How pilots change their takeoff preparation when it snows
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This week, the Met Office has forecasted that there could be up to 25cm of snowfall across the U.K. in the days leading up to Christmas. Whilst the idea of a white Christmas would put a shine on what has been a thoroughly depressing year, for those of us flying aircraft, the snow provides a different challenge.
Snow on the wings, ice in the engines and slush on the runway can all pose hazards to aircraft and must all be considered and evaluated by pilots before each takeoff. Changes to how we prepare the aircraft for departure, how we taxi to the runway and how we takeoff must all be incorporated into the flight to ensure we get the aircraft, and its occupants, safely off the ground.
At the Gate
Preparation for a flight in snowy conditions begins long before we make our way out to the runway. Even whilst sitting at home or in the hotel before heading to the airport, most pilots will be watching the snow coming down, thinking ahead to how it will affect the flight. If I haven’t flown in snowy conditions for a while, I’ll normally take this opportunity to refresh my knowledge of the winter operations specific to the 787 Dreamliner and the airport I’m flying from.
As we board the aircraft, we take advantage of windows on the jetty to get a good look at the surface of the wing and conditions on the ground. We’re looking for signs of ice or snow contamination, which could affect the takeoff performance.
An aircraft flies not because of the engines but because of the lift generated by air flowing over the wing. Aircraft designers engineer the wing in such an optimum way that should there be any impediment to the smooth flow of air over the wing, such as ice or snow, there will be a significant loss of lift.
Before each flight, one of the pilots will conduct an external safety check of the aircraft. During cold weather operations, there are certain areas of the aircraft that we must pay extra attention to.
The pitot tubes and static ports are situated towards the nose and play an important role in determining a whole range of information in the flight deck, such as airspeed and altitude. If these are blocked by snow or ice, they could cause erroneous readings.
The air conditioning inlets and exits, engines inlets, fuel tank vents and APU inlets are also checked to confirm they are free from ice build-ups and blockages. We also check the landing gear doors for build ups of ice.
After takeoff when we select the gear up, the movement of the gear doors could cause large ice build-ups to fall onto the ground below – not ideal if you live near the airport. It is for this reason that Japan has special rules in place for arriving aircraft to ensure that any ice fall resulting from the lowering of the gear does not fall onto a populated area.
In order to remove snow and ice from the aircraft surfaces, specialist deicing trucks spray hot fluid onto the aircraft. This not only melts the contaminant but it also prevents further build-up for a limited time period. This is usually a two-step process.
Preparing for de-icing
Before the aircraft is sprayed, we must make some changes to the configuration of the aircraft. For a sealed metal tube, an aircraft has a lot of (deliberate) holes. These are used to draw air into the aircraft but also some allow air out. If the de-icing fluid was to get into these ducts, it could cause damage to the aircraft systems and send fumes into the aircraft cabin.
As a result, before the de-icing procedure is commenced, we must configure the aircraft using the de-icing checklist. Each aircraft type has its own procedure owing to differences in how the various systems work, but for the most part, it involves closing all external inlets to stop the de-icing fluid getting into the aircraft.
This, obviously, includes the cabin doors. Whilst it would be nice to save some time and get the aircraft de-iced whilst the passengers are boarding, the risk of the fluid getting into the jetty and into the aircraft is too great.
De-icing can either be done on the gate or, at airports where de-icing is a regular occurrence during the winter, at remote de-icing bays.
On-stand vs. remote de-icing
Depending on the facilities available at the airport, aircraft de-icing can be done either at the gate or at a remote de-icing pad.
On-stand de-icing is the most straight forward method of clearing the aircraft of ice and snow. When all the passengers have boarded and the pilots have prepared the aircraft for spraying, one (sometimes two) de-icing trucks begin the process.
Whilst the actual de-icing procedure is no different to using a remote pad, it does cause problems for the airport. Spraying can only commence once all the passengers have boarded and there may not be enough de-icing trucks for all the aircraft needing spraying.
The result of this is delayed departing aircraft and, subsequently, delays for arriving aircraft that have to wait for the gate to become available.
At larger airports where de-icing is common during the winter, such as Munich and Montreal, there are specialist remote de-icing pads. These serve very much like a drive-through de-icing facility.
When ready to depart, we push back from the gate as normal and start the engines. We then taxi out to the remote de-icing pad where we are directed into one of the de-icing bays. Here, in radio communication with the de-icing controller, we set the parking brake and inform them when, like on the gate, the aircraft is ready for spraying.
The key difference here is that the engines are still running when the spraying takes place. This is obviously a major threat for those on the ground, so we have to remain absolutely vigilant whilst this procedure is taking place.
Moving the aircraft or increasing engine power before the spraying team have finished their work could prove fatal for those driving the trucks.
The de-icing process
Which areas of the aircraft are sprayed is down to the pilots. However, with limited visibility of the whole airframe, we will normally ask the operator of the de-icing truck for their assessment of the contaminant.
The bare minimum is normally just the wings and tail, however, if there is extensive ice or snow on the fuselage, we will get the whole aircraft de-iced.
For the first step, a hot mixture of glycol and water is sprayed onto the aircraft surfaces; this literally melts and blasts the ice and snow off the airframe. Once this is done, theoretically, the aircraft is good to go. However, if the temperature is close to or below freezing and there is still moisture around in the form of fog or falling precipitation, there’s always a chance that more could settle on the wings before takeoff. To stop this from happening, the anti-icing stage is carried out.
Anti-icing fluids are similar to the de-icing fluids except that they also contain polymeric thickeners. This results in a layer of what often looks like green or yellow slime on the surface of the wings, preventing any falling precipitation from settling. Whilst this is effective at the time of spraying, it’s only good for so long.
Depending on what type on anti-icing fluid was used and the current weather conditions, the anti-icing may be effective for anything up to a couple of hours down to only a couple of minutes. This is known as the holdover time. Once this time has expired, the pilots can not be sure that the wings are clear from snow and ice and the whole process must start again.
Push back and engine start
Pushing back from the gate during cold weather operations, there is a good chance that the stand and taxiway could be slippery. The pushback tugs are incredibly powerful, but when grip is reduced, they may struggle to push the aircraft back, especially if the engines are running.
As a result, we will quite often wait until the pushback is complete with the tug disconnected before we start the engines.
Normally on the 787, we start both engines at the same time. The autostart system monitors the process and alerts us to any abnormal parameters that may require our intervention.
However, when the outside air temperature is below 5 degrees C, Boeing recommends that we start one engine at a time. Using the same autostart function, we allow the first engine to stabilise before starting the second engine. In these conditions, the oil pressure may be slow to rise and the initial pressure may be higher than normal.
This all takes extra time but it is critical that we take care of the engines at this important stage of their operation.
Before taxiing away, we will make a conscious decision whether or not we want to deploy the flaps. The Standard Operating Procedure (SOP) states that we should set the takeoff flap before taxiing away from the gate area.
However, in winter conditions, if the aircraft has not been de-iced at the gate, the flap drive mechanisms could be iced up. Forcing them into action could result in damage, incurring a significant delay and cost to repair.
If the aircraft has been de-iced on the stand, slush and snow on the taxiways could get into the working parts of the flap system, once again causing them to ice up. Once airborne, this could cause the flaps to malfunction when we try to retract them.
For remote de-ing, we leave the flap up until we have been de-iced. Once this process has taken place, we then make an assessment of the taxiways and decide if we feel it safe to deploy the flaps immediately or wait until just before we takeoff.
In either situation, if we choose to delay setting the flaps, it’s imperative that we remember to do so before we enter the runway. History has shown that the consequences of taking off without the correct flap set are, more often than not, disastrous.
Engine care is critical in winter conditions so we have to keep a close eye on the engine conditions as we taxi out to the runway.
Before all flights, cold weather or not, the engines must be given five minutes after start to reach their optimum operating temperature. However, when it is cold, we must apply further procedures.
When the temperature is between 3 degrees C and minus 7 degrees C, if the engines have been running for an hour before we have taken off, we must perform an engine run-up. This involves stopping the aircraft and applying around 50% power for 30 seconds. This is to ensure that any ice which has formed in the engine is broken down before we take off. This must then be repeated every hour.
If the temperature is below -7°C, we only get one go at this. If after another hour we are still not airborne (not uncommon at places like JFK), we must go back to the gate, shut the engines down and have them manually de-iced by an engineer.
A snowy airport scene may look beautiful but it can be a real headache for all those involved with the operation. Fights can be delayed as aircraft waiting to be de-iced clog up the gates and aircraft may need de-icing again if their holdover time expires.
Ultimately, the pilots are responsible for the safety of the aircraft. It is up to us how we prepare the aircraft and what level of de-icing is performed on the aircraft. Ultimately, snow and ice will most likely cause delays. We appreciate that this is frustrating, especially when passengers have connections to make, but it is our job to ensure that the aircraft gets safely from A to B and we will let nothing jeopardise that.
Featured Image by Andreas Gebert/picture alliance via Getty Images
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