During climb, pilots change the altimeter to standard pressure (1013 hPa) when passing through the transition altitude. Thereafter, the vertical position of an aircraft is controlled by reference to flight level.
It is important to note that the aircraft is not really flying at FL 050 unless standard pressure is set (1013 hPa). Consequently, if the pilot does not change the altimeter setting and 1030 hPa remains set => when you reach an altimeter indication of 5 000 ft and level off, you will not be at the same pressure altitude as the surrounding traffic. In this case, the aircraft is referring to their indicated altitude rather than the pressure altitude they should be using.
Let’s start by determining the difference between local QNH and standard pressure:
1030 – 1013 = 17 hPa
Using the assumed barometric lapse rate of 30 ft/hPa:
17 hPa x 30 ft/hPa = 510 ft
The aircraft is 510 ft above or below the required flight level. to find this out, we can use the rules that state:
When QNH is lower than standard --> Indicated altitude is lower than pressure altitude.
The aircraft's indicated altitude is therefore higher than pressure altitude, so their actual pressure altitude is 510 ft lower than the required 5000 ft, making the pressure altitude 4490ft.
Note: According to the new syllabus (EASA 2020) a barometric lapse rate of 30 ft /hPa should be used unless specified otherwise in the question.
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