Mode reversion occurs slightly before reaching the limit speed. Both the AFDS and A/T have reversion modes which activate according to the condition causing the reversion.
Placard Limit Reversion
When one of the placard limit reversions (gear, flap or Vmo/Mmo) is reached, the overspeed limiting symbol appears in the MCP IAS/Mach display and the following occurs:
.  
If not in AFDS or A/T speed control and the A/T is armed, the A/T reverts
to SPEED and controls speed to the placard limit.

.  
If in AFDS or A/T speed control, no reversion is necessary. The AFDS or
A/T, whichever is controlling speed, holds speed slightly below the
placard limit.

.  
If the A/T is not available, no reversion response to gear or flap placard
speeds is available. The AFDS reverts to speed control for Vmo/Mmo
speed limiting.

Minimum Speed Reversion
The AFDS and A/T do not control to a speed which is less than minimum speed for the current flap configuration. This speed is approximately 1.3 Vs. Minimum speed, FMC speed, or selected speed, whichever is higher, becomes the AFS commanded speed. If actual speed becomes equal to or slightly less than the minimum speed, the underspeed limiting symbol appears in the MCP IAS/Mach display, and if operating in the V/S mode, the AFDS reverts to LVL CHG. The AFDS will also revert to LVL CHG from VNAV PTH, except when capturing or flying a level segment.
The AFDS commands a speed 5 knots greater than minimum speed. Selecting a speed 15 knots greater than minimum speed reactivates normal MCP speed selection control. The AFDS commands nose down pitch to increase airspeed if the thrust levers are not advanced. When actual speed becomes 15 knots greater than minimum speed, the underspeed limiting symbol disappears.
The A/P disengages, and the F/D command bars retract when in a LVL CHG climb with a command speed equal to minimum speed and a minimum rate of climb cannot be maintained without decelerating.
Copyright . The Boeing Company. See title page for details.
4.20.26   December 7, 2001

Automatic Flight -System Description

737 Operations Manual
Minimum speed reversion is not available when the A/T is OFF and the AFDS is in ALT HOLD, ALT ACQ, or after G/S capture. Minimum speed reversion is also not available when in VNAV PTH and capturing or flying a level segment.
Performance Limit Reversion
This function, present on airplanes without –4 FCC installed, occurs only while
operating in the V/S mode when the selected vertical speed and airspeed
combination exceeds the thrust capability to maintain the selected airspeed. If
airspeed becomes more than 5 knots below the MCP selected airspeed and is not
increasing, the AFS reverts to LVL CHG, and the underspeed limiting symbol
appears in the MCP IAS/MACH Display.

The underspeed limiting symbol remains displayed until a different speed or a
different pitch mode is selected.
Performance limit reversion can occur with the A/T OFF and the autopilot ON.

Copyright . The Boeing Company. See title page for details.
December 7, 2001  4.20.27
Automatic Flight -System Description 737 Operations Manual

Intentionally
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Copyright . The Boeing Company. See title page for details.
4.20.28  December 1, 2000

Controls and Indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5.10.1
VHF Communication Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5.10.1 HF Communication Panel  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5.10.4 Audio Selector Panel (ASP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5.10.7 Miscellaneous Communication Controls (Typical) . . . . . . . . . .  5.10.11 Interphone and Passenger Address Controls . . . . . . . . . . . . . . . 5.10.13 Cockpit Voice Recorder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10.14 Call System  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10.18 Selective Calling Panel (SELCAL)  . . . . . . . . . . . . . . . . . . . . . . 5.10.19
System Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5.20.1
Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5.20.1 Audio Systems and Audio Selector Panels  . . . . . . . . . . . . . . . . .  5.20.1 Speakers and Headsets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5.20.1 Microphones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5.20.2 Normal Audio System Operation . . . . . . . . . . . . . . . . . . . . . . . . .  5.20.2 Degraded Audio System Operation . . . . . . . . . . . . . . . . . . . . . . .  5.20.2 Airplanes with DigitalAudioSelector Panels . . . . . . . . . . . . . . . . . . . . . .5.20.2 Flight Interphone System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5.20.3 Service (Attendant) Interphone System . . . . . . . . . . . . . . . . . . . .  5.20.3 Passenger Address System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.20.3 Call System  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5.20.4 Selective Calling (SELCAL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.20.5 VHF Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5.20.5 HF Communications  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5.20.5 Cockpit Voice Recorder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5.20.6 ACARS System (as installed) . . . . . . . . . . . . . . . . . . . . . . . . . . .  5.20.6
　
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