I got tasked by my instructor pilot to find out why each country has a different transition altitudes / levels. For example in the US it's FL180 and in Saudi Arabia it's FL150.
Can someone help me with that, and if possible provide any document that talks in details about this subject.
Flight Levels are easier to work with for ATC and for pilots since everybody will use standard pressure setting. Effectively it means less work load and lower probability of making an error with a safety impact.
Typically the highest terrain elevation plus some margin is used as transition altitude. This gives the highest number of flight levels available.
For example in the Netherlands, being a flat country partly below sea level and the highest obstactle at about 1500 ft, has a transition altitude of 3000 ft for IFR flights and 3500 ft for VFR flights.
There is (was?) an initiative to create a common transition in Europe, but I haven't heard anything about it for a while. Some interesting reference documents were produced:
Towards a Common Transition Altitude - A Flight Deck Perspective
A Common European Transition Altitude - An ATC Perspective
(both files in PDF format)
The highest peak in Saudi Arabia is Jabal Ferwa at 9,856 feet above sea level. Add some padding for safe terrain clearance, instrument error and human error, and 15,000 feet looks like a reasonable transition altitude.
The highest peak in the contiguous United States is Mt. Whitney at 14,505 feet, so 18,000 feet for the transition altitude looks similarly reasonable. (Special rules apply in Alaska around Denali at 20,310 feet.)
In theory, the transition could be much lower for non-mountainous areas of both countries, but apparently their authorities believed that a uniform transition altitude was simpler and therefore less prone to errors.
