1. An economy performance data avionic system for a jet aircraft for producing the most economical engine thrust and speed information for the various phases of a flight, said jet aircraft including at least one jet engine and sensors and subsystems for producing an actual thurst signal for each jet engine, an actual speed signal representing the speed of said aircraft, and an altitude signal representing the altitude of said aircraft, said economy performance data avionic system comprising: speed indicator means including a first display means connected to the appropriate one of said sensors and subsystems of said aircraft for receiving said actual speed signal and producing a display of said actual speed of said aircraft, said speed indicator means including a second display means for displaying the most economical speed of said aircraft; thrust indicator means including thrust indicators equal in number to the number of jet engines of said jet aircraft, each of said thrust indicators including a first display means connected to the appropriate one of said sensors and subsystems of said aircraft for receiving said actual thrust signal related to one of said jet engines and producing a first display displaying the actual thrust of said one of said jet engines, each of said thrust indicators including a second display means for displaying the most economical thrust for the related one of said jet engines; a control display unit for receiving manually inserted mode of operation and parameter information and producing, in accordance therewith, mode of operation and parameter data signals, said parameter information including a dimensionless INDEX number that represents a chosen relationship between fuel costs and trip time costs; and, a data processor connected to said control display unit for receiving said mode of operation and said parameter data signals and connected to said sensors and subsystems of said aircraft for receiving said actual thrust signals, said actual speed signal and said altitude signal, said data processor including a memory having stored therein, in tabular form, data related to said aircraft, said data processor determining the most economical thrust and speed of said aircraft for the various phases of a flight based on said mode of operation, parameter data, actual thrust, actual speed and altitude signals and on said data stored in tabular form in said memory and, in accordance therewith, produce economy thrust and economy speed control signals related to said most economical thrust and speed, said economy thrust control signals being applied to said thrust indicators for controlling the second displays of said thrust indicators and said economy speed control signal being applied to said speed indicator means for controlling the second display of said speed indicator means.