Racing around the world at 43,000 feet, you may think that the biggest threat is hitting the ground below you. However, have you ever thought of what’s hitting the aircraft from above?
Cosmic radiation is all around us, but its effects are seen more at altitude, and particularly at the Poles, than on the ground. However, some of the most time and fuel-efficient flight routes take aircraft well into the Arctic Circle and close to the North Pole.
So how does this radiation affect us and how do pilots fly these remote routes?
Every time you get out of bed (and when you’re in it for that matter) your body is exposed to radiation. It’s all around us and is, for the most part, unavoidable. Some of this radiation is useful and other types are not so useful.
The effects of non-ionizing radiation, such as ultraviolet light, radio waves and microwaves, very much depend on the intensity of the radiation received. It can damage the skin and eyes (hence why we wear sunglasses and sunscreen) and if it penetrates the body, can cause damage to organs by heating them.
For the most part, this is the type of radiation we put up with day to day, utilizing the benefits to rapidly heat food in our microwaves and give our skin a much sought-after sun-kissed glow.
Ionizing radiation, such as cosmic rays, X-rays and that from radioactive material is the kind which tends to get peoples’ attention. The greatest worry about ionizing radiation is the increased risk of malignant diseases and genetic malfunctions.
Once again, the risks of ionizing radiation very much depend on the amount of exposure.
Cosmic radiation and the earth’s magnetic field
Cosmic radiation originates from two sources. Most of it originates from outer space, but some of it comes from the sun, which produces a constant stream of particles that billow out into space at almost one million mph. This is known as the solar wind and consists primarily of protons and electrons.
Meanwhile, back on earth, currents of electricity which flow deep in the molten core create a magnetic field. These currents are hundreds of miles wide and flow at thousands of miles an hour as the earth rotates. The magnetic field extends out thousands of miles into space where it acts as a shield against incoming radiation.
As the charged incoming particles hit the magnetic field, they are deflected away and are prevented from coming into contact with the atmosphere. However, there is a weakness in the magnetic field.
If you look at the image above, you’ll see that the magnetic field is thickest at the equator and virtually nonexistent at the poles. Added to this, parts of the magnetic field deflect the incoming particles to the areas around the poles.
As a result of this gap in the shield, a greater amount of radiation gets into the earth’s atmosphere at the poles than at the equator. The earth’s atmosphere does a good job of stopping most of the radiation from reaching the ground and it’s this interaction which causes the Aurora Borealis.
However, the amount of radiation in the upper atmosphere remains higher than on the ground.
Great circle routing
This is all very interesting but what relevance does it have to commercial aviation? Why would aircraft need to be flying by the North Pole anyway?
The answer lies in the curvature of the earth (sorry, flat earthers) and the shortest distance between two points on the surface. This is known as a Great Circle.
Everyone knows that the U.S. is west of the U.K. In fact, most of it is southwest. However, if you’ve ever flown from London to the west coast of the U.S. and watched the moving map, you will have noticed that the flight initially routes north toward Scotland.
This isn’t because the pilots are lost, its because it is actually the most direct route.
When planning the route of a flight, airlines will naturally try and take the shortest route possible, the Great Circle track. However, sometimes its beneficial to deviate off this route to take advantage of (or to avoid) strong winds. Even though the distance may be longer, the flight time (and subsequently the cost) will be reduced.
So with flights routing over the North Pole, what risk is there to the passengers and crew from cosmic radiation?
A December 2019 study looked to see whether the length of the flight or the routing of the flight had the greatest effect on the amount of radiation an aircraft was exposed to.
It sampled 15 of the longest commercial flights in operation, including four of which flew over the Arctic. These included flights to Los Angeles from Doha, Abu Dhabi and Dubai.
The image above shows the routes taken to Los Angeles by flights from the three Middle Eastern hubs and also the route from London for comparison.
Contrary to what the scientists were expecting, the study found that it wasn’t the duration of the flight but the route which has the greatest effect on cosmic radiation exposure. Aircraft that flew closer to the North Pole experienced greater radiation than those flying more southerly routes, even if they were airborne for longer.
Do I need to be worried?
A study by NASA found that polar flights during the solar storm of 2003 we exposed to 12% of the annual radiation limit recommended by the International Committee on Radiological Protection. Whilst this isn’t a problem for individual flights, it could start to pose problems for those who fly these routes frequently. Such as pilots and flight attendants.
Airline crew are categorized as “radiation workers” by the U.S. federal government, a classification that includes X-ray technicians and nuclear power plant workers. According to NASA, the average airline pilot receives more radiation a year than does a fuel-cycle worker in a nuclear power plant.
A survey of flight attendants in Europe and North America also found higher rates of skin, breast and prostate cancer, as well as acute myeloid leukemia than the average person.
For the average passenger, there is little to worry about. Even for the frequent flyer, the doses of radiation experienced on normal flights are not considered to be excessive. However, if you find yourself regularly flying between the Middle East and North America, you may want to give this some thought.
Flying across the Poles
So how do we fly polar routes and do we do anything different to avoid the cosmic radiation? Simply put, not really. As the radiation depends more on the route than the altitude, there is little we as pilots can do to reduce the exposure when flying these routes.
However, any flight across the Poles requires a bit more thought before departure. By definition, the routes are particularly isolated and careful consideration has to be given to diversion airfields. Cold air masses may affect fuel temperatures, potentially taking them below the minimum allowed temperature.
For aircraft to be able to take advantage of routes across the North Pole, very much like across the North Atlantic, a Polar Track structure has been created. However, unlike the North Atlantic tracks which move in location depending on the winds, the polar tracks are fixed. To ensure that there is no conflict between the two sets of tracks, the polar tracks are well north of the airspace used by the North Atlantic tracks.
The use of the polar tracks is similar to those crossing the Atlantic. Before reaching the start of the track, pilots must receive an ATC clearance. This includes the flight level, speed and track which the crew must adhere to.
However, as the polar tracks are less busy than the North Atlantic ones, pilots can normally plan on flying the track at the altitude and speed of their choice, normally those optimum for the flight.
Low fuel temperatures
While you’re seated enjoying a glass of wine in a pleasant 70 degree Fahrenheit cabin, outside your window it’s bitterly cold, normally around minus 67 degrees Fahrenheit in temperate regions. In Arctic areas, it can get even colder, minus 97 degrees Fahrenheit over Siberia is my personal record. When temperatures get this low, a conventional fuel would freeze.
The Jet A-1 powering the engines has a freezing point of minus 52 degrees Fahrenheit, so why doesn’t the fuel freeze when it’s minus 67 degrees Fahrenheit outside?
Take an average spring day out of London where it’s 59 degrees Fahrenheit. For this example, let’s say the fuel is also 59 degrees Fahrenheit. As the aircraft climbs, the outside air temperature decreases. Nominally by 35 degrees Fahrenheit every 1,000 feet. This means that by the time it reaches 35,000 feet, the outside temperature will be minus 67 degrees Fahrenheit. This is called the static air temperature, or SAT. This is the temperature you’d feel if you were stood on a passing cloud.
If the aircraft was just sitting on that cloud with you, the surfaces would chill to minus 67 degrees Fahrenheit, as would fuel in the wings. However, the aircraft isn’t stationary. It’s flying through this cold air mass at hundreds of miles per hour.
The speed of air over the wings creates friction, which actually heats the surfaces. By knowing the airspeed, you can work out what this heating effect will be. Adding this value to your SAT gives you your total air temperature or TAT. It is this TAT value that is chilling the wings and thus affecting the fuel temperature.
If I was to tell you that a typical TAT value at 38,000 feet is just minus 6 degrees Fahrenheit, you’ll now be able to understand why the fuel doesn’t freeze.
The wing material also has an effect on this chilling. The composite structure of the 787 Dreamliner wing means that it cools far slower than a conventional aluminum wing resulting in much warmer fuel temperatures.
What happens if the fuel temperature gets close to minus 52 degrees Fahrenheit?
It is possible that, if flying for prolonged periods in extremely cold air masses, the fuel temperature could drop toward the freezing point. However, pilots are alert to this possibility and will take proactive steps to ensure that this doesn’t happen. Each aircraft type has a threshold at which the crew are alerted to low fuel temperature.
On the 787, that threshold is around minus 35 degrees Fahrenheit. If this happens, the crew have two options. Either fly faster to increase the heating effect of the air or descend into warmer air. Since aircraft tend to fly as fast as they are designed, normally the only viable option is to descend.
Communications can also be problematic. Most areas of the world are well served by SATCOM. Pilots simply pick up the Sat phone, dial a number and can be connected to any telephone in the world in an instant. Unfortunately, when flying above 82 degrees north, SATCOM is unavailable.
In order to maintain communication with the ground, pilots must ensure that they establish high frequency (HF) communications with the relevant ATC unit. Fortunately, they do not need to listen to the painful static for the whole flight.
A system called SELCAL enables the crew to turn the volume off when they are not communicating with ATC. A SELCAL notification activates in the flight deck, very much like a phone ringing, to let the crew know that ATC needs to speak with them.
True versus magnetic
As the aircraft gets closer to the Pole, the magnetic compass becomes less reliable as the position of the aircraft relative to the Pole is changing so quickly. It gets to the point where pilots consider it totally useless. Instead of using magnetic headings and tracks, we use true headings and tracks.
Unlike the magnetic North Pole, the true North Pole doesn’t move. It is in effect the “top” of the earth. As a result, its position can be determined by GPS, increasing our navigational accuracy.
Whilst cosmic radiation should not be of concern to most passengers, it’s an occupational hazard of the job for the crew. There is greater exposure to radiation when flying routes over the Poles than those closer to the equator. This is down to the lack of protection from the earth’s magnetic field at the poles.
That said, the threat of radiation over the poles does not alter how pilots fly their aircraft. The cold temperatures and lack of communications do provide more of a challenge than on other routes but flight safety is never compromised. No matter what route your flight takes, your pilots will ensure that you arrive safely at your destination – leaving you blissfully unaware of the challenges such flying poses.