FLYING SAFETY

New Zealand Herald, Volume LXXVII, Issue 23748, 30 August 1940, Page 6

FLYING SAFETY

AIBCBAFT OVERHAUL EXHAUSTIVE ENGINE TESTS NO DETAIL OVERLOOKED Working to standards of fractional accuracy, specialist. technicians at the air base at Mechanics' Bay bear much of the responsibility of maintaining the safety and reliability of the Tasman Empire air service. With half a thousandth of an inch as a common basis for measurement and with a flaw in metal so minute as to be invisible regarded as a major defect, they perform their, work with the aid of scientific instruments which are almost human in their scope and ingenuity. Each flying-boat is powered by four Perseus sleeve-valve engines, made by the Bristol Aeroplane Company, -\jfhich operates the largest aero engine manufacturing plant in the world. With each unit giving an. output of 890 horsepower for take-off the aggregate of nearly 3600 horsepower enables the machine to be raised from the water in a few seconds. Great Precision The continual, examination, checking and overhauling of the engines represent triumphs of precision. The base workshops are in full operation, specialist departments dealing with such components as airscrews, navigational and electrical instruments, and carburetters, and a special building being equipped to test under actual service conditions engines assembled after overhaul. No flight is completed before a thorough engine check. The commander's report may—as it : usually does —record an uneventful crossing, but, none the less, the routine examination is made. The task of these skilled mechanics is to look for trouble. They start with the oil filters, where the presence of metallic particles would reveal undue wear in bearings or cylinders. They go over the whole boat from stem to stern; they put the spanner over nuts and bolts; they examine oil levels; thoy check magnetos. Minute Examination The routine gains in complexity when complete overhauls take place after an engine has done about 400 hours' running. Complete stripping is involved:, ana when dismantling is completed no two components are left attached. Apart from a keen visual examination there is a dimensional check, and sight testing gauges and micrometers, reveal errors of half a thousandth of an inch or less in the ovality of pistons, the imperceptible wear on sleeves or the clearance between moving parts. The very metal of. the components is subject to suspicion. Cracks may not be visible to the naked eye, but when [ the object is placed, across the magnet-

ised polos of an apparatus known as a magnaflux the application of a ferrous solution immediately shows , up even the slightest defect. There are specially-developed instruments to test the efficiency of carburetters and others to provide an X-ray-like impression of sparking plugs as they fire. In the airscrew section the pitch and balance of the propellers are minutely adjusted, and in tlie instrument room the delic.ate apparatus checks the instruments on which aerial navigation relies. Now nearing completion, the engine testing house is a remarkable example of engineering ingenuity. When an overhauled engine is re-assembled it is installed on a stand and its performance watched carefully and remotely controlled by engineers in the soundproof control room above and to the side of the xinit. For the first five hours it is run lightly at from 800 to 1400 revolutions a minute, then for two hours at 2250 revolutions, giving 680 horsepower, and finally for five minutes at full throttle, which represents 890 horsepower and 2700 revolutions a minute.

In the control room panels of instruments tell the full story of the test, revealing factors to the keenest eye. Horsepower being developed at any particular speed can be determined, the potrol flow is recorded, supercharger and oil pressures are shown, on the.-gauges and thermometers place before the operator the temperatures in such places as the oil tank, the oil outlet and the carburetter air intake. There is even an airspeed indicator to measure the airflow cooling the cylinders. No test would be complete without making it apply to varying altitudes. This is done ingeniously by the use of a shutter controlling the amount of air entering the carburetter. In this way it is possible to obtain accurate information regarding engine performance at varying altitudes.