An incident during flying last night reminds me of why a partial failure can be more complex than a total failure.

Our home airport, CYFD, is open day & night. In poor visibility conditions (fog or darkness), a lighting system assists pilots in locating and descending to the runways for landing. Part of this is the PAPI, which is a color-coded indicator near the runway threshold, which tells pilots whether they are above, on, or below a normal 3-degree glide slope.

Following a normal glide slope for the last ~30 seconds of the flight makes it more likely to have a safe landing. Following the PAPI (or its predecssor VASI) visual indicator at night is important when one cannot see ground obstructions, when the runway edge lights alone could confuse a pilot due to unusual width/length/flatness. The rule is to line up the airplane on an inclined path where of the lights in the PAPI, half are steady red and half steady white. (Too white: too high; too red: too low.)

What if the PAPI is broken? Of course one can still land without it, but with a smaller safety margin. The airport would usually issue a NOTAM - a "note to airmen" - a broadly electronically distributed notice that reads something like this:

1404130010 TIL APRX 1404132000
Even the layman might decode this compressed goo with some hints. The bottom numbers are YYMMDDHHMM timestamps in UTC; U/S means unservicable. In theory, pilots operating in the area are supposed to read all such announcements before a flight, and air traffic controllers may need to relay new/important announcements right on the air.

But this particular NOTAM is misleading. Last night, the PAPI light was not merely unserviceable. It was broken, but still lit. In particular, it stayed lit in the worst possible way. Take thirty seconds to think about how that could be before you read on.

Time's up. The PAPI lights were ... all white, regardless of glide slope angle. The red filters must have gotten broken off, or the lights were hit and misaligned, or something. The white told pilots that they were too high, implying a suggestion to descend faster. This false "too high" indication continued even if an airplane was well below the glide slope (I tested that part carefully). An unwary pilot could trust the lights and descend right into the ground. It got me worried enough to mention the problem to Toronto area ATC by radio.

Now it's clear why I was talking about a partial failure above. In this case, the broken indicator could be worse than no indicator at all. The power should be shut off, or lights completely blocked, if such a problem occurs.

It's a good reminder that indications of all sorts - whether on board the airplane, on the ground, or in one's head - can be erroneous. Cross-checking and scepticism are essential in critical phases of flight.