Introduction to Flight Planning: Route Selection and optimalization

After prefilling the planning parameters of the flight mission we are planning to undertake, being, in short:

1. Scheduled Time of Departure (STD)
2. Scheduled time of Arrival (STA)
3. ?Alternate Aerodromes, wherever needed
4. Zero Fuel Weight
5. Performance limited takeoff and landing weight constraints, including technical deficiencies which impact these.
6. Technical deficiencies en-route fuel flow implications
7. Selected runways at departure, alternate and destination airports
8. Contingency fuel policy to be applied.
9. Checking airfield NOTAMS (Notice to Air Mission) of our selected fields
10. Taxi times (especially in winter, with de-icing considerations)

which basically are also the constraints in which we operate out current flight mission, we can start to determine the optimum routing by letting our flight planning system calculate 5 types of routes where applicable/available:

1. A minimum cost track (MCT) Costs taken in account are fuel costs, (only trip and taxi), maintenance costs, en-route ATC charges, SID & STAR (standard Instrument Departure, Standard Arrival Route) restrictions and/or limitations (e.g., due to aircraft performance or because of a safety issue risk assessment). Unfortunately, crew costs are not taken in account as the calculation would become too complex, as working times are rooted in the work- and rest time regulations of the crew, but there is also a significant amount of customized ( union) agreements coming into play in the operational phase which have not yet been modeled accurately enough to count them in their entirety, impact-wise as a factor into the flight planning system. A manual check will need to be done to consider them in case of late arrivals due to this complexity. Other very important considerations are how much margin do we have to ensure our passengers including their luggage make their onward connections and fleet availability (E.g., How urgently is the aircraft needed for the onward flight)
2. A minimum Time Track (MTT) This option can be considered in case of excessive delays but panning an MCT with a high-cost index (planned at a high cruise (air)speed/Mach number.) usually still gives the best time gain vs cost. An airline cost index is defined as the ratio of time-related costs of operation of a plane to the total cost of fuel used at the same time. (High-cost index = high (air)speed) The difference between a route optimized for minimum time vs a high-cost index cruise system can be explained the easiest in the following way: An MTT route optimized for the minimum flight time actively looks for flight levels and coordinates and waypoints giving the best groundspeed (usually with the greatest tailwind component), without varying the cost index ( cruising speed)along the route when at the same flight level. Step climbs may vary the air speed/Mach number to be flown. A step climb is a higher optimum flight level as the flight progresses, caused by a lower gross weight of the aircraft as it burns fuel. The cost index principle just increases the airspeed (higher thrust settings of the engines). Of course, it is not as black and white as described here, there are many overlapping techniques depending among others on the mandated cruise techniques required for separation by ATC in certain airspaces, but this is to explain the essential difference between both techniques to get the aircraft home in the quickest way.
3. A minimum Fuel Track (MFT) This can be an option to investigate when we have more payload than we can accommodate. (Usually performance related)
4. Predefined routes along fixed waypoints. These have been checked in their entirety as complying with all airspace requirements their respective waypoints are in, as opposed to the above 3 options.
5. North Atlantic tracks: these are published daily and therefore are different every day. Their validity is in a certain time bracket and their function is basically to create highways in the sky to structure the eastbound and the westbound traffic peaks between north America and Europe according to the prevailing weather conditions for the time of their validity. On this link you can find a thorough explanation of the Organized Track System as they are also called.

Now that we have determined the optimal route for our mission, we will start to consider hazards enroute, and avoid them where possible by tweaking our route or rerouting it in its entirety if circumstances so dictate. Safety before everything else. Weather related safety hazards are often interdependent, which is why some phenomena are repeated in below examples.

Cumulonimbus clouds

Clouds with vertical development are a family of clouds that grow really tall in the sky. They include towering cumulus and cumulonimbus clouds. These clouds have supercooled water, which means water that is still liquid even though it's below freezing temperature, high up in the sky. When a cumulus cloud grows very tall, the water at the top freezes into ice crystals, and it becomes a cumulonimbus cloud.

These clouds can start forming at around 1,000 feet above the ground and can go way up above 10,000 feet. Two specific types mentioned are towering cumulus (TCU) and cumulonimbus (CB) clouds. Towering cumulus clouds indicate there's a thick layer of unstable air, and they look like big cauliflower with swelling tops. They can bring showers, and there's strong turbulence and icy conditions high up.

Cumulonimbus clouds are the most extreme form of unstable air. They are huge clouds with dense, boiling tops that sometimes look like an anvil. They come with all sorts of flying hazards:

• Turbulence. Vertical movement within a Cb can be as much as 50kt. The interaction between strong updrafts and strong downdrafts causes wind shear and severe turbulence within the cloud.
• In-Flight Icing. Moderate to Severe icing can be expected, especially in the higher levels of the cloud, which can be very dangerous when accumulated on the surface of the aircraft.
• Electrical disturbance. Aircraft flying in the vicinity of Cb clouds may experience electrical disturbances effecting communications and navigation systems. The electrical phenomenon known as St Elmo's Fire, while not a threat to safe flight, is an indication of nearby Cb activity. Aircraft in the vicinity of a Cb are at risk of being hit by Lightning.
• Lightning is a hazard by itself: When lightning strikes an airplane, the bright flash of light that comes with it can temporarily blind the flight crew, especially when it's dark outside. To prevent this, the flight deck lights should be turned to full brightness. After a lightning strike, a special inspection needs to be done once the airplane has landed. This inspection includes checking the compass to make sure it's working correctly. A lightning strike can cause small holes in the airplane's structure, and it can also make instruments, compasses, antennas, or radio equipment unreliable or damaged.
• Precipitation. Hail can cause significant structural damage to an aircraft.
• Turbulence. Turbulence occurs when air currents in the atmosphere vary greatly over short distances. These varying currents, from mild eddies to strong currents, can jostle an aircraft as it moves through them, causing changing accelerations. Turbulence can range from slight bumpiness that may be annoying to severe jolts that can damage the aircraft or injure passengers. The aircraft's response to turbulence depends on factors like wind speed differences, aircraft size, wing loading, airspeed, and attitude.
• There are several types of turbulence:

1. Convective:( caused by areas with differential heating properties landmass vs. sea, barren or with dense vegetation).
2. Mechanical or frictional turbulence:( caused by obstructions to wind flow (for example, steep hills, ridges, and buildings, when windspeed is higher than 20 kts).
3. Wake turbulence.: (Caused by preceding aircraft)
4. Clear Air Turbulence (CAT). For en-route flight planning hazard avoidance CAT is the most important.CAT is a sudden and severe turbulence that occurs in cloudless regions, causing strong shaking of aircraft. It is most common at higher altitudes and related to wind shear. CAT is encountered outside of convective clouds, including cirrus clouds and near thunderstorms, but excludes turbulence caused by thunderstorms, low-altitude temperature inversions, thermals, strong surface winds, or local terrain features.

Common causes and sources of CAT are:

Jet Streams.

A Jet Stream is a narrow, fast-moving current of air, normally close to the Tropopause and generated because of the temperature gradient between air masses. Although not all jet streams have CAT associated with them, there can be significant vertical and horizontal Low Level Wind Shear on the edges of the jet stream giving rise to sometimes severe clear air turbulence. Any CAT is strongest on the cold side of the jet stream where the wind shear is greatest. Near a jet stream, CAT can be encountered anywhere from 7,000 feet below to about 3,000 feet above the tropopause. Because the strong vertical and horizontal wind shear occurs over short distances, this jet stream related CAT tends to be shallow and patchy so a descent or climb of as little as 2,000 feet is often enough to exit the turbulence.

Terrain.

High ground disturbs the horizontal flow of air over it, causing turbulence. The severity of the turbulence depends on the strength of the air flow, the roughness of the terrain, the rate of change and curvature of contours, and the elevation of the high ground above surrounding terrain. Greenland and the Andes are prime examples of areas where this type of turbulence can be present. Another term for this type of turbulence is called Mountain Waves.

Mountain Waves are associated with severe turbulence, strong vertical currents, and icing, loss of Control and / or Level Bust. The vertical currents in the waves can make it difficult for an aircraft to maintain en route altitude leading to level busts and can cause significant fluctuations in airspeed potentially leading, in extremis, to loss of control.

The speeds of the vertical currents within the oscillations can reach 2,000 ft/min. The combination of these strong vertical currents and surface friction may cause rotors to form beneath the mountain waves causing severe turbulence.

Aircraft can suffer structural damage because of encountering severe clear air turbulence.

CAT can cause damage to the aircraft's structure and fittings, and it can also lead to injuries for passengers and crew members.

Impaired Flight Crew Performance: Moderate or Severe turbulence can make simple tasks, including reading instruments, near impossible.

In even moderate turbulence, damage can occur to fittings within the aircraft especially because of collision with unrestrained items of cargo or passenger luggage.

If caught unaware, passengers and crew walking around the aircraft cabin can be injured.

To avoid all these hazards Dispatchers and Pilots use multiple sources of SIGMET and of high altitude significant weather charts, like below, which can be obtained for different times of validity from this site.

The yellow contours are turbulence, red contours are cumulonimbus clouds, the green arrows are the jet streams, and the depicted volcanoes speak pretty much for themselves.

Space Weather

What is space weather and why is it a threat for aviation?

Space weather refers to the conditions in outer space, including the Sun and the space environments around planets. It covers various things, like slow-moving cosmic rays coming from outside our solar system and fast solar wind streams from the Sun's outer layer. Space weather can cause different effects, like eruptions and changes in charged particles, light, and magnetic fields.

For international civil aviation, space weather forecasts focus on specific disturbances, such as solar radiation, geomagnetic and ionospheric storms, and solar flares. They also predict slow changes, like cosmic rays and geomagnetic storms caused by high-speed streams. These forecasts help operators stay aware of the situation and make alternative plans if the conditions could disrupt normal operations.

From an aviation operations perspective, space weather events cause disruptions to:

• Communications, mainly HF (High Frequency) radio communication
• GPS navigation and surveillance systems
• Communications via satellite
• Radiation exposure at flight levels

Space weather events may occur on short time scales, with the effects occurring from almost instantaneously to over a few days.

Volcanoes

Volcanic Ash Advisory Centers (VAACs) are responsible for monitoring and responding to volcanic eruptions and ash reports in certain areas. They can be obtained from this site:

Here's how they do it:

They use satellite data (from geostationary and polar-orbiting satellites) and, if available, data from ground-based and airborne sources like Doppler weather radar, ceilometers, lidar, and passive infrared sensors to detect volcanic ash in the air. They use a special numerical model to predict how the ash cloud will move. They issue advisory information about the extent and movement of the ash cloud to various aviation-related entities, such as weather offices, air traffic centers, and other VAACs that could be affected. They also share this information with international databases and operators who need it.

They keep updating this advisory information to the mentioned entities every six hours until the ash cloud is no longer visible in satellite and other data, there are no more reports of volcanic ash from the area, and no further eruptions occur.

VAACs work around the clock to keep an eye on the situation. If a VAAC's operation is interrupted, another VAAC or meteorological center takes over its responsibilities.

The advisory information about volcanic ash is issued using approved abbreviations and numerical values that are easy to understand. If there are no approved abbreviations available, they use simple English text. This information is shared in a specific format to make it easy to distribute to everyone who needs it. An example of such an advisory can be found below, and there is also the possibility to plot the projected ash cloud in google earth via the downloadable KLM file and plot your intended flight route next to it to make sure, in an objective, quantifiable manner if you can clear the ash with the intended route with sufficient margin.

Hurricanes and Thunderstorms.

I have published an article already here for this phenomenon, so I will not elaborate further on it at this point in time.

Sandstorms

These are common in arid or semi-arid areas, particularly the Sahara and the Arabian Peninsula.

An advancing sandstorm associated with a gust front is a spectacular sight and looks like an advancing wall of swirling sand. The height of this wall can be 1 nm or more; in situations where there is significant atmospheric instability, dust can reach as high as 20,000 feet.

The frequency of sandstorms, particularly across the lands bordering the Sahara Desert, has increased dramatically over the past few decades, caused by and perhaps a major causal factor behind desertification. It is therefore becoming more commonplace for aircraft to encounter a sand or dust storm when operating in these regions.

Sandstorm activity results in reduced visibility, ingestion of sand and dust particles into engines, pitot static systems and air conditioning packs causing blockage and corrosion.

Compliance with local AIP Rules and Regulations

After avoiding or considering all natural hazards, like we did above and adjusting out flight routing accordingly we should ensure that especially routings overland comply with all local rules and regulations. For this purpose (partial or whole, from origin to destination) company routes are made, which ensure compliance with them.

Avoiding CTOT’s wherever possible.

Calculated Take Off Time (CTOT), or colloquially called Slot, is a crucial concept in aviation that refers to the specific time when an aircraft is scheduled to take off from the airport. It is calculated based on various factors to ensure safe and efficient operations in busy airspaces, especially during periods of high air traffic.

When an aircraft is preparing for departure, air traffic controllers and airline operators work together to manage the flow of planes in and out of the airport. CTOT is determined by considering factors like the aircraft's weight, its planned route, the current weather conditions, and the air traffic demand at that time.

More information about CTOT can be found on this link.

Here's how it works in a bit more detail:

Aircraft Weight: The weight of the aircraft affects its performance during takeoff. Heavier planes need a longer runway and more thrust to become airborne, so the CTOT considers the weight of the aircraft and calculates an appropriate time slot for takeoff to maintain safe distances between planes.

Planned Route: The CTOT also considers the aircraft's intended flight path. This helps to avoid congestion and ensure a smooth flow of traffic in the air.

Especially European airspace is very congested so en-route it can happen many times that within a certain area of responsibility of national air traffic control the traffic demand exceeds the capacity of that area.

Weather Conditions: Weather can have a significant impact on aircraft performance. Strong winds, storms, or low visibility might necessitate adjustments to the CTOT to ensure safe operations.

Air Traffic Demand: During busy periods, when many flights are scheduled to depart or arrive at an airport, air traffic controllers use CTOT to manage the flow of planes. By assigning specific takeoff times, they can spread out departures and prevent congestion on the runways and in the airspace.

By calculating the optimal takeoff time for each aircraft, air traffic management systems can enhance safety and efficiency in the aviation industry. This helps prevent delays, reduces the risk of aircraft getting too close to each other, and ensures that flights can operate smoothly even during busy periods.

While many times a slight reroute can avoid CTOT’s altogether it will not help if the restriction is at the departure or arrival airport, and/or if en-route, every other operator starts to avoid the concerned airspace as well causing the slot-free airspaces to be congested as well. And sometimes reroutes can be way too long to be still economically viable against the slot (which still can improve as the departure time comes closer, unlike a reroute, which is fixed unless changed again). Please find below a typical day in Europe from an ATC regulation perspective.

Luckily nowadays we as dispatchers have tools with a lot more information about alternative routings at our disposal to consider rerouting due to en-route capacity reductions like illustrated below. We can optimize both in a horizontal and vertical plane, wherever allowed.

Avoiding extremely low fuel temperature situations.

Another, maybe unexpected hazard we might encounter during flight planning is the risk of extremely low fuel temperatures.

Aircraft nowadays are less sensitive to this due to more extensive use of composite materials in the manufacturing process as opposed to full metal, but they are still at risk.

In aviation we have mainly 2 types of Jet Fuel:

Jet Fuel A vs. Jet Fuel A1:

The differences between the 2:

Freezing Point:

Jet Fuel A: It has a relatively higher freezing point, making it less suitable for use in extremely cold temperatures. ( -40 Degrees Celsius) because it contains fewer additives to improve its low-temperature performance.

Jet Fuel A1: This fuel has a lower freezing point and improved cold flow properties, because it contains additives that enhance its cold flow properties, reducing the risk of fuel waxing or gelling, making it therefore more suitable for use in colder climates. ( -47 Degrees Celsius)

In summary, low fuel temperature detection and prevention in aviation are critical for maintaining safe and efficient flight operations, especially in cold weather conditions. Jet Fuel A1 is the preferred choice due to its improved cold flow properties and broader usability across different aircraft types.

Actions we can take during the flight planning process to mitigate this risk:

• Consult the flight planning system to identify and avoid extremely cold spots along the route, either vertically or horizontally, where feasible. Lower Flight Levels are normally warmer.
• Consider planning the flight at a higher speed to maintain a higher total air temperature due to the so-called RAM rise, caused by increased friction to keep the temperature higher.

Rocket Launches.

Last but not least rocket launches and especially their re-entry from space pose a risk for air travel which will need to be mitigated by appropriate route selection.

Please find 2 articles below explaining the phenomena and schedules

https://medium.com/faa/lets-give-em-some-space-4028dea2e7d8

https://www.rocketlaunch.live/

This concludes in high level most of the aspects we need to consider in the route selection phase of flight planning.

Note: I would?like to thank my co-writer Terry Mitchell for his contribution to the design of the article and for ensuring readability also for people outside the aviation industry.