The following is general information and is not intended to be an exhaustive explanation of each subject.
Visual Flight Rules / Instrument Flight Rules
Aircraft and airports operate under two different rules or systems, mostly depending on the weather. Essentially, when weather and visibility are good, aircraft are permitted to operate under visual flight rules (VFR). When weather and visibility are bad, aircraft are required to operate under instrument flight rules (IFR). The FAA (Federal Aviation Administration) sets exact guidelines as to when each system is to be used.
Glide Path or Glide Slope
When an airplane comes in for a landing, it follows what is called a glide path. The standard glide path is 3 degrees up from the runway touchdown zone, and makes for a smooth transition from regular flight to landing. It is called a glide path because in the early days of aviation most airplanes would glide in for a landing using little or no power.
Many airports, including most that handle airline traffic, have landing aids that assist the pilot in setting up the aircraft on the proper glide path/slope. Some landing aids are designed for use in VFR conditions and some are designed for IFR conditions.
Visual Approach Slope Indicator
VASI (Visual Approach Slope Indicator) is a system of lights used to aid the pilot in landing. It consists of bars of red and white lights that tell the pilot whether the aircraft is on, above, or below the glide path. If all lights are red the aircraft is below the glide path, a combination of red and white indicates on the glide path, and all white tells the pilot that the aircraft is above the glide path. VASI will guide the pilot down to about 200 feet above the ground, and to continue landing, the runway must be clearly visible.
Precision Approach Path Indicator
PAPI (Precision Approach Path Indicator) is a newer and more precise light system that operates in a similar fashion as VASI. A system of red and white lights tell the pilot where he is in relation to the glide path. PAPI is much more precise than VASI, hence the name, and is accurate to about 50 feet above the runway. This enables the pilot to fly almost down to a landing using the system.
The diagram, shows the Precision Approach Path Indicator (PAPI) system of lights. These lights will be seen by pilots beside the runway as the plane approaches to land. As can be seen, if all the lights are white, the plane is above the glide path; if the lights are all red, the plane is below the glide path. Two red and two white lights indicate the plane is right where it should be on the glide path. Other formations of the lights are found, but this diagram describes the principal type of visual aid the pilots have with the PAPI system.
When using an instrument approach, or when visibility is poor, the ILS (Instrument Landing System) may be used to aid the pilot in making a safe landing. ILS is a radio system with components on the ground and in the aircraft. The localizer part of ILS will tell the pilot whether the aircraft is lined up straight with the runway, or to the right or left. The glide slope part of the ILS indicates to the pilot whether the aircraft is higher, lower than or properly on the glide path. Thus ILS is an accurate guide for the pilot down the glide path. An approach to landing using this system is called an ILS approach.
When an approach to landing is done using the ILS, the ILS can be and often is used with VASI or PAPI light systems as a cross check. The pilot flies the ILS approach until he can see the lights of the VASI or PAPI system, and uses both systems for final approach and landing.
The ILS can be used for a precision approach even in good weather with fine visibility. But an excellent approach and landing can be made in VFR (Visual Flight Rules) conditions without using either VASI, PAPI, the glide slope or the localizer.
The fact is that most every commercial flight is conducted IFR because of the altitude that these aircraft are flying. IFR flight is necessary for navigation at high altitudes. However, IFR flying is generally used all the way through the landing phase. Having said that, as has been discussed in the media, commercial airline pilots must be comfortable landing these aircraft using VFR rules and the associated systems.
Also, interestingly, airplanes still land with little or no "power" now and are, in a sense, gliding in. But, there is a need to have power available if needed. Power is used to adjust the airplane's altitude during the landing phase to keep it on the glide path; i.e. utilizing small power adjustments as they probably did in the early days of aviation. With piston engines, getting additional power is nearly instantaneous with an advance of the throttle. With jet engines, however, one must "spool up" the engines in order to have available to the crew any needed power adjustments, and that process takes as much as 10 seconds, or so. That is why pilots of jets should not attempt to "glide" in with the engines at flight idle all the way to touchdown.