A Real-Time Engine Model is defined as a transient performance computer program whose engine
outputs are generated at a rate commensurate with the response of the physical system it represents.
Real-Time models were first used in hardware and software development of engine and aircraft
controllers. The early models were relatively simple and primarily analog devices. As model complexity
increased to satisfy more demanding requirements, analog models became too costly and difficult to
use and digital models became predominant. The early digital models were simply digital
implementations of the analog models. As digital computer capabilities increased and costs decreased,
the number of real-time model types expanded to best utilize these resources. Current practices vary greatly in terminology and methods depending upon application. This document is intended to provide a vehicle for presentation of model types and definitions to be used as a basis for communication between supplier and customer. It is also intended to complement SAE AS681, Gas Turbine Engine Steady-State and Transient Performance Presentation for Digital Computer Programs, and ARP4148, Gas Turbine Engine Real Time Performance Model Presentation for Digital Computers.