Editing Instrument flight rules (section)

==Weather==
<!-- [[MVFR]] and [[Mvfr]] redirect here. -->
[[File:IFR on top.jpg|thumb|IFR flying with clouds below]]

It is essential to differentiate between flight plan type (VFR or IFR) and weather conditions (VMC or IMC). While current and forecast weather may be a factor in deciding which type of flight plan to file, weather conditions themselves do not affect one's filed flight plan. For example, an IFR flight that encounters [[visual meteorological conditions]] (VMC) en route does not automatically change to a VFR flight, and the flight must still follow all IFR procedures regardless of weather conditions. In the US, weather conditions are forecast broadly as VFR, '''MVFR''' ('''marginal visual flight rules'''), IFR, or LIFR (low instrument flight rules).<ref>[http://www.aviationweather.gov/adds/metars/description/page_no/4 Aviation Weather Center - METARs Help Page (4 of 5)] National Weather Center. Retrieved 29 January 2015</ref>

The main purpose of IFR is the safe operation of aircraft in [[instrument meteorological conditions]] (IMC). The weather is considered to be MVFR or IMC when it does not meet the minimum requirements for [[visual meteorological conditions]] (VMC). To operate safely in IMC ("actual instrument conditions"), a pilot controls the aircraft relying on [[flight instruments]] and ATC provides separation.<ref>{{cite web|last=McCloy |first=John |title=Safety Pilot in IMC |work=IFR magazine |url=http://www.ifr-magazine.com/safety_pilot.html |access-date=2009-05-20 |archive-url=https://web.archive.org/web/20070905150824/http://www.ifr-magazine.com/safety_pilot.html |archive-date=5 September 2007 |url-status=dead }}</ref>

It is important not to confuse IFR with IMC. A significant amount of IFR flying is conducted in visual meteorological conditions (VMC). Anytime a flight is operating in VMC and in a volume of airspace in which VFR traffic can operate, the crew is responsible for seeing and avoiding VFR traffic; however, because the flight is conducted under instrument flight rules, ATC still provides separation services from other IFR traffic, and can in many cases also advise the crew of the location of VFR traffic near the flight path.{{Citation needed|date=June 2015}}

Although dangerous and illegal, a certain amount of VFR flying is conducted in IMC. A scenario is a VFR pilot taking off in VMC conditions, but encountering deteriorating visibility while en route. Continued VFR flight into IMC can lead to [[spatial disorientation]] of the pilot which is the cause of a significant number of [[general aviation]] crashes. VFR flight into IMC is distinct from "VFR-on-top", an IFR procedure in which the aircraft operates in VMC using a hybrid of VFR and IFR rules, 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. Also possible in many countries is "Special VFR" flight, where an aircraft is explicitly granted permission to operate VFR within the controlled airspace of an airport in conditions technically less than VMC; the pilot asserts they have the necessary visibility to fly despite the weather, must stay in contact with ATC, and cannot leave controlled airspace while still below VMC minimums.{{Citation needed|date=June 2015}}

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 or to land; these 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, [[Reno-Tahoe International Airport]] (KRNO) in a mountainous region has significantly different [[instrument approach]]es for aircraft landing on the same runway surface, but from opposite directions. Aircraft approaching from the north must make visual contact with the airport at a higher altitude than when approaching from the south because of rapidly rising terrain south of the airport.<ref>See KRNO [[approach plate]]s for "LOC RWY 16R", "ILS RWY 16R", and "ILS or LOC/DME RWY 34L".<!--Links please--></ref> This higher altitude allows a flight crew to clear the obstacle if a landing is aborted. In general, each specific instrument approach specifies the minimum weather conditions to permit landing.

Although large airliners, and increasingly, smaller aircraft, carry their own [[terrain awareness and warning system]] (TAWS),<ref name="skybrary.aero">[http://www.skybrary.aero/bookshelf/books/2328.pdf "Terrain Awareness and Warning Systems—TAWS"]. Skybrary. Retrieved 11 July 2014</ref> these are primarily backup systems providing a last layer of defense if a sequence of errors or omissions causes a dangerous situation.<ref name="skybrary.aero" />