How pilots keep their wingtips clear of obstacles

This week, an American Airlines Boeing 737-800 reportedly struck a lamp post as it taxied out from the ramp area at Dallas-Fort Worth Airport. Video footage showed the left wing tip of the aircraft clipping the post, causing it to be uprooted from its base and falling to the ground. The aircraft then came to an abrupt stop.

It may seem surprising that it's possible for an aircraft to hit such an obvious structure, particularly during the daytime. However, incidents like this occur more often than most people realize. Taxiing an aircraft around a busy airport is often the most challenging part of a flight. At 39,000 feet, there is very little to collide with. However, the dynamic airport environment provides plenty of obstacles for even the most modest of aircraft to contend with.

What was a lamp post doing there in the first place?

Think of a lamp post and you probably imagine a dark road being lit from above, helping divers guide their cars safely down the street. On the highway, lights help illuminate the road ahead, enabling cars to travel safely at speed. So what purpose do they serve for aircraft at airports?

The reality of lamp posts at airports isn't so much for the aircraft, but more for the ground staff working around the gates and cargo areas.

Getting an aircraft away from the gate on time is more than just boarding the passengers from the terminal and closing the door. When a flight arrives on the parking stand, a team of ground crew swarms the aircraft to prepare it for its next flight. Baggage and cargo must be offloaded, toilet waste tanks emptied, potable water uploaded, engineering checks completed and fuel pumped into the wing tanks for the next trip.

The ramp area is a hazardous place during this hive of activity, with vehicles and people working closely in the same space. It's bad enough in the daytime, but in the darkness of night, it's even more dangerous. In order to enable the ground staff to do their job effectively and safely in the hours of darkness, large lamp posts are positioned around the ramp area to illuminate the parking stands and cargo areas.

However, these are not placed at random. Significant thought must go into where they are located so that they do not interfere with ground equipment or provide a hazard to aircraft.

Turning corners

Taxiing between the runway and the gate can be quite a journey in itself. Large hub airports such as Amsterdam (AMS) in the Netherlands and Dallas-Fort Worth, Texas (DFW) have multiple runways, with many of them often being used at the same time. The runway in use for arrivals will depend on a number of factors including the wind direction, local noise regulations and the direction in which the flight is coming from.

When there is more than one runway available for arrival, to keep things as straightforward as possible, air traffic control (ATC) will normally try to land aircraft on a runway located on the side of the airport from which the aircraft is coming. For example, Amsterdam Schipol airport has a combination of five runways that are used for landing and takeoff.

On a normal day, two of these will be used for arrivals and two will be used for departures. For aircraft arriving from the west, the runway on the western edge of the airfield (Runway 18R) is used, and for flights coming from the east, the runway farther to the east (18C) is used.

This leaves Runway 18L and Runway 24 for departing flights, as can be seen in the image below.

However, you will notice that once landed on 18R, you're still a significant distance away from the terminal buildings. In fact, it's around two miles away. Not only does this take a significant amount of time, but it also requires us to navigate a number of different taxiways, links and ramp entry points before we finally arrive at the gate.

How we find our way around

Taxiing around an airport is quite often the most challenging part of a flight. Vacating a runway in the dark, we are met with a myriad of lights and junctions, some routes leading us towards the gate, others towards active runways. As a result, we need to have a rough plan of our taxi route to the gate before we even touch down. This is done as part of the approach brief back in the calm of the cruise.

Related: Parking a 200-ton aircraft: How pilots move a plane around on the ground

An hour or so before we are due to land, our operations department will send us an ACARS message notifying us of our gate. With this information on hand, we then know where we need to end up. But how do we plot our route between the runway and the gate? The simple answer is by use of a map.

Every airport around the world publishes a collection of charts that detail the approach and departure procedures and also a map of the taxiways and gates. During the brief, we discuss the most likely route from the runway to the gate, particularly the exit point from the runway we plan to use and the first turn after that.

This enables us to vacate the runway expeditiously, allowing the aircraft behind us to land safely. Of course, this may all change depending on what ATC requires at the time, but with a little local knowledge built up from visiting airports regularly, we get an idea of what to expect.

Instead of being given names like roads, taxiways are given alphanumeric names, for example, A, B, D6 and E3. When instructing us where to go, ATC will read out a string of these letters over the radio and we must then find them on our charts to work out where to go.

However, finding the route on the chart is not the same as actually doing it. To help with this, airports have signboards on the side of the taxiways, indicating which routes are where. Bigger airports will also have this information painted on the taxiway itself.

Different markings, different instructions

To make things less confusing and reduce the potential for error, most airports around the world have standard markings and lights on the runways and taxiways. These vary from solid lines which tell us where the center of the taxiway is, to pairings of solid and dotted lines allowing access from one direction but not the other.

Related: Runway approaching: How pilots find their way safely to the ground in all elements

Taxiway markings are relatively simple with a solid yellow line indicating the centerline. During the hours of darkness, green lights are spaced along the yellow line to make the centerline clear. At some large airports, for example, London Heathrow, ATC is able to control the taxiway lighting, providing green lights for the route they want us to take and indicating where they want us to stop by use of red lights across our path.

However, some airports, particularly those in the U.S., also use yellow lines to indicate the edge of the taxiway, as can be seen in the image below.

However, not all taxiways are created equal. At some airports, particularly older ones, some taxiways are not suitable for larger aircraft types. The area around the gates can be quite tight, with parking stands, baggage facilities and lamp posts getting in the way. As a result, some taxiways may only provide enough wingtip clearance for certain types of aircraft.

Unfortunately, there are not always signs on the ground indicating that a taxiway is only suitable for aircraft with a wingspan below a certain value. The only way we can ascertain this is by looking in another section of our airport charts.

When using a taxiway, so long as we keep the aircraft on the centerline, we will be assured wingtip clearance from any obstructions on either side. However, as straightforward as this is when taxiing in a straight line, it can be a little more problematic when turning corners.

'Driving' a 787 Dreamliner

The 787-10 is 223 feet long, with a wingspan of 197 feet. This means that from our seats in the flight deck, we are unable to see the wingtips. Our eye level is 18 feet above the ground, and this creates a 46-foot blind spot ahead of the aircraft. As a result, accurate taxiing is essential.

This is done through two different sets of controls. Limited steering is possible using the rudder pedals, which sit underneath our feet. Unlike in your car, the pedals under our feet have very little to do with acceleration and a lot to do with braking and steering. These primarily control the rudder on the tail (mainly used in crosswind landings) and also control the brakes. However, the main source of steering comes from the nosewheel steering tiller, which can turn the nose wheel up to 70 degrees.

Taxiways can vary in width and load-bearing strength depending on the airfield. Depending on these factors, they are categorized so that pilots know which taxiways they are able to use for their aircraft type. The taxiways at some airports can be particularly tight, especially for bigger aircraft like the 787-10 and 777-300.

A gradually curved corner isn’t much of an issue, as we simply ease the tiller into the corner and the aircraft will follow the centerline. Where it becomes more challenging is when the corners are a tight 90°, for example, at Newark Liberty International Airport (EWR), outside of New York City.

On a 787-10, the main landing gear is nearly 100 feet behind the nose wheel. If we were to start the turn as we approached the yellow line of the new taxiway, the main wheels would cut the corner and end up in the grass. Not ideal.

To stop this from happening, what we want is for the main wheels to track the centerline of the taxiways, not the nose wheel. To do this, we have to ensure that we taxi "deep" into the new taxiway, before inputting a good amount of tiller application and starting the turn. To do this, we keep going until the new centerline is over our shoulder and then start the turn. This means that the nose of the aircraft will almost be overhanging the grass before we start the turn.

If judged correctly, the nose wheel will swing wide of the centerline but the main wheels will continue to track it.

How do we stop?

Getting moving is no good unless you can bring the aircraft safely to a stop. To do this, each wheel on the main landing gear has a brake unit. On most aircraft, the brakes are powered by the hydraulics system. However, on the 787 Dreamliner, the eight main wheel brakes are electrically powered instead. This means that the aircraft then produces fewer CO2 emissions due to the lower weight — 245 pounds for each 787-9 aircraft. The simplified system also benefits engineers, as the procedure to replace a brake unit is much quicker and easier.

Related: How do aircraft brakes work?

Being such a safety-critical part of the aircraft, the braking system has to work well under all conditions. This includes situations where a brake unit isn’t working. During the testing stage of the aircraft, engineers put them through a punishing process.

Bottom line

Taxiing around an airport is often the most challenging part of a flight. With airport service vehicles crossing taxiways and other objects in close proximity to the parking stands, pilots have to be uber alert when moving between the gate and the runway.

Yellow centerlines on taxiways show use the path that will keep us clear of most obstructions, but we have to ensure that we don't find ourselves on a taxiway for which our aircraft wingspan is too big. In addition, taxiway markings at some airports are more confusing than at others, using the same color line to indicate the edge as they do to indicate the centerline.