented Sep 14, 2020 at 20:30
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    $\begingroup$ still hit terrain because they intercepted the PAPI lights too far out Source? If you can see the PAPI, how can terrain later appear between you and the lights? Unless it's pitch black... and you're flying VFR? $\endgroup$
    – J...
    Commented Sep 14, 2020 at 21:29
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    $\begingroup$ The source is in the answer. Just because you can see the lights, that means there is no terrain between your eyesight and the runway. There is still 5 FT in a Cessna 172 that can hit terrain below you. It has happened before. $\endgroup$
    – wbeard52
    Commented Sep 14, 2020 at 21:42
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    $\begingroup$ @wbeard52: It might not be the terrain you hit, but the power lines on the 40 ft poles running along the crest of the hill :-) $\endgroup$
    – jamesqf
    Commented Sep 29, 2020 at 16:48
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When you're wondering about failure, usually one of the first things to consider is failure modes. Bulbs burn out, power can fail, etcetera. But the symptoms you mention do not match those failure modes. An earthquake could, but in that case the runway itself may have bigger issues.

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If you fly in cooler climate, frost on the PAPI lenses has been known to cause false signals. See Transport Canada Aerodrome Safety Circular (ASC) No. 2002-014, available here. Mitigation strategies make it fairly unlikely that you will encounter this.

In Canada, if ATC is operating, they will switch them on to warm them up in advance of an arrival. In places where pilot-controlled-lighting is used (called ARCAL in Canada), you might see the PAPIs left on continuously for the same reason.

I'm with wbeard52, these things have a maximum range, even though you might see them from way further out. Make sure you're close enough before you start using them.

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One thing to keep in mind when flying GA small aircraft is that often the papi is not ment for us but for bigger craft which touchdown point is often 1000ft after the threshold. Being on the papi to intercept the glide path far from the runway and then aiming for the threshold will alway gives you higher slope than expected. When learning to land you'll probably focus on the threshold and thus you'll most of the time have 4 reds on final even if you are following the right slope.

Otherwise if you're aiming for the right landing point, you can trust the papi as it is checked quite often for safety reasons.

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