IMC in Aviation

Aircraft · 4 min read · May 23, 2022
IMC Aviation

Getting certified for flying under instrument meteorological conditions is the next exhilarating achievement for pilots after pulling off their Private Pilot License (PPL). Getting used to flying with instruments has never been an easy task – student pilots fly with an IFR hood or specialized glasses to block outside vision and focus only on the instruments.

Additionally, the efforts made with the jargon added to their daily vocabulary go without saying. And for some people, instrument meteorological conditions, visual flight rules, instrument flight rules, and related terminology have created more ambiguity in their minds. This article is tailor-made to diminish all the grey areas while prioritizing instrument meteorological conditions. Hop on, let us dive in!

This entire article is based on meteorological conditions. There are two main types – Visual Meteorological Conditions (VMC) and Instrument Meteorological Conditions (IMC). Depending on the prevailing meteorological conditions, pilots change their flying style to comply with certain flight rules. The flight rules to be followed during VMC conditions are termed Visual Flight Rules (VFR), while rules to be followed during IMC conditions are termed Instrument Flight Rules (IFR).

Visual Meteorological Conditions (VMC)

As the name suggests in the first place, VMCs are weather conditions where pilots can fly with the aid of visual cues thanks to greater visibility. This is more like traveling on their own without a hand from the Air Traffic Control (ATC). Hence, it is quite obvious that clear meteorological conditions play an imperative role within this context.

VMC minima are the borderline between the VMC and IMC. When the prevailing conditions pass the minima values, VFR conditions are no longer applicable and the flight should be continued under IFR conditions. If by any chance, either pilot or the aircraft is not rated for instrument flight, a diversion is required to revert to VMC.

VMC Visibility Requirement Chart (Applicable for the United States)

In the United States, Federal Aviation Regulations (FARs) specified by the Federal Aviation Administration (FAA) are listed under title 14 of the Code of Federal Regulations (CFR). As per regulations, VFR minima for classes C, D, and E are as follows:

  • 3 miles of visibility at day or night (3 SM)
  • clouds clearance of 1,000 ft above, 500 ft below, and 2,000 ft horizontally (152).

In congested airspaces like type A, VFR flights are not allowed unless it is an emergency – all the flights are operated under IFR regardless of the weather conditions.

The above figures enacted by the FAA are an example of the minimum values and the applicable values for each country are governed by the respective aviation authority.

Visual Flight Rules

A pilot flying VFR shoulders the responsibility of keeping an eye on traffic and taking corrective actions to avoid intruders. VFR pilots employ the ‘See-and-Avoid’ strategy to a greater extent for mitigating collisions. Perfect weather conditions allow more room for pilots to actively search for other aircraft in the vicinity and fly with the aid of visual references.

All the training flights provided under a PPL are performed as VFR flights giving more practice for the pilots to understand how weather conditions, visibility, and separation from clouds affect the flight. VFR flights are more restrictive than IFR flights. Normally, a flight under the visual flight rule cannot be operated in bad weather conditions.

Different regulatory bodies have set out limitations on VFR flights by introducing operational minimums and special operating procedures. Here are a few key rules mentioned in ICAO Annex 2: Rules of the Air, Chapter 4: Visual Flight Rules:

  • When operating a VFR flight in controlled airspace, a special clearance has to be obtained by contacting air traffic control. Generally termed as a Special VFR flight.
  • Stick to VMC minimum values.

Change from VFR flight to IFR flight – can pilots do this?

Yes, it is possible, but here is a disclaimer!

When an instrument-rated pilot flying under VFR conditions cannot maintain VMC requirements due to bad weather, the pilot can contact ATC immediately and request an IFR clearance. However to execute the aforementioned switch from VFR to IFR conditions, both the pilot and aircraft should hold an instrument rating.

Such an event is termed Inadvertent Entry Into Instrument Meteorological Conditions (IIMC) and is considered one of the most demanding situations a pilot has to manage. Once a pilot encounters an IIMC, the surrounding ground and terrain no longer provide visual cues.

This may lead to spatial disorientation and finally, catastrophic accidents endangering the lives of passengers flying on two airplanes. Typically, pilots are instructed to avoid IIMC to the best of their ability by comprehending weather forecasts for departure, en route, and arrival.

Pilots can execute practices such as scud running to avoid clouds and deteriorating weather. In scud running, the pilots operate the aircraft to a lower altitude to avoid clouds and safely land without any incidents.

Instrument Meteorological Conditions (IMC)

By now you should have an answer to the question: what are instrument meteorological conditions?

Just like VMC, IMC conditions are also actual weather conditions where pilots are not allowed to fly on their own. Such conditions occur during bad weather when the minimum visibility is below a threshold value. From this point onwards pilots should seek support from flight instruments such as attitude indicators, altimeters, and radio for aircraft control.

During an IFR flight, the aircraft is maneuvered based on the instrument indications rather than outside vision. During the instruments training, pilots are trained to operate their aircraft by giving more focus on the flight instruments without ground reference.

Instrument flight rules

To fly IFR, the respective aircraft should be instrument-rated and the pilot should hold an applicable instrument rating. IFR describes the set of rules and requirements mandated by the relevant regulatory bodies to fly under IMC conditions.

When an IFR flight comes for a landing, pilots stick to the instrument approach procedure and take a note of the minimums set for the approach, and if the minimums are exceeded, pilots perform a go-around.

On the other hand, pilots can perform a visual approach if the weather conditions allow, and a different set of minums are there to govern a visual approach.

An aircraft flying during bad meteorological conditions.

The role of air traffic control

Air traffic controllers come into action during IMC conditions and pilots should communicate when flying IFR. Typically, even flying visually, air traffic controllers should be addressed in case of an IIMC. Then controllers look into the traffic and instruct the pilots to operate without any congestion.

Changing from IFR flight to VFR flight

This changeover can happen only if VMC is met. The pilot can contact ATC and request a flight plan change. Even though when an aircraft is flown under instrument conditions with normal weather, perfect flight visibility, and a safe distance from clouds, it is not mandated to go VFR, the pilot can stick to IFR conditions if they are willing to.

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Aeroclass Team
A team of professionals with a deep passion for the aviation industry bringing you the newest and the most striking industry-related news and content.

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