10 minutes ago, G550flyer said:

Greetings,

Going back to the original question, I'll say this. Yes, it is normal to change VNAV speeds on approach and here are some reasons why.

First, lets understand this wonderful flight computer. It has two concerns. One, get you down to a point at a designated airspeed, altitude or both. Two, do this as efficient as it can. The system will look at the end point and work it's way back up to your altitude and calculate performance using it's embedded digital performance manual. With this info, it wants to keep you as high as it can, as long as it can and at an efficient speed. Unless specified, it will give speeds and altitude based on performance and descent angle, which is normally 2.5 degrees. In it's performance, it wants to get you to a point at  speed and altitude. Altitude takes the priority if both can not be done. This is why a plane can get fast if there isn't a hard speed. Typically, aggressive arrival and approach procedures can cause you be be faster than desired. Which is one of the reasons you might change the VNAV speed on approach. This happens on aggressive procedures because the system will recalculate performance as it transitions points. If the points are close together or the angle between the points are steep, the jet starts to get behind. This is most noticeable when there are intermediate level offs or the angle varies along the points. As you approach the point, the system enters a sort of cone of silence as it recalculates the VPATH. It will hold current trajectory as the calculation is made and the new VPATH indicator starts you come down to re-acquire again. Unfortunately, when the points are close and angles vary, the jet starts to trend high above path because it can't keep up and calculate fast enough causing the AP to lag. I have this same discussion with new guys while we are descending. The point is to understand how the system manages VNAV so that you can intervene to help it out.

Here are some examples of situations where you might adjust those speeds:

1. You are on an arrival and ATC gives you a speed restriction due to traffic ahead of you. You can either intervene and control the speed through the guidance panel or go heads down and PERF the speed or do it along the points.

2. The arrival is a little aggressive and you want to slow things down to help it out. You can intervene, PERF it or change the descent angle to a lower one making the plane start earlier at a lower angle. I tell guys if the arrival is aggressive, use a 2.0 angle. As stated in other posts, VNAV descents are disrupted a lot by ATC in some form or fashion. Some will abandon VNAV all together and use V/S, but you have to be on top of your restrictions as you should in any case. I cross check the VNAV calculation with mental math using the ol, 15 thousand to lose, I'm at mach .80 so about 8 miles a minute and I have 25 miles to get there in about 3 minutes....will I make it?😬. I also keep up with the computer and throw in a direct to altitude to a point to force it along when I know it will have a hard time. Instead of letting it calculate a new VPATH from my current altitude, the direct to altitude function tells the jet to descend right now at this moment.

3. The approach is a constant descent or a little aggressive. Most jets will calculate your path based on flap settings, mine does. It uses what we call auto speeds even when not using the FMS. It looks at airport distance, jet's altitude and configuration to command speeds and use these speed in the VNAV calcs. If I were to suddenly change flaps outside of it's anticipation, it has to recalculate the VPATH. As I mentioned before, it wants to be fast and efficient. Changing these speeds along the VNAV approach would solve that issue and the aggressive approach issue. When doing a FMS approach, to me it is good practice to be at the FAF on approach speed and fully configured. This is where I change the VNAV speed on approach or intervene the speed. I prefer altering the speeds in the FMS on the VNAV approach instead of manual/intervene. In the event I go missed, I hit TOGA, the missed is part of the approach and the speeds will automatically transition to missed approach speeds and holding with out anything being done. This way my PNF can focus on getting the gear/flaps when I call and monitor with out having to worry about my manual speed.

Perfect example, going into ASPEN in a G550. The jet is really clean and efficient. It's hard to slow down while descending. The approach into ASPEN is aggressive. If I arrive at the FAF gear down with the flaps at landing at VREF + 5, the jet will VNAV the FMS approach perfectly. In this case, I program the speeds inside of the FAF at REF+5. If I wait till the FAF to slow and configure, it is hit or miss if I will get slow enough and configured to land. That's why I prefer to get to the FAF fully configured and at approach speed when doing FMS/GPS approaches. I picked that up when I was flying heavies.

In the end, it depends on the type of jet and the system's capability

Rick           

My small brain isn't capable of processing all that but I think you said that the VNAV speeds are not fixed and will vary in flight unless overridden? The last waypoints don't vary at all, they stay at 170kts until in on the G/S and the aircraft gives me control over the IAS knob

As for everything else you said I think I should learn how to do smoother approaches and descents, overall it goes fine but occasionally I need to apply some drag, things only get messy during the final approach.

3 minutes ago, Gabriel Victor said:

My small brain isn't capable of processing all that but I think you said that the VNAV speeds are not fixed and will vary in flight unless overridden? The last waypoints don't vary at all, they stay at 170kts until in on the G/S and the aircraft gives me control over the IAS knob

Yep, You can go in and change that speed to your approach speed. Do it when you load the approach.

13 minutes ago, Gabriel Victor said:

Yeah, maybe some skilled guys can decelerate the aircraft that fast but I'd just make a mess.

I did fly again on a route that went fine yesterday and I again was too fast. 1900 feet AGL at 170kts flaps 15, still had to reduce to 138 and flaps 30. Got the no AUTOLAND message but after I was stable I reset the a/p and it engaged autoland.

Try this, I picked it up in heavies and it works well. The DC10 was difficult to slow down once you were descending. It was easy to end up with flaps past 15 preventing you from using the speed break and too fast to get to landing flaps while descending. Passing 10,000ft, we throw out the slats. Once we were being vectored for the approach, we slow to 200. When on a intercept course to the localizer, we slow to 180. As the GS starts to move, we drop the gear and start dialing the speed back, no lower than minimum maneuver. When the G/S hits the one dot above mark, we check below 175 and drop the flaps to landing. By the time you were capturing the G/S, you're on speed and glide path. This allows you to get slow in a methodical fashion and have a standard way to slow for the approach. In the Gulfstream we use the 1, 3, 5, 7 technique. 7 miles flaps 10, 5 miles flaps 20, 3 miles drop the gear and 1 mile flaps 39. Again, it's a standard way to ensure you are configured and able to slow for the approach and stay ahead of the jet. Try the heavy technique and see how it works. I use it in all of my sim jets. 

Like others have said use speed intervention when you need to.  Another piece of advice is that flaps 15 usually isn't used as a flap setting during approach/landing in a 777*.  On an ILS you should plan for flaps 5 before localizer intercept, then gear down and flaps 20 as soon as the glideslope indication starts to move, then landing flaps when the glideslope is intercepted. There's only a 5 knot limitation difference between flaps 15 and 20 and the extra drag should help slow you down quicker.  

 

* of course with everything in aviation there are some situations where you might, an early speed reduction for example.