View Full Version : Mount Gambier T-VASIS ERSA caution

28th Nov 2003, 12:25
Some may recall the RFDS Super Kingair crash at Mount Gambier during a night approach to runway 18 in low cloud, mist and light rain. Date December 2001. The pilot was killed but the nurse aboard escaped minor injuries. The aircraft caught fire and was destroyed. The crash site was 3.1 miles short of the runway close to the extended centreline.

The ATSB report stated that the T-VASIS was operational and serviceable. ATSB were unable to find a cause of the accident although they hinted that perhaps the pilot flew below the DME (GPS) steps and crashed while not visual. One witness (not mentioned in the ATSB report) stated that he saw the landing lights of the aircraft shortly before it crashed.

There was no mention in the ATSB report of the possibility of erroneous T-VASIS glide slope indications although AIP AD 1.1-30 paragraph 5.1.1 Note 2 warns of the possibility of erroneous glide slope indications with any T-VASIS where weather consists of thin layers of ground fog or mist.

The BASIS Aviation Safety Digest (114/1982) revealed an incident at Perth where an RPT aircraft followed an erroneous T-VASIS fly-down signal while at the same time the ILS showed the aircraft low on the electronic glide slope. The pilot recovered in time but stated that the T-VASIS continued to provide false fly down presentation throughout. The T-VASIS was flight tested the next day and found to be operating correctly. BASIS stated in the article that the limitation of T-VASIS is that they are likely to produce erroneous indications in fog or mist and the systems cannot necessarily be relied upon in these conditions.

Fog and mist was present in both night approaches into Perth in 1982 and Mount Gambier in 2001.

Erroneous indications is an understatement. If not immediately recognised at night they can be deadly.

The latest ERSA (November 2003) now has the first acknowledgement that an erroneous T-VASIS indication may have occurred. It reads as follows: For Mount Gambier...Note 2..CAUTION: RWY 18/36 AT-VASIS susceptible to erroneous fly-up or fly-down signals under certain weather conditions.

It may go some way towards explaining why a highly experienced RFDS pilot with hundreds of black hole night approaches to his credit, would inexplicably fly wings level into the deck 3 miles on final approach. It was his first and last night landing into Mount Gambier.

29th Nov 2003, 18:12
The one and only purpose of T-VASIS is to provide the pilot with on-slope, fly-up and fly-down indications, especially in poor visual conditions. The concept that T-VASIS seem to provide exactly the opposite signals to those required under some poor visual conditions, thus creating a more unsafe situation than if they were switched off seems a bit stupid.

Why do they still exist when there are safer visual approach indication systems such as PAPIS available? Is it simply cost? Is it because the users (us pilots) donít complain enough about the limitations of such systems? And why is it that Australia seems to be one of the few (only?) countries in the world that still use T-VASIS?

Has anyone had any personal experience that might validate the possibility of erroneous T-VASIS glide slope indications at Mount Gambia?

The Hedge
29th Nov 2003, 18:16
Try Here (http://www.pprune.org/forums/showthread.php?s=&threadid=95367)

29th Nov 2003, 18:51
All quite amazing... I thought that it was common knowledge that the T-Vasis is subject to "errors" in conditions of particulate moisture due to refraction.

I would be surprised if any Australian pilot with much experience in night operations hasn't seen simultaneous fly up/down indications in conditions of ground fog. Not much different to all the other visual illusions with which we have to contend. I well remember one of my early night training sessions at Bankstown years ago .. ground fog rolled in from the other end and I was suckered into the illusion in such conditions which leads to a nose over .. increased descent rate .. and short landing... fortunately nothing was damaged other than my ego.

Perhaps the problem is using aids as command guidance rather than just another piece of information for the pilot to integrate in the puzzle's solution .. same can be said for ILS erroneous indications .. the Air New Zealand report into the Apia incident makes for very sobering reading with regard to this latter problem.

29th Nov 2003, 19:14
PAPI is still subject to erroneous indications in mist/moisture except that the colour is affected rather than the pattern. As some airline pilots are partially colour blind yet can still legally hold an airline pilot's licence, it becomes a serious matter when they cannot always interpret a normal PAPI because of difficulty in seeing the red lights. Imagine the problem if they see erroneous indications with colours merging in mist.

They have no problems with T-VASIS as it is not colour discriminatory -unless you are below 1.9 degrees when it goes all red - but you would have recognised a low approach before then. Unless the fog/mist bogey appears.

It is not that PAPI is more safe than T-VASIS. It is cheaper and easier to maintain, so I understand. That is why PAPI is appearing at more aerodromes in Australia and replacing T-VASIS.

29th Nov 2003, 19:29
Does anyone recal 'X VASIS'? Used to be one at Mt Isa; on slope showed a white X, low showed a red fly up symbol ^ and high showed a white fly down V.

PAPI has proved to be more reliable than T Vasis in the deceptive condition discussed here; its lower cost is a bonus.

30th Nov 2003, 07:24
The problems with false T-VASIS indications in certain weather conditions have been known for 40 years. Despite this, it is still considered to be the best visual approach slope guidance system in the world, cost notwithstanding.

If you want to learn more about the history of T-VASIS, go to www.airwaysmuseum.com (http://www.airwaysmuseum.com) and go to 'History', then 'Communications and Navigation'.

30th Nov 2003, 23:35
The comments both for and against T-VASIS are somewhat illuminating, to coin a phrase.

But, perhaps the comments regarding the known dangerous indications of T-VASIS and yet the continued support of T-VASIS seems to speak volumes about what we pilots are prepared to accept as the norm.

One wonders what the ill-fated and highly experienced RFDS pilot might have added to our debate if it were possible?

As Centaurus is hinting, maybe we owe it to this pilot upon whom fate has smiled unkindly, to pursue this problem to a deeper level and find a resolution before the System finds another black hole with attendant further loss to our ranks?