Instrument Flight Rules - Weather

Weather

The main purpose of IFR is the safe operation of aircraft in instrument meteorological conditions (IMC). The weather is considered to be IMC when it does not meet the minimum requirements for visual meteorological conditions (VMC). To operate safely in IMC, a pilot controls the aircraft relying on flight instruments, and ATC provides separation.

It is important not to confuse IFR with IMC. A significant amount of IFR flying is conducted in Visual Meteorological Conditions (VMC). Any time a flight is operating in VMC, the crew is responsible for seeing and avoiding VFR traffic; however, since the flight is conducted under Instrument Flight Rules, ATC still provides separation services from other IFR traffic.

Although dangerous and illegal, a certain amount of VFR flying is conducted in Instrument Meteorological Conditions (IMC). A common scenario is a VFR pilot taking off in VMC conditions, but encountering deteriorating visibility while en route. "Continued VFR flight into IMC", as it is known, is responsible for a significant number of light-airplane crashes due to spatial disorientation. VFR flight into IMC is distinct from "VFR-on-top," an IFR procedure in which the aircraft operates above IMC but remains in contact with ATC, and "VFR over the top," a VFR procedure in which the aircraft takes off and lands in VMC but flies above an intervening area of IMC, both of which are legal in the United States.

During flight under IFR, there are no visibility requirements, so flying through clouds (or other conditions where there is zero visibility outside the aircraft) is legal and safe. However, there are still minimum weather conditions that must be present in order for the aircraft to take off and land; these will vary according to the kind of operation, the type of navigation aids available, the location and height of terrain and obstructions in the vicinity of the airport, equipment on the aircraft, and the qualifications of the crew. For example, mountain airports such as Reno (KRNO) offer significantly different instrument approaches for aircraft landing on the same runway, but from opposite directions. Aircraft approaching from the north must make visual contact with the airport at a higher altitude than a flight approaching from the south, because of rapidly rising terrain south of the airport. This higher altitude allows a flight crew to clear the obstacle if a landing is not feasible. In general, each specific instrument approach specifies the minimum weather conditions that must be present in order for the landing to be made.

Although large airliners and, increasingly, smaller aircraft now carry their own terrain awareness and warning system TAWS, these are primarily backup systems providing a last layer of defense if a sequence of errors or omissions causes a dangerous situation.