Brakedwell
The quote below refers to RAF Brits 252 and 253 Series and may be relevant. I have further info somewhere, but not immediately traceable!
Anyway passed on FWIW.
EGVs On
The first line of defence was heating of the engine entry guide vanes (EGVs). Air entering the nacelle met these vanes whose purpose was to suitably adjust the flow to meet the next engine component. The heating of these vanes had to be switched on if the aircraft entered cloud at any temperature. Two questions might cross the mind. One: why not have them on all the time? Answer: there was a fuel penalty; consumption was increased by 5%, which could be critical. Two: what’s the problem? You can see cloud. At night? That is why visitors to the flight deck in the dark might have found one or both of the pilots, sitting on the edge of their seats peering through the windscreen, possibly with a torch.
Cowl Heat on
The EGVs went on, no matter what the temperature. The next protection device was very temperature, and speed, sensitive. This was known as the Cowl Heat and was designed to combat the problem caused by the reversing of flow in the engine of air with a high water content at particular temperatures, such as are met in the tropics. It has been mentioned how the Britannia development met bad luck here in that it was not previously realised that such sensitive conditions existed.
Lumps of ice
Cowl Heat did just what its name implies. The outer surface of the engine air intake was heated as well as the rear walls and corners of the individual ducts by hot air bled from the engine. All this to stop the wet cold air depositing ice on the bend where the flow reversed. If ice was allowed to accumulate there, then when it reached a certain volume, it would detach itself and pass down the engine. The finely engineered components there were intended for the flow of gases, not lumps of ice, so damage could occur, the engine be temporarily stalled and, in the extreme, the flame of combustion extinguished.
Critical temperatures
One can imagine crews under training, learning of this deficiency, being somewhat relieved to hear that this icing phenomena was only critical at certain temperatures. Such relief would be dashed when it was discovered that those temperatures were just the ones met at the aircraft’s normal cruising altitudes in the tropics. It was a real problem.
The Cowl Heat HAD to be switched on, in cloud, between indications on the outside air temperature gauge of +2°C and +12°C. However, if the temperature was between +2°C and +6°C, then the speed had to be reduced to 200 knots first and the temperature rechecked to see if it was still in the ‘Cowl Heat On’ range of +2° to +12°. Just its use carried a large fuel penalty and if slowing down was also required, the result could be critical.
All this concentrated the minds of crews at the Meteorological Briefing and the subsequent pre-flight planning. Choosing the height at which to fly was like some aeronautical brain teaser; flying at the optimum height for fuel flow, airspeed and range could have the temperature right in the critical range. Lower height and the temperature would be alright but the fuel flow too high; as high as possible might bring the temperature below +2°C but the airspeed might be unacceptably low to cover the distance with the fuel available. It was certainly a nightmare situation for crews.