Found the following:
The 717 autoflight system is adapted from the MD-11 system. It consists of two flight control computers (FCC) and a glareshield control panel (GCP). The FCCs provide autopilot, autothrottle, flight directors, stall warning, wind-shear detection and guidance, engine synchronization, and various other functions
And from Airliners.net regarding 737-700/800:
Below are some extracts from the SDS.
Display Electronics Units (DEU) The A/T computer sends mode data to the DEUs to show A/T modes of operation on the FMA on the CDS.
The FMC calculates engine N1 limits and N1 targets during each flight phase and sends the data to the DEUs. The DEUs show N1 limits on the engine display. The DEUs send N1 targets to the EECs.
FMC The FMC calculates thrust N1 limits and N1 targets for each flight phase. The data goes to the DEUs. The DEUs show the N1 limits on the engine display. The DEUs send the N1 targets to the EECs which calculate equivalent TRA targets to send to the A/T to set thrust. The FMC also sends N1 targets directly to the A/T. During takeoff and max thrust go-around, the A/T uses EEC TRA targets and FMC N1 targets to set thrust. During takeoff, climb, and max thrust go-around, the FMC N1 targets are the same as the N1 limits. During reduced thrust climb and cruise operations, the FMC N1 targets are less than the N1 limits.
EEC The DEUs send FMC N1 targets to the EECs. The EECs use the data to calculate equivalent TRA targets. The A/T uses the EEC TRA targets to set thrust during takeoff, climb, and max thrust go-around. For takeoff and max thrust go-around, the A/T initially uses EEC TRA targets to advance the T/Ls. As the T/Ls get to within 4 to 6 degrees of the FMC N1 limit, the A/T computer then uses FMC N1 targets to make final T/L adjustments to the FMC N1 limit.
Flight Management Computer (FMC) The FMCs send this data to the A/T computer: N1 targets Gross weight Minimum speed FMC altitude Static air temperature FMC modes GMT/Date.
BITE test information. The autothrottle computer converts the target N1 values from the FMC to an equivalent TRA target. The target N1 rating is dependent on the FMC engaged mode. Gross weight is used in the go around control logic and approach control logic. Minimum airspeed is the lowest airspeed that is acceptable during VNAV operation. FMC altitude from the FMC is used for anticipation of altitude acquire during VNAV operation. SAT is used to calculate a backup TRA limit value. The FMC mode discretes are used to determine control gains and limits.
Electronic Engine Control (EEC) Each EEC channel sends this data to the A/T computer: Thrust resolver angle (TRA) N1 command indicated TRA for max forward idle Estimated corrected thrust TRA for actual N1 TRA for N1 target TRA for N1 max TRA for 5 degree/sec response. Thrust resolver angle is used by the autothrottle computer to calculate an N1 command. N1 command indicated is used to set a throttle position using the error between the target N1 (from the FMC) and the commanded N1 from the EEC.
TRA for maximum forward flat is the throttle angle below which the engine is in the idle range. Estimated corrected thrust is used in the reduced go around control logic. TRA for actual N1 and TRA for N1 target are used in the N1 mode control logic. TRA for N1 maximum is used as a reversion limit in the event that the airplane is dispatched without an operative FMC and as a protection from excessive throttle angles. TRA for 5 degree/sec response is used in the retard control logic.
So with autothrottle engaged N1 should be very closely matched. However the beat frequency heard will be a function of the tolerance allowed by the control system (there will necessarily be hysteresis in the system I assume or the RPM will be in a constant chase mode).
This may be more significant with a larger engine (like the 777 donk) I'm guessing without thinking too hard.
- GY
