Instrument flight rules

The instrumental flight rules or instrument flight rules —better known by the acronym IFR, instrumental flight rules— are the set of rules and procedures contemplated in the Air Traffic Regulations that regulate the flight of aircraft based on the use of navigation instruments, which implies that it is not necessary to have visual contact with the terrain, as occurs in the navigation method under visual flight rules (or VFR, from English visual flight rules).

Aircraft separation

The most important thing about instrument flight is that it allows the continuous operation of aircraft at times when the pilot's ability to see and avoid collisions with other ships or obstacles is clearly reduced or even non-existent.

To do this, certain separation criteria are applied between aircraft and the terrain. The air traffic controller is responsible for the first, while the second is included in the aeronautical navigation charts. The controller establishes the separation between aircraft by calculating time, distances and speeds between them. It obtains such data in two ways: either through the speed, height, course or position transmitted by the pilot, or through the radar screen, which obtains the same data thanks to a radio transmitter in the aircraft called a transponder.

The air traffic controller

Generally, instrument flight requires controller authorization for each phase of the flight. The controller provides heading or airway, altitude and limit (the furthest point to which the aircraft can fly before receiving a new authorization. In the most traffic-overloaded areas it is common for these authorizations to be requested for visual flights as well.

In uncontrolled airspace, separations are established by navigation charts, which indicate minimum altitudes to avoid terrain and obstacles, and cruising altitudes to avoid the collision of aircraft flying in different directions. Likewise, in uncontrolled airspace pilots must report their intentions using a specific radio frequency to keep other pilots alert. In Spain, airspace above 24,500 feet (7,468 meters) above sea level is called Class A, and is restricted to instrument flights.

Meteorology

An advantage of instrument flight is the ability to fly in instrument meteorological conditions (IMC), that is, those conditions that do not allow visual flight. Under these conditions, the pilot controls his airplane based on the reading of the flight instruments, and entrusts the controller with his separation from other aircraft or the terrain (despite the fact that most airliners and increasingly light aircraft They incorporate their own ground proximity alarm systems (GPWS) and collision with other aircraft (TCAS).

In any case, it is important not to confuse instrument flight rules (IFR) with instrument meteorological conditions (IMC). Most commercial aircraft operate under instrument flight rules, even when conditions permit visual flight. As long as visibility allows, the pilot uses direct observation as a primary reference, even to avoid collisions with other aircraft, since in some cases (in areas with limited radar coverage), the controller lacks information about the aircraft in visual flight.

Usually, the pilot navigates using the flight instruments. In areas with radar coverage it can also receive vectors—that is, different headings and altitudes—from the controller. There must also be minimum visibility conditions to allow operation, which depend on several factors: the qualification of the crew, the equipment of the aircraft (some of them allow completely blind landings, thanks to the instrument landing system ( ILS, Instrumental Landing System), the prevailing winds and the existing radio aids in the vicinity of the airport.

Procedures

An instrument flight consists of four stages: departure, route, approach and arrival. Each of these stages has its standard procedures, published in aeronautical navigation charts and that allow instrument flights to move in an orderly and safe manner, from the moment the plane starts its engines until it turns them off. These procedures also allow the pilot, in the event of a communications failure, to complete a flight so that his intentions can be predicted, both by the controller and by the aircraft that may be in the area.

Instrumental departures and arrivals

The departures are described in the SID and describe the routes to be followed by the aircraft depending on its destination, from the airport until it joins the assigned airway. The STARs, for their part, detail the route from when the plane leaves the airway to the point where the final approach to the runway begins.

The function of both procedures is to allow simultaneous departures and arrivals in the airport control area without risk of collision. The departure and arrival routes are clearly detailed and are set from the existing radio aids (mainly the so-called VOR and NDB), from points referring to distances and routes from such radio aids, called waypoints or intersections, and other points marked by GPS.

Approaches

The instrument approach is detailed in the instrument approach charts (IAC), which describe the route, both vertically and horizontally, to a minimum point from which the pilot must have visual contact with the runway and complete the flight without help of instruments. This point is called Decision Altitude (DA) or Decision Height (DH).

The Decision Altitude refers to mean sea level and the Decision Height refers to the threshold elevation.^{[citation required]} The required visual reference means that section of the visual aids or approach area that should have been in view for sufficient time to allow the pilot to make an assessment of the position of the aircraft and the rapidity of the change in position, relative to the flight path desired.

If at this point the pilot does not have visual contact with the runway, he must abort the landing, in accordance with the procedures established in the same chart, and wait for instructions from the controller or go to an alternative airport specified in his flight plan. flight.

Precision approximation

We call precision approach one in which the airport has specific radio aids called ILS or instrument landing system. This radio aid provides the plane, in the final stretch, with horizontal and vertical guidance that allows it to head and descend safely to the landing strip.

In the most sophisticated versions, this system allows the plane to land without pilot intervention. This is known as autoland.

Non-precision approximation

Non-precision approaches are used at airports or runways not equipped with ILS. In these cases, the charts guide the aircraft to a point more or less close and more or less aligned with the runway, so that the pilot completes the landing visually.

Within this type of approach, ADF, VOR/DME and other approaches, both visual and radio aids, are distinguished.

Routes and airways

Likewise, the routes are also ordered by means of the so-called "route charts", which establish airways through which the planes must fly. Depending on their course, they will fly at even or odd altitudes, to avoid collisions. Airways are made up of reference points.

Instrumental qualification

Not all pilots, nor all aircraft, can fly under instrument flight rules. Pilots require a special license and also prove that they have recently flown under these conditions.

Similarly, aircraft must have the following equipment:

• Two radio stations/receptors for communications with controllers.
• Two VOR receivers to contact the radioayudas on the ground.
• At least one NDB receiver to contact ground radiofares.
• And a distance measuring equipment (DME) to know the distances with the radio helpers.
• If a GPS team is certified with the corresponding databases.