Pilot Counsel:

The hemispherical rule

February 1, 2008

John S. Yodice is the owner of a Cessna 310 and a Piper Cub.

Every so often when I am flying along listening on an air traffic control frequency, I will hear a controller gently chide a VFR pilot for cruising at the wrong altitude. I heard such an exchange just the other day, which prompted this column. Virtually all pilots are generally familiar with the rule that specifies VFR cruising altitudes, but there are some refinements and nuances to the FAR that are worth reviewing from time to time. Most important, it bears repeating that this regulation is a critical part of a collection of guidelines intended to help pilots see and avoid other traffic. Some other elements in this collection include the altimeter setting rule, the right-of-way rules, the VFR weather minimums, and speed limits. These FARs establish and supplement the most basic way that we operate in our national airspace system, even under IFR.

The rule is contained in FAR 91.159 and is titled “VFR cruising altitude or flight level.” Although this regulation doesn’t use the term, it has within it what has become colloquially known as the hemispherical rule. The commonly understood general expression is that when you are operating an aircraft VFR in level cruising flight in an easterly direction, you must maintain an odd-1,000-foot msl altitude plus 500 feet, and when cruising in a westerly direction, you must maintain an even-thousand-foot msl altitude plus 500 feet. An easterly direction is specified in the rule as a magnetic course of zero degrees through 179 degrees. A westerly direction is a magnetic course of 180 degrees through 359 degrees.

The hemispherical rule applies only to flights more than 3,000 feet above the surface. In some parts of the country, in hilly or mountainous areas, you could well be cruising at or higher than 3,000 feet msl and yet be less than 3,000 feet above the surface. Strictly speaking, the rule does not apply to this type of operation. The other side of the coin is that operating VFR at 3,000 feet agl or below, regardless of msl altitude, you should be aware that you do not have the benefit of the rule, and there could be conflicting aircraft at any altitude.

Also, in applying the rule, pilots should know that the direction of flight is determined by magnetic course. When cruising on a northerly or southerly heading, this could be confusing. A magnetic course is ordinarily not what you are reading off your compass or directional gyro or horizontal situation indicator, which is a heading. As tempting as it might be to use this easy reference, if you are flying or planning to fly close to a northbound or southbound heading, you need to determine your magnetic course to apply the rule properly. Magnetic course is your track over the ground corrected for magnetic variation. It is the VOR radial that you are tracking along an airway. If not on a radial, and you are heading roughly north or south, you may have to do some computing, taking into account your wind correction angle (or, thank heaven for GPS).

There are also some common-sense refinements in the rule. It does not apply to an aircraft while it is turning, and, of course, constantly changing course and heading. The rule also does not apply while the aircraft is in a holding pattern of two minutes or less. And because the rule applies to cruising flight, it does not apply to climbing and descending aircraft. These refinements should alert pilots that while we can generally enjoy the benefit of the rule (generally not to expect roughly “head-on” conflicting aircraft at our altitude), we should recognize that this “general” expectation has exceptions. We should expect that there could be perfectly legal conflicting traffic when we are operating at an appropriate VFR cruising altitude.

The VFR cruising altitude rule technically does not apply when operating above 18,000 feet msl, that is, in the flight levels. That is because VFR operations are not permitted in Class A airspace that begins at FL 180. The rule simply states that when operating above 18,000 feet msl, aircraft must “maintain the altitude or flight level assigned by ATC.”

The rule also has an effect on IFR operations below 18,000 feet msl.

The VFR cruising altitude rule generally provides a measure of separation from aircraft operating IFR, cruising at IFR altitudes (at least 500 feet with proper altimeter settings). Cruising IFR traffic will be at the cardinal altitudes (that is, not plus 500 feet). But they also could be climbing or descending through the VFR altitudes and require the same vigilance to see and avoid as for VFR climbing and descending traffic.

IFR pilots will recognize that, although controllers generally try to adhere to the hemispherical rule, they have the discretion to deviate when traffic conditions require. So, it is not unusual to find IFR traffic just 500 feet overhead of cruising VFR traffic, at what might appear to be head-on or nearly so.

There is one application of the hemispherical rule that does apply to IFR operations. When an IFR clearance assigns VFR conditions on top, the flight must observe an altitude specified by the direction of flight, as well as the plus 500 feet.

Bottom line—the hemispherical rule bears reviewing as an important part of a collection of regulations intended to help pilots keep clear of other traffic. At the same time, we are reminded of our overall obligation as pilots operating in the see and avoid environment to maintain vigilance for other traffic, even when operating under IFR.

John S. Yodice