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Jet streams – how wind can affect an aircraft

Recently I did one more flight from Oslo to Spain, which proved to be interesting. The day started a bit rushed as the shuttle bus from the hotel to the airport was overfilled and we could not take the bus we originally wanted and therefore were already slightly late when we arrived at the airport. However, my crew was very experienced and we managed to finish our preparations in time, however the boarding took longer than expected as a lot of people brought some very big items of cabin baggage which had to be stowed into the cargo compartment during the boarding and so we left Oslo with a delay of around 10 minutes. I was flying with the same First Officer as I did a few days ago and this time it was his turn to start as ‘Pilot flying’ while I would be ‘Pilot flying’ for the way back to Oslo. As the ‘Pilot Monitoring’ on the first flight it was my duty to conduct the so called ‘walk around’, which is a visual inspection of the exterior condition of the airplane for any obvious damage or other defects that is performed before each flight. It was raining heavily so I was soaked when I finally arrived in the cockpit again. According to our flight plan we were to expect an average of 65 knots (120km/h) headwind on the first flight, which added around 10 minutes of flight time. These so called ‘Jet streams’ are quite common and can sometimes be turbulent, but today we were lucky and managed to have a smooth ride. As the airplane was very heavy, being full with passengers, bags and fuel, we were initially limited to a relatively low altitude. We initially climbed to FL340, which is 34000ft (in pilots speak we remove the last two Zeros from the altitude in feet and call the result flight level – we only do this above a certain altitude, called transition altitude which is different from airport to airport). We could have barely made FL360 as well, but the French and Spanish Air Traffic Control uses a slightly different altitude allocation system than other countries, so once we reached French airspace we would be expected to move to an ‘odd’ level, either FL330, FL350 oder FL370. As we would be to heavy to climb to FL370 by the time we reached French airspace I discussed with the First Officer to stay at FL340 and then climb to FL350, instead of climbing to FL360 right away only to descend back to FL350 later on. Once over France, I checked the forecasted wind conditions one more time during the flight and noticed that we were right in the core of the jet stream at our current altitude (as expected we were asked to climb to FL350 at the French border) and that the wind should reduce by around 20 knots (around 40km/h) if we climb by 2000ft (600m). I showed this information to the First Officer on my iPad and asked him about his thoughts. He agreed that once we had used enough fuel to be able to climb to FL370 we should try to climb out of the core of the jet stream to save time and fuel. We had finally burned enough fuel and were light enough just after passing Paris and asked the friendly air traffic controller to climb to FL370, which he approved. Before starting the climb I noted down the current ‘ground speed’ to see how much faster we would get by climbing out of the headwind. We were both disappointed when we found out that the forecast was inaccurate and we actually even slowed down slightly by climbing. After landing we swapped the ‘Pilot flying’/’Pilot monitoring’ duties and prepared for the flight back to Oslo.

We were delayed by Air Traffic Control that informed us that we had a so called ‘slot’, a time window in which we had to depart. These ‘slots’ are issued and coordinated by the agency ‘Eurocontrol’. I will give you some more information about slots in one of my next posts. When we finally took of we were around 45 minutes late. This time there were a lot of thunderstorms over Italy, but luckily they were far to the east of our route and therefore they were not affecting us at all. It was not cloudy today, so we got a great view of Lake Geneva while crossing the alps.

The alps and lake Geneva

The headwind from the first flight obviously became a tailwind for our flight back to Oslo. When we passed the alps we noticed a special type of cloud, called lenticularis cloud (they are named like this because they look like lentils). These clouds are an indicator of a phenomenon called ‘mountain wave’. ‘Mountain wave’ can only occur under very special conditions, when a strong wind hits a ridge of mountains. This will cause the air behind the ridge to move up and down in a wave pattern. This can cause very strong up- and downdrafts, but generally a flight in mountain wave is very smooth. Skilled glider pilots have used this phenomenon to soar to great heights by using the updraft generated by the wave.

Lenticularis clouds above the alps in very strong wind conditions. Notice their smooth appearance compared to the other clouds visible on the picture.

The flight was progressing well and surprisingly we hardly encountered any turbulence in the jet stream. As we were still running late I decided to increase the speed to save some time at the expense of a slightly higher fuel consumption. I had just finished my meal over Denmark and was having a conversation with the first officer when I noticed the engines spooling down slightly (It would be very tiring and inefficient for one pilot to always control the airplane manually during long periods of cruise flight, the airplane is controlled by the autopilot and auto thrust system. The auto thrust works like a cruise control on a car, i.e. maintains a preselected or computed speed by changing the thrust setting of the engines). This made me look at the speed scale on my Primary Flight Display, short PFD. This screen displays all essential parameters necessary to control the aircraft and to monitor the autopilot. The auto thrust was supposed to maintain a ‘Mach Number’ (Speed measured as percentage of the speed of sound, Mach .80 means that the airplane is traveling at 80% of the speed of sound) of .80 instead of the usual cruise speed of Mach .78 as I had increased the speed to reduce the delay. The speed had increased to Mach .81 and I saw that it was still increasing even though the auto thrust system had already reduced the thrust. The maximum allowed Mach number for an A320 is Mach .82 – exceeding this Mach number by a small amount is not immediately dangerous, but – depending on the exact amount of exeedance – may require a maintenance inspection before the next flight and also needs to be reported to our safety department. During the certification of the aircraft type very brave test pilots have to prove during a test flight that the aircraft can fly up to the ‘design dive speed/mach number’ (called Vd/Md in manuals), which is Mach .89 for the A320. As you can see there is a lot a margin involved in airplane design, but we will never intentionally use this margin, as it is there for a reason. I instinctively reached to the control panel for the auto thrust system and reduced the selected Mach number to Mach .76, so the auto thrust would decrease the thrust even more. Afterwards, as the speed was still increasing and just about to reach the limit I pulled on the speed brake lever to arrest the speed increase (the speed brakes are panels on top of the wing that, when activated, will reduce the lift produced by the wing and therefore help to descend steeper or to slow the airplane down, depending on the situation). This finally proved effective and now the speed was decreasing rapidly. I immediately stowed the speed brake again as there is only a narrow speed band available at high altitude and it is possible to go from ‘going to fast’ to ‘going to slow’ in a few seconds if the reaction to an impeding overspeed is too aggressive. The airplane recovered to normal flying conditions, but I decided to leave the Speed at Mach .77 as a precaution, which is approximately in the middle of the allowed speed band and gives a good margin to both over- and underspeed. The First Officer remarked ‘That was quite close’ I remarked: ‘And totally unexpected.’ The First Officer noted that the jet stream reduced from 80 knots (150 km/h) to just 40 knots (75 km/h) within just a few seconds, which was probably what caused this speed excursion. He told me that he had experienced a similar situation already once while flying over Greece. Due to our quick reaction no limits were exceeded and therefore no report was needed.

We started our descend into Oslo soon afterwards and we were told to reduce our speed. For some reason Oslo Airport was very busy at that time. When we got close to the airport we counted at least 7 airplanes ahead of us on our screen (other airplanes are displayed on the ‘Navigation Display’, which gets this information from a collision avoidance system called ‘TCAS’, Traffic Collision Avoidance System [1]). We were held above our optimum profile by the air traffic controller. Usually airplanes enter the ‘glideslope’ from below, but in this case we were above the ‘glideslope’ and therefore had to apply a special procedure to get back onto our desired vertical profile. At this moment Air Traffic Control told us to reduce our speed to 160 knots as the aircraft ahead of us was just around 4 nautical miles (8 kilometers) ahead of us and getting closer. With the help of the speed brakes we managed to reduce our speed and to get back onto the glideslope at the same time (it is very difficult to descend and to reduce speed at the same time in a jet aircraft – the drag produced by the airframe is just not enough so the speed brakes are very helpful in these types of situations). The First Officer suggested that we could extend the landing gear earlier to further increase the drag and thereby reduce the speed faster. I decided that using the speed brake would be sufficient for now. While we were passing through 1000 feet (300 meter), which equates to slightly over one minute before touching down, the Airbus ahead of us landed. Air Traffic Control asked them to expedite vacating the runway which they luckily did. They finally cleared the runway while we were descending through 300 feet (90 meter) and we got landing clearance. After landing Air Traffic Control told us that there was somebody that was close behind us and we vacated as fast as we safely could. We were just turning onto the taxiway parallel to the Runway when the next airplane, a Boeing 737 was touching down. It is quite unusual to see this kind of tight spacing and in Europe only London Gatwick and London Heathrow work like this, but here in Oslo it worked perfectly as well. It was tight, but very efficient. I think if more airports were able to provide this quality of Air Traffic Control, delays could be reduced by a lot, but unfortunately in most airports the Air Traffic Control are not able to use this kind of procedure. There is no safety risk involved in this kind of operation as an airplane is always able to perform a go around (a maneuver that is considered ‘normal’ in aviation and is practiced by pilots many times, mostly in the simulator), if things don’t work out as planned. In most airports, especially big airports go arounds are very common and usually happens several times a day. In fact if there is any doubt if an approach should be continued to a landing or not for any reason, a go around is usually the safest option, as it gives everybody more time to reevaluate the situation and get back ‘into the loop’. All in all an interesting day for both of us.

If you are interested in some more details about flight testing at high speeds, I can recommend you this youtube video.

It shows the ‘flutter testing’ for the A380 where the test pilots push the airplane to the ‘design dive mach number’. As the A380 is certified for higher speeds than the A320 they had to fly the airplane up to Mach .96. For the A320, as mentioned above, the ‘design dive speed’ is Mach .89 only.

[1] Center of Gravity
[2] Fuel Planing
[3] Mount Teide
[4] Noctilucent clouds

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