I was flying the 757 today and I lined up to takeoff. Put in 50% power, then hit EPR, and it went as usual. At about 70 knots though, my throttles backed down to where my physical throttles were at, decreasing my power.

Then I manually increased my power again, but even at full thrust my 757 was not accelerating, and it was staying at 70 knots. I then braked and aborted takeoff, and then tried to taxi off the runway but I was stuck (it was like I had a parking brake on.) I went downstairs to check something and came back and I could only turn right. I'm wondering what just happened?

For the thrust reduction on takeoff:

This is how the aircraft is supposed to operate.  Passing through approximately 80 knots, the thrust FMA changes from EPR to THR HOLD.  This is an automatic function which disengages the autothrottle servo motors from the physical throttle levers, so that the captain isn't fighting the servo motors during a high speed abort.  As you can imagine, if the AT were to stay at EPR, when the captain pulls the throttles to idle the throttles would keep trying to "push up" to TO or FLEX EPR against him.

This system works in the actual airplane because the autothrottle physically moves the throttle, so that when the AT mode transitions to THR HOLD the throttle is already in the appropriate position and there is no change to thrust unless the throttles are moved.  In the simulator, however, this obviously is not so -- the AT does not physically move the throttles on your desktop -- and any amount of throttle "noise" is interpreted by the sim as a throttle command.  I would imagine that you left your throttle at 50%, and when the AT system went to THR HOLD some noise or spike directed the sim to reset the throttles to where your throttles physically were -- i.e. 50%.

There are a couple of things you can do to fix this:

1)  Follow the throttles all the way though the power advance.  Keep pushing the throttles up to the thrust carats, so that there is no "arc" on the EPR gauge.  Then, if a spike commands a throttle lever reset while in THR HOLD, the throttles are already where they need to be.  Note, be sure to engage EPR before 80 knots, or EPR will not engage.

2)  Set the null up and the sensitivity down for your throttles to try to eliminate the noise or spike.  The throttles should not reset in THR HOLD unless the sim detects a command.
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Regarding the "stuck brakes", assuming you had the autobrake in the RTO position for takeoff, in accordance with procedures (which it appears, you did).  During a rejected takeoff, autobrake RTO will provide maximum effort braking once the system has detected the throttles at idle, and will continue to do so until the autobrakes have been deactivated.  You can disarm them by applying manual braking or by rotating the AUTOBRAKE rotary switch to DISARM or OFF. 

In the real aircraft, applying manual brakes will automatically disarm the autobrakes, the switch will spring load to DISARM and an EICAS AUTOBRAKE caution message will illuminate.  I have not seen this on the CS 757, so it is visually unclear whether the autobrakes are still holding at RTO or are deactivated.

To taxi clear of the runway, then, the best thing you can do is manually move the autobrake switch to OFF or DISARM after you have stopped.  This should eliminate any possibility of the autobrake system holding you stopped when you are trying to taxi.

Note, you can taxi normally with AUTOBRAKES selected to RTO.  The autobrakes don't kick in to RTO mode until the system has detected the throttles set to takeoff power (with the aircraft in a takeoff configuration) and then subsequntly moved to idle.

I have no idea why it would only let you turn to the right.  Was the autopilot on and trying to join a course or something?

Cheers,

Jason

jbirdsal wrote on Apr 2^nd, 2020 at 12:45pm:

This system works in the actual airplane because the autothrottle physically moves the throttle, so that when the AT mode transitions to THR HOLD the throttle is already in the appropriate position and there is no change to thrust unless the throttles are moved.  In the simulator, however, this obviously is not so -- the AT does not physically move the throttles on your desktop -- and any amount of throttle "noise" is interpreted by the sim as a throttle command.  I would imagine that you left your throttle at 50%, and when the AT system went to THR HOLD some noise or spike directed the sim to reset the throttles to where your throttles physically were -- i.e. 50%.

Interesting observation, and analysis, of the problem. When I take off, I often use F3 to increase the throttle, eventually increasing it up to 100% (there are times when I simply press the EPR button right at the start of the takeoff run). When doing this, my joystick throttle (a VKBSim Gladiator MKII) is in the stop position (because I have pulled it all the way back after taxiing on to the runway so the 757 stops). Using this method, I have never had the 757 pull up/fail to gain enough speed to take off (V2) - VNAV and LNAV are always armed. Maybe I'm just lucky that there are no spikes from the joystick to cause it to go into an aborted takeoff. Those spikes are known to cause problems with the throttles, so increasing the null zone can help fixing the problem.


jbirdsal wrote on Apr 2^nd, 2020 at 12:45pm:

I have no idea why it would only let you turn to the right.  Was the autopilot on and trying to join a course or something?

This happens, or at least used to happen, with the 757 v4.x, but I only had this problem upon landing, so I was using Approach mode on the A/P. I don't recall it ever happening with the 757 III. Could it be that the A/P has locked onto the ILS? However. If that was the case, I would have thought that you wouldn't be able to turn left, or right, as happens with the 757 v4.x.