In the United States, 14 CFR 91.159 prescribes cruising altitudes for level flight under VFR:

  • (a) 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 + 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 + 500 feet (such as 4,500, 6,500, or 8,500).

In other words, planes going generally east use one set of altitudes, and planes going generally west use another set of altitudes.

Now, it's not obvious that these rules are the best possible rules for VFR cruising altitudes. Some alternatives would be:

  • No cruising altitude rules at all. All VFR flights select a cruising altitude arbitrarily.
  • VFR cruising altitudes are given as blocks instead of single altitudes. For example, planes going generally east cruise at 3,200 through 3,800, or 5,200 through 5,800, and so on.
  • Cruising altitudes are prescribed in such a way that aircraft with different courses are always given different cruise altitudes. For example, require aircraft to cruise at an altitude such that the 100-foot needle on the altimeter points in the same direction as the "N" on the heading indicator.

The Wikipedia article "Navigation paradox" mentions a couple of papers which state that random cruising altitudes would result in fewer mid-air collisions than the VFR cruising altitudes prescribed by regulations.

Is there any research suggesting that the current VFR cruise altitude rules do, in fact, improve safety? In other words, are there any studies which compare the current rules to at least one alternative, and show that the current rules are better?

(The specific rules I mentioned are the US FAA regulations, but I'm also interested in research about other countries' VFR cruise altitude rules.)

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    $\begingroup$ I think most mid-air collisions (at least with GA aircraft) occur at low altitudes, on clear days, and around points of interest (airports, VOR's, or other landmarks) that are below the 91.159 rule altitude. These happen at "random" altitudes... $\endgroup$ – Ron Beyer Nov 20 '18 at 21:26
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    $\begingroup$ For research purposes, the UK (and other European nations?) used to use the quadrantal rule, where heading 00-89 was odd-thousand feet (5000), hdg 90-179 was odd+500 (5500), 180-269 even-thousand (6000), and 270-359 was even+500 (6500). They switched to the hemispheric rule used by the rest of the world (?) a couple of years ago. Maybe a study suggested it was safer? Or maybe a study suggested that "the same thing everywhere" is safer, as opposed to the specific rules. {shrug} $\endgroup$ – Jimmy Nov 20 '18 at 21:46
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    $\begingroup$ I always presumed that vfr altitudes were simply to keep them from interfering with ifr traffic. If there's a vfr aircraft that atc can't see they would prefer it not be at the same altitude unless climbing/descending $\endgroup$ – TomMcW Nov 20 '18 at 21:52
  • $\begingroup$ @TomMcW That probably justifies the +500 but not the even/odd rule. $\endgroup$ – StephenS Nov 21 '18 at 2:13
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    $\begingroup$ Whenever the FAA proposes a rule they lay out their justification in a notice published in the Federal Register. The problem with 91.159 is that it was created with pretty much he same wording as today (at least for airspace below 18000 ft) within the first version of Part 91 published in 1963. This "FAR" version was a mass move/renumber from the old Civil Aviation Regulations (CAR) version without additional comment. So you need to find the old CAR reference and the associated Federal Register publication. Those records aren't easily searched. $\endgroup$ – Gerry Nov 24 '18 at 4:25

Searching Scopus for (VFR OR 'Visual Flight Rules') AND safety in title/abstract/keywords yields two publications (out of 99 hits) that might contain information (or references to other sources) on the safety performance of the current VFR system with alternatives.

Prinzel et al. (2011) - Flight deck interval management and delegated separation for equivalent visual operations

An emerging Next Generation Air Transportation System concept Equivalent Visual Operations (EVO) can be achieved using an electronic means to provide sufficient visibility of the external world and other required flight references on flight deck displays that enable the safety, operational tempos, and visual flight rules (VFR)-like procedures for all weather conditions. Synthetic and enhanced flight vision system technologies are critical enabling technologies to EVO. Current research evaluated concepts for flight deck-based interval management (FIM) operations, integrated with Synthetic Vision and Enhanced Vision flight-deck displays and technologies. One concept involves delegated flight deck-based separation, in which the flight crews were paired with another aircraft and responsible for spacing and maintaining separation from the paired aircraft, termed, "equivalent visual separation." The operation required the flight crews to acquire and maintain an "equivalent visual contact" as well as to conduct manual landings in low-visibility conditions. The paper describes results that evaluated the concept of EVO delegated separation, including an off-nominal scenario in which the lead aircraft was not able to conform to the assigned spacing resulting in a loss of separation.

Quinby (1980) - General aviation operating requirements for the 1980's

This paper presents a review of the use of the airspace and the air traffic control system. Safety, capacity and efficiency of this use under Visual Flight Rules (VFR) are compared with the evolving constraints imposed by operations under Instrument Flight Rules (IFR). The concept of Electronic Flight Rules (EFR) as developed by Topic Group III on Freedom of Airspace in the FAA’s 1978–79 E & D Initiatives Process is reviewed. The potential for conservation of air traffic control manpower and aircraft fuel is further developed. Promising areas for further technical exploration are noted. The paper concludes with some considerations for co-existence of VFR, IFR, and EFR traffic during a transition period.


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