Control Laws — Page 420, Lesson 515

We're now into the heart of the speed protection logic on this aircraft. I want to walk you through what happens when the airplane is being pushed toward its speed limits, and how the autopilot and autothrottle step in to save the day. First, the core principle: either pitch or thrust, whichever is engaged in a speed mode, attempts to hold the limit speed. So if you're in a speed mode, one of those two systems is actively working to keep you at that limit. The commanded limit speed and the MCP speed condition symbol remain displayed until another speed is selected which does not exceed the limit. And here's a specific number to remember: a speed 15 knots greater than the minimum speed must be selected to remove the under-speed symbol. That's the exact threshold for clearing that warning. Now, let's talk about Reversion Modes. During some flight situations, speed control by the AFDS or A/T alone could be insufficient to prevent exceeding a limit speed. AFDS is the Autopilot Flight Director System, and A/T is the Autothrottle. If this occurs, the AFDS and A/T modes automatically revert to a more effective combination. There are two reversion modes: Placard Limit reversion and Minimum airspeed reversion. Mode reversion occurs slightly before reaching the limit speed. Both the AFDS and the A/T have reversion modes which activate according to the condition causing the reversion. Let's break down Placard Limit Reversion. When one of the placard limits is reached—that's gear, flap, or VMO/MMO, where VMO is maximum operating speed and MMO is maximum operating Mach number—the over-speed limiting symbol appears in the MCP IAS/Mach display. MCP is the Mode Control Panel, IAS is Indicated Airspeed. Then three things can happen. First, if you're not in AFDS or A/T speed control and the A/T is armed, the A/T reverts to SPEED mode and controls speed to the placard limit. Second, if you're already in AFDS or A/T speed control, no reversion is necessary—whichever is controlling speed holds it slightly below the placard limit. Third, if the A/T is not available, there's no reversion response to gear or flap placard speeds. In that case, the AFDS reverts to speed control for VMO/MMO speed limiting. Now for Minimum Speed Reversion. The AFDS and A/T do not control speed to a value less than the minimum speed for the current flap configuration. That minimum speed is approximately 1.3 VS, where VS is the stall speed. Minimum speed, FMC speed, or selected speed—whichever is higher—becomes the AFS commanded speed. AFS is the Autopilot Flight System. If actual speed becomes equal to or slightly less than the minimum speed, the under-speed limiting symbol appears in the MCP IAS/Mach display, and if you're operating in V/S mode—that's vertical speed mode—the AFDS reverts to LVL CHG, which is level change mode. The AFS commands a speed 5 knots greater than the minimum speed. Selecting a speed 15 knots greater than the 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 under-speed limiting symbol disappears. There's also a critical failure condition here. The A/P—that's autopilot—disengages and the FD command bars retract when in LVL CHG climb with a command speed equal to minimum speed and a minimum rate of climb cannot be maintained without decelerating. So if you can't climb fast enough without slowing down, the autopilot gives up and the flight director bars disappear. Finally, a limitation: no minimum speed reversion is available when the A/T is OFF and the AFDS is in ALT HOLD, ALT ACQ, or after G/S capture. ALT HOLD is altitude hold, ALT ACQ is altitude acquire, and G/S is glideslope. In those modes, you're on your own for speed protection. Let me show you the schematic for the series yaw damper, which relates to how these control laws interact with the flight control system. That's the full picture of the reversion logic. The key numbers to hold onto: 1.3 VS for minimum speed, 5 knots above minimum for the commanded speed, and 15 knots above minimum to clear the under-speed symbol.