Business & Commercial Aviation magazine 14 September 2014 contains an excellent article at page 38 called "The Blurred Approach". It deals with visual illusions in heavy rain.
The author Patrick Vieillette (
[email protected]) starts the article by saying "It is tempting to think that the hardest part of an instrument approach is the portion conducted solely with reference to instruments, and the challenge ends once the runway is in sight. However, that transition to the visual portion of the approach can pose a new set of challenges resulting from reduced visual cues or visual illusions, and the spatial disorientation that can follow."
If any of the editorial staff of the CASA/ATSB journal Flight Safety Australia happen to read B&CA they should seriously consider reproducing the article in FSA so that Australian licence holders can gain valuable flight safety knowledge from it. The article quoted an incident reported to the NASA Aviation Reporting System that graphically illustrated that VASIS lights can give erroneous indications which may not be detected by pilots until short final. In this incident, a corporate jet was landing at an uncontrolled field at night. Unknown to the pilots, the VASIS had been knocked askew by a tractor mowing the grass. The pilots thought they seemed a bit low, but trusted the VASIS. They noticed the VASIS lights seemed a bit intermittent, and then realised through their landing lights that they were actually skimming the tops of trees. An immediate go-around followed.
Upon inspecting the VASIS in daylight, it turned out the " on-slope" indication put the flight path through the trees.
Back in the 1980's an Air Nauru Boeing 727 came close to over-running Runway 30 at Nauru one night on to rocks of the seawall at the northern end. The 727 was on a straight-in approach when the captain sensed the aircraft was higher than normal for a three degrees slope on the A (abbreviated) T-VASIS. At about four miles the aircraft was one dot high so the pilot adjusted the flight path to be on slope. In those days the runway was 5600 feet long with the sea at both ends. At approximately 500 feet, the 727 was still stabilised on slope on the T-VASIS indications although the pilot still sensed the aircraft was slightly higher than expected.
The aircraft touched down further into the runway than expected from a three degree approach causing the pilot to apply maximum braking and full reverse. The 727 was stopped with 50 metres to spare. Investigation revealed that two of the VASIS Fly-down boxes of lights were inoperative. When the pilot saw one fly down light (which was in fact the top light fly- down light, he was unaware that the next two fly-down lights were out of action. T-VASIS vertical lights are numbered one to six with No 1 light the top fly-down and No 6 the bottom fly-up light.
Thus, when the pilot adjusted his flight path from what he thought was one dot high to no dots high or low, he assumed that the correct glide slope of three degrees had been attained. With no other visual cues available because of the dark night over the ocean his assessment was understandable. In fact, the aircraft was following a two dot high approach since lights 2 and 3 of the fly-down lights were inoperative. The VASIS indication to the pilot was on slope.
On 10 December 2001, a Beech Kingair flown by an RFDS pilot crashed four miles short of the runway at Mount Gambier during the course of a VOR/GPS (DME) arrival at midnight and in light drizzle. The pilot had reported being in the circuit area and would cancel SAR after landing. The ATSB investigation was unable to prove the reason why the aircraft had flown into terrain in the approach configuration. Later it was revealed the T-VASIS at Mount Gambier was notorious for giving erroneous glide slope signals during weather conditions of mist or light rain and where dew bulb and dry bulb temperatures were close to each other. In fact, they were on the night of the accident.
During an ILS at Manila, a 737 flown by the writer, became visual at three miles and noted the T-VASIS light signals were quite confusing. Both undershoot and overshoot signals were showing with the bar signals having some lights red and others white or simply missing. The light signals made no sense whatsoever.
The crew reported the problem to ATC who replied that they were aware of recent earth tremors that had affected the alignments of various light boxes. No NOTAMS were issued. When it was suggested by the 737 crew that the T-VASIS should be turned off until the problem was rectified, the reply by ATC was that local ATC Standing Orders required the T-VASIS be turned on for all jet arrivals.
PAPI lights are not immune to certain weather conditions where moisture-laden air can affect light rays. While landing in heavy rain is difficult because of rain refraction on the windshield, the availability of a VASIS of some sort is vital. However pilots should be aware that instances of erroneous light signals are known to have happened both due to weather conditions and human factors. It is therefore good airmanship to be aware of why false light signals may occur particularly at uncontrolled airports where daily inspections may not be conducted.
Finally from the sublime to the ridiculous. In December 1971, Cyclone Althea severely damaged airport infrastructure at Townsville. Water flooded the aerodrome and damaged the T-VASIS installations. The decision was made to completely replace all light boxes and a few weeks later a DCA F27 navigation aid calibration aircraft was sent to Townsville from Melbourne to conduct in-flight tests of the T-VASIS. Part of the flight test schedule included level flight at 800 feet from three miles back to over the runway, in order to assess the correct progression of T-VASIS signals from three red dots undershoot (1.9 degrees) to three dots overshoot. At the completion of the first run, the two pilots looked at each other with amazed looks with one saying "Did you see what I saw?"
What they had seen was instead of the three red undershoot lights at the start of the run at 800 feet and three miles from the runway, they saw the three white overshoot lights. It turned out that in the days earlier when the driver of the vehicle who laid down each light box in the space where the previous lights had been located, he inadvertently put them down for the reverse runway. T-VASIS for Runway 19 instead of those for Runway 01.
