Flight Director Systems — Page 352, Lesson 407

I want to walk you through the go-around mode of the Flight Director on the 737-400, and then the concept of manoeuvre protection. This is a dense operational section, so let's take it step by step. First, the engagement criteria. For the 737-400, two conditions must be met before the Flight Director can engage in the GA — that's go-around — mode. The FD switches can be either on or off, and both of these must be true: you must be in-flight below 2000 feet radio altitude, and not in the TO — takeoff — mode. And you must press the TO/GA switch. TO/GA stands for takeoff/go-around. Once you engage, here's what happens. Command bars appear for both pilots. TO/GA is annunciated — that means displayed — for the FD pitch mode. The MCP IAS/MACH display blanks — MCP is the Mode Control Panel, IAS is indicated airspeed. And the Airspeed Cursors display the manoeuvring speed for the existing flap setting. Now, exiting the mode. Below 400 feet radio altitude, both FD switches must be turned from ON to OFF to exit the FD GA mode. Above 400 feet radio altitude, you can select other pitch and roll modes. Here's the sequencing logic: if you change the roll mode first, the FD pitch mode remains in the GA mode. If you change the pitch mode first, the FD roll mode automatically changes to HDG SEL — that's heading select. There's also a relationship with the autopilot. Engaging an autopilot following a FD go-around automatically engages both the autopilot and the FD in LVL CHG and HDG SEL — that's level change and heading select — for pitch and roll respectively. Now let's look at the actual commands. For a two-engine go-around, the FD commands 15 degrees nose-up pitch, and roll to hold the approach ground track at the time of engagement. After reaching a programmed rate of climb, pitch commands hold the manoeuvring speed for each flap setting. For a single-engine go-around, the FD pitch command is initially 13 degrees nose-up. But as climb rate increases, FD pitch commands maintain a target speed. Roll commands are the same as for the two-engine case. Now here's where the target speed logic gets detailed, and I want you to follow carefully. If engine failure occurs prior to GA engagement, then the MCP selected speed becomes the target speed. If the engine failure occurs after GA engagement, then the FD target speed depends on whether 10 seconds have elapsed since GA engagement. If it's prior to 10 seconds, the MCP selected approach speed becomes the target speed. If it's after 10 seconds, we look at the airspeed at engine failure relative to the GA engagement speed. If the airspeed at engine failure is within 5 knots of the GA engagement speed, the airspeed that existed at the GA engagement becomes the target speed. If the airspeed at engine failure is more than 5 knots above GA engagement speed, then the current airspeed becomes the target airspeed. And there's a floor on all of this. In all cases, the GA target speed is not less than V2 speed based on flap position — V2 being the takeoff safety speed — unless you're in windshear conditions. The FD target speed is displayed on the MCP and by the airspeed cursors. And importantly, no commanded acceleration can occur until a higher speed is selected on the MCP. Finally, let's touch on FD Manoeuvre Protection. The modern FDC — that's the Flight Director Computer — is configured for each particular aircraft type, so it has the aircraft performance parameters stored in its memory. Because it has inputs from the ADC — the Air Data Computer — and other systems, it can ensure that it never commands a manoeuvre which will overstress the aircraft. This is the beginning of the systems used for protection in aircraft with fly-by-wire controls, which is discussed fully in another chapter. So the key thread here: the go-around mode has strict engagement gates, a defined exit logic, and a target-speed algorithm that changes based on engine failure timing and airspeed. And the manoeuvre protection is the computer's built-in limit to keep you from overstressing the airframe.