Captain Lim Khoy Hing explains how pilots find their way, up in the skies.
Words: Captain Lim Khoy Hing
A traveller once asked, how does a pilot navigate in what looks like an empty sky? Even though the sky may look empty from the ground, it is a web of many invisible airways. Indeed, a flight, say from Kuala Lumpur to London traverses many routes or airways. Unlike highways where motorists can rely on signboards and exit markers, the skies have no road signs. As such, pilots are trained to navigate from point A to B from the day they begin learning to fly.
In fact, ‘navigate’ is one part of the first golden rule in the aviator’s mantra: ‘Aviate, navigate and communicate!’ In aviation terms, ‘navigate’ basically means to plan the course of a plane by using flight instruments or maps.
A pilot does not need to remember the airways he is flying in; he refers to airway charts and uses a flight management computer instead. In this article, I will explain the various navigation techniques, from the very basic to the more advanced forms of navigation today.
AT ITS MOST BASIC
From day one, a pilot trainee is taught the basics of navigation by recognising ground landmarks like buildings, rivers, shorelines, mountains, etc. This is typically mastered in flying clubs or schools that use smaller planes. Note that such means of navigation is only good for flying in perfect weather, low altitudes and short distances.
RECKONING ONE’S POSITION
For basic navigation, pilots learn to recognise ground landmarks like buildings, rivers, shorelines and mountains.
The next basic means of navigation is simply to calculate a plane’s position based on the aircraft’s known speed and direction. For example, if you know your position from 10 minutes earlier and you know that you have travelled due west at 480 knots (or eight nautical miles a minute), you can calculate that you are now 80 nautical miles west of your previous position!
This, in air navigation terms is known as ‘dead reckoning’. It requires an accurate fix or position of where the plane is and in what particular heading and speed it flies at. This does not require the aid of a computer, just some quick mathematical calculations. This is why mathematics is a mandatory subject that one must excel in to become an airline pilot. As a pilot progresses to more advanced flying in bigger planes, longer distances and higher altitudes, navigation by other means becomes necessary.
NAVIGATING BY RADIO
Radio navigation works by sensing the signals from a radio transmitter on the ground. When a plane is tuned to the frequency of a particular airport station down below, a pointer or dial on the instrument panel in the cockpit will home onto that station. So, in that sense, navigation from the air is easy; just fly toward the station!
Air navigation map showing airways, waypoints and radio aids
But, this technique of homing a plane toward a particular point or airfield by radio signals has certain shortcomings, as it is susceptible to weather interference, which can affect navigation. A more reliable radio navigation aid is the VOR (Very High Frequency Omnidirectional Range) beacons, which transmit stronger and more accurate signals from the ground. VOR stations are often located with a DME (Distance Measur-ing Equipment) transmitter, and together, they enable a pilot to know exactly how far the aircraft is from the VOR station. In that sense, a pilot – when within range (maximum is around 250 to 300 nautical miles) – is always able to determine the plane’s location.
INERTIAL NAVIGATION
If you understand the principle of ‘dead reckoning’, then, inertial navigation is easy to comprehend. While ‘dead reckoning’ requires the human brain to compute the position of a plane, the inertial navigation process of airborne calculation is performed automatically by a system of gyroscopes.
Put simply, this system uses three sensitive gyroscopes to measure changes in speed in three directions. The changes are then used to calculate the speed and position of the plane.
This inertial navigation system is used by larger planes. Like ‘dead reckoning’, it requires an accurate fix or position. However, this system has since been replaced by an even more accurate system, the global positioning system (GPS) – similar to those used in your car like Garmin or the ubiquitous ones in the smartphone, such as Waze.
GLOBAL POSITIONING SYSTEM
In aviation worldwide, the GPS has been further refined to provide very accurate guides that enable planes to land in difficult terrain. Using GPS, airplanes can now manoeuvre between canyons and make precise turns onto a runway, without ever seeing more than just clouds. The GPS guides the plane safely to break out of clouds and align it straight on the centre line of a runway. Such approaches can only be flown by aircraft that have been certified to a certain level of accuracy. The pilots must also be specially trained to fly with such navigational aids.
Close-up of navigational system in cockpit.
A good example is the use of the GPS approach (or correctly known as RNP AR approach) at Kathmandu’s Tribhuvan International Airport in Nepal. While the conventional arrival to land at this airport requires the pilot to skillfully descend steeply after crossing a mountainous range, the GPS approach takes the aircraft between valleys with a very gentle descent. It then positions the plane to land at a lower altitude, which is not possible when using the conventional approach.
EFFICIENT ADVANCEMENTS
Modern technology has seen advances in navigation techniques, making it ever more accurate and reliable. While passengers sleep on long journeys with nothing to look at during the night, pilots can actually see exactly where they are going all the time using the GPS–linked navigational computer. This same system guides planes to land on runways that can be just about 45 metres wide, touching down precisely at the right spot. This is largely due to the 24 satellites that orbit our globe 24 hours a day. These satellites make GPS navigation possible, which, in turn makes it possible for you to travel the world.
Captain Lim Khoy Hing is a former AirAsia Airbus A320 and AirAsia X A330/A340 pilot who also used to fly the Boeing 777. He has logged a total of more than 25,500 flying hours and is now a Simulator Flight Instructor with AirAsia X. In his spare time, he shares his opinion on aviation issues with others. For more air travel and aviation stories, check out his website, ‘Just About Flying’ at www.askcaptainlim.com.
Captain Lim’s first book, LIFE IN THE SKIES and its Mandarin version 【飞行日记】 are now available for purchase onboard all AirAsia and AirAsia X flights. Enjoy these great collections of articles from a veteran aviator.
