The basis of the DC-9/MD-80 engine synchronization system is a circuit that compares the rpm of one engine and adjusts the same parameter of the other engine to zero the difference. In the case of the MD-80, the system includes synchronization of engine pressure ratio (EPR).
The system was first installed on DC-9 Series 30 and 50 aircraft and its initial design objective limits it to synchronizing only N1 and N2 rotor speeds - parameters that are the sources of the annoying acoustical beat. This system cannot synchronize EPR. With these series aircraft, EPR equalization is achieved manually for all phases of flight.
The MD-80 engine synchronization system builds on the original design and incorporates the digital flight guidance computer (DFGC), full time autothrottle system (ATS), three position switch logic or an optional four position switch and an additional advisory light. As a result, the MD-80 system goes beyond N1 and N2 synchronization to offer EPR synchronization in all modes, including takeoff.
Establishing and maintaining even thrust and lining up N1 or N2 are normal engine operation procedures. Therefore, the engine synchronization system is not essential for flight and the Minimum Equipment List (MEL) does not require it to be operative.
The right engine is the master engine. The synchronization system fine tunes only the left, or slave, engine using the master engine as a reference.
Engine Sync Selector Switch. On the DC-9 Series 30 and 50 the switch provides a true OFF position, whereas the OFF switch position on the MD-80 is used for EPR sync. Several operators purchased a four position switch that has separately labeled EPR and OFF positions.
Advisory Lights. The ENG SYNC ON advisory light is common to the DC-9 Series 30 and 50 and the MD-80, and it pertains to N1 or N2 only. The light illuminates anytime the system is selected to N1 or N2 and the landing gear control handle is in the down position. Its purpose is to alert the crew to position the selector switch to OFF during takeoff or landing since attention to rotor speed sync is not permitted below an altitude of 1.500 feet AGL where maintaining appropriate EPR is most important. The advisory light will not illuminate when the landing gear control handle is at OFF (EPR) on the three position switch or when it is at OFF or EPR on the four position switch.
Some configurations have a SYNC FAIL advisory light that alerts the crew when the engine parameter synchronization range has been exceeded or when there is a failure in the system.
ENGINE SYNCHRONIZATION PARAMETERS. Engine fan section N1 and high-pressure turbine section N2 are selectively synchronized to reduce or eliminate acoustical beat and EPR is used to synchronize engine thrust.
There are no mechanical connections between the N1 and N2 sections of the engine and either can produce a beat if it is not in sync with its corresponding section of the other engine.
Due to its mass and thrust characteristics, N1 is the parameter most likely to cause acoustical beat and is therefore the parameter of choice for synchronization. N2 can produce acoustical beat as well, but analysis shows N1 to be the most prevalent cause. It is also quite possible for one parameter to be in sync while the other is not due to the differing cycles, hours and maintenance between and within engines. Consult the after cabin attendant to determine which parameter produces the most satisfactory results. It may take more than one attempt to produce the most comfortable noise level.
N1 and N2 synchronization and EPR synchronization are undertaken for separate reasons and should remain distinct operations.
EPR sync, which equalizes engine thrust, should not be used to enhance cabin comfort because differences in EPR do not necessarily produce acoustical beat. Further, pilots in the process of attaining correct thrust levels should not consider acoustical beat.
FIVE-SECOND PAUSE. Synchronization is more than a function of switching from one position to another. Regardless of the desired parameter, the engine synchronization actuator must be properly set to be able to bring the slave engine into sync with the master engine.
The actuator has a physical trimming capacity of several centimeters in either direction from a neutral position. When signaled it trims the left engine fuel control throughout this range. Obviously, if the actuator is positioned at or close to its limit of travel in one direction when a new parameter is selected, it will not have a full range of movement in that direction in which to accomplish its task and synchronization may not occur.
The solution is to place the synchronization switch in the OFF position for a minimum of five seconds prior to selecting a parameter. The five second pause is critical because it gives the actuator time to reposition itself in the neutral position.
The three position switch of the MD-80 requires disengaging the ATS before positioning the switch in the OFF position, this prevents EPR synchronization. Failing to give the system a five second pause all too often results in unwarranted maintenance write-ups.
ENGINE SYNCHRONIZATION RANGES. Engine parameters must be within the following approximate ranges for the system to achieve synchronization:
· N1 must be within
+/- 3 percent of the master engine.
· N2 must be within
+/- 2 percent of the master engine.
· EPR must be within
.1 to .3 EPR of the master engine.
The broad ranges ease alignment, but the selected parameter indications must be aligned as closely as possible before positioning the system switch to that parameter. Only then will the engine sync actuator make the difference.
It is important to understand that the system does not restrict throttle movement in any way. Manual or ATS control of the throttles is always available. Keep in mind, however, that any throttle adjustment will cause the system to attempt to remain synchronized until adjustment stops or the parameter exceeds its sync range.
This may cause the system to become inoperative. (a SYNC FAIL advisory light illuminates on aircraft equipped with indicator) To successfully sync the engines, start the procedure over again by switching the ATS to OFF, placing in the OFF position for a minimum of five seconds, aligning the selected parameters and the placing the synchronization switch in the selected parameter position.
ATS AND ENGINE SYNC. Throttle rigging resistance prevents throttle movement when the sync actuator trims the engine fuel control. The flexible shaft from the actuator is effectively piggy-backed to the end of the throttle cable, and it fine tunes a trimming linkage attached to the fuel control unit. As a consequence, there is no feedback to the throttle during the small amount of travel required in either direction. The only synchronizing indication is movement of the slave engine parameter toward the level of the corresponding master engine parameter indicator.
ATS does not have to be engaged as a prerequisite of synchronizing N1 or N2 on the MD-80. However, it must be engaged to sync EPR
On the MD-80 the ATS provides for EPR sync up to 60 knots on takeoff. At this airspeed, the DFGC - which receives input from the air data computer - tells the autothrottle mechanism in the throttle quadrant to “clamp” and it tells the EPR sync driver unit to freeze. These events stop further EPR sync.
PROTECTION AGAINST LOSS OF ENGINE POWER. It is systematically impossible for loss of power on the slave engine to affect the power setting of the master engine, but this is not the case if the condition is reversed. The physical trimming range of the sync actuator on the slave engine prevents it from following power loss of the master engine.
A worst-case scenario would be a loss of master engine power as the landing gear is being retracted during takeoff, with the engine synchronization switch erroneously selected to N1 or N2 in violation of procedures and the crew ignoring the advisory light. On the MD-80, the automatic thrust system (ARTS) would command the good engine fuel control to advance which immediately increases the power of that engine. The ATS has the throttles clamped, so both throttles stay where they are. Additionally, the engine sync system will not trim down due to the DFGC signal that freezes the actuator coincidental with the ATS clamp. If the left engine had lost power, it would have no influence over the right engine.