THE "six-pack" in aviation refers to six essential flight instruments in an aircraft cockpit: the attitude indicator (AI), heading indicator (HI), turn co-ordinator, airspeed indicator (ASI), altimeter and vertical speed indicator (VSI).

In the early days of aviation, pilots navigated from one point to another using rivers, roadways, hills and other landmarks that could be viewed from the air.

This limited the pilot to fly by visual flight rules (VFR) only. This is a set of regulations under which a pilot operates an aircraft in weather conditions known as visual meteorological conditions (VMC) in which VFR flights are permitted once the pilot has a very clear view of the surrounding environment including terrain.

However, during night time and periods of adverse weather with cloud ceilings, pilots could not fly as they had no visual reference to the ground and could not use visual means to avoid obstructions and other aircraft in flight.

The flight instruments provide the pilot with situational awareness data about the aircraft's position such as attitude, altitude, airspeed, vertical speed, heading and rate of turn.

Safety was vastly improved as pilots could fly the aircraft in level flight, and make turns, without reference to outside the aircraft such as the horizon.

When pilots fly using instruments only, it is called instrument flight rules (IFR).

Flying by VFR requires an airspeed indicator, an altimeter and a compass or other suitable magnetic direction indicator.

Flying by IFR additionally requires an attitude indicator, a heading indicator, a rate of turn indicator, an adjustable altimeter and a vertical speed indicator.

Instrument meteorological conditions are weather conditions that require pilots to fly primarily by reference to flight instruments, and therefore under IFR, as opposed to flying by outside visual references under VFR. Typically, this means flying in a cloud or poor weather, where little or nothing can be seen or recognised when looking out of the cockpit window.

Flights under IMC also require radio navigation instruments to navigate along the aircraft's planned route from the departure point to the arrival point.

If the weather is less than VMC, pilots are required to use IFR, and operation of the aircraft will be primarily through referencing the instruments rather than visual reference.

When operation of an aircraft under VFR is not safe because the visual cues outside the aircraft are obscured by weather, IFR must be used.

This permits an aircraft to operate in instrument meteorological conditions, which is essentially any weather condition less than VMC, but in which aircraft can still operate safely.

The following gives a brief description of the "six-pack" instruments:

• The attitude indicator (AI): Also known as an artificial horizon is an instrument that shows the aircraft's position in space in relation to the horizon.

It is a primary instrument for flight under IMC. The AI informs the pilot of the aircraft's orientation relative to the earth's