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Aviation History
1977
1977 - 0085.PDF
FLIGHT Internationa/, 8 January 1977 79 Stopping the GPWS wolf-calls SUNDSTRAND, one of the largest pro ducers of •-,. ground-proximity warning systems (GPWS), has announced a Mk II GPWS. The com pany claims that the Mk II will over come some of the problems revealed by service experience. Warning times are as much as doubled, and Sund strand claims that spurious warnings will have been reduced by 85 per cent. Maintenance costs will also be lower. Statistics gathered by Sundstrand show that since the system was intro duced the average annual number of US accidents attributable to controlled flight into terrain (CFIT) has been reduced from about six to four. Sund strand now believes that GPWS can assist by identifying potential dangers latent in standard procedures as well as by providing warnings when errors have been made and a CFIT accident is imminent. Observations stemming largely from 37 reported GPWS warn ings were presented recently to the Flight Safety Foundation by Mr Don Bateman (engineering manager for flight safety and performance systems, Sundstrand Data Control). Steep initial approaches, capturing the glideslope from above, have been the most significant cause of unexpec ted GPWS warnings. One carrier recorded warnings on one approach in every ten. In this case GPWS alerted pilots to the potential danger and over six months there occurred fewer warn ings. "High descent rate" warnings fell from<98 to'71 per cent/1,000 approaches and "glideslope alert" warnings fell from 32 to four per 1,000 approaches. Some system users have reported GPWS performance shortcomings and low crew confidence. Sundstrand attri butes some of the problems to the introduction of hastily developed systems when the equipment was made mandatory. Designs were prematurely frozen and pilots have reacted against dubious performance in some The warning envelopes of the latest Sund strand Mk II GPWS, of which one is shown, have dual boundaries to provide an aural identification of mode before the "pull-up" warning. On the envelope shown there is also an expansion of the upper limit, depending on aircraft Mach number 2000 4000 6000 Closure rate.ft/min areas. Now that production and teeth ing troubles are passing their peaks there is a chance to consider the development of systems which will produce more acceptable performance. One of Sundstrand's Mk II improve ments, a response to operational ex perience, is the introduction of voice identification of the warning mode and revised warning envelopes. "Cry Wolf" Sundstrand does not expect to see the end of CFIT accidents. Because the GPWS tends to "cry wolf" when there is no immediate danger (and some pilots treat "high descent rate" warnings when capturing the glide- slope from above as a nuisance) pilots tend to pull the circuit-breaker when the warning sounds. Almost all pilots scan their instruments to verify warn ings during instrument flight before responding to the "pull up" command. Sundstrand predicts that one accident will occur in the USA each year with the circuit^breaker pulled, and that one accident will occur because the pilot fails to respond promptly to a GPWS warning. The most common example of pilot hesitation is scanning the altimeter after a "pull up" warning, and on approach many pilots apparently scan the radio altimeter, vertical speed in dicator and gear and flap positions. According to Sundstrand, pilots want to determine warning causes before they take positive action. The company believes that in future warning systems the pilot should be able to verify the warning quickly, by cross-checking to raw data or to an independent source. The value of GPWS can be increased by changing procedure slightly. Lowering approach flap before the field is in sight during non-precision, circling or surveillance-radar-assisted approaches will provide earlier warn ing of terrain closure at a penalty in noise. Difficulties Sundstrand acknowledges that there are likely to be rare GPWS problems of which little evidence has come to light. One design change followed a night take-off incident with a Boeing 747. The aircraft was fully loaded and operating from a high-temperature airport. When the crew established initial climb attitude and retracted the gear, the stick-shaker operated. The crew lowered the nose and the shaker stopped momentarily, but it restarted again and the pilot further reduced the angle of attack. Both pilots realised that speed was building up despite the warning; when a GPWS warning was triggered the crew pulled the stall-warning circuit-breaker and re-established the climb. It was later found that a faulty gear-tilt switch caused the over-rotation warning. On the strength of this incident Sund strand recommends that GPWS should n • 3 i i FMonics not be automatically inhibited in the event of a stall warning, a feature which has been suggested in the past. Sundstrand expects that as in-service experience with GPWS increases, and as details of more incident reports are received, the total value of the sys tem will become more apparent and more modifications will be included. New antennas from CCC A HIGH-STRENGTH glass-fibre and epoxy VHF radio antenna has been designed for high-performance air craft by Communication Components Corporation (CCC) of California. The blade antenna can resist high side- loads, has a metal leading edge for erosion protection, and its radiating elements are encapsulated for shock and contamination protection. Opera ting frequency range is 225-400MHz. Also new from CCC is a series of broadband FM communications an tennas suitable for both fixed-wing and rotary aircraft. The three new aerials operate in the range 30- 80MHz, and can be either vertical or swept back 45° or 90°. The 90°-swept model is designed specifically for helicopters and low-speed aircraft. FAA updates American ATC TWO RECENT CONTRACTS from the FAA will provide new equipment at several American air traffic con trol centres. Direct-access radar channel (DARC) systems have been ordered from Raytheon, at a cost of $11-2 million, to equip the 20 National Airspace System (NAS) centres. A back-up system linking radar digitizing equipment and con trol positions, DARC permits air traffic services to be maintained in the event of a failure of the main data-processing system. The contract calls for delivery of prototype equip ment to Atlantic City 13 months after the initial order, and for a peak pro duction rate of one per month. A second contract, worth $820,000 to Bendix, will provide 15 secondary radar interrogator beacons for ter minal and en route radar sites cur rently operating with primary radar only. The beacons will be delivered between July and October of this year.
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