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Library Reference Number: 194

Chopper Blades and Cumulonimbus Rain

Sqd Ldr, Bill Campell, AFC, Scottish Saltire Branch, ACA

Searching for ideas for another article and re-reading ACA Saltire Branch’s first book I came across Ted Bracken’s story entitled ‘Monsoon’. I too served in the Far East and experienced the extremes of weather common in that part of the world. In October 1973 Hong Kong was still in the typhoon season during which the colony had an elaborate early warning system which occasionally brought all non-essential activity in the colony to a halt. When the black ball went up the pole at the Observatory in Kowloon and radio warnings went out everyone packed up work and went home to wait for the 90 miles an hour winds and torrential rain to hit.

Even without a typhoon in the area violent thunderstorms could appear with little warning; clouds 10,000 feet deep could form in half an hour and within these clouds vertical movement of air could be as great as 8,000 feet per minute.

On a recent night flying sortie I had seen a spectacular sight while flying through Hong Kong harbour by Star Ferry. What looked like a giant silver mop was illuminated by the Kowloon lights; the head of the mop was a ragged cumulonimbus cloud thousands of feet above and the handle was a shaft of rain which terminated in a frothing, boiling pool about a hundred yards wide. We gave it a wide berth.

On 2nd October 1973 the weather was sunny but with plenty of isolated clouds about when my pilot and I took off from RAF Kai Tak on one of the tasks of 28 Squadron’s Wessex, the re-supply of one of the isolated army posts dotted around the colony’s boundary with the Peoples’ Republic of China. We departed eastwards and began climbing over a densely wooded hillside when without any warning the heavens opened, like a power shower. Paddy the pilot immediately lowered the lever and flew towards the ground to stay visual but there was nowhere to land. The rain was so intense and had come on so suddenly that there was no time to think about escaping, we simply had to keep visual with the trees. Paddy got the aircraft down to about 10 feet above them but then, as the rain continued to increase in intensity, came down to 5 feet before telling me to give him directions as he could no longer see the trees. I talked him down until the starboard main wheel was in the leaves; I could just make out the leaves at the bottom of the wheel while Paddy, sitting no more than 5 feet above me, could see nothing.

What happened next was expected. The leading edge of the Wessex blades had a layer of thick neoprene bonded to them for protection against flying debris, birds and precipitation and a further thin layer of sacrificial tape to minimise erosion of the neoprene. This tape when damaged started a whistling noise and as more was lifted so there was an imbalance in the blades and the aircraft started to vibrate. This was not an uncommon experience in heavy rain but this was beyond heavy, it continued to thrash down. The scale of the bouncing vertical movement made it obvious that all four blades had been badly affected and not just the sacrificial tape, but more ominous was a vibration, heard and felt through the airframe, of a much higher frequency which could only be coming from the tail rotor or its drive. With no signs of smoke or overheating we had to assume that the tail rotor was being affected by the torrential downpour. Water was also finding any imperfection in the cockpit and cabin sealing and was getting everywhere. The noise of this rain even with our sound-deadening helmets, was tremendous - and then suddenly there was silence.

After about ten minutes the rain had stopped as if the ‘power shower’ had been switched off. We then saw a phenomenon straight out of the meteorology handbook or school physics lessons; so much water had passed through the surrounding air that it had become saturated and we were now in cloud. It wasn’t long, however, in the midday heat that the cloud began to thin and we were able to start our return to base. Paddy immediately asked me to get up into the left-hand seat, make a Pan call to Air Traffic and then help him on the controls.

When I got ‘upstairs’ and strapped in I quickly realised how difficult a time Paddy had had. The instrument panel and the coaming above were just a vibrating blur. Paddy had locked the collective lever using its small mechanical brake and had both hands wrapped around the cyclic stick trying to damp out its stirring motion. I couldn’t initially see the transmit switch on the vibrating coaming to make the Pan call but once that was done I followed Paddy’s instruction to take over control of the collective lever by easing off the brake. The vertical jumping of the lever was so strong I had to lean over and put my weight on it and with both hands try to damp out its movement and at the same time lower it to start the descent. Soon we were established on final approach to the old defunct runway at Kai Tak where a disconcerting array of fire trucks awaited us, but we made an un-eventual running landing.

After shut-down following an interesting 25-minute sortie we were met by the squadron engineering officer who shook his head and said “ten thousand quid minimum for a new set of blades”. The neoprene on the main blades had delaminated from the metal and was covered in large bubbles which did not really surprise us but the state of the tail rotor blades amazed and puzzled us. The tail blade protection came in the form of a 1-inch inset wrap around alloy strip bonded to the blade leading edge. It’s front face was now flat and the trailing edges were standing out at right angles by ¼ of an inch. It was difficult to work out how they got like that (probably like a metal-worker using a peening hammer) but it certainly explained the severity of the vibration. ‘Walter’ Wessex could take it though; I flew this rugged helicopter on various tours from 1966 to 1990 and it never let me down, and it was not long before a new set of blades was fitted and this particular aircraft was back on line. Thinking back to the ‘silver mop’ experience, maybe we had been ‘shafted’ by something similar.

Source: Weather information from The Boat People, Bruce Grant, Penguin (1979).

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