When quitting Slew mode and your aircraft zips away at mach speed it is usually the result of one of two things. Either the aircraft contact points (which may include the static pitch) is slightly off and the sim detects a "crash", thus sending the aircraft into mach flight...or...the sim detects "jitters" from your joystick/yoke.
Anymore, with much better configured aircraft, it is usually the tell-tale leak of electrical impulses from our joystick/yoke....."jitters".
*One tip that I recommend in all slewing scenarios, is after settling the aircraft to ground level with the "A" key (or F1), tap the numlock #5 key, to ensure that the "Freeze All Movement" command has been received by the sim before exiting slew with the Y key. (as presented above, if the contact points are whacked or your joystick is emitting jitters, then you might still be jettisoned into flight, regardless)
There are fixes we can do to correct the jitters:
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(my preference)
Turning off your joystick/yoke while in Slew Mode........ permanent
FS9.......open/active
1. Options/Controls/Assignments
2. Select "Joystick Axis" tab
3. Select radio button: Slew mode
4. Select Joystick type: your joystick or yoke from drop list
5. Delete all the joystick assignments (be sure you are on the Slew mode page).
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Turning off your joystick/yoke while in Slew Mode........ per session
FS9.......open/active
1. Options/Controls/Disable Joystick
2. Slew using numpad and keyboard commands
3. After slewing is complete, reinstate joystick per line #1.
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SLEW COMMANDS
Slewing allows you to rapidly reposition your aircraft without flying in real time. Use these commands to slew.
Num Lock -- off
Slew Mode On/Off...........Y
Freeze All Movement...................Num Pad 5
Move Forward............................Num Pad 8
Move Backward..........................Num Pad 2
Move Left.................................Num Pad 4
Move Right................................Num Pad 6
Rotate Left................................Num Pad 1
Rotate Right..............................Num Pad 3
Bank Left..................................Num Pad 7
Bank Right.................................Num Pad 9
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Set Heading True North/Attitude Straight-and-Level..... SPACEBAR
Move Up Slowly.................Q (or F3).......tap the Q key for increments/hold for increase
Move Down Slowly.............A....................tap the A key for increments/hold for increase
Tilt Nose Up......................9
Tilt Nose Down..................0 (or F7)
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Move Up Quickly......................F4
Move Down Quickly..................F1
Freeze Vertical Movement.........F2
Move Nose Up Quickly..............F5
Move Nose Down Quickly..........F8
Freeze Pitch...........................F6
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Hope this may be helpful