Plane & Pilot
Tuesday, January 1, 2008

VMC Vs. VFR


What’s technically legal isn’t necessarily safe


vmc cs vfrIn basic flight training, student pilots memorize the cloud clearance and visibility criteria for operation under visual flight rules and instrument flight rules (VFR and IFR). Flight schools and instructors drill into students the cloud clearance and visibility requirements for VFR operations in various categories of airspace, all the while neglecting to mention that none of this has much to do with the ability to keep an airplane upright during periods of restricted visibility and/or lack of terrain definition." />

vmc vs vfr
Marginal VFR conditions can very quickly transition into IMC.
It’s winter in northwest Alaska. The forecast for a flight eastbound from Kotzebue calls for VFR conditions along the route. Current visibilities are at least five miles, and ceilings are 3,000 feet or greater at all reporting stations along the route. It looks like a go.

But first I need to verify that the instrumentation in the aircraft is functional and that the airplane is approved for instrument flight. As the pilot, I also need to be current and competent (not necessarily the same thing, as the NTSB accident database graphically illustrates) on instruments. At least the first portion of the flight I’m about to embark on will be through weather conditions that I refer to as PVFR. If you haven’t found that acronym in the Aeronautical Information Manual, it’s because I made it up.

My definition of PVFR is: “A meteorological condition in which visibility and ceilings meet the minimum required values specified in the Federal Aviation Regulations for flight under VFR…but there’s nothing within the prevailing visibility to use as a visual reference.” The P in PVFR stands for “pretend,” in case you haven’t guessed by now.

The snow-covered and treeless landscape that I’m preparing to traverse provides no visual definition, and the low winter-sun angle produces what’s referred to as “flat light.” The combination of flat light and absence of visual references produce a phenomenon described by many pilots as flying inside a milk bottle.

The conditions for this flight meet the requirements for a basic VFR operation, but, in fact, IMC will prevail along a significant portion of the route, and the only way to safely conduct this flight is by reference to instruments.




0 Comments

Add Comment