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  • Flight Planning: How to map the route

Flight planning is no easy task. It is not in vain that airline companies have a whole department specialise in this undertaking. It’s not at all just about drawing a line from one airport to another as there is rather a lot more to it.

In order to plan a flight, it is important to keep in mind areas over which they cannot fly, operational restrictions or meteorology, amongst many other factors.

In fact, it is of such importance that one of the subjects of the commercial pilot licence is precisely based on this.

In this post, we tell you everything behind good flight planning: how to do it and the points to keep in mind. Let’s go for it!


How to plan a flight

Flight planning requires a series of precise calculations; these must be accurate for a safe operation. Following is a list of them:

  1. Calculating PET or Point of Exact Time.
  2. Determining PSR or Point of Sure Return.
  3. Establishing Point of No Return.
  4. Calculating TOC or Top of Climb.
  5. Calculating TOD or Top of Descent.

Following we look at each of them in detail.

Point of Equal Time PET

The point of equal time or PET is established with respect to the total duration of a specific piece of the flight. PET is the point at which the aeroplane will take as long to fly towards its destination than to return to its origin.

If wind is calm, the point of equal time will always coincide in the middle of the route, but in actual fact, it is rarely the case that wind is calm.

For this reason, to calculate PET, the following formula is needed so that the effect of wind is taken into account:

PET= DIST*GSH/GSH+GSO

– END: Autonomía expresada en horas.
– GSH: Is the speed of an aeroplane when returning from its airport of origin.
– GSO: Is the speed of an aeroplane on course to the destination airport.

To calculate PET in flight planning, we need to know the direction of the aeroplane as well as the wind direction. The wind component is added or taken away from that of the aeroplane.

Point of Safe Return PSR

At the point of safe return, or PSR, an aeroplane can either return or continue onto its destination airport taking just as long and with reserves intact.

To give you an idea, while the PET takes the distance between points as a reference, the PSR takes into account the range of the aircraft.

To calculate the PSR, it is also necessary to take into account the wind component along the route. The formula is as follows:

PSR=END*GSH/GSH+GSO

– END: Autonomy expressed in hours.
– GSH: Is the speed of an aeroplane when returning from its airport of origin.
– GSO: Is the speed of an aeroplane on course to the destination airport.

Point of No Return

The point of no return is the point beyond which it is not possible to return to the airport of origin. The point of no return is often used when flying to isolated airports or those with difficult weather conditions.

Top of Climb in flight planning

Top of climb or TOC, is the point at which the aeroplane will reach cruising altitude and it is necessary to calculate it for navigation.

To find out TOC, the elevation of the airport has to be taken into account, as well as knowing the wind component, vertical speed of ascent and horizontal speed of the aeroplane.

But let’s take a look at a practical example.

How to calculate the Top of Climb

A Diamond DA40 NG takes off at the airport of Granada (2000 ft elevation) doing an instrumental departure towards the Bailen VOR.

According to the navigation departure, it must ascend to 13000 feet. The rate of ascent is 1000ft/min and the speed of the aeroplane is of 100 knots.

If shortly after take off the controller advises of a tailwind of 20knots, when will TOC be reached?

In this exercise, to calculate TOC, the first thing to do is work out the vertical distance to which the aeroplane has to ascend to. To do so, we subtract the altitude of the airport of origin (2000ft) to the total altitude the aeroplane must go up to (13000ft). The result is 11000 feet.

The next step is to calculate how long it will take to reach this distance. If we have a vertical speed of ascent of 1000ft/in, the DA20 will take 11 minutes to ascend 11000 feet.

Finally, we calculate the horizontal distance the aeroplane will go to reach the Top of Climb. In this case, if the speed of the aeroplane is 100 knots and there are 20 knots of tailwind, the speed with respect to the ground will be 120 knots. Our DA40 will be going on for 22 nm (nautical miles).

How did you like this example? It was quite an easy one. In real life, factors such as the difference between pressure at the airport and standard pressure or the deviation of temperature with respect to the ISA atmosphere come into play.

Calculating TOC correctly is essential, though, for flight planning.

Top of Descent

TOD or top of descent is the point at which an aeroplane must begin descent. To figure it out, we do it in the same way as TOC.

As when calculating TOC, it is obviously essential to be thorough when working out the calculations.

In case of going over the TOD, flight crew would have to use air brakes or wait to descend to the required altitude which would result in delays, higher fuel consumption as well as other inconveniences.

By contrast, if descent began before reaching TOD, the aeroplane would be flying low for longer, resulting in a higher use of fuel.

  • By Andrzej Otrębski – CC BY-SA 4.0 on Wikimedia Commons

Green approaches, a new concept in flight planning

Spain is a pioneer in green approaches, a term which is closely related to Top of Descent.

Descents to airports are usually done in phases, that is, by taking various steps. In contrast, this new concept consists in doing a continuous descent from cruising altitude to the airport, without “steps”.

In order to do so, maximum and minimum altitudes are eliminated and TOD is slightly delayed.

What is all this for? Well, green approaches reduce fuel consumption at this stage by 25%. Considering the small change made, it’s a major saving!

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