On many flights, the descent via VNAV PTH from TOD starts right on the mark and follows the FMC flight plan with no issues.

 

However, on some flights, the descent begins with the 777 well above the path and, without an altitude intervention, will stay well above the planned path for the entire descent.

 

I have read the introduction, tutorial, and looked in the detailed manuals but have not found anything that I can identify as obviously wrong.

 

Is this something I'm doing wrong in my flight planning for the flights where I have the problem, or just part of flying the 777?

 

Thanks,

 

Stephen Bickford

Be sure to check the FMA to see what the pitch mode is in.  For example, the normal pitch mode of vnav path will change to vnav speed if the aircraft needs to use pitch to control speed instead of staying on the vertical path.  This usually calls for the generous use of speed brake.

 

Always keep your eye on speed, pitch and power with in automated mode. It cannot replace  a pilot.

 

As a related side note, I usually change my descent speed to either 310/.84 or 280/.84 depending on speed constrains on the star.  If the star has a 280 constraint then I will descend at mach until 280 and hold 280, removing the need for the aircraft to slow down in descent.

Hi Dan,

 

Thanks for your suggestions on managing 777 descents and what to look for and do when a descent is going off the planned path.

 

However, my question is whether I can predict/avoid descents that are immediately too high right from the beginning of the descent sequence at TOD. For example, does changing the descent speed (I assume on the  VNAV Descent page before TOD) help the descent begin and stay right on track?

 

Thanks,

 

Steve Bickford

However, my question is whether I can predict/avoid descents that are immediately too high right from the beginning of the descent sequence at TOD. For example, does changing the descent speed (I assume on the VNAV Descent page before TOD) help the descent begin and stay right on track?

 

Make sure you get a wind forecast loaded for the descent.  During descent, keep an eye on IAS, I like to stop an increasing speed when it is greater than 5 kt above target by nudging just enough spoiler to freeze the speed.... too much and speed decreases. It is normal if speed is below target, this happens as often as it being higher, the FMC will command power when the speed drops about 10 or 15 kts below target (I'd have to look up the exact amount in the FCOM).  Maintaining target speed is a guarantee to keeping her on the path.

 

I don't understand why you want to avoid a situation where you have to intervene with the automatics.... it's normal. Stay in the loop when flying.

I would start to get worried if the plane got 600-700 above profile and continued to increase. It might be time for V/S and the speedbrakes to get back to 300 above profile. At 300 feet above profile, I would engage VNAV again. Waiting until you are on profile and engaging VNAV is going to cause you to go below profile. The T7 has a lot of momentum. On one flight I got distracted messing with the radios. I looked up and I was around 4000 feet above profile. Fortunately this happened at a about 23000. A lot of speedbrakes and V/S of 3500 FPM got me back on profile at the correct speed. Sometimes, you are going to be operating outside the envelope.

Hi Dan and Michael,

 

Based on your helpful responses, it appears that my expectation that every descent would begin right on path was not realistic and that "normal" would include  descents already in need of some intervention. 

 

Thanks for convincing me that I haven't been doing something wrong and encouraging me to and helping me develop my piloting skills

 

Stephen Bickford