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?