Cold Weather Operations

Cold weather operations cover everything an aircraft and its crew do differently when the temperature is low: preparing an airplane that has spent the night in the cold, starting engines whose oil has thickened, taxiing on snow, slush and ice, keeping the fuel above its freezing point and correcting altitudes for cold air. Removing frost, ice and snow from the critical surfaces before takeoff belongs to the same season but has its own article, aircraft ground de-icing and anti-icing.

Most of the detail is type-specific and sits in the flight manual's cold-weather procedures, which take precedence over any general guidance; the figures quoted here for named types illustrate the principles, which are examined from the PPL, with its questions on oil and carbon monoxide, to the ATPL.

Cold soak and preflight

Cold soak is the chilling of an aircraft and everything in it through long exposure to low temperature. On the ground, a night in the cold brings the airframe, oil, hydraulic fluid and batteries down to ambient temperature. In cruise, the fuel in the wing tanks cools toward the total air temperature and can reach between about −20 °C and −40 °C on a long flight at altitude; it warms only slowly after landing (see fuel quantity and temperature indication).

A cold-soaked wing collects frost in humid air even when the air is above freezing: cold-soaked fuel frost. The A220 cold-weather data warn that with the fuel at 0 °C or below, clear ice can form on the wing with the OAT above freezing, and call for a tactile check when the humidity is high or moisture is visible.

The walkaround on a cold morning looks for contamination where it does the most harm. All frost, ice and snow must come off the wings and control surfaces and the area around the static ports, and pitot tubes, intakes and fuel vents must be clear. The FAA's winter checklist asks pilots to inspect static wicks, which can turn brittle, before pulling off the covers. In a piston airplane the crankcase breather is checked for ice, and fuel samples are drained with care: water in a sump can freeze and block the fuel lines. Even the thermometer needs thought: on the ground the A220's OAT and TAT probes are warmed by their own heating, so the temperature is taken from the ATIS.

Engine and APU starts in the cold

Cold oil is thick. In a piston engine it makes the oil pressure read high just after start, while the oil temperature takes several minutes or longer to rise. The pressure should still register within about 30 seconds of start, or up to a minute in very cold weather; if it does not, the engine is shut down. The engine is then warmed at low power before high power is used. In extreme cold, oil lines and tanks may need insulating to keep the oil from congealing, and congealed oil can stop an oil-controlled propeller from unfeathering after it has been feathered (see engine lubrication systems).

Starting is harder too. There is too little heat to vaporize fuel in the carburetor, so the primer sprays fuel straight into the cylinders. Over-priming can score the cylinder walls, make the start harder and cause an engine fire, and the primer must be locked afterward. If the engine fires briefly and stops, ice on the spark plug electrodes is a likely cause. A cold battery delivers starting current less readily, so ground power is used where it is available, and a battery start is made with the electrical load kept to a minimum (see aircraft batteries). Preheating the engine and cabin, ideally in a heated hangar, limits the oil's thickening, helps the battery and keeps instruments from sticking. In moderate cold an engine may start without it, but with care, because the oil is partly congealed. Once airborne, the cabin heater brings its own hazard. Most light-aircraft heaters warm air over the exhaust, so a cracked manifold puts carbon monoxide into the cabin. Carbon monoxide has neither color nor smell, so a headache, nausea, dizziness or sluggishness means heater off, fresh-air vents open and a landing as soon as possible (see carbon monoxide and cabin air hazards).

Turbine engines carry their own cold-weather limits and checks:

Ice on the A220's spinner, fan or inlet may be removed only with heated air, never with de-icing fluid, and never by chipping or scraping (see gas turbine engine starting).

Taxi on snow, slush and ice

On a contaminated taxiway or runway the brakes and the nose-wheel steering lose much of their grip, and temperature matters. In the ICAO runway condition codes, compacted snow at or below −15 °C is RWYCC 4 but warmer compacted snow is 3, ice is 1 and wet ice 0 (see wet and contaminated runways). Taxi speeds come down accordingly. The E190's cold-weather table gives 10 kt in a straight line and 5 kt in turns on a wet or contaminated surface, against 30 kt and 10 kt on a dry one, and the A320 training manual recommends 5 kt throughout a 180° turn on a wet or contaminated runway.

Standard operating procedures may rule out single-engine taxi on snow or ice. ATR's 1992 all-weather brochure, quoted in the NTSB's Roselawn report, allowed it only with a friction coefficient of at least 0.3 and without large steering deflections, and recommended both engines and differential power on icy taxiways or in slush. Slush thrown up by the wheels can freeze in the wheel wells, on the brakes and on the landing gear, which is why the FAA's winter checklist advises against taxiing through it. Airbus advises delaying slat and flap extension while taxiing, especially through slush, and then checking that they have extended before takeoff.

Engine anti-ice is part of the taxi. On the A320, icing conditions exist on the ground whenever the OAT is +10 °C or below with visible moisture, or with snow, slush or standing water on the surfaces the engines can ingest; on landing, its reversers are stowed when the speed reaches 25 kt on snow-covered ground. Snow also deceives the eye: in blowing snow with a tailwind the flakes can seem to fall vertically, giving the illusion that the aircraft has stopped while it is still moving.

Ice-shedding run-ups and engine anti-ice

Engine anti-ice heats the intake lip. Air accelerates into the intake and cools, so ice can form there with the outside air a few degrees above freezing, and ice shed from the lip can damage the fan. The system is therefore selected before entering the moisture, not when ice appears. On the A320 it must be on whenever icing conditions exist or are anticipated, on the ground and in flight, except in the climb and cruise with the SAT below −40 °C (see thermal anti-icing).

The lip is not the whole engine. During long periods on the ground in icing conditions, ice can still build up on the nacelle and the fan, and pieces that break off are ingested. Manufacturers therefore specify ice-shedding run-ups that throw the ice off while it is small:

• A320. With the OAT at +3 °C or below in icing conditions for more than 30 minutes, about 70 percent N1 for 30 seconds, brakes held, at intervals of no more than 30 minutes and again just before takeoff. In freezing rain, freezing drizzle, freezing fog or heavy snow, extra momentary run-ups to 70 percent N1 are made at intervals of no more than 10 minutes.
• E190-E2. In one operator's procedures, 60 percent N1 for 10 seconds, with the OAT at 3 °C or below, visible moisture and 30 minutes or more at ground idle; for a takeoff with anti-ice on, at least 60 percent N1 and the engine parameters checked before brake release.
• E190 (E1). 54 percent N1 for 30 seconds, or until the vibration indication is normal.

A run-up needs a surface on which the brakes will hold and a clear area behind the aircraft.

Fuel temperature and freezing point

Kerosene does not freeze at one temperature. As it cools, its heaviest hydrocarbons come out as wax crystals that can block the engine's fuel filter, and the specification fuel freezing point marks that limit: −47 °C for Jet A-1, −40 °C for Jet A and −50 °C for Jet B. Aircraft limits keep a margin above it. The Boeing 737 limits tank fuel to −43 °C or 3 °C above the freezing point of the fuel in use, whichever is higher; the A320 to −43 °C for Jet A-1 and −36 °C for Jet A, with the fuel temperature watched once the TAT reaches −34 °C on Jet A. The E190-E2 gives a FUEL TANK LO TEMP caution at −37 °C. A fuel system icing inhibitor acts on water, not wax, and does not change these limits.

Because the fuel cools toward the TAT, not the static temperature, the remedies for cold fuel in flight are to descend into warmer air or to increase the Mach number, which raises the ram rise. Some airlines print the freezing point of the fuel uplifted at each station on the flight release. Engines warm their fuel ahead of the filter, usually in a fuel-cooled oil cooler (see aviation fuel grades and properties). Cold fuel also hides water: on British Airways Flight 38 ice that had built up in the fuel system during a long cold flight restricted the fuel flow to both engines on final approach.

Cold-temperature altimeter corrections

A pressure altimeter assumes ISA temperatures. In colder air the pressure levels lie closer together, so the altimeter over-reads and the aircraft is lower than indicated. The error is zero at the elevation of the altimeter setting source and grows with height above it. A rule of thumb from PANS-OPS gives about 4 percent of that height for every 10 °C below ISA, safe only above −15 °C; colder than that, the ICAO table is used. Some of its values, in feet to add:

Cold temperature corrections are added to the barometric minimum altitudes relied on for terrain clearance: minimum sector altitudes, approach fix and step-down altitudes, DA/H or MDA/H and missed approach altitudes. The ILS glide path and an SBAS vertical path are fixed in space, although the altimeter will read higher than the published crossing altitudes while the aircraft follows them; the radio altimeter is unaffected. Barometric VNAV is not, so RNAV approaches carry limits such as "Baro-VNAV NA below −15 °C". An FMS that compensates for temperature must not be corrected again by hand (see minimum safe altitudes).

In the United States, Cold Temperature Airports carry a snowflake and a temperature in °C on their approach charts. At or below it, pilots correct either all the approach segments or those the FAA lists for that airport. Corrections on any segment except the final are given to ATC; a correction in the final segment need not be reported. The method is worked through in cold-weather altimetry.

Parking and overnight protection

Protection for the next flight starts at the end of this one. After an approach in icing conditions or a landing on slush or snow, the A320's flaps stay down until the engines are shut down and the ground crew confirm that ice does not obstruct the flaps and slats; the E190-E2 keeps them extended until they are free of ice, snow and slush. When icing conditions exist at +3 °C or below, A320 crews log the taxi-in time, or the time since the last ice-shedding run-up, because it limits the taxi-out time allowed on the next flight.

Covers go on the engines and probes, and come off again before start. Batteries are kept fully charged or removed when a light aircraft is parked in the cold, and a heated hangar is the preferred way to keep one warm. A light aircraft is refueled soon after landing, which leaves less room for condensation in the tanks, and wing covers go on when snow or a clear, cold night is forecast. In the morning the clean aircraft concept applies in full: no frost, ice or snow on the critical surfaces at takeoff (see aircraft ground de-icing and anti-icing).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.