Pilots have instruments that help them land. But in low visibility they are required to use auto land. Why can't they rely on their instruments at those times?
Pilots are required to operate by two separate sets of regulations, protocols, and procedures when flying an aircraft. They are Visual Flight Rules (VFR) and Instrument Flight Rules (IFR). The rules are dependent upon what type of airspace you are flying in as well as cloud ceilings and visibilities outside the aircraft. Most commercial flights, including virtually all scheduled airline services are conducted under IFR.
Landings conducted under VFR must be done by flight under visual references and are subject to specific limitations on cloud ceiling and visibility minimums for the specific airspace surrounding an airport. If the local weather drops below these minimums, you cannot takeoff or land under VFR and the airport can only accommodate IFR traffic in this weather.
IFR operations require pilots to fly specific procedures during departure and approach to an airport. These procedures themselves set weather limitations on the flight depending on aircraft and airport equipment, NavAid availability and GPS reception and the specific regulations in which the flight operation is being conducted under eg. Part 91, 121, 135, etc. If the weather does not meet these minimum requirements, the flight is not permitted under IFR either, which is why airline flights get cancelled in bad weather such as snowstorms, etc.
IFR Instrument Approaches into airports are broadly divided into 2 categories: Non-precision approaches, which provide lateral guidance to either the runway threshold for a straight in approach or to the airport itself for a visual circling approach and landing. Precision approaches provide the pilot with both lateral guidance to the runway center line and a glide path to the runway touchdown zone. Both types of approaches have minimum ceiling and visibility requirements along with specific minimum altitudes and decision points based on your aircrafts’s approach speed. If you do not have the runway environment in sight when flying these approaches at these points and altitudes you cannot continue the approach to landing under visual conditions and must fly the missed approach procedures specific to this instrument approach.
The most common type of Precision Instrument Approach is the Instrument Landing System or ILS which uses terrestrial radio signals to guide the pilot along an approach path and identify specific points in the approach. ILS approaches fall under 3 different categories, or CATs. CAT I approaches are standard ILS approaches and can be flown by most IFR Certified aircraft. CATs II and III approaches require specific additional flight crew training, and aircraft equipment.
CAT III approaches are the most precise ILS approaches and have 3 three additional subcategories: IIIa is the most precise approach that still allows the pilot to hand fly an aircraft equipped with an approved HUD. IIIb, and IIIc require the use of a coupled autopilot and allow for even lower weather minimums. CAT IIIc approaches are “zero-zero” meaning they can be flown with ceilings of 0 ft AGL and visibilities less that 1/16 of a mile, though they do require the visibility be great enough to adequately allow the crew to taxi the airplane off the runway environment to parking. Again these are limited to specifically trained flight crews, specifically equipped aircraft and specific airport infrastructure.
While CAT IIIb and IIIc ILS approaches do require a coupled autopilot to perform them, all other Non-precision and Precision Instrument approaches can be flown by hand - and all aircrews should be able to do this in the event of an autopilot failure.