Is it normal to change VNAV speeds on approach?

[Gabriel Victor 1]

Hello everyone!

While I was on approach with the PMDG777 I noticed that on the last waypoints before the runway the speed was too high, at about 210kts 10-20nm away from the airport. So I changed them to 180kts to give me time to decelerate and deploy flaps. On one of the final waypoints it was automatically reduced to 170 but that was already too close to the runway. VNAV only gives me manual control of IAS knob when I'm on app mode and on this particular runway I intercepted the glideslope 10nm away from the runway at 170kts with flaps 15. It immediately gave me a NO AUTOLAND message on the eicas as I tried to reduce the speed to 139kts and deploy flaps 30. I switched off the autopilot and suddenly i lost altitude and had to pitch up 20° to maintain altitude, got several "glideslope" warnings but eventually landed.

I did something wrong here and from the manuals I imagine it's because I was too fast despite following the FMC. So I'm repeating this flight right now and I intend to edit the waypoint speeds to about 160 on the final waypoints, but I don't know if this is normal, needed or correct at all.

If anyone can tell me what I'm doing wrong that'd be greatly appreciated.

 

Landing on KDEN RWY 34L

[JasonPC 173]

Real world procedures rarely use VNAV on descent and definitely not all the way to final approach. So changing speed manually is completely normal. V/S control with manual speed input is often used.

[marc1184 42]

Manual intervention using the MCP is standard when transitioning to approach.  VREF+5 is the speed you need to plan to be at on final. So if VREF is 145, plan for 150 on final.  Then I add 20kts to every 5 miles beyond that point.  So, at about 20mi out, I typically set speed to 210.  Then I reduce 20kts every 5mi while deploying flaps in increments until I'm at my VREF+5 speed, usually at about 5mi from the runway.

[thibodba57 462]

13 minutes ago, JasonPC said:

Real world procedures rarely use VNAV on descent and definitely not all the way to final approach. So changing speed manually is completely normal. V/S control with manual speed input is often used.

I would disagree.  I use VNAV if I’m reasonably close to my lateral path for VNAV to be effective and if I’m not I use FLCH I almost never use V/S. 
 

@Gabriel VictorYou don’t have to program a speed at any waypoint you get VNAV to slow down early.  Open the Speed window (Speed Intervene) by depressing the Speed knob. Then select your desired speed. You can also enter a desired speed on VNAV Des page (VNAV page 3). 

[JasonPC 173]

FLCH is used a lot as well but in real word videos I see V/S mode used a lot. VNAV can be used in some cases but sometimes depending on other traffic the controllers may not exactly stick to the STAR procedure. The problem is if you are higher than the calculated flight path altitudes the Boeing a/p will slow the plane down. For keeping the flow of traffic the controllers usually like to keep you at a higher speed.

[downscc 1,327]

2 hours ago, Gabriel Victor said:

Hello everyone!

While I was on approach with the PMDG777 I noticed that on the last waypoints before the runway the speed was too high, at about 210kts 10-20nm away from the airport. So I changed them to 180kts to give me time to decelerate and deploy flaps. On one of the final waypoints it was automatically reduced to 170 but that was already too close to the runway. VNAV only gives me manual control of IAS knob when I'm on app mode and on this particular runway I intercepted the glideslope 10nm away from the runway at 170kts with flaps 15. It immediately gave me a NO AUTOLAND message on the eicas as I tried to reduce the speed to 139kts and deploy flaps 30. I switched off the autopilot and suddenly i lost altitude and had to pitch up 20° to maintain altitude, got several "glideslope" warnings but eventually landed.

I did something wrong here and from the manuals I imagine it's because I was too fast despite following the FMC. So I'm repeating this flight right now and I intend to edit the waypoint speeds to about 160 on the final waypoints, but I don't know if this is normal, needed or correct at all.

If anyone can tell me what I'm doing wrong that'd be greatly appreciated.

 

Landing on KDEN RWY 34L

The B737 FMS will automatically reduce the speed for the approach such that the final segment is your Vref+5.  The 777 does not do this but just slips a 170 in there.  It is important to set those speeds on the LEGS page when you set up the approach if you intend to use VNAV such as on a RNAV(GPS) approach where LNAV/VNAV is used down to mins.

Don't rely on the computer to fly the  airplane.  Some will advocate FLCHG or V/S but the important thing is to be in command.

210 kt 10-20 nm out is not too high, in fact one might keep it at 240 up to the final heading to intercept final, and then a good target might be 190 at 9 nm.  On GS intercept drop the gear and start dropping flap out on schedule such that you are stabilized on speed and configuration not later than 1000 ft AGL (500 ft if it is CAVU and you are visual).

You will get a no autoland if you are not in landing configuration and on speed about 1300 ft AGL.

 

[thibodba57 462]

1 hour ago, JasonPC said:

FLCH is used a lot as well but in real word videos I see V/S mode used a lot. VNAV can be used in some cases but sometimes depending on other traffic the controllers may not exactly stick to the STAR procedure. The problem is if you are higher than the calculated flight path altitudes the Boeing a/p will slow the plane down. For keeping the flow of traffic the controllers usually like to keep you at a higher speed.

Just so we are on the same page.  I fly the 74 for a major cargo company and am well versed in when VNAV will perform adequately and provide its protections and when you need to move to a different mode.  I can also tell you 99% or the pilots at my company don’t use V/S. While it’s a functional tool, it can get you sideways more easily than other modes.  That being said I used to fly an MD-80 that didn’t have VNAV so all we had was V/S or IAS/MACH modes of climb and descent.  I wish people would just not reference videos they’ve seen on YouTube as a reference as to what the ‘norm’ is.  It might be common for what you’ve seen, but it’s usually the same people posting videos of themselves. It will lead to a misconception of what standard procedures really are.

[richjb2 195]

3 hours ago, JasonPC said:

Real world procedures rarely use VNAV on descent and definitely not all the way to final approach. So changing speed manually is completely normal. V/S control with manual speed input is often used.

huh????  I don't think so....

We design procedures so that VNAV can be used all the way to the FAF/PFAF. 

No all airplanes have coupled VNAV, e.g., ER-RJs and CRJs.  However, for Boeing and Airbus, procedures especially parallel runway operations are dependent on VNAV descents.

Rich Boll

Wichita, KS 

[JasonPC 173]

Ok I misspoke. What I meant is there are often traffic/controller situations where your vertical profile would deviate from the STAR or calculated descent and would require intervention from flying the VNAV Path. FLCH in the 737/777/747 is usually used in that case. In other older planes with not as advanced autopilots or no vertical profile control V/S is the preferred method. If you're cleared to descend via STAR then obviously you can use the profile that's calculated in the FMS. Some STARs don't even have much vertical guidance and are dependent on the FMS to calculate a descent path. I guess in that case you could enter the expected altitude on a waypoint and let the FMS calculate the descent path. But entering a V/S FPA when instructed to descend will get you where you need to be with some monitoring.

The main point being be prepared to take control of the plane and follow ATC instructions and don't allow a pre-programmed FMS always determine your speed (use speed intv when instructed or necessary). I misunderstood the original post and thought he was already using manual speed intv (rather he was trying to place a speed restriction in the FMS). I thought he was saying that he thought not using speed intervention was normal and using VNAV alone all the way down was standard operating procedure.

Edited December 6, 2019 by JasonPC

[Gabriel Victor 1]

Thanks for all the info guys.

I'm used to the 737 and QW787 where they insert vnav speeds really precisely so I don't have to worry about editing them so I'm glad to know that it's normal to change those speeds, I thought I was setting the FMC wrong.

Also I read that it's fine to be at 210 when intercepting the g/s? How do you have time to decel?

[G550flyer 1,009]

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           

[mikeymike 357]

7 hours ago, marc1184 said:

Manual intervention using the MCP is standard when transitioning to approach.  VREF+5 is the speed you need to plan to be at on final. So if VREF is 145, plan for 150 on final.  Then I add 20kts to every 5 miles beyond that point.  So, at about 20mi out, I typically set speed to 210.  Then I reduce 20kts every 5mi while deploying flaps in increments until I'm at my VREF+5 speed, usually at about 5mi from the runway.

V REF + 5 ???

I was under the impression that depending on runway wind speed. Crosswind etc

you would have to do a small calculation  to get the appropriate + kts???? 
but I forget the formula.

 I typically fly vnav till about transition altitude or 10 000ft  between 220 and 240 kts then I would either use FLCH 

or V/S  Depending if I’m on vatsim using ATC and work with the arc.

perhaps I’m doing it wrong ??? But hey it works!!! 
 

Cheers 

mike 

[G550flyer 1,009]

1 hour ago, Gabriel Victor said:

Thanks for all the info guys.

I'm used to the 737 and QW787 where they insert vnav speeds really precisely so I don't have to worry about editing them so I'm glad to know that it's normal to change those speeds, I thought I was setting the FMC wrong.

Also I read that it's fine to be at 210 when intercepting the g/s? How do you have time to decel?

One, you have to know your jet's performance capability and two, that sounds like a rushed approach. I want to be no higher than 180 at intercept and that's behind for my liking.

Rick

[G550flyer 1,009]

3 minutes ago, mikeymike said:

V REF + 5 ???

Just standard approach criteria, of course you would have your additives.

[Gabriel Victor 1]

2 minutes ago, G550flyer said:

One, you have to know your jet's performance capability and two, that sounds like a rushed approach. I want to be no higher than 180 at intercept and that's behind for my liking.

Rick

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.