Hi folks,I've always been wondering, how is cruising altitude determined? All I know is something about the FL being odd-numbered if you're flying west and even-numbered if you're flying east. Are there any other factors? For example, in an Airbus A319, what would be the ideal cruising altitude between Lisbon (LPPT) and Funchal, Madeira (LPMA) (About a 1.2 hour flight)?Thanks!

It would also depend on aircraft weight and weather. After all, you don't want to be at an altitude with all sorts of turbulence. Not sure if anything else would dictate cruising altitude besides direction, but these are the only two I'm aware of so far.

If your example flight from Lisbon is a scheduled flight, there is a good chance that the airline will simply use a regular flight plan, which, as you say, would probably incorporate something similar to the quadrantal/semi circular rule. Many countries have broadly similar quadrantal rules, but they are not universally identical and can - and often are - overridden by ATC to maintain better separation, because if the rules change when crossing a border, it can cause problems.A good example of that, is when going from Western, to Eastern Europe, where meters are used as opposed to feet for flight levels. So the cruise altitude decision will not necessarily be one the crew makes on the day independent of other factors, and could even change a little mid-route, simply to assist ATC. Crews have the discretionary permission to alter things should circumstances dictate, such as for odd weather, temporary airspace restrictions, or unusually light or heavy take-off weights, but the decision is not always entirely in their hands if flying under ATC control. All of that planning, based on reading daily briefings, NOTAMs, company directives, ATC directives etc before the flight planning begins, is the 'uncool' part of flying airliners, which is something the general public don't imagine dashing aircrews have to indulge in. The 'posh' airlines often have software applications the crews can use which take a lot of that sort of thing into account for them and it can even dump that info into the FMC on the aircraft, but the lesser airline crews still have to do things the old fashioned way.Such restriction can change daily, for example, the US has some quite considerable temporary airspace restrictions in place a lot of the time (following the 9/11 attacks). These steer aircraft away from things such as flights that Air Force One is making and large gatherings of people, such as the Superbowl or some other such event that would be a big target for a suicidal airliner crew. The UK has 'Purple Airspace' corridors designated when members of the Royal Family are flying a route over the UK, and other countries also indulge in this kind of thing too. Likewise, military exercises often jam aircraft systems and GPS reception and these are just a few of the things that appear on daily NOTAMs (notices to airmen), from organisations such as the CAA and FAA, to assist crews in making safer and legal flight plans.When regular flights are made, they are often something that has evolved into being over some time, for example, if an airline always gets cleared to 28,000 feet for a particular regional flight they make every Wednesday, they will clearly incorporate that knowledge into their operations and have a company route which is based around such an expectation that they will be flying at that height. ATC likes that sort of thing, because if they know a 737 will be appearing at a certain time, at a certain location and height on, say, the Eastern edge of their area of control every Wednesday, it can help them shuffle less scheduled stuff safely aound such a flight, and long-term planning is made easier too. This is important, because ATC is expensive - a lot of staff are required, since frequent breaks and short shifts are a built in safety measure for ATC staff.Sometimes prevailing winds can be a factor in determining cruise altitude too, this is most often seen on transatlantic flights, where westbound flights tend to use northerly placed tracks in the daytime, and eastbound flights tend to use more southerly tracks at night, to take advantage of the typical winds over the Atlantic which tend to rotate anti clockwise over it. The day/night directional separation also builds in an additional safety margin. The actual location of those tracks is often determind on the day by several meteorological survey aircraft making trips out into the Atlantic to report on actual conditions and is a large responsibility for countries such as the US, Canada and the UK. The information being diseminated to places such as the Oceanic Control Centres at Gander and Shanwick, which approve cruise altitudes and tracks for transatlantic crossings.Similar stuff occurs for the Pacific and Indian Oceans too, with the relevant nearby countries being responsible for such constraints, although reliable satellite imagery that can make infra red scans of clouds too in order to learn a lot about what they are made up of, is making the need for survey flights less of a necessity these days.Back with the Atlantic, it is not unknown for flights from quite high and low latitudes to fly a long way north or south to take advantage of this weather aspect before crossing the Atlantic, and that of course means that if they follow some sort of quadrantal ruling, then they would need two cruise altitude assignments, one for each main directional leg. This prevailing wind factor is why the infamous Pan Am flight 103 which was destroyed by a terrorist bomb, was over Lockerbie in Scotland at 31,000 feet, when one would expect it to be at 31,500 feet for such a trip, and also why it was a few hundred miles north of a direct route, despite being on a flight from London, bound for New York.Such transatlantic flights will invariably have a stepped climb to cruise planned into the flight, so they might set off at 30,000 feet, but gradually go up to nearer 40,000 feet as they burn off fuel, become lighter, and can make it up to a more economic cruise altitude, but they might on the other hand find that there is a headwind up at higher cruise level from other aircraft further across the Atlantic or a company message on SELCAL, and choose to take the hit on fuel economy in favour of staying on schedule by remaining lower down.They would typically make such a decision known to their company to seek approval, since it would possibly burn more fuel, and to let other aircraft know they were doing so, since when mid-Atlantic, you are typically out of regular ATC contact, although other responsibilities for radio discipline are often mandatory/traditional, such as monitoring 121.5Mhz on a spare radio channel, to listen for mayday and pan calls. Some radio channels are used by numerous airlines to make informal courtesy announcements about such operational changes, and to assist other airlines in keeping on schedule by advsising on weather and best cruise altitudes, sort of in the manner of a friendly pilot's club.Al

In controlled airspace it is the responsibility of ATC to maintain separation between IFR flights. This means that ATC assigns cruising altitudes taking account of pilot's requests.Also, in the UK the quadrantal rule only applies only when outside controlled airspace (ie in Classes F and G) and then only to flights at levels below FL245.

Just thought I'd add that Quadrantal Rule flight-level assignment is not used in controlled airspace, and it is almost only used in the UK. It is the Semi-Circular rule that would apply, which explains the altitude of the Pan Am flight over Locherbie. Additionally, seperation of 2000ft for "same direction" flights only applies upto FL290, except for RVSM (Reduced Vertical Separation Minima) approved aircraft operating in RVSM approved airspace. Above that height seperation of 4000ft for "same direction" flights applies to non RVSM flight, using odd numbered flight levels only. Whether the semi-circle is split E/W or N/S depends on which country you're in. The UK splits traffic E/W, whereas France splits traffic N/S.HTHDan