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The Jet Stream

The Jet Stream
Have you ever wondered why is it faster to fly from America to Europe than the other way around? The answer is the Jet stream. This ‘river’ of air flowing through the Earth plays a big role in aviation. Airlines have learned how to take advantage of it and plan their flights accordingly.

DISCOVERY OF JET STREAM

The discovery of the Jet Stream is often attributed to Wasaburo Ooishi, a Japanese meteorologist. Back in 1923, he observed that strong winds aloft would deviate atmospheric balloons as they gained altitude. By tracking their position, he was able to determine the wind speed. Although it was only a prediction, he was able to record a trend over the years and determined that these winds followed a pattern. His observations remained mostly unnoticed as he published them in Esperanto.

During World War II, Ooishi’s observations allowed Japan to launch “Operation Fu-go”. Using his prediction on winds aloft, the Japanese launched nearly 10,000 hydrogen balloons carrying bombs over the Pacific Ocean to America. Predictions of 190 knots (350 km/h) in wind speed (between 30,000 and 38,000 feet) would carry the balloons to the United States west coast in 3 days. His predictions turned out to be inaccurate and only some balloons arrived and didn’t cause the intended effect.

Balloons loaded with bombs. Photo: warhistoryonline.com
Balloons loaded with bombs. Photo: warhistoryonline.com

American aviator Wiley Post is also attributed the discovery of the Jet Stream. Post achieved the first around-the-world solo flight in 1931, developed the pressure suit and explored the limits of high- altitude flying. In 1935, while flying at 30,000 feet in his supercharged Lockheed 5C Vega “Winnie Mae”, he experienced speeds up to 340 miles per hour. Flying in the Jet Stream he was able to cover 2,035 miles between Burbank, California and Cleveland, Ohio in 7 hours and 19 minutes, proving the benefits of high-altitude flights. The same distance, at sea level, would have taken 12 hours and 42 minutes.

Wiley Post en su "Winnie Mae"
Wiley Post and his Lockheed 5C Vega “Winnie Mae”. Photo: Hulton Archive

A WORLDWIDE EFFECT

The air acts like a fluid, just like water. And so, it flows and it is affected by external forces, modifying its behavior and shaping its patterns.

On earth, due to the differential heating along its latitude, the vertical development of the atmosphere changes. Close to the Equator the air is warmer, so it ascends creating an area of low pressure near the surface. The surrounding air tends to fill in this ‘gap’, so it flows from the higher pressure area to the lower. The ‘gap’ created by the air moving to the low pressure area at surface, creates a sinking motion for the air that is up at the Tropopause. Thus, creating a circulation.

Circulation of the Hadley, Ferrel and Polar cell. Photo: NASA – Wikimedia
Circulation of the Hadley, Ferrel and Polar cell. Photo: NASA – Wikimedia

There are three circulation cells per hemisphere. The Hadley, the Ferrel and the Polar cells. These cells find their top at the Tropopause, the layer at which the air stops rising. Close to the equator, the air is warmer and ascends much higher, elongating the Tropopause further up. The average (it varies during the year) altitude of the tropopause at the equator is 56,000 feet, and 30,000 feet at the poles.

Cross-section of the Cells and its circulation. Photo: Sleske – Wikimedia
Cross-section of the Cells and its circulation. Photo: Sleske – Wikimedia

The Jet Stream originates at the boundary of these cells. Due to Earth’s rotation, the air traveling to this boundary is forced sideways. This is called the Coriolis effect. In the Northern hemisphere, the air traveling North will be forced to flow East. This is why the Jet Stream flows primarily in that direction. The greater the air velocity, the greater the deviation. If the difference in temperature is high between cells, the Jet Stream speed increases, up to 200 Knots (370 Km/h).

The Jet Stream is like a continuous ‘river’ of air, meandering. This is due to the difference in Coriolis effect at different latitudes. These are called Rossby waves, and it’s the reason why we often see Jet Streams that are not directly heading East.

Rossby waves
Rossby waves. Photo: NASA

THE JET STREAM IN AVIATION

With all this knowledge, aviation can take advantage of a given atmospheric condition at any time. By using meteorologic and satellite information, we can predict future weather phenomena, winds aloft and more. Airlines use hourly updated information to plan their flight avoiding dangerous weather worldwide.

When dealing with Jet Streams, flight planning departments take into account the position, height, extension and speed of the wind on their planned route. Therefore, anticipating themselves and being able to modify the route to, for example, avoid a strong headwind o a turbulence area associated with it.

SIGWX
Significant Weather Chart of the Atlantic Ocean. Photo: Crewbriefing.com

Pilots also receive weather information in form of SIGWX (Significant Weather Chart) and wind charts. Then, they can also judge the situation, discuss and decide the better course of action. On long haul flights, a route deviation implies a great deal of considerations: Fuel planning can be affected, ETOPS operation may restrict certain deviations, en-route alternate airports may have to be adjusted, the airline may have to consider schedule affectations on connecting flights, etc.

TURBULENCE ASSOCIATED, CAT AREAS

As we have seen the Jet Stream is a fast-flowing stream of air. The air surrounding it is, by comparison, slower or even still. When an aircraft is approaching a Jet Stream area and the wind suddenly increases, it suffers from what is called Windshear. This is a sudden change in relative speed between two adjacent air masses. A change in wind speed causes instability within the air mass. As it flies through it, the aircraft is subject to those disturbances, suffering from turbulence. Also, as the Jet Stream flows right next to the border of the Cell, we find a transition between air temperatures, thus changing the density of the air and creating instability as well.

This type of turbulence is not associated with clouds, that’s why it’s called CAT, Clear Air Turbulence. Usually the turbulence is reduced to a mere bumpy road-like feeling. With short and repetitive shaking, this kind of turbulence is unpleasant for passengers rather than dangerous for flight safety. Nevertheless, there have been situations in which moderate and severe turbulence has been encountered as a result of CAT.

Depiction of a cell boundary, Jet Stream and Area of CAT
Depiction of a cell boundary, Jet Stream and Area of CAT

From the flight planning stage, with help of the aforementioned maps & charts, pilots do their best to avoid such areas. CAT areas are marked and their vertical extent also advised. As we can see in the map, the blue dotted line over Sardinia (Italy) represents an area of potential clear air turbulence. In the legend we can see it ranges from FL210 to FL410. We can also see how it is associated with a Jet Stream that is flowing from North to South (Red line) at 120 knots (Every triangle represents 50kt and every line 10kt).

Significant Weather Chart showing the Jet Stream and associated CAT areas. Photo: Crewbriefing.com
Significant Weather Chart showing the Jet Stream and associated CAT areas. Photo: Crewbriefing.com

When an aircraft is subject to moderate and severe turbulence, the pilots shall report it to the ATC —Air Traffic Control— to help other traffic in the surrounding area and raise awareness of potential hazardous areas. A simple change of Flight Level (Altitude) shall suffice to get out of the turbulent area. Often times, pilots ask the ATC for an altitude change to avoid the uncomfortable shaking.

CASE SCENARIO

As a little example of the big influence the Jet Stream can have on a given flight, we are going to take a look at a flight from Los Angeles to Tokyo-Haneda, and how we can take advantage of our knowledge.

We can see that the Jet Stream, the same that instigated the Japanese balloon project, flows along the Pacific Ocean. If we were to follow the standard routing, we would find ourselves right in the middle of the Jet Stream. Let’s take a look at the effect of it.

When flying through the Jet Stream, we are experiencing a sustained headwind of up to 120kt along he whole route. This would result in a flight time of 12 hours and 45 minutes to cover 4835 Nautical Miles, and an estimated fuel burn of 94,800 Kg of fuel. Allowing for contingency, final reserve and alternate fuel we would need roughly 108,000 Kg of fuel on departure.

Conversely, if we decide to deviate further North, even though we will fly a longer route (123 Nautical Miles longer) we will avoid the Jet Stream and the effects are very noticeable. Let’s see:

Flying the Northern route we would fly a distance of 4958 Nautical Miles, it would take only 11 hours and 25 minutes. 1 hour and 20 minutes and nearly 10 tones of fuel less. This represents a massive time and fuel cut, even when flying a longer route. Of course, this is a perfect example, some days the difference would be less. But, all in all, it means millions of dollars in savings when thousands of flights are scheduled on a yearly basis. This is how important is to have an effective operations & flight planning team supporting the flight operation.

How the ADS – B works. The Future’s technology is already arrived.

The increase in density of air traffic in Europe, United States, and remote areas such as the North Atlantic Ocean, made the necessity to implement Single European Sky programs (SESAR) and NEXTGEN. To achieve this goal, technology plays a vital role. The ADS-B is one of them.

According to stablished critera by every civil aviation administration, as of 2.020, aircrafts must be equipped with ADS-B system. In Australia, pioneers in remote airspace management is already implemented since December 2.009 above FL300. Thanks to this technology they were able to reduce aircraft separations from 30 NM to only 5 NM, increasing airspace capacity significantly. But, What is it? And How does it work?

Principles of working.

ADS-B (Automatic Dependent Surveillance – Broadcast), is a surveillance system which will replace information already obtained from radars.

ADS-B Schematics.

This new system allows navigation systems onboard of an aircraft to obtain its position from GPS signals. Signals are joint together with other flight data gathered from other aircraft’s systems and broadcasted. Signal broadcasted is received by ground stations, inflight stations or satellite’s receivers and represented on a screen.

ADS-B Definition.

Nowadays, to be able to supply air traffic control with radar, air traffic controllers have one o more radar stations on ground to provide aircraft’s position. This information is obtained from radar echoes PSR (Primary Surveillance Radar), or exchanging information between aircraft and ground station, thanks to the transponder. This is known as SSR (Secondary Surveillance Radar). Between the two systems, SSR is the most accurate with MODE S transponder.

System and capacity.

The ADS-B has two basic capacities known as “OUT” and “IN”.

ADS – B “OUT” defines its capacity to broadcast ADS – B information. As an example of what kind of information broadcasts, the A330 with “OUT” capacity sends out the next information automatically and in a continuously manner:

  • Latitude and longitude, Horizontal Integrity Limit (HIL), the difference between barometric altitude and geometric altitude and ground speed (GS). All obtained from GPS signal;
  • Barometric altitude is obtained from ADIRS;
  • Track and vertical speed given by IR’s;
  • The ATC flight number introduced in the preflight check is given by FMS;
  • Emergency status; and
  • Selected altitude and heading, and barometric pressure (QNH/QNE) from the FCU.

This last function allows ATC, if they are equipped with proper system, to see on their screens their clearance and what pilot selected on the same radar tag. Very similar to what it happens with MODE S “enhanced” which uses “Down – link of Airborne Parameters” (DAP). This is what happens in airports such as London Heathrow. But this is another story…

On the other hand, ADS – B “IN”, defines its capacity to receive information from other ADS – B “OUT” stations which broadcast information.

Sounds pretty obvious that an aircraft equipped with both functions will be able to broadcast and to receive ADS – B information, to and from other ADS – B stations.

For an aircraft to able to be equipped with ADS – B technology, is necessary to have datalink equipment in VHF band. To do that, aircrafts use mainly two different equipments: 1.090ES and UAT978.

The UAT978 (Universal ADS Transceiver) is an equipment only used in United States below 18.000 feet. Created for general aviation, if it’s equipped with “IN” capacity, will be able to receive weather information free of charge. However, the rest of the world Will be using 1.090ES complying with ICAO requirements, what in fact has higher data transmission capacity.

But, What is 1.090ES all about? Basically, is a mode S transponder modification, currently on board of aircrafts. As the mode S transponder does, transmits on 1.090 Mhz., broadcasting information instead of waiting for the interrogation from a SSR station. This transponder has a group of extra capacities added to mode S, that the reason for its name: “Extended Squitter”.

Besides, because it’s working on the same frequency, it’s able to comply with airspaces where SSR radar service is provided and with ADS at the same time.

Advantages.

Regarding all conventional radar information which air traffic controller receives, ADS – B is more reliable. All data is sent directly from the aircraft’s navigation equipment.

There is other factor which affects service which is the transmission speed. Nowadays, secondary surveillance radars use interrogation/response of onboard transponders to obtain data information from aircrafts. Let’s say for a moment and aircraft flying under radar coverage equipped with transponder. The SSR antenna begins with an interrogation on 1.030 Mhz. and aircraft through its transponder will response on 1.090 Mhz. with information asked. Once this information is received on ground, is presented to air traffic controller’s screen. To the contrary, ADS – B broadcast twice per second automatically without the necessity to be interrogated by any other equipment. An ADS – B “IN” antenna receiver is only needed.

With ADS – B, all radar antennas could be easily replaced by ADS – B receivers, simpler to install, easy to maintain, more energy efficient, and in the end, cheaper.

Functions.

If I explained it correctly until now, maybe you were able to guess other advantages or capacities this technology has.

If we add an ADS – B receiver antenna to an aircraft, we would give it ADS – B “IN” capacity. We only would need a way to represent this information in the cockpit: CDTI (Cockpit Display of Traffic Information).

All this will be translated into being represented on places we’re all familiar with, such as TCAS screen, on a MFDU (Multifunction Display Unit), or on a ND (Navigation Display). We would be able to see on one of these screens onboard of our aircraft the same information as the air traffic controllers on their radar screens. There is no doubt how this increases pilot’s situational awareness in highly congested airspaces.

As we mentioned before, he appearance of ADS – B has brought about some new applications: TIS – B (Traffic Information Service) and FIS – B (Flight Information Service).

TIS – B allows information regarding aircrafts with transponder but not ADS – B equipped, flying under radar coverage, to be broadcasted by ADS – B “OUT” stations. This information is received by ADS – B “IN” equipped aircrafts being able to see on their screens onboard other aircrafts around them which are not equipped with ADS – B.

FIS – B allows aircrafts equipped with ADS – B “IN” to receive weather information, ATIS, or NOTAM from ADS – B “OUT” ground stations. This type of service is well known as FIS – B.

New Procedures stablished on NAT – HLA oceanic airspace.

Of course, ADS – B is a very substantial improvement in air traffic control. The ITP (In Trail Procedure), allows aircraft to choose optimum flight levels without being “blocked” by other aircraft flying at a distance with no radar further than “ITP Distance”.

This is, if an airplane wish to climb or descend crossing other airplane’s flight level, and both are ADS – B (“IN” & “OUT”) equipped, sending the request via CPDLC to ATC, it will show  distance from other aircraft, their flight level and callsigns before being sent to ATC on CPDLC’s screen.

Thanks to ADS – B, ATC will receive more accurate information and will have a total picture of airplanes instantly with no ADS – B equipped and ADS – B airplanes. So, it will be easier for ATC to see if they have separation enough to give the clearance for level change request.

Near and future developments.

On the other hand, over remote areas and over oceans, receiver antennas on ground is not possible or feasible. Because of that, for some time companies have been working on a low altitude satellite constellation able to receive ADS – B signals from aircrafts.

This constellation called Iridium is made of 66 active nanosatellites and 9 as spare. They are in an orbit just at 785 km from earth’s surface, being able to receive ADS – B signal and send it to ATS centres. It’s expected to be fully operational in the end of 2.018.

Curiously, and to mention flight MH370 disappearance, the company FlightAware has signed an agreement with Aireon (Owner of Iridium Constellation) to supply airlines with a fleet management and tracking capabilities based on this system. This is the solution to ICAO’s system called GADS (Global Aeronautical Safety System) to continuously monitor aircraft’s position.

In Europe, comparing to United States, there are no immediate plans of integrating ADS – B position to ATC’s system in all regions, and to provide ATC service. That’s the reason why Iridium is fastest solution. In Italy, for example, will be implemented.

The implementation of ADS – B is, as you have read, a very remarkable change in what we already knew about surveillance. There is no doubt the reduction on aircraft’s separation in remote areas will be very valuable, especially in HLA airspace in the North Atlantic Ocean, as it happened before in Australia with no effect on safety at all.  

Besides, being able to handle information rapidly and accurately, crews and air traffic controllers will be able to increase their situational awareness which will lead to take decisions easily.

Finally, implementing ADS – B procedures like ITP, will allow an optimization of airspace. Aircrafts will be able to fly closer to their optimum flight level, reducing fuel burn and CO2 emissions.