Unusually Cold Weather Operations
I was going to write about flying in the winter and all its pros and cons; then thought better about it because in the UK we normally have mild winters. Generally, it is the poor visibility, low cloud base and wet, windy weather that stops us.
However this spell of very unusual, easterly winds have brought the temperatures to below freezing in late February and early March, while the day gets longer. This is the time of the year we start to pull our aircraft out of hibernation and plan to go flying. But the unseasonal cold winds have made our local country roads more treacherous because we are not prepared with snow tyres or chains on our cars. Our normal driving skills and duty of care to our passengers and other drivers are no match for icy roads and stressful school runs nor the pressure of reaching the office on time.
I write this blog for my student pilots and old pilots who may not be as familiar with airframes and engines as I would like them to be. I am making a broad generalisation and assuming that most new pilots have not much hands-on experience of dealing with Rotax engines and modern glass cockpits fitted to composite aircraft. Having made this bold statement, I will venture further to say that many qualified and long experienced pilots on light aircraft have never seen the inside of the engine compartment nor looked behind the instrument panel facia of certified aircraft (because they are not allowed to touch anything, but the oil dip stick, yoke, rudder pedals, throttle, mixture control and perhaps the propeller pitch lever).
The human body performs best within a certain range of temperatures. The athlete or the ordinary person must perform a suitable warm up before exercise to prevent cramps and allow the body to cool down after exercise to reduce aches, pains and muscular spasms. Similarly, the internal combustion engine must be warmed up to the correct operating temperature before take off…….
PREPARING THE AIRCRAFT
The aircraft has been parked outside (or inside a non heated hangar) and will have cooled down to ambient temperature along with the engine with below freezing temperatures…
The ropes and webbing used to tie down the aircraft will be cold and brittle. When frozen, rope knots are impossible to undo. (I know from my experiences of climbing in the Himalaya), so I use webbing tape with camlock type buckles. The fabric covers over the aircraft must be removed carefully without damaging the pitot tube and the wind screen. Ice crystals are liable to scratch the windscreen quite easily.
The airframe must be cleared of ice and / or hoar frost to prevent the reduction of lift generation by the airfoil. It is very easy to scratch the composite or metal surface when clearing it of ice and frost. It may be that the moving parts and bearings of the control surfaces are frozen solid and must be defrosted and dried, or the water may refreeze in flight and jam them. The wheels and brakes will have frozen from standing outdoors and you may have no braking power available. Very cold temperatures make the lightweight rubber tyres and tubes vulnerable to cracking and can be punctured very easily by solidified frozen grass.
It is quite likely that de-icer liquid may be required in large quantities to be sprayed over a metal aircraft, but we do not know how the de-icer chemicals may affect a wood/fabric/composite structure. So be warned before using these sprays. During pre-flight checks ensure your pitot tube and static ports are clean and dry, or they may freeze in flight and give you incorrect readings on your pressure instruments. It is very likely that your pitot tube will freeze in flight when flying through damp air or rain and you will lose your airspeed display. Have you a heated pitot tube and is your aircraft battery charged enough to supply the high current required to heat the pitot?
Cold instruments with moving dials may freeze if there is the slightest of moisture inside the chambers. Glass cockpit display screens will need warming up to operating temperatures before they indicate any information correctly.
The windscreen and windows must be dry outside and inside. It is certain during cold weather conditions that simply sitting in the cockpit and breathing out will mist up your windows and obscure your vision.
The most wear and tear inside an engine is done when starting cold, using the human body analogy, the engine must be warmed up. This can be done with an electrical hair dryer or warm air gun blowing warm air into the air intake. This may take more than 30 minutes of blowing warm air. Care must be taken to NOT blow hot air, which may melt some of the electrical wiring or soften composite parts.
The oil container of the Rotax engine should be warmed up to make the oil less viscous and more runny so that it lubricates the cylinder walls and pistons immediately and correctly. Recall that we normally wait for the oil temperature to warm up to 50o C before applying full power.
The battery when cooled below 10oC loses more than 50% of its output amperage, which results in the starter motor not being able to turn the engine at more than 450 rpm required to fire the sparks. It is always a good move to apply a charging current to the battery during cold weather to keep it topped up and in good condition.
The cold body of the carburettor prevents the petrol and air from mixing adequately to combust in the cold engine. The cold temperature makes the air thicker, thus the Density Altitude may be very much lower than when the carburettor was set for idle mixture. This will make engine starting difficult, thus draining the already straining battery.
PLANNING THE FLIGHT
Human factors play a big part in this scenario. Extreme cold will make the pilot more likely to make the wrong decisions.
Ensure that you, the Pilot, are dressed up warmly. You may have lost a lot of body heat while checking out the aircraft. The airfield café will probably be closed as their water pipes will have frozen. Take a large flask of hot chocolate for you and your passenger.
If you have cabin heating in the aircraft, do not depend on it to keep you warm as the air entering the intake may be well below freezing and the hot exhaust pipe may not have enough time to warm it up before it enters the cockpit. I find flying in temperatures below Zero Celcius, the warm air entering the cockpit is only about 10oC warmer. Seal off all the places the ambient air might enter the cockpit; for example the joint between the wing and fuselage can be sealed with glider tape. Ensure the tape is warmed up before applying, as the adhesive on the tape is useless below 10oC.
Please do check the correct TAF/METAR, paying special attention to the temperature and dew point temperature. When these are 2 degrees or less apart, you will certainly have poor visibility. Check your spot wind chart (Form 214) for temperature aloft. A cold airframe going through rain will accrue ice and your normal lift capability will be degraded, if not totally destroyed. Your pitot tube will freeze when going through rain. If you do have a heated pitot, check it works before flying. Is your battery powerful enough to supply the current required for this ?
If you plan to land at another airfield, call and ask them about their runway conditions. Don’t be surprised if they tell you not to arrive there. And remember to check the same for your alternate airfield.
It is always a good idea to walk the runway to check for snow depth and quality. Dry powder snow will behave differently from icy runway. The powder snow will fill up the wheel pants, jam the steering mechanism and freeze the brake pads. The aircraft will make deep tracks in the snow and you may not be able to steer. Treat this as a soft field operation; so when you start moving from your taxi point, do not stop anywhere on the runway or you may not be able to move once stuck in soft snow. Full power may not be sufficient to move you. If this happens, you know that take off may be impossible!
Warming up the engine to operating temperatures may take more than 15 m