I am about to start my PPL, still flying in my FSX.

I have a question, let's say I am flying from airport A to airport B. Both airports are in G airspace and I would like to stay in G airspace and don't go over 1200' AGL to class E. Let's say I would like to fly at 1100' AGL and keep this altitude from A to B. And let's assume we are in un-congested area, so we do not violate minimum safe altitude rule. But I do not understand if terrain in uneven, I mean its elevation is constantly changing, how do I keep my altitude 1100' AGL. I don't have radio altimeter like in Boeing. I understand that at the airport you can get your elevation, also you can have minimum safe altitude from sectional chart and also you can find any obstructions, but how do I know in a middle of my flight (if I need to).

If this question sound stupid and pilot don't fly in G airspace please let me know.

  • $\begingroup$ You could look out the window? With experience, you get fairly good at judging altitude. $\endgroup$
    – jamesqf
    Jan 10, 2018 at 20:21
  • 2
    $\begingroup$ Why are you avoiding class E? Flying at 1200 AGL mostly is not safe (assuming fixed wing aircraft). $\endgroup$
    – Steve Kuo
    Jan 10, 2018 at 21:06
  • 7
    $\begingroup$ This is a great question! I had a job where this was a big concern for me. I was always trying to check my altitude AGL (I was usually 300-500 AGL) and used a GPS that had an approximate AGL readout based on a terrain database. $\endgroup$
    – J Walters
    Jan 10, 2018 at 22:43

4 Answers 4


You can't know with 100% accuracy, even radio altimeters have limitations, and if you're flying on a PPL the aircraft probably wouldn't have one anyway. The VFR pilot flies with a map which shows the height of the highest terrain and noteworthy landmarks, but unless you are directly over one of these it is a matter of your experience in judging vertical distances.

I'm not sure if it's different wherever you come from, but where I fly in Australia, all airspace is measured in sea level altitudes or flight levels, never in reference to ground height, so we would never encounter your situation. Trying to stick to one height AGL might sound like a fun challenge to some but to me it sounds like less time to save my life when the engine quits.

  • $\begingroup$ @Michael Kjörling: Not in my experience, anyway. As long as you don't have to clean sagebrush out of the wheel pants, at least :-) $\endgroup$
    – jamesqf
    Jan 11, 2018 at 5:58

I bumped into this thread and was disappointed to see that no one was able to actually answer your question. Why you are avoiding Class E, whether 1200' AGL is unsafe, and the ins and outs of your flight planning are all subjective. After 19 months I figure you probably know the answer yourself, but I wanted to post a solution for anyone who might run into this same issue while learning to fly.

The best way that I know of to determine your en-route AGL altitude with no GPS, albeit not the most accurate, is by finding obstacles along/near your route of flight which have an MSL and AGL altitude posted on VFR sectionals. Subtract the AGL altitude from the MSL altitude of the obstacle and this will give you the MSL altitude at the ground at that location. Add to that MSL altitude your desired AGL elevation and the answer is the MSL altitude you need to be at a specific AGL altitude over that point. This method does require you to have a VFR sectional, have a generally good idea of your location (the need for accuracy of your location depends on the type of terrain you're flying over), and a bit of mental math. You also need a current altimeter setting for improved accuracy.

Anyhow, I hope this helps, for anyone in the future who might have this question and can't find the answer.


As a practical matter, sans radio altimetry or synthetic vision, you don't have any way to determine your AGL en route accurately. Why do you want to use Class E for en route flying? In the U.S. neither Class E nor G require an ATC clearance, but both require adherence to VFR flight rules. However, the VFR daytime flight visibility for Class G is only 1 mile versus 3 miles for Class E. There's a nice table showing that at the beginning of Chapter 15 of the FAA's Pilots Handbook of Aeronautical Knowledge.

  • $\begingroup$ It sounds like he's trying to avoid class E $\endgroup$
    – TomMcW
    Jan 10, 2018 at 19:07
  • $\begingroup$ @TomMcW I agree, but I'm curious as to why. $\endgroup$
    – Terry
    Jan 10, 2018 at 21:42
  • $\begingroup$ I guess in case you have visibility >1 but < 3 miles. I think he's really just looking for how to know how to figure out where 1200 agl is from your altimeter msl reading $\endgroup$
    – TomMcW
    Jan 10, 2018 at 22:01

You should be able to use the Quadrental altitudes on a VNC chart. That is all you need to determine your 3000 ft AGL for direction of flight. If u are in mountainous terrain it can be quite high

  • 5
    $\begingroup$ The OP isn’t trying to determine how high “right altitude for direction of flight” begins; he is trying to determine an actual AGL altitude (without having a radio altimeter, which is really what he needs). This response doesn’t answer the question as asked. $\endgroup$
    – Ralph J
    Apr 13, 2018 at 3:21

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