The Sterile Cockpit/Flight Deck concept involves the restriction of flight crew member activity to that which is operationally essential during busy phases of flight - taxi out, take off, initial climb, intermediate and final approach, landing, and taxi in.
The International Civil Aviation Organisation (ICAO) Manual on the Prevention of Runway Incursions defines Sterile Flight Deck as "any period of time when the flight crew should be not be disturbed, except for matters critical to the safe operation of the aircraft."
In the USA, the Federal Aviation Administration (FAA) introduced a formal requirement to be applied to all commercial flights in 1981, after reviewing a series of accidents that were caused by flight crews who were distracted from their flying duties, by engaging in non-essential conversations and activities, during critical parts of the flight.
FAR 121.542, often referred to as the "Sterile Cockpit Rule", reads:
(a) No certificate holder shall require, nor may any flight crewmember perform, any duties during a critical phase of flight except those duties required for the safe operation of the aircraft. Duties such as company required calls made for such non safety related purposes as ordering galley supplies and confirming passenger connections, announcements made to passengers promoting the air carrier or pointing out sights of interest, and filling out company payroll and related records are not required for the safe operation of the aircraft.
(b) No flight crewmember may engage in, nor may any pilot in command permit, any activity during a critical phase of flight which could distract any flight crewmember from the performance of his or her duties or which could interfere in any way with the proper conduct of those duties. Activities such as eating meals, engaging in non-essential conversations within the cockpit and non-essential communications between the cabin and cockpit crews, and reading publications not related to the proper conduct of the flight are not required for the safe operation of the aircraft.
(c) For the purposes of this section, critical phases of flight includes all ground operations involving taxi, takeoff and landing, and all other flight operations conducted below 10,000 feet, except cruise flight. Note: ‘Taxi’ is defined as “movement of an airplane under its own power on the surface of an airport.”
In Europe the Sterile Cockpit concept is addressed by EU-OPS 1.085 paragraph (f)(9) although in less explicit terms than the FAR:
The commander shall...not permit any crew member to perform any activity during take-off, initial climb, final approach and landing except those duties required for the safe operation of the aeroplane;
Regardless of regulation, many operators have chosen voluntarily to apply similar rules within their company. Where introduced, these proactive moves have been widely adhered to and recognised as a valuable contribution to operational flight safety.
Some operators use surface to 18,000 feet as the "sterile zone", while others apply the sterile cockpit principle from Top of Descent/Pre-Descent checklist, in the descent, and up to Top of Climb/En-Route checklist, on departure. Exact interpretation of what constitutes the "sterile zone" is influenced by aircraft type, role, and operating environment.
In military transport operations, a similar philosophy is applied from "combat entry" to "combat exit".
Flight Safety Foundation