FlightDesk Aero / Articles / Operational procedures

PILOT · WINTER OPERATIONS

Winter flying for PPL pilots: frost, cold engines and carbon monoxide

The clearest air of the year, smooth and dense, with great climb performance. Winter can be the best flying you do. It also hides a few traps that only bite in the cold, and every one of them has a simple fix.

  • Pilot
  • EASA 081 · 050 · 021
  • 14 min read
  • 25 October 2026

The trap of a perfect winter morning

High pressure, no wind, CAVOK, visibility you never get in summer. You arrive at the airfield, the aircraft has been outside all night, and the wings are covered by something so thin you can still see the paint through it. The engine is cold soaked at −4 °C, the battery spent the night outdoors too, and sunset is before five in the afternoon.

None of this stops you flying. Winter can be the best flying of the year. But each of those details has caught out experienced pilots, and each has a simple fix if you know why it matters. This article goes through them in the order you meet them: the wing, the engine, the cabin heat, the runway, the clock and the air you fly through.

1. Frost on the wing: thin layer, big effect

The UK CAA's winter Safety Sense leaflet puts a number on it: a very light film of frost can reduce lift by up to 30 % and increase drag by up to 40 %. That sounds out of proportion until you remember where lift comes from.

The air right next to the upper surface flows in a thin boundary layer. Near the leading edge it is only a millimetre or so thick, and it has to climb against rising pressure towards the trailing edge. Frost turns a smooth surface into sandpaper exactly there. The roughness steals energy from the boundary layer, it separates earlier, and the wing reaches its maximum lift at a lower angle of attack and a lower value.

0°5°10°15°20°0.40.81.21.6 Angle of attack α Lift coefficient CL Clean wing Light frost Frost: CLmax about 30 % lower, stalls at a lower angle
Schematic lift curves of a typical light aircraft wing. Frost lowers the maximum lift and the angle at which the wing stalls. Indicative shape, not data for a specific aircraft.

What that does to your stall speed

Stall speed depends on the maximum lift coefficient: VS ∝ 1/√CLmax. If frost takes away 30 % of CLmax, the new stall speed is 1/√0.70 ≈ 1.20 times the old one.

Worked example

Clean stall speed in take-off configuration: 48 kt. With 30 % less CLmax: 48 × 1.20 ≈ 58 kt. If you rotate at your usual 55 kt, you are asking the wing for lift it can no longer give.

Three things make this worse on the day:

  • The stall warner is set for a clean wing. It triggers at a fixed angle of attack. A frosted wing can stall before that angle, so the warning may come late or not at all. In freezing weather the vane itself may be stuck.
  • Ground effect hides the problem. Close to the runway, induced drag is lower and the aircraft may lift off, then refuse to climb as it leaves ground effect, with extra drag and a long take-off run already behind it.
  • Frost is rarely even. One wing in the sun, the other in the shade of the hangar: different lift on each side, a wing drop at the worst moment.

The rule is short. Part-NCO, the EASA rulebook for private flights, says the pilot-in-command may start the take-off only if the aircraft is clear of any deposit that could affect its performance or controllability, unless the flight manual allows it. In practice: clean wing, or no take-off. Frost does not blow away on the take-off run; often it is still there after landing.

How to get it off

  • Sun or a hangar. Turning the aircraft into the sun or pushing it into a hangar works, if you have time. Dry it afterwards: water in hinges and control gaps can refreeze once you are back outside, and a wet aircraft in an unheated hangar can freeze overnight.
  • Brush and cloth for light frost and dry snow. Never a car scraper on the windscreen or wing skin.
  • De-icing fluid for light aircraft (Type I) from a hand sprayer. Type II and IV fluids are made for airliners and do not shear off at light aircraft rotation speeds. Type I gives little or no hold-over time, so fly soon after.
  • Plan the time. If the overnight forecast shows frost, add half an hour to your departure plan, or cover the wings the evening before.

2. The cold engine

At −5 °C engine oil is thick, the battery has less to give, and fuel vaporises poorly. That is why cold starts are harder, and why the first minutes after start do the most wear.

Preheat

The CAA suggests considering preheat, with a heater designed for aircraft, when the temperature is below 0 °C. Your engine manufacturer and your flight manual give the limit that applies to your aircraft; many piston engines call for it at lower temperatures. Preheat makes starting easier, gets oil to the bearings sooner and reduces condensation inside the engine. Never leave a heater running unattended.

Priming: enough, not more

Prime exactly as the checklist says. If the engine does not start, pumping more fuel in does not help: it floods the cylinders, washes oil off the cylinder walls and can leave raw fuel in the induction system, where a backfire turns it into a fire. Know the flooded-start and engine-fire-on-start procedures for your aircraft before winter starts. If the engine fires for a second and then quits, the spark plugs may have frosted over; it is time for preheat, not for another long crank on the starter.

After the start

  • Keep electrical load low and the engine a little above idle until the alternator has put some charge back.
  • A long warm-up at idle is not kind to the engine: oil circulates poorly and plugs foul. Warm up at the RPM your flight manual gives, and make power changes smooth.
  • Wait for oil temperature in the green before the run-up, and for the minimum take-off values in your flight manual. If the engine stumbles at high power, it is not warm enough yet.
  • Check the crankcase breather pipe on the walk-around. The CAA warns that if ice blocks it, crankcase pressure can rise and cause a severe oil leak in flight.

3. Cabin heat and carbon monoxide

In most light aircraft the cabin heat is air warmed by passing over the exhaust muffler, inside a metal shroud. That is free heat, and it works well, as long as the muffler has no cracks. A crack lets exhaust gas into the air going to your feet, and with it comes the one exhaust gas you cannot notice: carbon monoxide (CO).

CO has no colour, no smell and no taste. It binds to haemoglobin in your blood more than 200 times more readily than oxygen does, so even small concentrations take the place of oxygen for hours. It acts like hypoxia, and like hypoxia it takes away the judgement you would need to notice it.

CO: symptoms and actions

Symptoms: headache, dizziness, nausea, lethargy and sleepiness, blurred vision, poor judgement; in severe cases convulsions and loss of consciousness. Passengers often feel it first.
Actions: cabin heat off, all fresh air vents open, oxygen if you carry it, land at the nearest suitable airfield, and tell ATC. See a doctor: the effects last long after you breathe clean air again.

Exhaust cracks grow fastest in winter, exactly when the heat is on for whole flights. Two habits help. First, ask your maintenance organisation about the heat exchanger at every inspection: a pressure test, soot traces on the shroud, an unusually large RPM drop with carb heat. Second, fly with an active CO detector that beeps or flashes. The orange spot cards change colour slowly, only warn if you look at them, and expire once opened. The UK has required active detectors for many passenger-carrying piston flights since January 2025; elsewhere in Europe they are not compulsory, but they cost less than a tank of fuel.

4. Snow, slush and the runway

Winter runway problems in Europe are less often deep snow and more often ice on hard surfaces, slush on the taxiway and soft, wet grass. Since November 2021, airports report the runway state with the Global Reporting Format (GRF): a runway condition code from 6 (dry) down to 0 (for example wet ice, with almost no braking), published in a SNOWTAM or on the ATIS. It was designed for airliners, but the code and the contaminant description tell you a lot about your own take-off and landing.

The flight manual figures assume a dry, hard, level runway. For anything else, multiply. These are the factors from the UK CAA Safety Sense leaflets, the same ones our take-off and landing calculator uses:

ConditionTake-offLanding
Dry grass× 1.20× 1.15
Wet grass× 1.30× 1.35
Soft ground or snow× 1.25 or more× 1.25 or more
Safety factor, always× 1.33× 1.43
Worked example

Flight manual take-off distance: 280 m. Snow on the strip: 280 × 1.25 = 350 m. Safety factor: 350 × 1.33 ≈ 466 m. On a 450 m strip, that is a no-go, before you even think about landing back there.

  • Landing is the harder half. You can often get out of a contaminated strip; stopping on it is another matter. Brakes do little on wet grass and can make you slide on ice.
  • Ice may not lengthen the take-off run, but it removes your ability to steer and brake. Think about the crosswind and how you would reject the take-off.
  • Taxi slowly, test the brakes early and avoid slush. It splashes into wheel fairings and onto brakes, then freezes in the climb, ready to lock a wheel on landing.
  • The surface changes through the day: frost in the morning, mud at midday, refreezing at dusk. Ask the aerodrome what the runway is like before you leave.

Two more things live on the ground. Water in the fuel: cold nights condense water in half-empty tanks, so drain every sump and look for water and ice, and refuel to full after the last flight of the day. And blocked holes: pitot, static ports, fuel vents and air intake must be free of ice and snow.

5. The short day

For EASA, night starts at the end of evening civil twilight, when the centre of the sun is 6° below the horizon. In Bucharest around the winter solstice the sun sets before 17:00 local time and civil twilight lasts about half an hour more. Without a night rating, that is your hard limit, and it arrives early.

  • Under an overcast sky, light goes much faster than the tables suggest.
  • A delay for de-icing or a slow start eats directly into your margin. Re-plan the arrival time after every delay, not just once at the briefing.
  • The low winter sun sits right in your eyes on some approaches all day long. Several accidents involved pilots who lost the runway in the glare. If the wind allows, choosing the runway that puts the sun behind you is a legitimate decision.

6. In the air: ice you can see and ice you cannot

Carburettor ice does not take winter off

Carb icing can form across a wide range of temperatures, from below freezing up to about +30 °C, whenever the air is humid. Grey, damp European winter days between 0 °C and +10 °C sit right in the middle of it. The signs, the risk chart and the carb heat technique are in our article on carburettor icing. In short: check carb heat at the run-up, use full heat before reducing power and in descents, and switch it off before the go-around decision.

Airframe icing: stay out of visible moisture below zero

Ice on the airframe needs liquid water that is colder than 0 °C: supercooled droplets in cloud, freezing drizzle or freezing rain. Most airframe icing happens in the first 10 to 15 degrees below zero, where supercooled droplets are most common. Part-NCO allows you to fly into expected or actual icing only if the aircraft is certified and equipped for it, and to leave without delay if icing exceeds what it is certified for or, in an aircraft not certified for icing, as soon as it meets any icing. For a typical PPL trainer that means: no cloud and no precipitation wherever the outside air temperature is at or below 0 °C, and never freezing rain or drizzle. Read the freezing level on the low-level forecast chart and check that you can always descend into warmer air without hitting terrain.

The altimeter lies in cold air

Cold air is denser, so the pressure levels sit closer together. Your altimeter, calibrated for the ISA atmosphere, reads higher than your real height above the station. The rule of thumb is about 4 % of the height for every 10 °C below ISA.

Rule of thumb

Error ≈ 4 ft per 1,000 ft per °C below ISA. At 3,000 ft above the airfield with ISA −20 °C: 4 × 3 × 20 = 240 ft lower than the altimeter shows. Over hills, add that margin to your minimum safe altitude.

Flat light and whiteout

Over fresh snow under an overcast sky, the ground loses texture and the horizon can disappear. If you lose your visual references, you are flying in instrument conditions whatever the visibility says: fly on the instruments, climb if it is safe, and ask for help.

Your winter checklist

Tick the items as you go: from the forecast the evening before to the covers after landing. Your progress is saved in this browser, so you can come back to it on the day. It complements your aircraft checklist, it does not replace it.

Go or no-go?

Six winter situations. For each one, decide: go as planned, fix something first, or don't go (or land as soon as possible).

Common myths

“It's only a thin frost”

Thin is the problem: the roughness sits in the boundary layer, where it hurts most. Up to 30 % less lift from a film you can see through.

“It will blow off on the take-off run”

Frost is frozen to the skin. At light aircraft speeds it stays, and you find out at rotation.

“The stall warning will tell me”

It is set for a clean wing. A frosted wing stalls at a lower angle of attack, possibly before the warner triggers.

“A long idle warm-up is good for the engine”

At idle, oil circulates poorly and plugs foul. Follow the flight manual warm-up RPM and wait for the oil temperature.

“I would smell exhaust”

CO has no smell. You may smell other exhaust gases, or nothing at all. Trust an active detector, not your nose.

“Carb icing is a summer thing”

It needs humid air, not heat: it can form from below freezing up to about +30 °C. Damp winter days around 0 to +10 °C fit perfectly.

Test yourself

1. Frost reduces CLmax by 30 %. By how much does the stall speed rise?

By about 20 %: 1/√0.70 ≈ 1.20. A 50 kt stall becomes about 60 kt.

2. Mid-flight, with the heat on, you get a headache and your passenger is sleepy. What do you do?

Suspect CO: heat off, fresh air vents open, oxygen if available, land at the nearest suitable airfield, tell ATC, then see a doctor.

3. Flight manual take-off distance 300 m, snow on a paved runway. What distance do you need?

300 × 1.25 × 1.33 ≈ 500 m, and treat 1.25 as a minimum.

4. ISA −15 °C, you are 2,000 ft above the airfield on the altimeter. How high are you really?

About 4 × 2 × 15 = 120 ft lower, so around 1,880 ft.

5. Without a night rating, when must you be on the ground in EASA land?

Before the end of evening civil twilight (sun 6° below the horizon), and in practice with a margin, especially under overcast.

Check your take-off and landing distance

Our free calculator applies the grass, snow, slope and safety factors from this article to the figures in your flight manual.

Open the calculator ›

FlightDesk Aero articles are for training and reference. Always follow the flight manual (POH/AFM) of your aircraft, your engine manufacturer and the rules that apply where you fly. Sources: UK CAA Safety Sense Leaflet 03 Winter Flying (2025) and 09 Weight, balance and performance, EASA Part-NCO, FAA GA Winter Weather Checklist (2026).