9.3 The Flight Planning Problem

The Flight Planning Problem in Context

The basic problem of flight planning is to find out, for a given origin, destination and payload:

1. Which route should I take? 

2. How much fuel is required?

3. How much time is required?

The answers to these questions can be summarized succinctly with: it depends. 

For these three questions, we may come up with multiple sets of valid answers. The answers to these questions are interdependent. To help narrow down the possible answers, we may want to introduce more constraints, or introduce additional questions such as:

• how can I minimize the fuel required?

• how can I minimize the time required?

• how can I minimize the cost required?

To get started on the flight planning problem, it may be helpful to consider a simpler problem most people are familiar with: trip planning for car trips. On any given day, millions of people around the world use Google Maps as they go about their business. Most of the time, users want to know two of the three outputs in our mini diagram:

• what route to take

• how long it will take

The third, fuel, is less important because gas stations are common along the road network, and there isn't a huge performance penalty for carrying around a full tank of gas. Using Google Maps is straightforward, and besides choosing some customization options like avoiding toll routes and unpaved roads, the user can let the app take care of crunching all the numbers.

With Google Maps as a starting point, we can approach the flight planning problem by considering what additional complexities the flying aspect brings into the trip planning problem. 

Fuel

The penalty for running out of fuel in an aircraft is obviously severe; unlike a car, you can't pull over and wait for help to arrive. Less obvious is the fact that there is also a considerable penalty for carrying too much fuel. An aircraft in flight has to support its own weight, and the energy to do this comes from the engine(s) consuming fuel. The more weight that has to be held aloft, the higher the fuel consumption. And, since fuel is being burned off as the flight progresses, the rate of fuel consumption changes over time. 

Winds and Weather

In a car, unless you are paying really close attention, it's unlikely that you will notice an impact of winds on your fuel consumption and trip time. However, up high in the sky, winds flow along at much greater speeds, uninhibited by the friction encountered near the Earth's surface. For an aircraft cruising at, say, 400 knots relative to the air, a 100 knot headwind or tailwind will have a significant impact on the its resultant groundspeed (the sum of the airspeed + wind speed), and accordingly on its total trip time and total fuel consumption. 

Cars and aircraft are both subject to weather hazards, but the variety and significance of the weather hazards planes face is arguably greater. For example, suppose an A380 is flying from Toronto to Dubai in December. In a single flight the flight crew may be managing icing conditions on departure, turbulence en-route, and high heat and dust at arrival. 

The Vertical Dimension

Cars are constrained to the Earth's surface; but for planes, the vertical dimension of altitude opens a huge range of possibilities for trip planning. Choosing the right combination of climb rates, cruising altitudes and descent rates to suit the given mission is an art.

Performance

Aircraft performance has more dimensions than car performance. For a discussion of this aspect please see 9.5 - Aircraft Performance in the Real World.

The "Road Network"

Cars operate within a clear physical network of roads, with posted speed limits and directions. In aviation, there is a similar network but it's largely invisible. It exists in navigation databases and can be viewed on specialized aviation maps. Some of the key components include:

• waypoints: fixed points in space defined by a name and coordinates

• navaids: ground-based radio navigation aids like VORs, DMEs and NDBs that act as beacon points for nearby aircraft 

• airways: can be thought of as equivalent to highways in the sky, connecting various cities and airports together

• terminal procedures: short navigation procedures that connect airports to airways; think of them as highway on-ramps and off-ramps. These would include SIDs (Standard Instrument Departures), STARs (Standard Terminal Arrival Routes), and Instrument Approaches (precise navigation instructions guiding the plane to land at specific runways)

• airspace: the sky above the Earth is divided into volumes, and the volumes are classified according to different uses and entry requirements. 

Traffic

If you have a car, for the most part you are free to enter and exit the road network whenever you like. If there is heavy traffic, you grit your teeth and wait it out. Airplanes do not have the luxury of stopping for traffic congestion. Therefore, use of the airspace has to be more carefully regulated. This is the job of Air Traffic Control (ATC), and it includes not only active surveillance and control of active flights, but also pre-approval of flight plans before the engines have even started. 

In the next section we will look at how these flight planning aspects are managed in real world operations.