Selecting a cruising altitude is not simply a matter of aircraft performance or pilot preference. In the United States, the appropriate altitude depends on the aircraft’s magnetic course, whether the flight is conducted under VFR or IFR, the type of airspace, terrain clearance, weather conditions and Air Traffic Control instructions.
These rules create predictable vertical separation between aircraft traveling in opposite directions. However, the pilot must understand an important distinction: the rule is based on magnetic course—not the magnetic heading indicated on the directional gyro or heading indicator.
Magnetic Course Is Not Magnetic Heading
Magnetic course represents the aircraft’s intended path over the ground, referenced to magnetic north. Magnetic heading is the direction in which the aircraft’s nose must point to maintain that course after correcting for wind.
For example, an aircraft may follow a magnetic course of 090° while maintaining a magnetic heading of 083° because of a crosswind. The cruising-altitude rule is determined by the 090° magnetic course.
This distinction becomes especially important when strong winds produce a significant wind-correction angle.
VFR Cruising Altitudes in the United States
Under 14 CFR § 91.159, the VFR cruising-altitude rule applies during level cruising flight when the aircraft is operating more than 3,000 feet above the surface and below 18,000 feet MSL.
The rule follows this pattern:
| Magnetic course | Required VFR cruising altitude |
|---|---|
| 000° through 179° | Odd thousands plus 500 feet |
| 180° through 359° | Even thousands plus 500 feet |
Magnetic Course From 000° Through 179°
The pilot should select an odd-thousand altitude plus 500 feet, such as:
- 3,500 feet MSL
- 5,500 feet MSL
- 7,500 feet MSL
- 9,500 feet MSL
- 11,500 feet MSL
Magnetic Course From 180° Through 359°
The pilot should select an even-thousand altitude plus 500 feet, such as:
- 4,500 feet MSL
- 6,500 feet MSL
- 8,500 feet MSL
- 10,500 feet MSL
- 12,500 feet MSL
The additional 500 feet helps separate VFR traffic from IFR aircraft, which normally operate at whole-thousand altitudes.
The FAA rule includes exceptions while the aircraft is turning, operating in a holding pattern of two minutes or less, or when another altitude is authorized by ATC. 14 CFR § 91.159
Does the Rule Apply Below 3,000 Feet AGL?
The hemispheric VFR cruising-altitude rule does not apply when the aircraft is operating at or below 3,000 feet above the surface.
That does not mean the pilot may select an unsafe altitude. The flight must still comply with minimum safe-altitude requirements, airspace restrictions, cloud clearances, terrain and obstacle clearance, local procedures and any applicable ATC instruction.
The pilot must also distinguish carefully between:
- AGL — Above Ground Level: height above the terrain;
- MSL — Mean Sea Level: altitude indicated with the correct local altimeter setting.
The 3,000-foot threshold is measured above the surface, while the selected cruising altitude is expressed in feet MSL.
IFR Cruising Altitudes
The IFR situation requires another distinction.
IFR in Controlled Airspace
In controlled airspace, an aircraft operating under IFR must maintain the altitude or flight level assigned by ATC. The pilot cannot independently change to another altitude merely because it appears to correspond with the magnetic course.
The assigned altitude must also satisfy applicable minimum altitudes, aircraft performance limitations and the published structure of the route.
IFR in Uncontrolled Airspace
Under 14 CFR § 91.179, IFR aircraft operating in level cruising flight in uncontrolled airspace below 18,000 feet MSL follow the whole-thousand pattern:
| Magnetic course | IFR cruising altitude |
|---|---|
| 000° through 179° | Odd thousands |
| 180° through 359° | Even thousands |
Examples:
- Magnetic course 045°: 3,000, 5,000 or 7,000 feet MSL;
- Magnetic course 225°: 4,000, 6,000 or 8,000 feet MSL.
This is an important difference from VFR operations: IFR altitudes do not include the additional 500 feet. 14 CFR § 91.179
What Happens at 18,000 Feet MSL?
In the contiguous United States and Alaska, Class A airspace generally begins at 18,000 feet MSL and extends up to, but does not include, FL600.
Normal operations in Class A airspace must be conducted under IFR. Aircraft operate according to flight levels using the standard altimeter setting of 29.92 inches of mercury.
Therefore, a pilot climbing through the transition altitude must understand the difference between:
- 17,000 feet, referenced to the current local altimeter setting;
- FL180, referenced to the standard setting of 29.92 inHg.
Depending on atmospheric pressure, FL180 may not always be the lowest usable flight level. ATC accounts for this condition when assigning flight levels.
RVSM Airspace
Reduced Vertical Separation Minimum airspace generally extends from FL290 through FL410, inclusive. Within RVSM airspace, approved aircraft may operate with 1,000 feet of vertical separation between adjacent flight levels.
The aircraft and operator must satisfy the applicable RVSM requirements. A pilot cannot select an RVSM flight level solely because it matches the direction of flight.
In controlled airspace—which includes normal high-altitude operations—the actual flight level remains the one assigned by ATC.
Altitude Selection Requires More Than Direction
The hemispheric rule only identifies the proper altitude family. It does not determine whether a particular altitude is safe or operationally appropriate.
Before selecting or requesting an altitude, the pilot should consider:
- terrain and obstacle clearance;
- cloud bases and tops;
- icing conditions;
- winds aloft;
- turbulence;
- aircraft weight and performance;
- oxygen requirements;
- airspace limitations;
- minimum en route altitudes;
- communication and navigation coverage;
- direction and density of traffic;
- ATC restrictions.
A legally correct altitude may still be a poor operational choice. Strong headwinds, icing or terrain may make another altitude safer and more efficient.
A Practical Example
Consider a VFR flight with a magnetic course of 275°.
Because the course falls between 180° and 359°, and the aircraft will cruise more than 3,000 feet above the surface, the pilot should select an even-thousand altitude plus 500 feet—such as 6,500 or 8,500 feet MSL.
Now suppose the wind requires the aircraft to maintain a heading of 285°. The applicable altitude family does not change because the rule is based on the magnetic course of 275°, not the corrected heading of 285°.
If the same flight were operated under IFR in controlled airspace, the aircraft would maintain the altitude assigned by ATC.
Final Considerations
The American cruising-altitude system is built around a simple principle: aircraft flying in generally opposite directions should occupy different altitude groups.
The basic memory aid is:
- VFR, 000°–179°: odd thousands plus 500 feet;
- VFR, 180°–359°: even thousands plus 500 feet;
- IFR in uncontrolled airspace: odd or even whole thousands according to magnetic course;
- IFR in controlled airspace: maintain the altitude or flight level assigned by ATC.
The rule may appear simple, but safe altitude selection also requires weather evaluation, performance planning, terrain awareness and a clear understanding of the airspace structure.
By Marcuss Silva Reis
Economist | Commercial Pilot | Aviation Expert Witness | Civil Aviation School Instructor | Former University Professor of Aeronautical Sciences | Postgraduate in Aeronautical Sciences, Civil Aviation Safety and Higher Education | Optical Technician
Founder of Instituto do Ar
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