Joined: 11 Dec 2005
|Posted: Thu Aug 02, 2012 4:10 pm Post subject: Engines / APU (Technical Notes)
Vibration source displayed is the one with the highest vibration: N1 rotor vibration or N2 rotor vibration. If the vibration source BB (broad band vibration) is displayed, the source is unknown and average vibration is displayed.
High N1 vibration indication would most likely be accompanied by tactile vibration. This is not the case with high N2 vibration indication.
Certain engine malfunctions can result in airframe vibrations from the windmilling engine. As the airplane transitions from cruise to landing, there can be multiple, narrow regions of altitudes and airspeeds where the vibration level can become severe. In general, airframe vibrations can best be reduced by descending and reducing airspeed. However, if after descending and reducing airspeed, the existing vibration level is unacceptable, and if it is impractical to further reduce airspeed, the vibration level may be reduced to a previous, lower level by a slight increase in airspeed.
The EGT indication has a maximum continuous limit represented by an amber band. The maximum continuous limit does not apply during takeoff or go–around. EGT indications are inhibited from changing to amber during takeoff or go–around for five minutes. The EGT indication is often in the amber band during takeoff; this is acceptable. The inhibit is extended to ten minutes for single–engine operation.
If an N1, N2, or EGT red line is exceeded, the box enclosing the digital readout remains red after the exceeded limit returns to the normal range.
The red box color can be canceled to white or recalled to red by pushing the cancel/recall switch on the display select panel.
The EEC uses thrust lever inputs to automatically control forward thrust and reverse thrust. The EEC has two control modes: normal and alternate. In both normal and alternate modes, the EEC uses N1 RPM as the parameter for setting thrust.
EEC Normal Mode
In the normal mode, the EEC sets thrust by controlling N1 based on thrust lever position. N1 is commanded by positioning the thrust levers either automatically with the autothrottles, or manually by the flight crew. Maximum N1 represents the maximum rated thrust available from the engine. The EEC continuously computes maximum N1. Maximum rated thrust is available in any phase of flight by moving the thrust levers to the full forward positions.
EEC Alternate Mode
If the required signals are not available to operate in the normal mode, the EEC automatically uses the alternate mode. In the alternate mode, the EEC schedules N1 as a function of thrust lever position. The alternate mode provides soft and hard levels of control:
• Soft – When the EEC automatically switches an engine to the alternate mode and the EEC mode switch remains in NORM. At a fixed thrust lever position, thrust does not change.
• Hard – When ALTN is manually selected on an EEC mode switch, that engine is switched to the hard alternate mode. Thrust may change to set the commanded N1 when ALTN is manually selected.
Maximum N1 represents the maximum rated thrust available from the engine. The EEC continuously computes maximum N1. Maximum rated thrust is available in any phase of flight by moving the thrust levers to the full forward positions (in normal EEC mode). Thrust protection is not provided in the alternate mode and maximum rated thrust is reached at a thrust lever position less than full forward. As a result, thrust overboost can occur at full forward thrust lever positions. The EICAS caution message ENG LIMIT PROT (L, R) is displayed if the thrust lever position commands an N1 greater than the maximum rated thrust (maximum N1). N1 and N2 red line protection is still available in the alternate control mode.
If the EECs are in alternate mode, advancing the thrust levers full forward provides some overboost and should be considered only during emergency situations when all other available actions have been taken and terrain contact is imminent.
The autostart system does not monitor oil pressure and temperature.
The EICAS advisory message ENG REV LIMITED (L or R) is displayed if the reverser cannot deploy when commanded, or can deploy only with reverse thrust limited to idle. Not all conditions limiting or preventing reverse thrust can be detected before reverse thrust selection. For these conditions, the reverse thrust levers cannot be moved beyond the interlock position. The EICAS advisory message ENG REVERSER (L or R) is displayed on the ground to indicate a reverser system fault.
In flight, if both AC transfer buses lose power, the APU automatically starts, regardless of APU selector position. The APU can be shut down by positioning the selector to ON, then OFF.