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darth_damian_000

VNAV Trust Issues, or am I doing something wrong?

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This happens a lot, but I will use one particular flight as an example.

 

I am flying from KORD to KLAX on a 777-300ER. I am cruising at FL380. My AIRAC is 1605, and I am going to take the RIIVR2 approach to land on RWY25L. Just after the HECTOR VOR is my top of descent, and after that, a waypoint, GRAMM, had a restriction: FL210B / FL170A. GRAMM is 74nm away from the airport.

 

Once I get the "RESET MCP ALTITUDE" message, the plane, I set the MCP to FL170 and NOT press the selector. As I pass the T/D point, the plane does not descend (to be honest, the plane never automatically started to descend, I always had to push the selector). Mind you all, LNAV, VNAV, and A/P are all on. When it does descend, I am at FL170 at GRAMM flying at 280kts (is this normal?). On another occasion, I pressed the selector 2NM before T/D, and the plane failed to descend to FL210, instead, it was descending at a very slow rate. At GRAMM, I was at FL260. Any reattempts were performed after shutting down P3D, and reloading it at a save point about 40nm before the HECTOR VOR.

 

So I learned, and started to use V/S and speed management (with spoilers) to plan a safe descent, and it has worked very well for me.

 

I would appreciate if someone explained to me these two things:

 

1) Pertaining the the given scenario, I want to descend to FL180 by the time I reach GRAMM. My goal is to reach the speed of Vref(+5) + 80 at 4200 feet. So on this initial descent, how fast can I allow the plane to fly, so that I safely lose speed throughout my descent? I would like to stop relying on my speedbreaks. They are an important component and but by now I was hoping I can make it through a flight without a CAUTION warning.

 

2) Again pertaining to the last scenario, am I having issues with my autopilot? After all, it doesn't start to descend at T/D after the MCP has been reset, and furthermore, the speeds are ridiculous as I try to reach the GSIA (glide slope intercept altitude).

 

Thanks for your time and help!

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It is strange because at top of descent, if the MCP altitude is reset to a lower altitude, then the aircraft should start descending. If you get a caution with the speedbrakes, it means your speedbrakes are extended with a significant amount of thrust on (about 55%).

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1) Pertaining the the given scenario, I want to descend to FL180 by the time I reach GRAMM. My goal is to reach the speed of Vref(+5) + 80 at 4200 feet. So on this initial descent, how fast can I allow the plane to fly, so that I safely lose speed throughout my descent? I would like to stop relying on my speedbreaks. They are an important component and but by now I was hoping I can make it through a flight without a CAUTION warning.

 

There is no set speed. What's important is that you use the tools in front of you to address the situation properly.

 

 

 


2) Again pertaining to the last scenario, am I having issues with my autopilot? After all, it doesn't start to descend at T/D after the MCP has been reset, and furthermore, the speeds are ridiculous as I try to reach the GSIA (glide slope intercept altitude).

 

Sounds like a technique issue. Were you actually at your cruise altitude (i.e. did you step above your cruise altitude without updating the cruise altitude in the FMC)? Have you been setting your hardware throttles back to their lower stop prior to T/D? Have you flown the tutorial?

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Were you actually at your cruise altitude (i.e. did you step above your cruise altitude without updating the cruise altitude in the FMC)?

 

Ahha, you nailed it. It was flight that I saved and loaded. So when I started the flight (2 days ago), the cruise altitude was set to FL380 as planned, and it remained at 380 all throughout (at this point, I didn't pick an approach yet) on the FMC. I had to leave, so I saved the flight, and reloaded it when I was able to fly again. I just loaded that flight right now, and the FMC values were changed. For example, I was to be at FL301 at the BCE VOR (which is the 2nd waypoint behind T/D), whereas when I plugged in the Approach (BEFORE I saved the game), it was FL380. The FMC values changed after loading the game. I also saved the panel state under the same name as the flight, and the values remained the same (as in, FL301, and yes, this is all BEFORE the top of descent). Is this some load flight issue?

 

 

 


Have you been setting your hardware throttles back to their lower stop prior to T/D?

 

I don't use hardware throttles. I disabled the throttle axis and I just use autothrottle during flight and the F keys if I have to take off / land (reversers, idling)

 

 

 


Have you flown the tutorial?

 

I will fly it tonight, as a refresher perhaps.

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