For the first time this winter I felt like I was in a winter wonderland with full use of Winter operations. As we descended down over Eastern Europe, I could see that the ground was covered in snow – as far as the eye could see. With the temperature being -11C, and snow everywhere, it reminded me that we would have to adjust our operations to the weather.
We started by adjusting the Minimum Safe Altitudes (MSAs) and any altitudes below this, to compensate for temperatures below 0°C. This is because any temperature that differs from 15°C, which the instruments are calibrated to, introduces an error in the altimeter. Warmer temperatures will make the altimeter under-read, whilst cooler temperatures will make the altimeter over-read. This is because when the air temperature changes, the air expands or contracts. If the air is cold, it contracts and so the pressure levels bunch up closer together. The opposite happens when the air is warm. The altimeter works by being sensitive to the air pressure – consequently it causes errors in the reading when air temperature diverges from the calibrated temperature of 15°C. We don’t make any temperature corrections for warmer temperatures because an under-read does not affect safety (the aeroplane is actually higher than what the altimeter is indicating) and everyone else’s altimeters would be reading the same in any case and there wouldn’t be any issues with traffic separation.
We found the runway was cleared from snow, giving us plenty of room to land safely. Once we landed we ensured we taxied slowly and cautiously, as to not skid or slip, and followed our winter procedures. It did add to the workload, so everything had to be done without rush as usual and meticulously. During the turnaround we checked the wings to ensure they were clear of ice. This reminded me of an Air Crash Investigation episode I was watching a few days ago – where a Boeing 737 crashed into the Potomac river in Washington, shortly after takeoff. The aircraft crashed because the the wings were not de-iced and consequently the ice changed the aerodynamic characteristics of the wings, leading to an early stall. The episode gave a bit more detail and there were other factors involved, however it shows how import it is to remove ice off the wings and take off with a clean wing.
When we de-ice we usually apply fluid to the wings which removes any existing ice and protects them from accumulating any more ice for a certain period of time. This period of time is often referred to as the “holdover time”, which is dependent on the concentration and type of the fluid sprayed onto the wing. So, next time you are in cold and icy weather and you see some fluid being sprayed onto the aircraft from a large hose, then you know your aircraft is being deiced!
Thankfully it’s not snowed much in Western Europe this Winter, so we’ve not been afflicted by the consequential delays that come with winter operations. We still have a couple of months to go though, and it can still snow in April! I was looking at the news and saw the weather in North America – let’s just say that I’m glad we aren’t experiencing the same here!