T-VASIS (& AT-VASIS) are quite maintenance heavy, and it also needs to be done by techs who know what they are doing. This generally means ex Departmental guys, who are all retiring about now. The average country airport nowadays probably has to get the local sparky in, or relies on importing specialist expertise to do the job when it's required. AsA kept maintenance of all the navaids, but all aerodrome lighting was given back to the aerodromes, so rarely did they have the quite specialised knowledge to maintain airport lighting, especially precision lighting. PAPI is much cheaper to maintain. Basically once installed, the light units can be unmounted, and a replacement clipped into place without disturbing the alignment. Thus a stock of spare units, (which are all the same I think, just depends on the angle of mounting) may be kept, or the units can be sent away for maintenance, or even just repaired inside at the airport workshop. T-VASIS are very susceptible to ground movement, have to be worked on on-site (2 man job for most tasks), and have many different unit types, thus needing lots of different spares. The lamps are also very expensive (a 100W "Day" lamp was over $100 back in the nineties - there's 4 to each box, 160 in an installation if double end, double side, and for a while, almost unobtainable), short-lived, and require focusing each time one is replaced - EVERY lamp had a different beam characteristic!). They also require specialist tools (ie the long accurate level) which I imagine are getting harder to keep in service, and constant maintenance - spiders love to spin webs inside the dark boxes because lots of insects are attracted to the light - and the webs catch dust and debris which affect the light output. If the airport has a double ended, double sided VASIS system, that's 40 boxes of headaches vs 16 for the same system if PAPI. It's no wonder they are being replaced.
As per flight-testing, I think the Dept used to do annual or bi-annual flight tests of T-VASIS, which consisted of flying in a ground party with a tracking theodolite, and then monitoring the aircraft flight slope with that. Back in the day, I seem to recall that any more than half a turn needed on a levelling screw to bring it back level was required to be reported back up the chain, and would result in an adhoc flight test being scheduled, even if just an approach to check symmetry, which is something ground maintenance can not check. Nowadays they probably only do it randomly, or after a report that the system is out of alignment or needs to be checked after major repair. The airport probably gets charged for that too, which is another reason faults may go unreported.

From my days at the DCA Flying Unit my guess is you won't get too many written reports by pilots observing perceived PAPI light signal problems. Human nature being what it is, if they see what they think may be a problem with PAPI lights, most pilots will think "not my problem" and leave it to the next guy to report it. The process gets repeated and nothing happens.

I was referring more to T-VASIS alignment/symmetry problems, rather than PAPI, but much of a muchness. Things get reported directly at TWR'd airports all the time. It's the unmanned ones that are the problem I guess. Faults are noticed on the approach when things are busy, put on the back burner for comment during the turn, then probably forgotten during the start and takeoff.

In this modern age of high intensity long lived LED. What would it take to make a solid state box as a single fit replacement? Given the choice...VASIS or PAPI?

YLEC has AT-VASI.

DF.

Yeah ok fair enough.

DF.