777/787 VNAV approach
 

Any idea why Boeing require using VNAV with speed intervention active (ie the speed window open) during the final approach. And

2) why they don't recommend adding extra speed constraints to the final approach waypoints.

I recall in the 737 we simply leave the speed window closed in VNAV approach and then the A/T would maintain the Vref + wind correction (set in the approach page) when we select landing flaps. Wouldn’t the similar behavior occur in the 777/787?

Unlike the 737 NG the FMC will happily
command a speed below the current flap speed, requiring you to open the speed window. The FMC on the 787 is more like the old Pegasus FMC on the 767 - in many ways a step backwards from the FMC on the 737 NG.

It may well be running that FMC as a virtual machine, which would explain a lot (not least as to why it assumes that it has the aerodynamics of an AGA when it comes to descent planning).

I also believe that it should fly to either the UP speed or the green flap speed shown on the PFD when in the intermediate stages of the approach with the speed window closed. It’s an easy thing to miss given that it will close itself when you activate VNAV.

I think it’s the difference between the Smiths and Honeywell FMCs. They also allowed you to enter a destination QNH, if I remember correctly, that alleviated the step changes in path when going from QNE to QNH. Giving the FMC the ability to drive the commanded speed well below flap speeds has always seemed a questionable decision to me...

(on the 767) It means you can MEL the speed window and still use auto throttle. It has has to have speeds in the box to build the path backwards, so it assumes you're going to put some flaps out at some point.

But… but… you would just have it command the lower speed when you take the next stage of flaps. So if you’re at UP, and select 1, the target speed is 1. Then you select flaps 5, which becomes the next target speed. You don’t need to slow below the flap speed until you get to 25 or 30, and then it’ll just use FMC speed + wind correction.



Yes, that’s what I liked about the FMC on the earlier 737. You could, if you wanted, program it to fly the entire approach with the window blank and the speed would adjust to sync with what flap you put out. Lower workload and protection from going slower than the flap speed, compared with the extra button pressing and risk exposure from the other way of doing it. Seemed a bit of a no-brainer at the time?

Now I'm curious about where the FMC commands the speed to go. For example, if I select flaps 1 and the speed window is closed, what will be the target speed if it's not the flap 1 speed?

The speed in the FMC at the next waypoint surely?

Having spent upwards of ten years on 737's with FMC's, and now having spent a bit of time on the 777, I'm honestly baffled at some of the decisions Boeing made on the 777 FMC - to me, and I say this knowing it could just be my lack of knowledge, but a lot of it seems like a significant step backwards? Why do I (for example) have to input a speed AND altitude constraint at a point if all I wanted was a speed constraint? For example - in the 737, at Waypoint X, I could input 250/ if I wanted an FMC commanded speed in VNAV of 250 KIAS. The FMC would then show the predicted altitude at that point (let's say I'm in a descent) in small numerals. In the 777 I am forced to enter an at, 'A' or below, 'B' hard limit, which sometimes involves a bit of guesswork/predictions that the computer is supposed to be making for me (and is, in the background!) I also can no longer see the predicated altitude at that point assuming I remained in VNAV PATH on the profile.

If memory serves me right, I could also enter a Mach number in the 737 FMC as '0.78' or '.78' (the zero could be omitted, or not) whereas in the 777 the zero MUST be omitted (but NOT in our pre formatted ACARS messages, where the zero MUST be entered because we hate standardising), maybe my memory is playing tricks on me again though.

There are a few other points that baffle me as to why they are the way they are, but.....well, it just is what it is.