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Winter Weather


Author Captain Keith

Winter operations

I know that for many fearful flyers the onset of winter brings reminders of the dangers of flying this is because we think that flying is like motoring and we are only too familiar with the mayhem that occurs as soon as a drop of snow settles on the roads.

When we assess the riskiness of something we use very general rules of thumb according to how much we know about the subject.  The less we know about a subject the riskier we tend to believe it is, the more we know the less risky we think it is. But when we are experts in a subject we then are able to assess the risks as they really are and what’s more important we can see them in context. There is a short article on threats and risks in the library.

To think of flying in weather that we normally think of as being hazardous is bound to distort what we think. So if you think that snow and ice is dangerous (because of what you know about motoring) you’ll transfer that belief to other areas of your life. It’ll be even more damaging if you are predisposed to anxiety about the subject anyway,

So it doesn’t ever surprise me that anxious flyers have an extra thing to worry about as winter approaches. However, before I explain about the precautions and actions we take in flying let’s have a more detailed look at how we think about winter hazards anyway. And of course to a downhill skier and snowboarder snow is good.  So snow and ice have to be seen in context.

Let’s start by looking from a new pilot’s point of view. One of the things that we learn early in our careers is about the weather. We need good visibility when we first learn so that we can learn the attitudes of the aircraft when it is climbing flying level or descending. By attitude, I mean the position of the nose in relation to the horizon …remembering of course that when we are flying we normally see a flat horizon because we are usually above everything. Another thing we learn about early in our careers is the icing level because that is the height at which the temperature is at zero and thus any moisture hitting the aircraft will freeze. Without contact with anything solid,  moisture in the air can still be liquid below zero degrees. If we fly into these conditions the moisture will freeze on the plane and form a ridge of ice on the front edge of whatever enters the moisture first.

So the first two lessons about weather are about visibility and icing levels. It’s unlikely that a pilot will ever fly without thinking of these two things. And of course on a hot summer’s day while you’re basking in the sun, 7000 feet above you the temperature will be below freezing. On such a day any aircraft entering cloud or flying through any moisture will need to safeguard against ice forming on the wings or engine inlets. So while snow and ice may only occur to you during the winter … to a pilot, it’s encountered everyday.

So that’s the background from a pilot’s point of view … but there’s more to come from the pilots.

Let’s go back to how we consider risk. We assess risk by using a rule of thumb our knowledge of the subject, the number of times we hear the information, the impact of the information versus the effect on our lives if we were involved.

It’s called the availability heuristic it operates on the notion that, “If you can think of it, and think of an example of it, then it’s a high-risk situation.”  You think of bad weather and think of risk.

And so it is with winter operations, the combination of our views on motoring to suggest that, ‘winter is unsafe’. Press stories will persuade us to believe that more incidents occur than do and that they are more serious than they really are.

Let’s look at an airport during ‘bad’ weather.  The first point is that it’s easier to clear a runway with a snow plough than a road. The runway can be closed and there are no obstructions on a runway. The runway is straight. There are no access problems … the snow plough and sweepers don’t have to negotiate their way through traffic, and so there is no travelling time. The ploughs can be in action as soon as they are needed

Airlines pay for high fees for using the runway so it is the interests of the airport to keep it operating.

Unlike cars planes do not bump into each other on the runway, one doesn’t land until the runway is clear. Aircraft cannot taxi to the runway unless it is likely that they can take off straight away, in case they get covered in snow again … so there’s never any knock-on effect of congestion …i n fact there is no congestion at all.  All in all the organisation at an airport means that generally a runway and its taxiways can be cleared relatively quickly. This is very different from the roads where problems mount up.  I shall talk about delays because of bad weather later.

Let’s think now about the aircraft. If there is ice or snow on the wings, they lose their streamlined shape and do not generate as much lift as they do normally.

The law says that aircraft cannot take off when there is snow or ice on certain parts of the plane, this means that the plane is aerodynamically capable of flying as well as it does when it hasn’t been covered in snow or ice.

De-icing procedures are very strict because the effect of ice is considerable. Even a thin film of ice can ruin the efficiency of a wing. Each aircraft will have regulations about the de-icing needed prior to flight. Every aircraft has to have its wings free of ice at the time of take-off. De-icing fluid which is carefully manufactured stored and delivered is extremely expensive but even so, has a limited time of effectiveness.  The fluid can be applied either as it is or may be heated. Hot fluid normally has a longer period of effectiveness than cold fluid.

The procedure for de-icing a plane would either be the responsibility of the captain directly or an airline may have a department responsible for it, which could be engineers or a ground handling department. Regardless of who actually de-ices the aircraft, it is the captain’s responsibility to make sure that it has been done to the standard required. This standard is laid down in the operations manual for the aircraft and is part of the Airline’s Operators certificate. The airworthiness certificate of the aircraft would be invalid if it operates outside the limitations imposed by the manufacturer and aviation agency for that country. Although of course all these procedures are agreed at international level with the International Civil Aviation Organisation.

The procedures with de-icing a plane are complex and usually before de-icing commences those involved with the procedure will consult their manuals to see exactly what has to be done. The aircraft’s certificate of airworthiness, as I have said determines which parts of the plane have to be free of contamination. The next thing to consider is the type of fluid to be used and whether or not it is to be used hot or cold. These factors determine the holdover time … that is to say, the length of time that the fluid will melt any snow, sleet or, hail that settles on the aircraft. This will be affected by the actual air temperature at the time of de-icing

This information is then entered into the aircraft’s technical log with the time de-icing was started, fluid type and any other relevant information. The aircraft must now get airborne within the expiry time of the holdover period.

But of course, the matter doesn’t end there. We need to think about the condition of the taxiways and runways.

Pilots and airlines use a performance manual to determine the conditions and the maximum weight at which an aircraft can take off and at which it can land. When the runway is described as ‘contaminated’ restrictions are imposed so that the same safety margins are achieved. Usually, this means a reduction in the maximum weights not just because of the slowing effect of snow but because of the reduced braking effectiveness on the runway if the plane has to abandon its take off, or when it’s landing.

On a slippery runway, the reverse thrust has a better stopping effect than ordinary brakes. On most flights, the reverse thrusters do not have to be working however when landing on a snow-covered runway there is a legal requirement that they must be working.

How well can a plane slow down on an icy runway?

The braking effectiveness on the runway is measured and reported to the pilots via the radio the pilots will make the calculations accordingly. This measurement is known as the braking coefficient.

As the captain of the aeroplane, it is my responsibility to collect all this information before I take off. During the starting engines phase, I will discuss the de-icing precautions that are needed during taxi and take off. Normally this will be engine anti-icing on immediately after starting then wing anti-icing go on at a height of 500 feet after take off. This would be a normal procedure for most modern jet passenger aircraft.

Imagine then, that I’m your pilot and operating your flight. I’ll check in at my operations centre an hour before my flight, the operations team will give me my briefing sheets for the flight that will include ALL the weather information and all the extra information about any snow and ice. There will be information on the state of all the runways at my destination and any diversionary airports on the route.

These reports are updated hourly so I will always know the conditions regarding the landing runways. At the aircraft, the engineer will have been supervising the de-icing and be making the appropriate entries in the Technical Log. The co-pilot and I will be confirming the maximum take-off weight and the dispatcher will be loading the aircraft accordingly. We will confirm that we shall be able to taxi and take off within the holdover time of the fluid and continue with the normal procedures for departure.

At the take-off point, we will check again that we meet all the criteria for take-off under the prevailing conditions and then take off. We have to take into account the possibility that there may be a snow shower while we take off which may infringe our takeoff requirements … if the visibility conditions for take-off are not met then we cannot go. This, in turn, could have an effect regarding holdover times and may result in having to return to the terminal to de-ice the plane again.

My message to you is that under these conditions there may be delays but there are very good reasons for them … keeping everything as safe as possible for you.

Here are some more aspects of winter operations that you may find interesting.

Aircraft anti-icing systems.

I have mentioned that the aircraft have a deicing system that I haven’t describe how they work. Commercial jet aircraft all have similar deicing systems. Hot air is taken from the engines and directed along the front of the wings and tailplane. Hot air is also directed around the front of the engine casing. Some aircraft have detectors that show if ice is building up on the plane when this happens the pilots select the system on. The engines normally have their anti-icing systems operating if there is moisture in the air and the temperature is below 10°C, depending upon the aircraft type.

In-flight icing.

At the normal cruising altitudes of commercial jet aircraft the temperature is about -65°C. this means that any moisture will already have turned to ice and will be unable to settle on the aircraft. Anti-icing precautions are most significant during the climb up to cruising altitude and then the descent and landing.

Descent thru cloud.

When a plane encounters cloud during its descent ice is likely to build up if the conditions are right. I have already mentioned that if the temperature is below 10°C most aircraft require the engine enticing to be selected on, that you may find it interesting to know that the temperature change is quite significant in accordance with the speed of the aircraft. At high speed, the temperature difference may be as much as 5°C on subsonic aircraft

Landing conditions.

I have already mentioned some of the requirements for landing but it may be of interest to you to know that planes will land more firmly on contaminated runways then on dry runways. This is so that the tyres can break through any contamination and grip the runway is so that breaking will be more effective. A small film of water between a tyre and any surface may mean that the tyres could aquaplane, this can happen on a car as well.

Effects of ice and snow on a wing.

Back in the 1950s little was known about the effects of runway contamination and snow or ice on the wing of the aircraft. There was an accident involving a famous football team taking off from Munich, which led to considerable investigations into the effects of snow and ice on aircraft. When there is ice or snow on a wing, the air does not flow over it smoothly and causes a loss of lift.

Fuel freezing.

Although we associate low temperatures with the winter as I have already mentioned an aircraft flies in an environment where the outside temperature can be as low as -65°C. Fortunately, the passengers are insulated from this. However an aircraft on a long flight gradually get colder and colder and this will eventually have an effect on the fuel temperature because the main fuel tanks are in the wings.  When the captain signs for the fuel uplift he will check its freezing temperature. When cruising the captain may decide to descend into warmer air or to increase speed to keep the fuel above its freezing point.

Finally, I want to show you the sort of information that is available to operating pilots. One type of information is the FODCOM, the Flight Operations Division Communication. Here is an example of one reminding pilots of the precautions and practices that they should consider during winter operations.


1.1.1 The purpose of this FODCOM is to review and refresh some of the procedures and best practice that operators should adopt during winter operations.

1.2 Braking Action – General

1.2.1 Runways that are dry or wet with less than 3 mm of water will normally provide good braking.

1.2.2 Braking action is assumed to be poor on a wet runway that is notified as one that may be slippery when wet. Operators should ascertain from aerodrome operators the location and dimension of the part of the runway that has fallen below the minimum friction, ‘slippery when wet’ trigger level, in order that they can assess whether aeroplane performance is affected.

1.2.3 Braking action will not be measured or reported at UK licensed aerodromes on contaminated runways except on those covered in compacted snow and ice as described in the UK AIP AD 1.2.2 Snow Plan.

1.2.4 There is no reliable correlation available between the readings of Continuous Friction Measuring Equipment on a runway contaminated with water, slush and snow and aeroplane braking performance. Performance calculations must not be based on such readings. They will not be made available at licensed aerodromes in the UK.

1.2.5 Contaminants that aerodrome operators are unable to clear will be reported as depth and type of contaminant. These can be used with approved contaminated performance data to make a calculation of the landing or take-off distance required.

1.3 Aeroplane Performance Calculations for Operations on Contaminated Runways

1.3.1 Operators should avoid using contaminated runways whenever possible. All performance calculations for both take-off and landing on contaminated runways should be based on the depth and type of contaminant on the runway in accordance with approved contaminated performance data in the Flight Manual or approved supplement. The maximum depth of contaminant for operations can be found in AIC (Aeronautical Information Circular)15/2006 ‘Risks and Factors associated with Operations on Runways, affected by Snow, Slush or Water’. However, the aircraft Flight Manual limit should be used if more restrictive.

1.3.2 Where the contaminated runway performance data for the aeroplane has been provided by a third party data company other than the aeroplane manufacturer, operators are reminded that it is their responsibility to ensure that the correct data is used by the flight crew. Furthermore this service provider should be included in the operator’s quality assurance programme established under JAR-OPS 1.035 ‘Quality System’ and the performance data production process audited as necessary.

1.3.3 Take-Off Flight crews should be aware that in changing winter conditions the performance calculation carried out at the planning stage may no longer be appropriate at the time of take-off. A further calculation, based on the latest prevailing conditions, may be needed. Flight crews should also be made aware that using Electronic Flight Bag products for performance calculations on a contaminated runway often produces optimum flap setting performance where the computer uses the available runway length to accelerate the aeroplane to a higher speed in order to improve the climb performance. This is unlikely to be appropriate in such conditions where a shorter ground roll would be preferred.

1.3.4 Landing To compensate for operational variability JAR-OPS 1.515 and 1.520 specify factors that must be applied at the planning stage to the Flight Manual landing distance. In addition JAR-OPS 1.400 requires that prior to commencing an approach to land commanders must satisfy themselves that the weather and condition of the runway do not prevent a safe approach, landing or missed approach having regard to the performance information in the Operations Manual. If the flight proceeds as planned, the planning stage safety factors should remain valid. However in winter operations a scenario of rapidly changing conditions is possible. If conditions (including runway, meteorological, surface, aeroplane weight and configuration, and planned usage of decelerating devices) change or the aeroplane is required to land on a different runway, or in conditions that were not expected at the planning stage, it is still necessary to comply with JAR-OPS 1.400. JAR-OPS 1.400 does not specify additional factors in the same way as JAR-OPS 1.515 and 1.520. Operators should ensure that policies and procedures are in place to enable flight crews to assess whether sufficient landing distance is available at the time of arrival. The landing distance available should be the longer of that derived under paragraph above or that required by a non-normal configuration. However, in emergencies or abnormal configurations the flight crew needs to know the absolute landing distance (unfactored Flight Manual distance) for the aeroplane configuration in order to evaluate whether to land immediately or to divert to another aerodrome.

1.4 Contaminated Runway Clearance and Reporting Runway State

1.4.1 Aerodrome operators are responsible for clearing contaminants from runways and manoeuvring areas and keeping them clear as far as is reasonably practicable. Aerodrome operators should also measure and report the depth and type of contaminant present.

1.4.2 Contaminant is measured every 300 metres, between 5 and 10 metres either side of the runway centre-line and away from the effects of rutting. The measurement is reported in millimetres as a mean for each third of the runway. The contaminant will be described as Ice, Dry Snow, Compacted Snow, Wet Snow, Slush or Standing Water.

1.4.3 UK AIP AD 1.2.2 Snow Plan requires runway conditions to be reported every 30 minutes for as long as such conditions prevail, catering for a scenario of changing weather. In addition, the AIP requires the contaminant depth and type measurements to be carried out every 30 minutes. However, flight crews should be extremely cautious in rapidly changing conditions. Snow depth can increase rapidly and in typical UK conditions a slight thaw can also turn wet snow rapidly to slush. Flight crews should use the most adverse report available in such conditions.


Captain Keith.


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