Weatherwatch with Irv Lee

Dangers of Ice

Remove all traces of ice before flight. The red painted cowlings on this aircraft cleared a full hour before the white wings

The start of the year in the UK has obvious freezing implications. To begin on the airfield, all traces of morning ice on parked aircraft have to be removed before flight. It's not only consideration for the adverse weight and balance factors - there's also the change to aerodynamics which has an impact on performance and stalling speed.

A cloudless overnight sky allows ground to cool and is the starting point for dew, hoar frost or, with a light wind, radiation fog (Weather Watch #1). Conduction cools the air, less water vapour can be held, and in calm conditions, dew is deposited. In freezing temperatures, hoar frost forms directly onto parked aircraft.

With clear skies and unlimited visibility, waiting for the hoar frost on aircraft to melt is frustrating. The day will be short anyway due to early winter sunset, but patience is a safety virtue here.

Turning parked aircraft to allow the morning sun to work directly on the frost, can speed melting. Only use very soft scraping tools when attempting to help the process. If wet aircraft surfaces refreeze when turned back into the shade, retreat to the clubhouse for another cup of tea and to re-check your plans.

Winter high pressure, few isobars, clear skies, and lack of fronts are advance warnings of frosty mornings. Ask for hangarage the day before your trip, in case any of the regular occupants are away.

Planning can be even more fundamental. Aircraft colour choice makes a real difference on hoar frost days. Last winter I abandoned plans for an awayday in perfect weather, at a time when such opportunities were rare. My white winged aircraft would not defrost in time to make the trip worthwhile.

A neighbouring aircraft, of exactly the same type but darker in colour, was completely cleared of ice and airborne long before mine. The colour scheme absorbed the sun's energy more efficiently. It was never planned for that reason, but it made an enormous difference.

Once all frost has been removed, check the freezing level, available directly on Metform 215, or by interpolation from Metform 214. If the aircraft control surfaces or cables are still wet after de-icing, be very wary of immediately flying above the freezing level.

On cloudier days, aircraft without de-icing equipment should be kept in clear air when in freezing temperatures. The next Weather Watch will explain the problems of flying into supercooled rain under warm fronts above the freezing level. Icing conditions in that situation are extremely dangerous.

IMC rated pilots, descending to land through stratus cloud layers, will often see the appearance of white ice on wing leading edges. Small droplets freeze onto the aircraft with minute bubbles of air entrapped, causing brittle opaque 'rime ice'. Ice shedding can cause problems if another part of the aircraft is hit.

However, the main danger with rime ice comes from aerodynamic and weight changes causing a considerable loss of lift and the raising of the stalling speed.

Pilots unadvisedly flying into cumulus clouds in icing conditions can experience 'clear ice' build up - a dangerous form, transparent to the eye, spreading back rapidly from the wing leading edge. Each large raindrop initially freezes onto the aircraft, but in doing so releases latent heat, and partially re-melts. The liquid, free of trapped air, then smears itself across the wing before re-freezing. The lack of trapped air gives the ice transparency and strength, the latter resulting in heavy, solid, dangerous build up.

Carburettor icing and hail, quite possible all year round, but worse in warmer seasons, will be covered in a further article later this year.