Altitude, Height and Flight Levels – What is the difference?

Well let’s start by looking at the definitions:

Indicated Altitude:

Indicated Altitude is what we can see on our instruments. We need to make sure the QNH in the subscale is correct, or else it will be useless to us.

Height (Absolute Altitude):

Height is the height/distance between you and the ground.

Flight Level (Pressure Altitude):

Flight level is the height above the standard reference QNH of 1013.25 hPa (hectopascal) or 29.92 in Hg (inches mercury). Is also known as QNE.

So how do we use this information and when would we use it?

Lets dive in to Altitude, Height and Flight Levels – What is the difference?

Altitude (Indicated Altitude)

First of all we need to know where to find our Indicated Altitude, this will be on your altitude indicator. It is also important to know where and how to set your QNH. I will assume you know where and how to do this, if not ask your instructor.

Altitude is a height above the MSL (Mean Sea Level). This is only true if we have the correct local QNH set into the sub-scale, if not it will give us an inaccurate reading.

Altitude is represented just as we read it, for example 3500 ft.

When do we use it?

We will use it for all flying under the Transition Altitude or Level (This will be discussed in a future post).

Height (Absolute Altitude)

Height is the height above the ground. If you are flying straight and level over uneven terrain, this height will be changing constantly with the elevation of the ground.

Height is also represent by the way we read it, for example 1200 ft, generally the only instrument to give us this information would be a radar altimeter.

When do we use it?

Generally we only use it for Approaches and maybe doing some sort of survey work when we are require to stay a particular height above the ground. We will see it a lot though as it is good to know how much clearance you have between yourself and the terrain below you.

Flight Levels (Pressure Altitude)

Flight Level is an altitude flown on the standard QNH setting of 1013.25 hPa or 29.92 in Hg. This just makes it easy to keep everyone flying with reference to the same setting, so there is less chance of flying into each other.

Flight levels are represented by a FL, followed by a 3 digit number. For example FL035, this would be 3500 ft. The first number is 10,000’s, the second is 1,000’s and the third is 100’s of feet.

When do we use it?

We will use it for all flying above the Transition Altitude (this will be discussed in a future post).

Working out the difference between Altitude, Height and flight levels (pressure altitude)

So how do we do it?

Let’s look at some examples:

Example 1:

You are flying at 3500 ft (Indicated Altitude), the QNH is 1020. What is your Pressure Altitude?

If we look at the example above we can see that the elevation of the runway is 1200 ft. The aircraft is flying at an FL 035  (3500 ft above the standard atmosphere) and the QNH at the airport is 1020 hPa.

Note: 1 hPa is equal to 30 ft change in altitude (rule of thumb, it changes with altitude) but for our purposes and for exams we will use 30 ft.

Lets work through this example together. It is always a good idea to write it all out first.

It is always a good idea to write out and draw the question as you go.

Runway elevation – 1200 ft
Indicated Altitude – 3500 ft
QNH – 1020 hPa
Standard Atmosphere – 1013 hPa

First lets get the change:

1020 hPa – 1013 hPa = 7 hPa
7 hPa x 30 ft = 210 ft

The highest number will be below:

So 1020 hPa will be below 1013 hPa, this is because it is a higher pressure than 1013 hPa.

3500 ft – 210 ft = 3290 ft

We are therefore have a pressure altitude of 3290 ft.

Example 2:

 You are flying at FL044. The QNH is 1006 and the Airfield Elevation is 600 ft. What is your height above the Airfield?

So what information do we have?

FL 044 (44oo ft above standard)
Airfield Elevation – 600 ft
QNH – 1006 hPa

First lets get the difference in pressure.

1013 hPa – 1006 hPa = 7
7 x 30 = 210 ft

Lets get the FL to QNH, this will give us the height above MSL.

4400 ft – 210 ft = 4190 ft (above MSL)

Then all we do is minus the airfield elevation from the height above MSL.

4190 ft – 600 ft = 3590 ft

So our height above the runway would be 3590 ft.

 More Examples:

I have made a video with 2 more examples below. Feel free to have a look. And i hope you found the info on “Altitude, Height and Flight Levels – What is the difference?” useful.

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