FLIGHT SAFETY :: Winter Flying

AIRCRAFT PREPARATION

During the pleasant days of summer, items of equipment may have 'disappeared'. Make sure the aircraft has serviceable pitot head covers, static vent plugs, control surface locks and, if parked outside, proper tie-downs. Having made sure you have got them – use them.

Some engines may need the aircraft manufacturer's approved winter cooling restrictor to allow the oil and cylinders to reach and maintain correct operating temperatures. After fitting, keep an eye on the oil temperature/ cylinder head temperature, especially if the weather turns warmer.

The grade of engine oil may need to be changed when operating in colder conditions. Consult the Manufacturers Manual or Maintenance Organisation.

Carbon monoxide is colourless, odourless, tasteless, insidious in its effects and lethal. One of the first symptoms may be a severe headache, drowsiness or dizziness. ‘Spot’ type carbon monoxide detectors only have a limited life when unwrapped. Use a ‘fresh’ one and read the instructions.

The pitot-static system should be checked for water which can freeze and block the system. If static drains are fitted, know where they are and how to use them.

The battery is worked harder in winter, so make sure it is in good condition and well charged. If you’ve had to make prolonged attempts to start the engine, when it does start allow plenty of time for the battery to re-charge before using heavy electrical loads. In a single-engined aircraft it's all you are left with if the electrical charging system fails in flight.

Check that all the airframe, propeller and windscreen systems are operating correctly. De-icing systems suffer from neglect and may prove faulty when required.

Make sure engine crankcase oil breather pipes are clear and free from deposits which can freeze, causing a pressure build-up that could force engine oil seals out of their housings.

Control cable tensions may need to be adjusted.

FLIGHT PREPARATION

If you are planning to visit another aerodrome, make sure it is open as mud, snow, flooding or frozen ruts may have necessitated closure. Remember also that daylight and airport operating hours are much shorter in winter.

Never fly in icing conditions for which the aircraft is not cleared. Do not be misled into thinking that because an aircraft is fitted with deicing, or anti-icing, equipment, it is necessarily effective in all conditions. Microlights are not cleared for flight in icing conditions, although some protection may be given.

Continued flight into bad weather is the number one killer in UK general aviation. Get an up to date aviation weather forecast.

The most likely temperature range for airframe icing is from 0 to –10°C; it rarely occurs at –20°C or colder. Pay attention to any icing warnings. Note the freezing level, it can be surprisingly low even in Spring and Autumn; you may need to descend below it to melt an ice build-up; but beware of high ground. Remember also that altimeters over-read in very low air temperatures, by as much as several hundred feet. You can be lower than you think.

If you are likely to encounter ice en-route, have you room to descend to warmer air? Will the airspace or performance allow you to climb to cold, clear air? (Note that any ice build up may not melt and will degrade cruise performance). Can you land safely at your destination? If the answers to these questions are NO, don’t go.

Prepare an accurate route plan with time markers, including an alternative in case you do encounter ice/snow. The countryside looks very different when covered by a blanket of snow and familiar landmarks may have disappeared.

Have a cloth handy for de-misting the inside of the windows while taxying.

Dress sensibly, (you should spend some time outside whilst pre-flighting the aircraft), and have additional warm clothing available in case of a forced landing.

Some parts of the UK will be pretty inhospitable in winter (eg much of Wales and Scotland) so file a flight plan and carry a few survival items in case of a forced landing, eg warm clothing, silvered survival bag, torch/ mirror and whistle for signalling.

Be prepared to divert and carry a night-stop kit. Don't put pressure on yourself to get home if the weather deteriorates.

PRE-FLIGHT

There may be a greater risk of water condensation in aircraft fuel tanks in winter. Drain fluid from all water drains into a clear container so that you can see any water.

When refuelling, ensure the aircraft is properly earthed. The very low humidity on a crisp, cold day can be conducive to a build-up of static electricity.

Tests have shown that frost, ice or snow with the thickness and surface roughness of medium or coarse sandpaper, reduces lift by as much as 30% and increases drag by 40%. Even a small area can significantly affect the airflow, particularly on a laminar flow wing.

Ensure that the entire aircraft is properly de-iced and check visually that all snow, ice and even frost, which can produce a severe loss of lift, is cleared. This includes difficult-to-see ‘T’ tails. If water has collected in a spinner or control surface and then frozen, this produces serious out-of-balance forces. There is no such thing as a little ice.

The most effective equipment for testing for the presence of frost and ice are your eyes and your hands.

The best way to remove snow is by using a broom or brush. Frozen snow, ice and frost can be removed by using approved de-icing fluid in a pressure sprayer similar to a garden sprayer. An alternative is to melt the ice with hot water and then leather the aircraft dry to prevent refreezing. Make sure that control surface hinges, vents etc are not contaminated. Use of a scraper might damage aircraft skins and transparencies.

Do not rely on snow blowing off during the take off run. The ‘clean aircraft concept’ is the only way to fly safely – there should be nothing on the outside of the aircraft that does not belong there.

Water-soaked engine air intake filters can freeze and block the airflow.

If hand-swinging a propeller move the aircraft to a part of the airfield which isn’t slippery. Don’t try it unless you’ve been trained. Use chocks.

During the engine run-up, check that use of carburettor heat gives a satisfactory drop in rpm.

DEPARTURE

Remember that taxiways and aerodrome obstructions may be hidden by snow, so ask, if you are not certain.

Taxy slowly to avoid throwing up snow and slush into wheel wells or onto the aircraft's surfaces. Taxying slowly is safer in case the tyres slide on an icy surface. Stop well clear of obstructions if there is any doubt about braking effectiveness.

You may consider using a 'Soft Field' take off technique – if so be sure that you are fully aware of recommended procedures.

EN ROUTE

Monitor VOLMET and turn back or divert early if the weather deteriorates. Don't wait until you are in a blinding snowstorm or covered in ice.

Carburettor icing is one of the worst enemies. Carburettor ice forms stealthily, so monitor engine instruments for loss of rpm, which may mean carb ice is forming.

Apply full carb heat periodically and keep it on long enough to be effective. The engine may run roughly for a short period while the ice melts.

Use carb heat as an intermittent ON/OFF control – either full hot or full cold. Do not use full or partial carb heat continuously unless the Handbook/Flight Manual allow it.

Use full carb heat for a time before a descent. During a descent, when using small throttle openings, apply full carb heat and increase rpm periodically to warm the engine.

Remember carb heat increases fuel consumption.

At low rpm, use full heat but if appropriate cancel it prior to touchdown in accordance with Manual/ Handbook instructions.

If you are flying just above clouds to stay clear of airframe icing, remember that the cloud tops will quickly rise as you fly:

- across high ground;

- towards a warm, cold or occluded front;

- towards a low pressure area.

If you fly into the top of clouds, the concentration of water droplets is often greatest near the cloud top and ice could build up quickly.

Airframe Icing is most frequently encountered within convective clouds, Cumulus or Cumulonimbus (CU/CB) where the build up of ice can be very rapid. In these clouds the icing layer can be several thousand feet thick and a dramatic change of altitude will be required to avoid climbing above the cloud layer..

In the absence of dewpoint information assume high humidity when:

– the ground is wet (even dew)

– in precipitation or fog

– just below cloud base

If you’ve got a big build-up of ice, the drag and weight are increasing while the climb performance is decreasing so you can’t climb to get above it. High ground may prevent you from descending.

Tell ATC so that others can be warned.

Most of the time snow, which is already frozen, will not stick to an aircraft, but occasionally wet snow with a high moisture content will stick. Treat it like ice.

Freezing rain can occur during the winter months either at or near the ground, or in a layer above the ground. It occurs when warm moist air is moving into a cold region. The invading warm moist air may cause a layer of air, where the temperature is higher than zero°C, to overrun a much colder layer beneath where the temperatures are below zero°C. Under these conditions precipitation forming in the high cloud layers will melt to form rain as it falls through the warm air which will then fall into the sub-freezing layer beneath. This rain will quickly freeze again in the cold air forming a solid layer of clear ice over everything. This clear ice will build up very quickly and be difficult to ‘shake off’.

Freezing rain is the most severe form of airframe icing. It can be encountered in flight up to altitudes of 10 000 feet, or it may be encountered on the ground or when flying close to the ground. Aircraft parked outside will be quickly coated with a layer of clear ice, and similarly aircraft in flight. If such conditions are encountered in flight near the ground it is best to land as soon as possible, or if the severe icing is encountered at a higher altitude descend, if possible, into a warmer layer below.

Ice forming on an aircraft can cause odd vibrations and noises. An aerial iced up may begin to vibrate (and can fall off). Don’t panic, remember AVIATE, NAVIGATE, COMMUNICATE.

LANDING

Most icing accidents occur when the pilot loses control during approach or landing. Even a thin coat of ice on the aircraft justifies a 20% increase in approach speed. It will extend the landing run – perhaps on a slippery runway. The handling may be different, don’t make large or abrupt changes in power or flap settings.

If you suspect, because of changed stick forces or vibration, that there is ice on the tailplane, a flapless or partial flap landing may be advisable (the handbook/manual give flapless-approach speeds). This reduces the tailplane load and the likelihood of tailplane stall, which can result in a VERY severe pitch down. Recovery is by REDUCING THE FLAP angle and by pulling hard – over 50 kg (110 lbs) may be necessary.

When landing on a very wet or icy runway, particularly in a crosswind, the aircraft may aquaplane or slide and directional control can be lost. In such circumstances an alternate runway or diversion is necessary. Aircraft with castoring nosewheels may be more vulnerable.

Remember that ground temperatures fall quickly during the late afternoon on an exposed airfield and by dusk ice may be forming on any wet runways. The ice may form as a clear sheet which is invisible and has a coefficient of friction of zero!

AFTER FLIGHT

Take care when getting out of the aircraft. Jumping from the aircraft walkway onto an icy apron could lead to a painful tumble.

If parked outside, use control locks and proper tie-downs to guard against winter gales. Face into the prevailing or forecast wind. Put proper pitot and static covers on – make sure the pitot has cooled down!

If it is muddy or slushy, inspect wheel fairings, landing gear bays, flaps and tailplane for loose mud or slush. They are easier to remove when soft than when frozen.

Notify Air Traffic if the actual weather was different, or worse, than forecasted. It might be important for other pilots to know.

SUMMARY

- Stay out of icing conditions for which the aircraft has NOT been cleared.

- Note freezing level in the aviation weather forecast. Don’t go unless the aircraft is equipped for the conditions.

- Have warm clothing available for pre-flight and in case of forced landing.

- Mud, snow and slush will lengthen take off and landing runs. Work out your distances.

- Remove all frost, ice and snow from the aircraft – there is no such thing as a little ice.

- Be extra vigilant for carb ice.

- If ice does start to form, act promptly, get out of the conditions by descending (beware of high ground), climbing or diverting.

- If you encounter ice, tell ATC so that others can be warned.

- During the approach if you suspect tailplane ice, or suffer a severe pitch down, RETRACT THE FLAPS.

- If you have to land with an iced up aeroplane, add at least 20% to the approach speed.

- Snow covered, icy or muddy runways will make the landing run much longer and crosswinds harder to handle.