by Eric Stearns
There are several factors for a pilot to consider when deciding what altitude to use for cruise flight. In this article we will discuss several of these factors and how they affect both VFR and IFR flights. This is meant to be a basic survey of altitude planning. Some of the subjects discussed are complex; other articles cover some of these subjects in greater detail.
The first concern for any pilot in choosing a cruise altitude is avoiding terrain and obstacles along the route. Most navigation charts include at least some terrain and obstruction information. It's usually a good idea to fly at least 1000' above the terrain along your route; however, it is sometimes permissible to fly closer to the ground and in some cases you be required to fly higher than 1000' AGL. The regulations on this subject vary based on what you are flying over (generally, you must fly at a higher altitude over congested areas than you would over unpopulated areas) and the country you're flying in. Also, each country establishes a maximum altitude at which VFR flight may be conducted. This maximum altitude varies considerably across the world; in the U.S., VFR flight may not be conducted at or above 18,000' MSL.
VFR flights also must remain clear of clouds (in some cases a minimum distance from clouds is prescribed) and maintain a certain visibility. These weather minimums for VFR flight vary based on several factors and also vary between countries.
Just like a VFR pilot, IFR pilots must consider the terrain along their route of flight. Since they may not be able to see the terrain, regulations specify a minimum height above terrain for IFR flights. In this U.S., this minimum altitude is 1000' AGL; in mountainous areas (in the U.S., this is defined by FAR 95), pilots must fly at least 2000' AGL.
If your IFR route will be on airways, those airways will provide the minimum enroute altitude (MEA) for each segment. The MEA assures terrain clearance and also the ability to receive the ground-based navigation aids which define the centerline of the airway. If you plan to operate along an airway, SID or STAR, review the charted minimum enroute altitudes and choose a cruise altitude that is at/above the MEA.
There is also another altitude sometimes published known as the minimum obstruction clearance altitude (MOCA). When published, it applies within 22 NM of the navigation aid defining the airway centerline; at points more than 22 NM from the navigation aid defining the route, the MEA applies.
With the advent of GPS and other forms of area navigation, it's very common to conduct flights directly between fixes (other articles cover route selection and when direct routes are appropriate). These direct routes will not have an MEA like an airway. In this case it is up to the pilot to determine an appropriate altitude. Some low altitude IFR charts give a "Grid MORA" (MORA stands for Minimum Off Route Altitude). Those MORAs are very helpful and if you fly at or above the MORA you will always maintain a safe altitude above terrain and obstructions; however, it might be permissible to fly at a lower altitude. The best source for additional terrain information (in the U.S. at least) is sectional charts. Sectionals are generally thought of as VFR charts; however, they provide the best terrain detail of any aviation chart available to a pilot.
Other articles will cover charting and how MEAs, MOCAs, and MORAs are depicted.
Controllers also have minimum IFR altitudes available to them that they must use for flights not operating on published routes (airway, SID, STAR, etc); in some cases, the controller's minimum altitude might be higher than the grid MORA. These altitudes are not directly available to pilots, but the controller will advise you if his or her minimum IFR altitude is higher than your planned altitude.
On longer flights, most pilots will choose cruise altitudes near the service ceiling of their aircraft. Flying higher will almost always result in a decreased fuel burn. There are times this is not true; the first is with a much stronger headwind at the higher altitude. This is intentionally vague, since it will vary based on several factors; an additional ten knots of headwind at an altitude 4000' higher would likely not be enough to offset the higher fuel flow at the lower altitude; an additional 60 knots of headwind likely would be enough difference to make the lower altitude more fuel efficient (this large of a wind difference over 4000' of altitude is very rare). At very high weights in transport jets, the aerodynamic effects at high altitudes can make those higher altitudes less fuel efficient; this lower fuel efficiency is why longer range flights often "step climb" over the course of the flight as they burn off fuel (and the aircraft's weight decreases). For example, a 747 flying from Chicago to Hong Kong might initially level off at 30,000', then every couple of hours climb 2000' until reaching 38,000 or 40,000'. This is a fairly complex topic and further discussion is beyond the scope of this article; the documentation with your virtual aircraft might provide some additional guidance.
Sometimes a pilot will be more concerned with a quick flight than with fuel burn. In this case you'd generally want to choose the "fastest altitude" for your aircraft. Determining the fastest altitude will take some research; use the documentation provided with your aircraft or just fly and experiment to find what works best. Obviously, headwind and tailwind components can make this calculation more complex, the following assumes that there is no wind.
For jet aircraft, the fastest altitude will usually be the altitude at which the maximum indicated airspeed equals the maximum mach number for your aircraft. For most transport jet aircraft, this will be somewhere between 26000 and 30000 feet. For turboprop aircraft, this will usually be the maximum altitude at which the maximum cruise torque can be maintained. For turbo/supercharged reciprocating powered aircraft, the maximum speed can usually be attained at the maximum altitude which permits maximum cruise manifold pressure to be maintained. For normally aspirated (non-turbo/supercharged) reciprocating aircraft, consult the documentation with the aircraft to find the maximum true airspeed.
Direction of flight
Each country establishes general rules regarding proper altitudes for direction of flight. In the U.S., eastbound IFR flights (defined as a magnetic course of 0-179 degrees) generally fly at odd thousands of feet (e.g. 7000', 15000', 33000', etc.); whereas, westbound IFR flights generally fly at even thousands of feet. Other areas of the world divide even/odd altitudes so it's split between north and southbound aircraft instead of east and westbound aircraft. Air traffic controllers can approve deviations from these altitudes, but will usually try to get pilots to comply since it makes traffic separation easier. At altitudes above 41000', 2000' of vertical separation is required between aircraft, so the east/west altitudes alternate (e.g. Westbound flights use FL430, FL470, FL510, etc.; Eastbound flights use FL450, FL490, etc).
In the U.S., VFR flights follow the same rules as IFR flights except they add 500' to their altitude (e.g. Westbound VFR flights could cruise at 8500', an eastbound VFR flight might cruise at 5500'). An exception to this is for operations at/below 3000' of the surface; in these cases, VFR aircraft may operate at any altitude consistent with safe flight.
Oxygen / Pressurization requirements
For flight above a certain altitude, unpressurized flight requires the use of supplemental oxygen. While it's not necessary to use oxygen in front of your computer, if you choose to simulate all details of real world operations, you should consider the oxygen rules when choosing a cruise altitude. Each country will have its own rules; in the U.S., all unpressurized flights conducted above 14000' require the use of oxygen (it is required at lower altitudes for some flights beyond a certain amount of time). On an unpressurized flight conducted for hire (air taxi or air carrier), the pilots must use oxygen for flights at altitudes above 10000' (oxygen must be available for passengers at higher altitudes depending on several factors).