How do pilots know where to go? The fascinating precision of flight

May 21, 2022

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Making your way to the airport to catch a flight, I’m sure most people will have a plan. If you’re driving to then park your car for the trip, you may well already know the way to go. If you’re taking public transport, you’ll no doubt plan your route before you leave the house.

No matter how you make your way to the airport, you’ll know exactly where you are at any point and how to complete the rest of your journey.

So isn’t it a little odd that once you step onboard your aircraft and settle into your seat, you have next to no idea of how exactly you’re finding your way to your destination? Sure, there’s the moving map but how exactly does the aircraft take off from a 60m wide strip of concrete on one continent and end up on another 60m strip of concrete thousands of miles away?

Handing control of your destiny over to the crew in the flight deck is a large part of what gives people anxiety about flying. When you break it down, it’s actually quite understandable.

Having been in control of how you got to the airport, all of a sudden you have no idea how exactly you’re going to be getting to your destination. It’s all in the hands of people you’ve never seen, let alone met.

So how do those unknown faces know how to get you safely to your destination and how do they know the way?

On the Ground

The journey to the destination begins the moment the aircraft starts moving backwards from the gate. However, we are not actually able to do this on our own. In order to get the flight started, we need a little help from the ground staff in the form of a pushback tug,

Once pushed back from the gate, it’s time to make our way to the runway. At smaller airfields, there is often just a single taxiway to the departure point so it’s pretty straightforward to see where to go. However, at bigger airports with a maze of links routes and taxiways, things can get incredibly complicated.

Modern aircraft have a moving map display in the flight deck to show pilots where their aircraft is in relation to the taxiways (Image Charlie Page/TPG)

To help us, taxiways are given an alphanumeric name to help differentiate them from one another. These are then depicted on a map of the airport, which is provided to the pilots.

Modern aircraft have gone one better than a map and have a moving display — very much like the SatNav system in your car. Pilots are able to see exactly where the aircraft is in relation to the runways and taxiways around it. This is particularly useful at night or when the weather is bad, giving pilots greater spatial awareness and reducing errors as a result.

In the Air

Once airborne there are no paved surfaces for us to follow, no marked lanes to guide us. Like seafarers on the oceans, our external environment is our point of reference.

With that in mind, the simplest form of navigation is still by map. However, instead of roads and street names, we must use physical features that can be seen by looking down.

Visual Flying

The charts used by pilots to navigate visually depict the ground below us, giving an accurate representation of rivers, lakes, towns, motorways, and railway lines.

By comparing what we can see out of the window to what we have on our charts, we can fly extensive routes around the country without the need for any other help. This is known as Visual Flight Rules (VFR) and must be conducted only by reference to what the pilot can see out of the window.

However, whereas driving a car is arguably in a 2D environment, flying an aircraft is a 3D environment as we not only have to deal with where we are going laterally but also where we are vertically.

VFR flying is mainly the domain of small aircraft (Photo by: aviation-images.com/UIG via Getty Images)

As a result, airspace is divided up into different segments which all have a lateral and vertical limit. This means that the airspace around and above a major international airport can be protected but will still enable aircraft to fly safely over the top of it, so long as they are above a certain altitude.

VFR flying is perfect if you’re flying a small, relatively slow aircraft mainly overland on a day with good weather. The problem comes when you must fly through clouds or cover larger distances such as the flights conducted by commercial airliners. For this, we need a different method of navigating.

Instrument Flying

When flying an airliner, we can’t rely on rivers and roads to work out where we are going. Not only are they difficult to see when we are several miles above them, but they disappear completely as soon as we are in or above the cloud. They also tend to be few and far between over the oceans.

One of the key skills pilots require when flying an airliner is the ability to fly safely when in cloud and the majority of basic and continual training is devoted to this.

We take for granted that the world which we understand on the ground takes a majority of clues from what we see around us. Buildings go vertically up, the sky is above us and we can often see the horizon. It’s how our bodies have evolved over millions of years to enable us to walk, run and generally not fall over.

Pilots of large airliners have to be able to navigate when they can see nothing out of the window (Photo by Robert Smith/MI News/NurPhoto via Getty Images)

However, when all these visual references are removed and all you can see is white (or black at night), within seconds we become disorientated. We have no idea what is up and what is down. To stop us from spiralling the aircraft into the ground, we must trust and use our instruments.

As part of initial flight training, after mastering visual flying, cadets then move onto the instrument flying stage where they learn to fly purely with reference to the instruments in the cockpit.

Navigation Systems

To be able to work out where we are going when we can not use visual references from the ground, we have a navigation system that uses a combination of ground-based signals and aircraft-based systems.

Beacon Navigation

In the period after WW2, navigation beacons were established around the world that would send out a signal at a particular frequency. Pilots could tune their navigation radios onboard the aircraft to these frequencies to determine where the beacon was and, with some of them, how far away it was.

As a result, aircraft could fly from beacon to beacon, creating a system of roads in the sky with each beacon acting as a junction.

A VHF Omni direction Radio range (VOR) beacon with a car for scale(Image – www.nats.aero)

 

However, this method of navigation has its problems.

Firstly, due to the curvature of the earth, the range of these beacons is limited to around 150 miles. As a result, aircraft could only fly around 300 miles between beacons (150 miles away from the first beacon and then 150 miles into the next one).

This meant that hundreds of beacons had to be installed to enable pilots to ‘chain’ them and fly long routes. The other obvious flaw is that as beacons could not be installed on water, aircraft had to fly predominantly overland resulting in long, drawn-out journeys.

Inertial Navigation Systems

The next major step in navigation came from the invention of the Inertial Navigation Systems, or INS. Using a series of gyroscopes and accelerometers, the INS is able to determine how much the system has moved relative to a known starting point.

Early INSs required the pilot to enter the longitude and latitude of the parking gate and then as the flight progressed, the INS would be able to calculate the aircraft’s position based on how much its calculated position differed from that start point.

However, the longer the flight, the more inaccurate the system would become as the gyroscopes started to drift.

Modern Systems

With the introduction of GPS, aircraft were able to pinpoint their location with an accuracy of just a few metres. Aircraft like the 787 Dreamliner has 2 separate GPS units and uses the average of both systems to pinpoint the aircraft’s exact position.

However, to make the overall navigation accuracy even greater, the 787 uses a combination of sources to calculate its position. It also has a modern-day INS called an Inertial Reference Unit (IRU) that uses lasers instead of gyroscopes and an Attitude Heading Reference Unit (AHRU) that provides a stable source of attitude (nose up/down) and heading.

The GPS data is fed into the IRUs and AHRUs to create a hybrid GPS-Inertial position which is then fed into the Flight Management Computer (FMC). The FMC then uses all 4 inputs to calculate an accurate aircraft position which is then fed into various systems in the flight deck.

The way in which modern aircraft calculate their position is incredibly complex but it not only ensures great accuracy but also great redundancy should one system fail (Image Charlie Page/TPG)

Crossing the Oceans

When flying across continents, the ability to fly direct from point to point is made possible because ATC is able to keep tabs on aircraft on their radar screens. This ensures that we are always safely separated from other aircraft both vertically and laterally.

However, when crossing the oceans, there is no radar coverage so things have to be done in a different way.

To keep this flow of traffic safe, each night, airlines send ATC their preferred routings for their westbound flights during the day. Controllers collate this data and create a set of routes using GPS positions. This is known as the Organised Track System (OTS). It’s the same for the predominantly eastbound flights overnight.

There are usually around six to seven of these tracks at a time and they are then given an individual designator. Westbound flights during the day use tracks labelled Alpha, Bravo, Charlie, etc. So, to avoid any confusion, eastbound flights overnight start at Zulu and work backwards. Any flights planning to fly in this area of the Oceanic airspace must use the OTS.

North Atlantic Tracks. Image via Skyvector.

This works fine but the main problem is that they force aircraft to fly longer routes to fit in with other traffic. Sometimes these can be hundreds of kilometres longer than the direct route. As a result, airlines are obliged to create more carbon emissions than they need or want to. What would be better would be if flights could fly random routes to make the most of the tailwinds and reducing emissions – and this is where technology is changing the way we fly.

In 2019, NATS and NAV CANADA become the first air traffic services in the world to use satellite-based surveillance, which gives a real-time view of aircraft over the Atlantic. This enables them to safely space aircraft more closely together, resulting in more flights being able to route through oceanic airspace each hour.

With the downturn in air traffic over the last two years, ATC was able to trial a system where no OTS tracks were published. Instead, airlines were able to plan direct routes for their flights, based on their optimum speed and altitude.

With these trials progressing well, the end goal will be to enable more and more aircraft to fly direct routes. Not only will this massively reduce carbon emissions, but it will also result in smoother flights for passengers as we will not be confined to set routes and altitudes where the flying conditions may be sub-optimal.

Finding the Runway

There’s no point in making your way halfway around the world if you can’t find the last few miles to the runway at the destination.

As we get closer to the airport, ATC use our position on their radar screen to navigate us toward the runway. By instructing us to fly a series of headings called vectors, they guide us towards the final approach. Here, we use one of several types of guidance systems to enable us to find the 60m wide strip of concrete, even in the worst weather conditions. The most used is the Instrument Landing System or ILS.

The ILS consists of two radio beams which project up from the area around the runway up into the approach path. These signals are then picked up in the aircraft by the ILS receiver which displays them on the screens in the flight deck.

The first signal is the localiser, radiating from antennae which sit at the end of the runway, lined up with the centreline. This shows us where the aircraft is in relation to the centre of the runway.

The second signal comes from antennae to the side of the runway, around 1,000 feet in from the threshold abeam the touchdown zone. This is the glideslope, and it sends another beam into the sky, normally at an angle of three degrees, to guide the aircraft down vertically to the correct touchdown spot.

The brilliance of the ILS means that we can land the aircraft even when there is thick fog. In fact, we only have a legal limitation of 75m visibility in these conditions so that we can find the runway exit and then taxi to the gate.

Bottom Line

Whilst on the ground pilot uses maps to find their way around airports, when there are no physical roads to follow in the air, knowing how to navigate through the skies is incredibly important for slight safety. The use of GPS and other high tech onboard systems gives pilots incredibly accurate real-time information about their position.

We then use this information, along with signals generated by ground-based navigation systems to guide us from the runway, across thousands of miles of sky, down to the runway at our destination.

Featured Image by Getty Images.

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