Instrument meteorological conditions

Simulated IMC can be achieved for training purposes by wearing view-limiting devices, which restrict outside vision and force the trainee to rely on instrument indications only.

The VMC minima tend to be stricter in controlled airspace, where there is increased air traffic, therefore greater visibility and cloud clearance is desirable.

It is possible to be flying under VFR in conditions that are legally considered VMC, but still be forced to rely on flight instruments for attitude control because there is no distinct external horizon; for example, at night over water, which may create a so-called black hole effect if the sky and ground are equally dark, or when lights on the water cannot be distinguished from stars in the sky.

[6] In the 180—Degree Turn Experiment conducted in 1954 by the University of Illinois, twenty student pilots flew from VFR into simulated IMC; after entry, all of them eventually reached a dangerous flight condition or attitude[a] over a period ranging from 20 to 480 seconds.

[7]: 16  The average time to reach a dangerous condition was 178 seconds, echoed in the title of the "178 Seconds to Live" article distributed by the Federal Aviation Administration in 1993;[8] however, the original 1954 study was noted for simulating an aircraft the subjects had little to no experience with, and only providing a partial instrument panel.

[4] In addition, the "178 seconds" average time was extracted from the preliminary evaluation; after training for a standardized procedure to exit IMC, each student pilot was tested three times, and 59 of the 60 resulting simulated flights successfully resulted in a controlled descent out of the cloud deck without reaching a dangerous condition.

A pilot's view of the runway just before landing in thick fog at night
Summary of United States VMC minima ( 14 CFR 91.155 ), providing requirements for minimum visibility and separation from clouds for different airspace classes . In this example, for airspace classes C/D/E up to an altitude of 10,000 ft (3,000 m) AMSL , whether day or night, VMC minima require 3 miles (5 km) of visibility (denoted "3 SM") and cloud clearance of 1,000 ft (300 m) above, 500 ft (150 m) below, and 2,000 ft (610 m) horizontally ("152").