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Aviation History
1958
1958 - 0436.PDF
452 FLIGHT, 4 April 1958 Chasing the Ice Gremlin By W. F. GIBB "In the cabin was an automatic observer panel. . . . An automatic camera was also set up to take photographs of the panel at one-minute intervals." WE were wished godspeed at London Airport by theMinister of Transport; and at seven minutes past six onMonday evening, November 25 of .last year, Britannia G-AOVA left the runway and turned on to a heading of 160 degbound for Bahrein. There were 46 of us on board—five aircrew including myself, four B.OAC. representatives and a comple-ment of technicians from Bristol Aircraft and Aero-Engines. Our final destination was Singapore, where we were to be based forabout three weeks engaged on a series of weather exploration flights, the main object of which was to test the operation of amodification to the Britannia's Proteus engines and clear the air- craft for flight in conditions of mixed wet and dry ice. It is perhaps worth mentioning in outline the series of eventswhich led to the exercise on which we were engaged. As a result of direct experience with the Proteus engine during the course ofthe past two years we had come to recognize three distinct types of icing condition which can exist in the atmosphere. The firstof these is the classical variety in which super-cooled water drop- lets freeze on impact with cold surfaces. Extensive tests inCanada during the winter of 1954-55 showed the engine and powerplant icing systems to be completely effective in preventingice accretion, and following tests in an Ambassador aircraft the Proteus was granted unrestricted clearance for flight in severeicing conditions. At the time this glazed ice was the only type of icing which wasregarded as a likely cause of problems where the operation of aircraft was concerned. In October 1955, however, while wewere carrying out route proving flights in Africa, we came across a new type which caused malfunctioning of the engines. Occurringin ice-crystal clouds at temperatures generally below —15 deg, it came to be known as "dry" ice, since it produced no visible accre-tion on either the wings or the engine cowlings. Modifications to cope with this problem were incorporated by the end of thefollowing year, and in February 1957 B.O.A.C began their scheduled Britannia services to South Africa. All went well untilon the Australian route later in the year a third type of icing was encountered which again caused malfunctioning of the engines.Tests showed that the latest difficulty was caused by a mixture of water with dry ice conditions, and that it was not fully overcomeby the modification introduced earlier to deal with the dry type of ice on its own. A limitation was therefore placed in the Britannia flight manual.This recommended that prolonged flight should be avoided in cloud above 16,000ft where the indicated outside air temperaturewas between + 1 deg C and + 9 deg C. As we climbed into the night sky on that Monday during lastNovember it was our intention during the next few weeks to contravene this recommendation as much as the weather con-ditions would permit. In the two outboard engines of G-AOVA was, we believed, an answer to the "wet and dry" problem. Ouraim was to see that the icing limitation was removed from the Britannia's flight manual. Two thousand five hundred miles and 10 hours 45 minuteslater we touched down in dirty weather at Bahrein. The flight had been uneventful, though the route we had taken was slightlydifferent from usual: political disturbances in the area had pre- THE AUTHOR W/C Walter Gibb, D.SO, D.F.C., A.F.R.Ae.S., is chief te»t pilot of Bristol Aircraft, Ltd, With the company before the war, he rejoined them in 1946 after a distinguished R.A.F. career, mainly on night fighters. In the Olympus- Canberra, in 1953 and 1955 he twice raised the world's aeroplane altitude record, on the second occasion to 6S,890ft. vented our overflying Syria and we had been obliged to go byway of Corfu, Istanbul, Ankara, Baghdad and Basra. The mem- bers of the flight crew were all ready for bed after having been onduty through the night, so we retired to get some sleep before departure that evening. After several hours of welcome sleep and having had a high teaat the airport we were ready to set off on the last stage of the journey. Our course took us directly across India by way ofBombay and Madras. Just about the time we were crossing the west coast the torquemeter on No. 1 engine began to show a10 per cent power loss. It didn't seem to indicate anything very serious, so I decided to carry on as we were. After passing overthe island of Car Nicobar we turned south-west down the Malacca Straits and were about 120 miles out of Singapore when theoverspeed limiter cut No. 1 engine. We landed at Paya Lebar airport, Singapore, just before noon on the 27th. - Inspection showed that the cause of the engine failure had beena cracked breeches-piece.* This had apparently resulted in over- heating of the powerplant wiring followed by a short circuit inthe overspeed limiter, which had then cut the engine. The next two days, November 28 and 29, were spent in rewiringthe faulty powerplant and making last-minute preparations before starting the test programme. Before we left England, tenhours' flying on G-OAVA had been completed to check the functioning of the modifications installed in the outboard engines.Basically, the new device consisted of a number of jets fed from a compressor tapping and arranged to inject air through the outerwall of the powerplant intake just ahead of the elbow where ice tended to accumulate. This had the effect of speeding up theboundary layer in this area, sufficiently, it was hoped, to prevent the ice from building up. Having established at home that thesystem as such functioned correctly, it was now up to us to prove 1 that it would do the trick in the worst possible mixed icing con- ditions we could find. On Saturday, the 30th, we got down to some serious work witha seven-hour flight, the main purpose of which was to complete the necessary engine calibration. During the afternoon heavymonsoon clouds began to build up and I decided to make a preliminary excursion into some heavy frontal cloud that wasforming over the east coast of Malaya. This first spot of cloud flying gave us a chance to test the various items of meteorologicalequipment which were to be used for measuring water concen- tration and detecting ice. A simple device, christened the oxo-meter, was used to establish the total water-content of the atmosphere. It consisted of an open-ended tube curved into aquadrant so that the open end faced forward in the line of flight. The closed end was mounted in the escape hatch above the flightdeck and connected to a system of cocks enabling timed samples to be taken from which the total water-content could be estimated.In addition we had five electrically operated ice-detectors on various parts of the aircraft. Three of these were mounted on anaerofoil mast, alongside the oxometer, at distances of one, two and three feet from the fuselage skin. The other two werepositioned midway between the starboard pylons in the cowl intakes of Nos. 3 and 4 engines. Met. Office in London had told us that the most likely place tofind the type of cloud we were after was on the east coast—where, in fact, we had found it during that flight. We established thatthe necessary amount of cloud did not build up until during the afternoon, so it was decided to plan the day accordingly. Pre-flightpreparations were carried out in the mornings, and lunch was scheduled for 1145 hr, allowing us to be on board by 1245 fortake-off at 1300. The next five days were devoted to extensive cloud flyingoperations during which we averaged about six hours daily. Fortunately the weather we wanted was fairly close to base—and, *Pan of the ducting for hot gas tapped from the Proteus turbine sectionand used to provide heat for de-icing and other purposes.
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