What are the key differences between AFM and FCOM and where they will be used?
The simple answer is that the AFM is a certification document whose content (procedures, limitations, and performance data) is fundamental to the airplane's certification requirements and is approved (both original and all revisions) by the regulating authority that certified the aircraft. You could say that the AFM is part of the airplane's type certificate.
The FCOM/AOM/Operating Data manual is just a company publication. Its content is not approved. It takes AFM procedures and expands them into a kind of company-recommended procedural framework, provides descriptive information, and provides non-certification related performance data used for planning, like cruise data, optimal profiles etc.
AFM = Aircraft Flight Manual
FCOM = Flight Crew Operating Manual
The AFM contains data about the plane like performance, procedures, limitations and sometimes data about weight and balance. The AFM is made by the manufacturer and it is specific to a particular MSN.
The FCOM contains the systems, the procedures, the performance. The FCOM is the "guide" to know how to use the plane: it's the main reference for pilots.
Both the AFM and the FCOM can contain information applicable to a specific variant of tail number, but the AFM is specifically designed for a given aircraft.
The AFM is the manual designed by the manufacturer, on how to use the aircraft, it has been submitted to the certification authority. It is the reference.
AFM: An aeroplane flight manual (AFM) is a document produced by the aircraft manufacturer containing detailed information on the operation of the aircraft.
The AFM details the recommended aircraft operating technique for normal, abnormal and emergency operation together with the Aircraft Performance that should be achieved when the aircraft is operated in accordance with these procedures.
The AFM is a vital part of the aircraft inventory and must be carried on all flights unless the National Airworthiness Authority (National Aviation Authority (NAA)) of the aircraft operator has formally accepted that the Operations Manual of the aircraft operator replicates all relevant AFM information for an aircraft.
The AFM is specific to each aircraft and reflects the precise equipment and modification state of that aircraft. In practice, National Aviation Authority (NAA) dispensation not to carry the AFM on board an aircraft is commonly obtained by commercial aircraft operators.
The FCOM is a substitude to the AFM, for how to conduct day to day operations, and is usually designed by the operator (based on the AFM and other manufacturer information). The FCOM can be used for training.
FCOM: Aircraft Operating Manuals/Flight Crew Operating Manuals (AOM/FCOM) constitute the primary flight crew reference for the operation of an aircraft under normal, abnormal, and emergency conditions.
These publications include system descriptions, normal and emergency procedures, supplementary techniques, and performance data.
Along with the initial training course, the AOM/FCOM constitutes a trainee's first introduction to their new aircraft. This is normally followed by fixed-based or full-flight simulator training and, ultimately, operating the actual aircraft. Operating manuals must meet the needs of initial training, transition training, and line operations.
There are other words to name these two manuals, source:
As is often the case in aviation, different manufacturers, different National Aviation Authority (NAA), or different users often refer to a given item by different names. The naming of aircraft manuals is not an exception. This article uses the terms Aircraft Operating Manual (AOM) and Flight Crew Operating Manual (FCOM). However, each of these designations is interchangeable with other terms. As examples:
Aircraft Operating Manual: This manual might be referred to as Aircraft Flight Manual, Airplane Flight Manual or Aeroplane Flight Manual (AFM)
Flight Crew Operating Manual: Also referred to as Aircraft Operating Manual (AOM), Aircraft Operating Instructions (AOI) or, more simply, Pilot's Manual.
Further confusion arises with the introduction of terms such as Operations Manual or Company Operations Manual (COM) which deal with how the organisation conducts flight operations and are a requirement imposed by the NAA.