Flight Director Systems — Page 352, Lesson 407

We're starting a new topic now: the Flight Director, or FD, go-around mode, specifically as it's implemented on the 737-400. This is a dense, procedure-heavy part of the system, so let's take it step by step. First, the fundamental question: what has to happen before the Flight Director will even engage in the go-around, or GA, mode? For the 737-400, two criteria must be met. The FD switches themselves can be either on or off — that's not a condition. The two hard requirements are: first, the aircraft must be in-flight below 2000 feet radio altimeter, and it must not be in the takeoff, or TO, mode. Second, the TO/GA switch must be pressed. So, in-flight, below 2000 feet RA, not in TO mode, and you press TO/GA. That's the gate. Once it engages, here's what you see. Command bars appear for both pilots. The annunciator shows TO/GA as the FD pitch mode. The MCP IAS/MACH display blanks — that's the Mode Control Panel airspeed window going blank. And the Airspeed Cursors display the manoeuvring speed for the existing flap setting. So the system is telling you, "here's the speed to fly for this flap configuration." Now, how do you get out of this mode? It depends on your altitude. Below 400 feet radio altimeter, both FD switches must be turned from ON to OFF to exit the FD GA mode. That's the only way out down low. Above 400 feet RA, you have more freedom: other pitch and roll modes can be selected. But there's a sequencing logic to be aware of. If you change the roll mode first, the FD pitch mode remains in the GA mode. If you change the pitch mode first, then the FD roll mode automatically changes to HDG SEL — heading select. So the order of your selections matters. There's also a specific interaction with the autopilot. Engaging an autopilot following an FD go-around automatically engages both the autopilot and the FD in LVL CHG and HDG SEL — that's level change for pitch and heading select for roll, respectively. So the system sets up a clean climb-out configuration for you. Now let's talk about the actual pitch commands, because they differ between a two-engine and a single-engine go-around. For a two-engine GA, the FD commands 15 degrees nose-up pitch, and roll commands hold the approach ground track at the time of engagement. So you're climbing at 15 degrees nose-up while tracking the runway heading. Then, after reaching a programmed rate of climb, the pitch commands switch to holding the manoeuvring speed for each flap setting. So it transitions from a fixed pitch attitude to a speed-hold regime. The single-engine case is more complex. Initially, the FD pitch command is 13 degrees nose-up — slightly less than the two-engine case. But as climb rate increases, the FD pitch commands maintain a target speed. The roll commands are the same as the two-engine case — hold the approach ground track. Here's where the timing logic comes in, and it's critical. The target speed depends on when the engine failure occurs relative to GA engagement. If the engine failure happens prior to GA engagement, then the MCP selected speed becomes the target speed. If the engine failure occurs after GA engagement, then it 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 compared to the GA engagement speed. If that airspeed is within 5 knots of the GA engagement speed, then the airspeed that existed at the GA engagement becomes the target speed. If it's more than 5 knots above the 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 — unless you're in windshear conditions. So V2 is the safety net. The FD target speed is displayed on the MCP and by the airspeed cursors. And one more operational note: no commanded acceleration can occur until a higher speed is selected on the MCP. So the system won't accelerate you on its own; you have to ask for it. Finally, let's touch on FD Manoeuvre Protection. This is a broader concept. The modern Flight Director Computer, or FDC, is configured for each particular aircraft type, so it has the aircraft's 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 protection systems used in fly-by-wire aircraft, which is covered fully in another chapter. But the key idea here is that the FD isn't just giving you guidance; it's actively preventing you from exceeding structural limits. Let me show you the annunciation and display layout so you can picture what the pilots see. That's the full go-around logic. The key takeaways to hold onto: the two engagement criteria, the 400-foot exit altitude, the order-of-selection logic, the 15 versus 13 degree pitch difference, the 10-second and 5-knot timing rules, and the V2 floor.