Control Laws — Page 420, Lesson 515

We're starting a new topic now: the control laws of the autopilot and autothrottle system, specifically the reversion modes. Let's get into it. We've been talking about how the autopilot and autothrottle work together to control speed. Now, I want to walk you through what happens when things get critical—when the aircraft is approaching a speed limit it must not exceed. This is where the reversion modes come in. First, let's set the scene. The system has a concept of a "limit speed." This is a speed the aircraft must not exceed, like the maximum operating speed, or a speed it must not go below, like the minimum speed for the current flap setting. The key idea here is that either pitch or thrust—whichever one is engaged in a speed mode—attempts to hold that limit speed. So, if the autopilot is controlling pitch to hold a speed, it will try to keep the aircraft at that limit. If the autothrottle is controlling thrust for speed, it will do the same. Now, there's a visual cue for the pilot. The commanded limit speed and the MCP speed condition symbol remain displayed until another speed is selected that does not exceed the limit. So, if you're at the limit, that symbol stays. To remove the under-speed symbol—the one that warns you're near the minimum speed—you must select a speed that is 15 knots greater than the minimum speed. That's a specific number: 15 knots. Now, let's talk about the reversion modes themselves. The system is designed to be smart. During some flight situations, speed control by the AFDS—that's the Autopilot Flight Director System—or the A/T—the autothrottle—alone could be insufficient to prevent exceeding a limit speed. If this happens, 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. The mode reversion occurs slightly before reaching the limit speed. Both the AFDS and the A/T have their own reversion modes, and they activate according to the condition causing the reversion. Let's break down the Placard Limit Reversion first. A "placard limit" is a speed limit that's published on a placard in the cockpit—these are the gear limit, the flap limit, and VMO/MMO, which is the maximum operating speed and maximum Mach number. When one of these placard limits is reached, the over-speed limiting symbol appears in the MCP IAS/Mach display. The MCP is the Mode Control Panel, and IAS is Indicated Airspeed. Now, what happens next depends on the state of the autopilot and autothrottle. There are three scenarios. First, if you are not in AFDS or A/T speed control, but the A/T is armed, the A/T reverts to SPEED mode and controls speed to the placard limit. So, the autothrottle takes over and holds that limit. Second, if you are already in AFDS or A/T speed control, no reversion is necessary. The AFDS or A/T, whichever is controlling speed, holds the speed slightly below the placard limit. It doesn't touch the limit; it stays just under it. Third, if the A/T is not available, there is no reversion response to gear or flap placard speeds. In that case, the AFDS reverts to speed control for VMO/MMO speed limiting only. So, without the autothrottle, the autopilot can only protect against the maximum speed limits, not the gear or flap limits. Now, let's move to the Minimum Speed Reversion. This is about the lower limit. The AFDS and A/T do not control speed to a speed which is less than the minimum speed for the current flap configuration. This minimum speed is approximately 1.3 times VS, where VS is the stall speed. So, the minimum speed is about 1.3 times the stall speed. The commanded speed becomes the highest of three values: the minimum speed, the FMC speed—that's the Flight Management Computer speed—or the selected speed. Whichever is higher becomes the AFS commanded speed. AFS is the Autopilot Flight System. If the 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 here's a specific behavior: if you are operating in the V/S mode—that's the vertical speed mode—the AFDS reverts to LVL CHG, which is Level Change mode. So, it switches from holding a vertical speed to a level change to manage the speed. Now, the system commands a speed that is 5 knots greater than the minimum speed. So, it doesn't just hold the minimum; it commands 5 knots above it. To reactivate normal MCP speed selection control, you need to select a speed that is 15 knots greater than the minimum speed. That's the same 15-knot figure we saw earlier. The AFDS commands nose-down pitch to increase airspeed if the thrust levers are not advanced. So, if the throttles aren't adding power, the autopilot pitches the nose down to gain speed. When the actual speed becomes 15 knots greater than the minimum speed, the under-speed limiting symbol disappears. There's also a critical condition to be aware of. The autopilot disengages and the FD command bars retract—FD is the Flight Director—when you are in LVL CHG climb with a command speed equal to the minimum speed, and a minimum rate of climb cannot be maintained without decelerating. So, if the aircraft can't climb at the minimum rate without slowing down below the minimum speed, the autopilot gives up and disengages. Finally, there's a limitation on when minimum speed reversion is available. No minimum speed reversion is available when the A/T is OFF and the AFDS is in ALT HOLD—that's altitude hold—or ALT ACQ—that's altitude acquire—or after G/S capture, which is glideslope capture. So, in those specific modes, without the autothrottle, the system won't protect against the minimum speed. Let me tie this together. The reversion modes are the system's automatic safety net. When speed control by one channel alone isn't enough, it switches to a more effective combination. The placard limit reversion protects against exceeding the maximum speeds, and the minimum speed reversion protects against going too slow. Each has specific triggers, specific symbols, and specific responses depending on what's armed and what's available.