From Skybrary:
SOP is a Standard Operating Procedure.
Many industries use SOP’s as a common way of ensuring tasks or operations are completed correctly, however SOP’s are essential in aviation.
They ensure that aircraft are flown correctly in accordance with the manufacturers guidelines, but also it allows 2 pilots that have never met before who may be from different crew bases and different cultures or backgrounds to fly together as a flight crew team on the same aircraft fully understanding what the other pilot is expected to be doing for the whole flight.
Different types of SOP’s are as follows:
A memory flow of arranging switches and levers in the correct position for a particular phase of flight. For example it is normal that the PM / PNF (Pilot monitoring or Pilot not flying) will complete the before start flow and then read the before start checklist which the PF (Pilot flying) will respond to.
A call or acknowledgement of an event. For example most EASA airlines have to acknowledge an automated callout of 1000ft which would be followed by PM / PNF stating whether they are stable or not for the subsequent landing.
A procedure that requires completing with certain criteria. For example in visible moisture below 10 degrees pilots will be required to taxi and take off with engine anti-ice systems on.
SOP’s can also be developed as time goes by to incorporate improvements based on experience, accidents, near misses or innovations from other manufacturers or operators to suit the needs of a particular organization.
SOP’s should not be designed too detailed and exhaustive that the pilot does not provide any form of cognition to the process and not be too relaxed where the crew have too many options to decide between.
If a pilot is not conforming to SOP’s he/she can be expected to be challenged by the other pilot.
However there may be an occasion where it is preferably or vital to ignore or not carry out an SOP. This would normally be in an emergency situation.
An example of this would be continuing to land the aircraft below the operating minima where the pilots had not become visual with the runway as they had an uncontrollable cabin fire.