Legal Briefing: VFR Cruising Altitudes

One of the fundamental rules governing the operation of aircraft in flight is the "see-and-avoid" rule of FAR 91.113(b).

As a pilot, you are required to know and abide by the General Operating and Flight Rules of Part 91 of the Federal Aviation Regulations (FARs). One of the fundamental rules governing the operation of aircraft in flight is the "see-and-avoid" rule of FAR 91.113(b), which places the responsibility on pilots to spot traffic and to avoid it. But that rule does not work alone. Other FARs work with the see-and-avoid rule to make it easier for you to avoid a collision.

One of those helpful operating rules sets acceptable VFR cruising altitudes, which allows you to generally anticipate where other aircraft may be in the sky and provides some measure of separation for aircraft traveling in different directions.

The rule, which is found in FAR 91.159, tells you at what altitudes you should fly, depending upon your direction of travel. As it relates to our discussion, the rule states that "each person operating an aircraft under VFR in level cruising flight more than 3,000 feet above the surface shall maintain the appropriate altitude or flight level prescribed below, unless otherwise authorized by air traffic control:

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 ground track of the VFR pilot's aircraft is in an easterly direction (ground track equates to course, as opposed to heading, which may contain an easterly or westerly correction for wind drift), that aircraft should be at an odd altitude plus 500 feet, e.g., 3,500 feet. When the ground track of the VFR pilot's aircraft is in a westerly direction, the aircraft should be at an even altitude plus 500 feet, e.g., 4,500 feet. When I'm flying, the way that I remember this is that east is odd, but west is so much odder as to be even, and then add 500 feet.

There are a few things worth emphasizing about this rule. First, you only need to comply with the VFR cruising altitudes if you are flying more than 3,000 feet above ground level (agl). Below 3,000 feet agl, you are not required to follow the altitude rule (although it's a good idea) and neither are other aircraft. That means other aircraft could be flying at any altitude that could potentially conflict with your flight path.

Second, to determine that you are at the correct VFR cruising altitude, you need to have your altimeter set correctly. 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, this rule requires VFR traffic to operate at even or odd altitudes plus 500 feet, while IFR traffic is generally assigned to fly the cardinal altitudes, e.g., 2,000, 3,000, 4,000, and so on. Therefore, you shouldn't count on there being a 1,000-foot safety separation between aircraft because that safety margin may be, at most, 500 feet. And, traffic could be climbing or descending and, therefore crossing through your altitude.

The rule makes sense. It provides that traffic operating above 3,000 feet agl and traveling in opposing directions should be separated by at least 500 feet 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 crossing through your altitude, but should not be cruising at your altitude. However, an aircraft flying on a course of 359 degrees, and one flying on a course of 181 degrees, would each properly be at an even altitude plus 500 feet-in other words, on a potential collision course, even though they are flying in nearly opposite directions.

Although these rules work together to help keep aircraft safely separated, they do not relieve you of your responsibility to maintain a proper scan outside your aircraft in order to see and avoid any traffic. As many a pilot will tell you, there is no knowing where an aircraft could show up, so it's vital that you take your instructor's advice and "keep your head on a swivel."