The rule provides that traffic operating above 3,000 feet above ground level, traveling in opposite directions, be separated by a margin of airspace. When you're flying at the appropriate VFR altitude, any opposing VFR or IFR traffic should be above or below your altitude--or possibly passing through your altitude--but should not be cruising at your altitude.
Here is the language of the rule. Unless otherwise instructed by air traffic control (ATC), if you are flying VFR and cruising at more that 3,000 feet above the surface, FAR 91.159(a) states that
When operating below 18,000 feet msl and--
(1) On a magnetic course of zero degrees through 179 degrees, any odd thousand foot msl altitude plus 500 feet (such as 3,500, 5,500, or 7,500); or
(2) On a magnetic course of 180 degrees through 359 degrees, any even thousand foot msl altitude plus 500 feet (such as 4,500, 6,500, or 8,500).
One way to paraphrase this rule is to say that when the VFR pilot's aircraft is flying in an easterly direction, that aircraft should be at an odd altitude plus 500 feet--3,500 feet or 5,500 feet, for example. When the VFR pilot's aircraft is in a westerly direction, the aircraft should be at an even altitude plus 500 feet--say, 4,500 or 8,500.
There are a few points worth emphasizing about this rule. First, the requirement to comply with the VFR cruising altitudes applies only to VFR flights conducted higher than 3,000 feet above ground level (agl). Below 3,000 feet agl, you and any other VFR or IFR aircraft could be at any altitude, which could potentially create converging flight paths.
Second, your VFR cruising altitude is determined by the altitude reflected on your altimeter after it has been set to mean sea level (msl). So, you will need to have the appropriate altimeter setting entered into your altimeter to make sure you are operating at the correct altitude. FAR 91.121 states that your altimeter must be set to the current reported altimeter setting of a station along the route and within 100 nautical miles of your aircraft, or if there is no station within this area, the current reported altimeter setting of an appropriate available station.
Third, while the rule requires VFR traffic to operate at even or odd altitudes plus 500 feet, know that IFR traffic is generally assigned to fly the cardinal altitudes--3,000, 4,000, 5,000, and so on. Therefore, you shouldn't count on a 1,000-foot vertical separation between aircraft; that safety margin may be, at most, 500 feet. And, other traffic could be climbing or descending and, therefore, may cross through your altitude.
Finally, the rule refers to magnetic course, so your ground track is what governs the altitude you are required to fly.
While the VFR cruising altitude rule may assist you in detecting opposing traffic, it may not help you to see and avoid aircraft that may be traveling in the same easterly or westerly direction that you are traveling and, thus, at the same altitude. Usually, in that circumstance, you may be overtaking another aircraft or another aircraft may be overtaking you. In that event, FAR 91.113 specifies that the aircraft that is being overtaken has the right of way, and the pilot of the overtaking aircraft must alter course to the right to pass well clear of the overtaken aircraft.
We've seen how at least three regulations work together to keep the operation of aircraft orderly, and thereby safe, provided we all are following the rules. Each of us retains individual responsibility to maintain a proper scan outside the aircraft in order to see and avoid any traffic. Whether VFR or IFR, if you can see outside the aircraft, you will be expected to look outside the aircraft.
Kathy Yodice is an attorney with Yodice Associates in Washington, D.C., which provides legal counsel to AOPA and administers AOPA's legal services plan. She is an instrument-rated private pilot.