Plasi Glide Angle Approach Indicator

Recently, the Australian agent, ATS Australia Pty Ltd, invited AA to view a PLASI which has been operating over the last two years as a demonstration unit at the Hazelton Airlines’ Cudal Airport. Hazelton and ATS are jointly promoting the PLASI system in Australia. Having flown deck landing approaches using the mirror landing sight, I was keen to see for myself what the PLASI could offer in the way of glide slope guidance. Accordingly, on a perfect winter’s day, I flew up to Cudal in my trusty Musketeer, to shoot a few approaches and to discuss with Hazelton the merits of the equipment.

The Plasi Concept

The PLASI system, which has an effective visual range of about 5nm (9.3km) during the day and up to 20nm (37km) at night, projects a single light signal from an installation next to the runway, located adjacent to the touchdown point. When on the correct glidepath, the pilot will see a steady white light. If above the correct glidepath, the pilot will see a pulsing white light, increasing in intensity the further the aircraft deviates above the glideslope. If the aircraft descends slightly below the glidepath the pilot will see a steady red light. The steady red light is one half of the vertical white light segment. If the aircraft continues to descend further below the glidepath, the steady red light will commence to pulse. The further the aircraft deviates from the glidepath the faster the pulses appear to the pilot.

The on-glidepath steady white light is an angular wedge of 1/3° which translates to approximately 1° dots above or below the glidepath on an ILS glideslope indicator. The horizontal beam is a wedge of 16°, 8° either side of the centreline. The vertical approach angle can be adjusted to provide the desired glideslope taking into account obstacle clearance requirements, environmental considerations such as noise abatement, or to accommodate a particular aircraft type.

Technical Aspects

Keith O’Callaghan, who had installed the equipment, was my guide and mentor throughout our visit, and following a general briefing on the setup at Cudal, took me out to the end of runway 27 to view “the box”. The unit is relatively small, measuring less than a metre high and half a metre wide and sits on adjustable legs on a concrete base. An electric motor drives the mechanism, which generates the four horizontal bands of light through optical components and moveable shutters, to provide the pulsating effect. A 900w tungsten halogen lamp is positioned behind a condenser lens as the light source. A lamp changer table containing five lamps automatically moves a new lamp into the operating position if the one in use fails; this is controlled by an electric photo sensor.

The basic power supply required is 115 volts AC but a portable generator of 2.5KVA can be used as a power source when PLASI is set up as a temporary installation. Once the required brightness has been adjusted through a manual control, a light sensor then automatically controls the day/night voltage. Remote control of the equipment can be accomplished by hard wiring or radio, with the latter capable of being keyed by an aircraft transmitter. A tilt switch provides a safety function to cut the power off if the unit is jolted sufficiently to move the beam 0.7 to 1.0 of a degree up, or 0.4 to 0.5 of a degree down. A time delay prevents the tilt switch reacting to transient disturbances.

Operational Aspects

From my brief discussion with Max Hazelton I gathered that Hazelton Airlines’ pilots have enthusiastically accepted PLASI as a valuable aid to their operations into the airport. The unit is particularly useful at night when approaching from Orange when it can be picked up almost immediately after clearing the high ground between the two airports. An obvious question to ask was, was it of any value in carrying out a circling approach from an NDB letdown? Unfortunately because the beam width is only 8° either side of the runway centreline, glideslope guidance is only obtained when almost lined up. Digressing slightly, this was one of the problems that had to be solved in the development of the mirror deck landing sight where it was necessary that glideslope guidance be available from a position approximately 45 degrees off the ship’s centreline on a curved base leg. It was achieved by projecting a source light into a mirror which was concave in the vertical plane, with datum lights either side for reference.

The technical implementation of the PLASI concept limits the degree to which lateral guidance can be achieved, but it would be comforting in less than favourable weather such as rain to know that once lined up precise information as to the aircraft’s position in relation to the glideslope is available. This would certainly help to relieve the workload under such circumstances. The cost of a PLASI ex-works USA is approximately $A24,000 and the Australian agents estimate installation and commissioning costs at about $6000. This does not include cabling and trenching which will vary from airport to airport. The manufacturer claims significant annual operating cost savings over Precision Approach Path Indicator (PAPI) and Visual Approach Slope Indicator Systems (VASIS) with PLASI half that of PAPI and a quarter of VASIS.

This is because PLASI is a single box installation with significantly lower power and maintenance requirements. PLASI is FAA approved; however the CAA has stated that since the equipment is not endorsed by ICAO, and not presently being considered for approval, it cannot approve its use at an aerodrome where a VASIS is a mandatory requirement. In Australia, a VASIS is a requirement where RPT operations involving jet turbine powered aircraft are conducted. Apart from that exception, the CAA has authorised the installation and use of PLASI at any other licensed or unlicensed aerodrome, noting that the aerodrome operator bears the responsibility for the integrity of the system. Details of any PLASI installation are required to be provided to the CAA for inclusion in the Enroute Supplement (ERSA).

Other Applications

A development of PLASI which has gained worldwide endorsement is the Helicopter Approach Path Indicator (HAPI-PLASI). The HAPI-PLASI is set to a nominal glidepath of 6 to 9° and can be installed on a powered rotating turntable to accommodate various predetermined approach paths. This can be activated by remote control. The system has proved to be particularly useful for oil rig installations and trials have been conducted on rigs in the Bass Strait. Two other developments are the MINI PLASI and the IR PLASI. The MINI PLASI is a lightweight portable unit which has found favour with the USAF for use in conjunction with its emergency airfield lighting system. The IR PLASI is an infrared unit for military use with night vision goggles.

Flying The Plasi

Having finished the technical brief I flew a couple of approaches to try it out for myself. My immediate impression was that PLASI was simple to use, operated in a natural sense, and required nothing in the way of practice to qualify. The demarcation between on-glide slope and divergence, either high or low was sharp with absolutely no blurring in between. An aspect I particularly liked was the active presentation of off glideslope information, i.e.; the light beam pulsing provides an added urgency to make a correction.

Comment

With such ready acceptance of the safety enhancing value of the PLASI concept overseas, it seems strange that it hasn’t apparently yet found favour in Australia. A possible explanation of course is our mostly good weather, particularly the generally excellent visibility we enjoy. Whilst it is true that an experienced pilot will instinctively know whether he/she is high or low by the runway aspect, both by day and by night, in rain or less than favourable visibility, when the workload can be high, I believe there would be an added safety factor in having PLASI as a backup. Perhaps this is a matter for consideration by regional airport operating authorities. My thanks to Hazelton Airlines and ATS for the opportunity to view and assess the PLASI concept.