Question on descent for real 737 Drivers.

[Bobsk8 12,594]

Been flying the NGX for a couple of years, and one thing I am confused about is speed settings for descent's. I use an ATC program, which tells me when to begin descending, so I never use VNaV.  I use level change all the time for descending. My question is, what is the proper speed that should be sent for descending. If I am at FL 360 for instance, and told to descend to say FL 240, I always switch the speed IAS/MAC setting to IAS, and set in around 270, unless 270 is too high at that point for the altitude, so set it to bottom of red arc, and then as I descend I gradually move it to 273. The reason I use 273, is because the FMC descent speed recommends either .77 Mac or 273 Knots. Once I get to the point that 273 is below overspeed indicator, I just leave it there until ATC tells me to slow down usually around 12-14,000 feet. 

Now I notice, that at high altitudes, I could actually move the airspeed up much nigher, because the overspeed indicator goes well over 300 knots as I descend, so this would increase my descent rate as well as ground speed. Here is my question in two parts:

1. Should I leave the airspeed indicator in Mac until it switches automatically to Knots, or should I continue doing it the way I am doing it? 

2. If I leave the airspeed setting in Knots, should I increase it as I descend for a faster descent, or should I just leave it at 273? I want to do this the right way, and right now I am not certain what I am doing is correct. 

Edited August 3, 2018 by Bobsk8

[downscc 1,327]

Not using VNAV because you are using an ATC program doesn't make sense.  If ATC clears you to descend from FL360 to FL240 the  important piece of missing information is is this the start of your descent to destination or is this a descent to a new cruise altitude?  In either case, there are ways to use VNAV and I suspect most operators in fact do use VNAV..... at least until they are in the terminal environment or get vectored off of the LNAV path.

Normal descent are at cruise Mach until indicated increases to 280, 290 or even 310 depending on if ATC has given you a speed.... if you are in a flow of traffic there is usually a speed or it is printed on the arrival chart.  In lieu of anything else, I go with 280.

Last comment, using these ATC programs are not going to provide you with realistic scenarios.  Not even the best ATC program is anything resembling realistic... just wanted to mention that in case you believed otherwise.

[Bobsk8 12,594]

20 hours ago, downscc said:

 

Normal descent are at cruise Mach until indicated increases to 280, 290 or even 310 depending on if ATC has given you a speed.... if you are in a flow of traffic there is usually a speed or it is printed on the arrival chart.  In lieu of anything else, I go with 280.

 

That is what I was looking for, thanks. 

[vc10man 732]

Having just got back into the 737NGX scenario after being out of flightsims for almost a year, give or take, both your original post, Bob, and your reply Dan, are just what I was looking for.,

Now............if only the NGX's FMC scratchpad would accept digits without wiping any input I try to type in.....it would be great!

 

Edited August 4, 2018 by vc10man

[Iceman2 110]

FWIW, PF3 ATC program sometimes says descend at pilots discretion, meaning you can descend at the FMC calculated TD position. Which is way more realistic. (Providing you have setup all the flight planned altitude values correctly and accurately in PF3)

In real life when ATC make you descend early for traffic reasons we usually use vertical speed (VS mode) and descend the aircraft much slower (between 500fpm & 1000fpm) gradually getting back to the FMC desired profile. However once back on profile we then go back to VNAV.

Some airlines have ATC approved descent speeds for example my company uses ECON CRZ into 280 kts on descent for the 737. (Eg .78/280/250) The 250 is below 10000ft.

trust this helps

IM

Edited August 4, 2018 by Iceman2

[Stearmandriver 2,253]

More and more large and medium sized airline airports are using RNAV arrivals these days, which contain not only a published lateral track but a vertical one as well, in the form of altitude and speed restrictions at various waypoints.  More often than not, in the U. S. these days, your descent clearance will be in the form of "descend via the HAWKZ6 arrival landing south, altimeter 29.87."  You are then expected to meet all the restrictions (unless ATC includes exceptions, which is common, but another story.) 

VNAV is your friend for these procedures.  I'd recommend trying it out. 

In lieu of anything else, our average default descent schedule is .78 mach to 280 KIAS.  

Edited August 5, 2018 by Stearmandriver

[Bobsk8 12,594]

Guess I will have to give Vnav a shot, haven't used it up till now. 

[vhufo 35]

On 8/4/2018 at 8:51 PM, Iceman2 said:

In real life when ATC make you descend early for traffic reasons we usually use vertical speed (VS mode) and descend the aircraft much slower (between 500fpm & 1000fpm) gradually getting back to the FMC desired profile. However once back on profile we then go back to VNAV.

Why would you use "VS Mode"?

 

Why wouldn't you simply use the "DESC NOW" function in the FMS which descends you at 1000fpm until reaching the VNAV descent path?

 

Andy Baird

[Iceman2 110]

5 hours ago, vhufo said:

Why would you use "VS Mode"?

 

Why wouldn't you simply use the "DESC NOW" function in the FMS which descends you at 1000fpm until reaching the VNAV descent path?

 

Andy Baird

Why would you use DESC NOW when you can use VS?

Most Aircraft have two or three ways of accomplishing the same task so yes that is another option in getting down and a good option too if you want a 1000fpm.

No right or wrong there just different ways of managing you job on the day.

Good input thx

IM

 

[Bobsk8 12,594]

Well I tried the VNAV mode tonight on a descent, after ATC gave clearance to descend, on a Star , and it worked perfectly.   

Edited August 5, 2018 by Bobsk8

[JoeDiamond 67]

13 hours ago, vhufo said:

Why would you use "VS Mode"?

 

Why wouldn't you simply use the "DESC NOW" function in the FMS which descends you at 1000fpm until reaching the VNAV descent path?

 

Andy Baird

That’s what I would do for a couple reasons.  Using VS in that situation would work but it’s going to increase your workload. If your intent is to capture and descend on the path why switch modes just to have to switch back later?  Also, should you happen to get distracted by something VS will blow through the path whereas VNAV will not.

I will us VS in situations where descending at 1000 FPM in VNAV would capture the path close to the level off altitude which results in an awkward dive/level off maneuver.  In that situation you want more than the 1000 FPM VNAV Path would give you, but something less than the idle dive VNAV SPD would do.  Using VS with a rate that reaches level off before the path results in a smother descent.  Another option would be to stay in VNAV and open the speed window which will put you in VNAV SPD and just bump the power up a little once the throttles go to ARM.

It’s all just technique in the end.  As long as you comply with the clearance there isn’t a right or wrong way to do it, but some techniques are more efficient than others.

[Spin737 102]

I second Joe's technique.

Half of the safety reports I read are guys getting violated for busting altitudes because they were cleared to descend via a STAR but decided to use V/S.

STARs are often designed with VNAV in mind.

 

[Orlaam 306]

12 hours ago, Spin737 said:

I second Joe's technique.

Half of the safety reports I read are guys getting violated for busting altitudes because they were cleared to descend via a STAR but decided to use V/S.

STARs are often designed with VNAV in mind.

 

In reality, isn't the use of LVL CHG, V/S, and VNAV dependent on the pilot and SOP?  Unless restricted by a company, I would think whatever gets the job done is okay.  A lot of corporate pilots seem to use whatever the scenario calls for. 

I find in the sim that VNAV often can't meet restrictions. Close in on that 250/10,000 mark and struggle to meet the subsequent crossings.  Even with spoilers extended, I find it hard to believe that meeting the STAR requirements is that difficult.  I realize that a lot of pilots remark that the old 3:1 ratio has be come 4:1, but most new STARs are really hard to follow and still close in on the IAP IAF without going too fast or too high.  When I see jets around here at 210 knots around 7,000 feet and slowing to 170 at 8 miles out.  What's the overall secret?  I have yet to follow a STAR to an IAP and truly slow to the speeds I see in large cities.  I've almost always managed to slow to the appropriate APP speed, but it's rarely what I consider a comfortable affair.  I'm often extending spoilers for ages and using other drag methods to accomplish it.  Like the SERFR3 arrival into 28 at SFO.  I feel like that should be plenty of space to get down and slow, but in the 744 it's like a panic to get to a good speed and be stable.

[skelsey 765]

1 hour ago, Orlaam said:

most new STARs are really hard to follow and still close in on the IAP IAF without going too fast or too high. 

(snip)

I can't speak too much to the 737, but in general my experience of VNAV is that it is dumb and as such it will do dumb things like get slow on one segment, put a load of thrust in and then get high on the next segment because all that extra energy it added by pushing the thrust levers up now has to "go somewhere".

The idea of most modern STARs seems to be toward a continuous descent with near idle thrust - so it is necessary to watch the VNAV carefully and intervene if it starts doing dumb things like the above. It's all about energy management - imagine a rollercoaster. If you shallow off the descent you will bleed off some speed but if the next segment is much steeper you'll end up at the bottom of that segment much faster. You always have to be thinking several steps ahead and making a judgement about whether on a shallower segment you really want to be adding more energy to the equation (ie - am I high *now* or overall - remember the V/DEV pointer is very much instantaneous so you have to cross-reference the total track miles to run and use the three times table to establish where you are overall). Varying the speed is an excellent way to adjust the descent profile but obviously not always possible with ATC requirements in which case it becomes very tricky and really all you have to play with is drag.

Use whichever modes get the job done, but if you are not using VNAV then it is wise to be defensive in how you set the MCP (ie don't set it below the next restriction until you have passed it or are just about to pass it and it will clearly be met) in order to avoid busting any levels.

Personally I'll almost always use all three modes at some point during every descent - VNAV, FLCH and V/S depending on the requirements at that moment (and frequently in that order - VNAV initially, FLCH a bit closer in and V/S to finesse the final stages of the descent and glide slope intercept). V/S is a particularly useful mode when you need to slow down gradually whilst keeping a consistent rate of descent going as opposed to FLCH which will near level you off if you wind the speed right back.

[scandinavian13 5,192]

While this one seems pretty academic, and well-discussed, I love/hate these topics. People seem to get into these heated discussions about "which mode is best mode" (yes, I realize that isn't grammatically correct...it's an RCR reference), but in the end, I think it comes down to this:

Pilots are up front for a reason. Different modes exist for a reason. Pilots should use the modes in the proverbial toolbox to best suit the needs of the situation presented to them.

Done.