In our last post, we talked about controlled and uncontrolled airspace. Controlled airspace is further split based on the locality it covers.

The vicinity of the aerodrome is upto 2,500ft agl. This is surrounded by control zones, terminal control areas and airways (airways – between 5,000 – 24,500ft). So what covers the airspace above the airways? This area is covered by what is called the Upper Air Routes.

Airspace above FL245 is? C. Any airspace above FL245 is Class C.

How is a flight information region (FIR) managed?

As we covered in our earlier posts, a flight information region (FIR) is an area of airspace where flight information service is provided. In the UK we have 3 FIRs: E.g. London, Scottish and Shanwick Oceanic (Eastern Atlantic) and ATC provide assistance to aircrafts in these areas.  

~ FIR are split into sectors ~

A typical FIR is split into sectors by the dedicated Air navigation service provider, e.g. NATs in the UK. A number of air traffic controllers and flight officers are then requested to cover each sector. The advantage of sectors is this can be changed based on how busy the airspace is –  in a busy season – more sectors can be added and more controllers to a specific FIR.

In an upcoming post I will talk more around the Oceanic Shanwick FIR, the type of winds and the Jetstream. And why a specific wind direction (Jetstream/tailwind) means EVERY airline want their aircrafts to be routed right up against it for each and every flight!

Earlier in this post, I briefly mentioned flight levels so I’ll clarify the difference between flight levels and altitudes.

Let’s clarify the difference between altitude and flight level (FL)

You may have noticed some people say 2,000 ft whereas others say FL350 (Flight level 35,000) – so what do this exactly mean? Why can’t we say 35,000 ft and be done with it? First, let’s look at altitude and the issue we have.

Altitude: this is the vertical distance of the aircraft in question to the mean sea level.

Now, altitude is measured using a calibrated barometer. The barometer measures the air pressure. And as we know air pressure decreases with altitude – so every pilot in every aircraft has the tedious task of re-calibrating the altimeter (barometer) according to the local air pressure.

Altitude -> Altimeter -> Barometer -> Air pressure – mean sea level

Flight levels solve this issue of re-calibrating the altimeter by defining altitudes based on the standardised atmospheric pressure of 1013.2 mb. Regardless of the actual sea level pressure all aircrafts will standardise their aircrafts based on this atmospheric setting once they pass a transition altitude (differs with each country).

The procedure in the UK is as follows:

In the UK, the transition altitude is 3,000 ft.

Climb and you’re cleared to FL – now set to standard pressure setting (1013.2 mb)

Descend and you’re cleared to altitude – now set to QNH (regional)

Why don’t we use FL on the ground?

This is because of ground obstacles – such as tall trees and buildings are fixed to the ground. Their absolute height needs to be known – in effect we need to know how far the distance between us (the aircraft) and the ground is. Otherwise safety will be compromised and we will hit an obstacle assuming we’re at safe height that is based on sea level not ground level.

What happens if all the aircrafts in the local region set their altimeter to QNH?

That’s exactly what should be done. After departing an airfield (you will need to change the altimeter setting from QFE to QNH). And if all the aircrafts in the region are using QNH there will be a safe, true separation between aircrafts.

If you’re doing cross country or travelling internationally – you will need to change the regional QNH to the new QNH or QNE.

I’ll see you on the next blog. Happy Flying + Tailwinds!