Winter operations

Aberdeen is slowly moving across to winter and we have already had the first snowfall back in October and a few times we have been de-icing early in the morning. The start of November was reasonably mild and it was only the first flight of the day that the aircraft needed de-icing; for those that do not now what happens, a truck which is filled with a mixture of hot water and deicing fluid similar to that which you would spray on your car, has a large crane on the end of it with a guy controlling a high pressure hose that fires the hot deicing fluid onto the aircraft.  It is basically blasting any traces of ice or snow or even frost off the aircraft.  It is particularly important to remove any traces from the tail, elevators and the wing because even the slightest small trace upsets the flow of air over the surfaces, reducing optimum lift and hence the ability to fly according to the way it was designed. In extreme cases aircraft have failed to climb away to a safe altitude on take off, perhaps the most famous example is the florida air crashing into the Potomac River where the wings had ice all over them and the engine instruments also iced up giving false indications to the amount of power available.

The Dash 8 which I am flying has a T-tail and these aircraft are particularly vulnerable to icing and what is known as a ‘tail stall’.  A tail stall is when there is enough disruption of air over the surface and the surface rapidly loses any lift, because it is the tail surface that has lost lift, there is a rapid pitch-down nose movement and high speed and power is required to recover from the stall.  The symptoms and the recovery are slightly different from a conventional stall, but the principles are the same, reduce the angle of attack and increase speed.

Now that the days are getting much colder and we are starting to have regular snowfall we must perform what is known as a two-step deicing procedure.  This is where the aircraft is first deiced using the hot fluid, and once this is completed the aircraft must then be coated in an anti-icing fluid which has a consistency similar to maple syrup, the anti-icing fluid prevents snow and ice from re-forming on the aircraft before it can get out of the icing conditions, eg on the ground upon which the inflight mechanical and electric deicing and anti icing systems take over.  Depending on the temperature and the type of fluids used (there is a list of about 30 fluids available), the active time of the anti icing fluid will vary from anything as high as 90 minutes to as low as 10 minutes. With the extreme low case, this means the aircraft has 10 minutes from the anti icing fluid first being applied to the aircraft rolling for takeoff, a very short time indeed and it is the reason that there are so many delays in the uk when it comes to winter operations, that and the lack of deicing rigs available, although the latter is less of a problem here in Aberdeen.  In Europe the airports are far more geared up for winter operations, in Germany and Sweden they have what looks like a giant car wash that the aircraft just drives through.  It is often located close to the runway and hence increasing the chances of being able to takeoff within the calculated time available, it is also much quicker and hence reduces delays vastly.

Having lived in Sweden for two years, one major difference that I have noticed is that once the big freeze has come, they do not bother clearing the runway any longer and instead use big machines to cut grooves into the snow and ice so that aircraft can grip the surface.  This is in stark contrast to how things work in the uk where the runway must be clean enough to eat your dinner off it, the recent example of how not to do it was with heathrow in 2011 where the airport was effectively shut for almost a week due to the lack of resources and equipment.

Unlike the jets such as A320 or 737 of the world, the Dash8 flies at a maximum altitude of 27,000ft which often means that it will remain within some form of cloud, it doesnt have the performance (or the need) to climb above all of the weather all of the time.  It is therefore far more susceptible to icing on the airframe as it will often be flying through some sort of cloud at some point in its journey.  The aircraft had many different forms of ice detection and prevention, one of them is a small spigot attached to the (almost useless) windscreen wipers.  This spigot which has been engineered to give a visual clue as to what is happening to the tail when airborne and if in icing conditions for long enough, it can form what looks like a golf ball attached to the spigot.  Obviously whatever is on the spigot is manifested on the tail of the aircraft, therefore I am quite paranoid about icing on the airframe, maybe that is because I have watched just about every air crash investigation going, but I don’t think there is anything wrong with being paranoid about a weather condition which if left unchecked, can cause a potential catastrophe.

One advantage that the Dash8 does have over jet aircraft is that the deicing and anti icing systems do not drain a huge amount of power from the engines.  With jet engine aircraft, they have hot air pumped around the wings to prevent ice and numerous electrical system to prevent icing elsewhere on the aircraft.  All of this power means that the engines must turn over at enough of a rate to power the systems, this becomes a problem when trying to descend because the engines will no longer run at idle and hence a descent must be started much further out and the approach must be much quicker. The Dash8 must still fly at an increased speed of 15kts and use full flap which makes for a rough and turbulent approach, but the speed for landing is much slower than that of a jet.

The big disadvantage of the Dash8 is that with the extra 15kts of speed it carries combined with the large flap setting makes for a nose-down attitude and this makes flaring the aircraft to land on the back whEels quite tricky.  A chunk of power must be removed and a pitch of around 1-2 degrees to let it settle onto the Tarmac without wheelbarrowing down the runway and damaging the nose landing gear.

It’s only December at the moment, but we are sure to get worse weather throughout December, January and february.  The ski slopes have started to open all of their runs so at least that means I can get the skis out and go and try enjoy some of the snow, although more often than not Scottish slopes are covered in ice.  I have brought my spiked snow tyres up for my mountain bike too to that I can continue riding in the bad weather.

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~ by globalste on November 22, 2012.

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