When I did my NPPL training, there was a module on flying at minimum altitude, but nothing around high altitude flying, in fact, I've never flown over 3,500ft.
This weekend, I intend to fly up to around 9,000ft just for fun. Obviously I'll do this in an area that doesn't infringe any airspace, i.e. class G airspace.
Is there anything important to know before doing this?
There's really not that much difference between flying at 3,500ft and 9,000ft, however yes there are a few things you should be aware of. The most important, as usual when operating an aircraft, is check your POH. That is your definitive guide to some of the things you'll want to know. There are also some human factors to be aware of.
Service Ceiling: Check the POH for the service ceiling for the aircraft to ensure it is certified for flight at the altitude you expect. From previous questions I'm aware you fly what we call microlights in the UK (ultralights in the US and possibly elsewhere). A quick look for an example POH shows the EuroFOX has a service ceiling of 14,500ft.
Operating environment: Again the POH might establish an ambient temperature range that the aircraft can operate in. The above-linked POH specifies an ambient temperature above -10°C. Keeping in mind that you lose roughly 1.5°C per 1000ft, what temperature on the ground would take you beyond that limit at 9,000ft?
Engine management: If the aircraft you fly has a manual mixture control, then you will want to lean the mixture when cruising at altitude. As the air becomes less dense there is less oxygen available for combustion, so you will not be able to burn all of the fuel at a rich mixture. More information here: Why do we lean the mixture when the air become less dense?
Human factors: Hypoxia is the medical term for when the body is deprived of oxygen. One way this can occur is the lower pressure of oxygen at altitude. Typically people begin to start talking about those effects above about 10,000ft, but it's not like there's some invisible line where below it you're safe and above it you're not. One of the major problems with hypoxia and aviation is that often by the time you're compromised it's too late. So you need to be hyper-aware of the symptoms, and if you have any reservations then carry supplemental oxygen (or just don't push your limits in the first place!). If you're a heavy smoker or asthmatic then you might be more susceptible so speak to an AME if you're concerned.
Something to consider outside of your question. Most people think of height as danger. The higher you are off the ground the more dangerous whatever you're doing is. Notwithstanding the above answer, in an aircraft, altitude equals safety. The higher you are, the further you can go if the engine quits, and the longer time you have to deal with the problem. In a typical light aircraft a rule of thumb is roughly 1.5nm per 1,000ft glide range. So from 3,500ft you might be able to glide 5nm. From 9,000ft that is 13nm. In time, it's the difference between having 4 minutes to deal with the problem before your wheels are on the ground and having 11 minutes. That's a lifetime in an emergency situation!
The main thing you will notice is, if the propeller is fixed pitch, you will have the throttle all the way in to maintain cruise power (9000 ft will limit you to about 65% and the engine will be wound up close to, or at the engine RPM redline, and throttle setting might be limited by redline if the prop is pitched too fine.
The other one is it will be cold and the heater had better be working.
Indicated speeds you use are unchanged, but just be aware that the difference between indicated and true are starting to become significant and your indicated cruising speed will be lower than what you are used to, with less margin between operating speeds and stall (whose indicated airspeed remains the same).
Biggest one is to be aware that your overall energy margin that you get from excess power is declining off to zero as you approach the airplane's service ceiling. If the service ceiling is 12 or 14000 ft, you should still have some energy margin at 9000 ft, but nothing like down low. This applies to maneuvering margins as well; it's easier to stall the thing in a steep turn say.
If you don't have altitude compensating carbs, mixture leaning is an obvious one, but that really applies to all altitudes as soon as you reduce power after takeoff (a full rich mixture is strictly for cylinder cooling at full power, at lower altitudes where full power or close to it is available).
Otherwise, not such a big deal. Don't over-think it. Just learn the facts and keep them in mind.
