Wednesday 22 May 2013 01:17 UTC
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With a shiny new IMC rating tucked into my wallet and access to an IFR equipped Archer I was on the lookout for suitable weather to stay in practice. Before long the right day arrived. Five miles vis, 1500ft cloudbase 4500 tops, freezing level 6000ft and no CBs. A friend also with a new IMC rating agreed to share the flight.
The plan was to fly from Edinburgh to Newcastle and request radar vectors to the ILS. We would have lunch in the excellent aeroclub restaurant. (This was twenty years ago. Don't go looking for it now.) We would swap over for the return trip.
We requested SVFR out of Edinburgh departing the zone via the ski slopes and Dalkeith. Once clear of the zone we requested radar information and climb to flight level 55. Climbing through FL45 it was still solid IMC, 50 it's getting lighter and at FL55 we popped out between layers. We were just touching some of the higher whispy bits of cloud but revelling in the impression of speed.
We were transferred to Scottish control then shortly after transferred again to Newcastle radar. A check for ice revealed the first signs around the temperature probe at the top of the windscreen. There was also a just detectable line of frost on the leading edge of the wing. Normally a climb to clear the whispy bits of cloud would have been the answer but with only twenty miles to go I asked Newcastle for descent.
Thirty seconds later there was a short clatter and a severe vibration. Carb heat was already on, temperatures and pressures were normal and the engine was still producing power. Ed pressed the button and declared an emergency. Newcastle radar asked the usual questions. Why do they use the phrase "how many souls on board?"
It sounds so morbid. The next development was a single line of black oil running slowly back along the top of the cowling towards me.
We broke out on the ILS at four miles in light snow and as suddenly as it started the vibration stopped. We landed uneventfully on runway 07 and took the first right off the runway with the airport fire service in pursuit.
So what happened?
The tiny bit of ice I saw must have been picked up on the prop first. The clatter was the ice which had been shed from just one of the prop blades hitting the airframe. As we decended into warmer air the ice came off the other blade curing the vibration.
The oil breather pipe protruded below the cowling and as a result froze over creating pressure in the crankcase. This caused some oil to be pumped out of the front crankshaft bearing. Thankfully not much.
This all happened twenty years ago. It was only recently I was reading about a similar incident. Neither aircraft had received a simple modification mandated by Piper to simply cut a "whistle slot" in the breather tube further up inside the cowling.
The other lesson here is not to expect the forecast icing level to be accurate within a few feet. Somebody once said a weather forecast is only a horoscope with numbers.
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