11 hours ago, Orlaam said:

In reality, isn't the use of LVL CHG, V/S, and VNAV dependent on the pilot and SOP?

Yes.

However, I see a lot of not understanding VNAV SPD by some of the, ahem, more seasoned pilots at my airline. The younger guys and gals I fly with now seem to be okay.

Also, the big gotcha at my airline is we set "The bottom" of the STAR in the MCP and let VNAV PATH handle all of restrictions down to the bottom. If a pilot decides to exit VNAV, we are supposed to change the MCP back up to the next restriction. Some pilots think, "Oh, I'll just put it is V/S for a few seconds to rejoin the PATH and not reset the MCP."  But then they forget and a restriction gets busted.

Matt, Joe or anyone flying type:  How does the VNAV handle the transition altitude when the barometer is high?  The PMDG VNAV responds as if there is a step change in altitude, where the barometer is high the aircraft is suddenly high above the path and will dive to the needle resulting in an increase in speed of 10 kts or more.  Low barometer is not so bad because the descent rate is simple decreased until getting back on path.  I've started switching to V/S during the switch at the TA, where I dial in a couple of hundred feet increase in descent rate to catch the new path rather than letting the aircraft dive at it.  I was wondering if this is a PMDG limitation or do the RW aircraft do this too?

37 minutes ago, downscc said:

Matt, Joe or anyone flying type:  How does the VNAV handle the transition altitude when the barometer is high?  The PMDG VNAV responds as if there is a step change in altitude, where the barometer is high the aircraft is suddenly high above the path and will dive to the needle resulting in an increase in speed of 10 kts or more.  Low barometer is not so bad because the descent rate is simple decreased until getting back on path.  I've started switching to V/S during the switch at the TA, where I dial in a couple of hundred feet increase in descent rate to catch the new path rather than letting the aircraft dive at it.  I was wondering if this is a PMDG limitation or do the RW aircraft do this too?

In the descent forecasts page, there is a place to enter the landing altimeter setting.  In theory, this should take care of the problem you describe.  In reality, if you don't enter an altimeter setting on a high pressure day it'll do what you describe, and if you do... MAYBE it'll be smoother.  The performance can vary from plane to plane and even flight to flight in the same plane, with the same altimeter setting. 

9 minutes ago, Stearmandriver said:

In the descent forecasts page, there is a place to enter the landing altimeter setting.  In theory, this should take care of the problem you describe.  In reality, if you don't enter an altimeter setting on a high pressure day it'll do what you describe, and if you do... MAYBE it'll be smoother.  The performance can vary from plane to plane and even flight to flight in the same plane, with the same altimeter setting. 

Yeah, entering the altimeter setting on the descent forecast page has no effect that I can tell, and the 747/777 FMS don't even bother with that field.  So do you let her do the dive or do you intervene?

1 hour ago, downscc said:

Yeah, entering the altimeter setting on the descent forecast page has no effect that I can tell, and the 747/777 FMS don't even bother with that field.  So do you let her do the dive or do you intervene?

I always enter the altimeter setting on the descent forecast page and while you might get a momentary burble in the path around transition altitude it works itself out.  I haven't found it necessary to intervene.  Unless it was wanting to over speed badly I'd just let it work itself out.

Agreed, when I said you might still have the problem, I just meant the problem of ending up instantaneously high at transition.  The plane doesn't really dive; it may revert to VNAV SPD for a bit but you're already on something close to an idle descent path so that doesn't result in much of a pitch change.  It does usually work itself out, and if not, you can simply add a little drag. 

As stated above, coming out of VNAV has the potential to introduce more serious problems than the minor one you're trying to solve, in this case... So I leave it alone as well. 

LNAV VNAV PTH to LOC GS is a very satisfying experience, and one I strive for, especially when flying without active ATC. 

VNAV is strategic, FL CH and SPD VS are tactical (Bulfer)- there's room for both, but I find with proper descent forecast input, Anti-ice considerations and QNH and ISA Deviations, VNAV descents make up the vast majority of my arrivals.  Of course there are times where intervention becomes necessary- but on the whole I tend to use VNAV much more than anything else in my arsenal.

Good to see you here Andy!

C

Edited August 8, 20187 yr by cavaricooper

Good day all. Right then. Lets put this to bed with some basic background, theory and practice.

1. The Basis of real operations, is a decent briefing and look through the decent planning from PM, so both guys or gals have it all understood. Part of this is the setup on the FMC. Entering your STAR transition and approach means having the ATIS. On descent page, you can input both the Transition level and airfield QNH. Also ISA temperature deviation. You can also enter predicted altitudes between which you may be using ANT-ICE systems.

Based on your input COST INDEX on the PERF page, your FMC will calculate and draw your profile back from the Initial approach fix back up to the Top of Descent point. It uses all entered parameters. So, use of ANT ICE will mean a higher idle, less FPM and shallower angle during that phase. 

If you can't get the info until in the descent, an inputted QNH of say 1020hPa  will cause the VNAV PATH to jump maybe 80-100ft. It's nice to get it in place, and if you are about to enter it, then that's when I go to V/S to transition smoothly back onto path.

Note: VNAV will keep all intermediate altitude restrictions in place. If you are descending into somewhere like LTAI, a direct to a centrefix from ATC will lose these restrictions, so using ABEAM POINTS to retain that FL110A as you cross the Mountain range is IMPORTANT. Think CALI......

VNAV PATH or SPEED will also automatically transfer from MACH to IAS. 

V/S WILL NOT make this transfer and you may well overspeed. Doh.

V/S is a nice smooth tool for transitioning modes, but unless you are pretty switched on, it's a recipe for a trip to the Base Captain....

The 737NG has a M.82 limit. This is limiting at high cruise levels but once lower, the IAS at a set MACH number will rise as pressure and temperature rise. The VMO then rises until 340kts. Below 10,000ft the 250kt limit is procedural and for ATC and Bird Strike reasons. you can exceed it if approved.

 

Golden nugget. If you want to scream down at 280 or 320kts, the facility is there in the FMC to enter this in the Descent page. It will then redraw the descent from the 250/FL100 point back up and VNAV can fly this as well. 

Our mob like to descend at ECON, often 250kts or so. If ATC requests a speed, we have lots of available performance to comply. Its a death dive at high speed that is often the start of a panicky and desperate attempt to slow down far too late. Why put yourself under that much pressure? If you are faster than 200kts at 10NM from the runway, sort it out now. 190kts at 9nm, 180kts at 8nm. 210 at 11, 220 at 12, etc ,etc. Best tip I ever got from a trainer that.

So, VNAV can be your friend if you give it the tools to make a sensible calculation. V/S is a useful transition tool, but a trap for the unwary. Know your airplane basically.

 

 

 

That was beautiful Mark, cheers.

 

So your 'mob' descend at 250kts indicated from TOD?

 

Andy Baird

Sometimes. Usually down route or into non congested airports we will use ECON, often 247-248kts as our cost index is usually 6 or 7, but into the UK, invariably they'll give us "270kts on conversion" instructions, so the M.77 cruise given by that Cost index will translate to about the same IAS ECON speed. By selecting M.77/270 in the speed LSK 2L on the Descent page, the aircraft will begin descent at the T/D point in VNAV PATH and gradually pick up speed until it gets to 270 then lock to that. The MCP window changes over automatically and we sit and just watch. 

To be honest, a lot of us, myself included, will press the intervene (or DESC NOW prompt on LSK6R on Decent page if we don't have intervene buttons on the MCP), about 5NM prior to T/D so that the aircraft starts a gentle 1,000fpm rate then transitions into the steeper rate shortly after. It's just subtler and smoother for the guests.

Sometimes when late or for sequence we get told to go faster, but to be honest, if there is a possibility of a shortcut I don't like going faster because it limits your options.

 

At 250kts, I can cut 20nm out easily, at 300kts I'm a bit stuck. The idea is to sort out profile issues (it's always high and fast isn't it) as early as you can. Less embarrassing in the long run. it's all about good Situational Awareness.

Coming up UN864 with a MIRSI arrival into Manchester is a great example. They'll give you a conditional descent, effectively to get you under another ATC sector and outbound Westerly traffic going via Strumble Then another conditional descent to be FL200 by MONTY.

Here's the rub, at 8pm, or even 10pm, you'll be going via WAL, MIRSI and possibly holding, before routing east and round over the Pennines to a long 12-15NM approach to 23R.

At 02:00, you "MIGHT" be routing via DAYNE or if on 05L be given a straight in from MONTY. 

How do you plan for that? Some 50NM+ difference in descent planning.. 

 

Er, well, it's partly feel, partly guesswork and mostly just asking ATC at the time!

If likely I'll get to FL200 early and look to slow down ASAP, then I can pull the "Stick of Shame", select 300kts in LVL CHG, and state "TORA TORA TORA" to my colleague with a grin. On a ROSUN arrival from the east to 23R, I've had to use the gear to get down, in such cases, lifting the nose, selecting 220kts LVL CHG and then getting gear down, Flap 5, speed brakes out works well.

However, the main caveat.....

Most importantly, if it looks doubtful you can comfortably get under the glide by 12NM to slow it all down in time- THROW IT AWAY, ask for delaying vectors and get rid of the speed then chill out. It's not a race, aerobatic display or willy measuring exercise. There's always the argument that saving time, is of little use if you end up waiting for a stand, if the passengers have to wait for a bus, or steps, etc.