Spoilers & 737NG behaviour

[mauriceb]

This likely has nothing to do with PM (although it might), but I'm just wondering if anyone flying any 737 flight models has any success with speed brakes when it comes to slowing down the airplane.

I use the PDMG 737-800 flight model (no panels) and I cannot see any evidence that the spoilers have any effect at all when it comes to slowing down a descent. In fact, most times when I extend the spoilers, the engines spool up to maintain the same speed I guess. Even when the engines are at idle, the plane does not slow down. I'm almost never able to be at 250 knots or less when I descend below 10,000'.

Also, in a somewhat related question, I read that the preferred & most economical way to descend from cruise altitude is to have a descent with idle power. I have yet to see that in any of the flights I made. I'm not sure if PM CDU is not calculating properly or whether something is not quite the way it should be.

In a nutshell, the descents are always the most problematic for me. Is anyone out there able to control their descents properly (especially the speed), and if yes, could you please share how you do it?

Thanks,
Maurice

[BillTee]

Hi Maurice,

I too, use the PMDG models. The -800 is much more difficult to slow down than the -700.

I had a conversation with a Continental Capt where he said " it's not easy to go down and slow down."

Seems like the speed brakes don't seem to act on the -800 as much as the -700 PMDG ( must be inertia, as the -800 weighs much more than the -700 ), I just begin my descent farther out with a lower rate of descent.

Maybe you might have to play around with the aircraft config file to make it work more to your liking.

Let us know how it goes.

Bill T.

[mauriceb]

Thanks Bill. I'm going to try the 700 to see if that works better for me. There is still the issue of seldom seeing idle power only during the descent, especially when the speed setting on the CDU is much less than the actual speed. This may be a flight model problem, but I would think the CDU should certainly be commanding idle power descent once the speed is exceeded (or is that not a function of the CDU?)

Thanks,
Maurice

[stefanloss]

Actually there is a rule of thump that you can either slow down or descent, but not both at the same time.

[Westozy]

Hi Guys, I fly the PMDG 800NG and find the speedbrake to be quite effective especially in manual flight. Are you having an autothrottle problem? Is there a conflict between PM & PMDG? Just curious!

A tip I picked up from a visiting NG captain about descents - press the MTRS button on the EFIS to display the altitude in meters above the imperial altitude readout, use the first two digits as a guide in miles from destination to commence descent at flight idle.

[Bob Reed]

Westozy.. That is GREAT info! Another thing to check in the ini file of the MCP or CDU I do not remeber, make sure that the pilot controls the speed is turned on and not the AP.... If the throttles are at idle they will not spool up.

[mauriceb]

Actually there is a rule of thump that you can either slow down or descent, but not both at the same time.

I'm not sure that is true. I have felt a deceleration in a real plane when the spoilers are deployed during a descent, so they are effective to a certain degree.

Maurice

[mauriceb]

Hi Guys, I fly the PMDG 800NG and find the speedbrake to be quite effective especially in manual flight. Are you having an autothrottle problem? Is there a conflict between PM & PMDG? Just curious!

A tip I picked up from a visiting NG captain about descents - press the MTRS button on the EFIS to display the altitude in meters above the imperial altitude readout, use the first two digits as a guide in miles from destination to commence descent at flight idle.

This is a great tip indeed. But that means you don't let the PM CDU decide when the descent should start. If it is not calculating that properly, then the code is not doing what it should be doing. Also, if you want idle power, then you have to disconnect VNAV otherwise you can't control the power (since VNAV seldom commands idle power, for me anyway). In that case, VNAV would only be good for climb & cruise.

I do believe though this is at least partly an auto-throttle problem. As I said, I seldom see the power at idle for any descent that is triggered by the CDU. However, I have also noticed very little difference in the speed if the A/T is not on and I extend the spoilers after I retard the throttles manually to idle.

The power not being at idle during a descent I believe is the worst problem and I think FS9 is to blame for that, or the PDMG flight model if it models the same behaviour as FS9. There is no way there should be anything but idle commanded during a descent if the speed is already above the speed that is required.

If I was flying semi manually (with MCP only). I'm sure I could manage to not exceed the maximum speeds by controlling the start & rate of descent. And that is fine is that's what you want to do. But what is the point of having a CDU if you can never use its full ability.

By the way, I fly the PDMG-800 with no panels, so I can't see how there could be a conflict, but who the heck knows? :)

Maurice

[NicD]

Interesting! We've been experimenting with this during sim development. We too find that the PM CDU does not initiate descents that allow it to have the aircraft over fixes (or at 10,000) at or near the CDU commanded speed. Usually it is around 40-60 kts IAS faster.

From my experiments I believe that its caused by 2 things. 1) The PMDG 737-800 spoiler does have some effect but not as much as in the real aircraft, and 2) the PM CDU logic is oriented toward altitude control first, speed control second (which is probably same as real life it's just that the balance is not quite right).

When we're about to commence a tight STAR (ie no manouvering room to bleed speed on the profile) we use the commanded TOD as a guide but we manually commence the descent around 10 miles before TOD. Sometimes we also disengage A/T and pull the throttles back to idle for much of the descent, re-engaging A/T before the IAF on approach.

A 737NG skipper once told me that this bird is a slippery beast and speed control is one of the greatest challenges to master when they undergo type training/conversion (especially when coming from larger turboprops where the prop control range can be used to quickly create massive drag and reduce speed).

Agree with your thoughts on A/T idle Maurice ... doesn't make sense that it should be around 35% N1 when going too fast. Might be an engine preservation issue in the logic ... e.g. not safe or good for the engines to be at full idle for long periods airborne? Dunno - but I'll ask around and try to find out.

[mauriceb]

Sometimes we also disengage A/T and pull the throttles back to idle for much of the descent, re-engaging A/T before the IAF on approach.

.....
....

Agree with your thoughts on A/T idle Maurice ... doesn't make sense that it should be around 35% N1 when going too fast. Might be an engine preservation issue in the logic ... e.g. not safe or good for the engines to be at full idle for long periods airborne? Dunno - but I'll ask around and try to find out.

Nic,

I think you are right & disengaging the A/T for the descent and starting the descent early may be the only way to operate the SIM if the software does not do it properly. I guess that will keep me busier and in a way, it's more 'interesting' that way :). I'm not sure if setting the MCP altitude at a certain value would stop the descent at that altitude as it does if VNAV is engaged or if the descent would continue until you select 'alt hold'. Will have to check on that one.

As far as 35% N1 minimum, you may also be right. Maybe too much cooling could damage the engines. If you find out for sure, please let me know. I'm very curious about that.

Thanks,
Maurice

[BillTee]

Hi guys,

I have found that the VNAV descent does not work for me, it usually does not calculate the distance and speed properly. So that being said, I always figure my own descent and do the descent ( and ascent ) manually ( ala Southwest. ) I do find that there is no problem with speed control issues at low or high altitudes. I also find that my own descent with autothrottles set to speed, does call for idle and the throttles come back to that OK.

Some time back, I had a retired 747-400 capt tell me that " if you need to use the speed brake on descent, you planned it wrong." ( Except for an ATC " slamdunk ", I'm sure )

Best,

Bill T.

[Westozy]

[quote=mauriceb;36750]
I do believe though this is at least partly an auto-throttle problem. As I said, I seldom see the power at idle for any descent that is triggered by the CDU. However, I have also noticed very little difference in the speed if the A/T is not on and I extend the spoilers after I retard the throttles manually to idle.

One thing you might have missed is that flight idle is different to ground idle. Flight idle should be around 30-32%, this represents the windmilling effect of the airspeed on the fans. My NG Captain mate says this is normal and in case of an engine fail the windmilling engine still drives the generators and hydraulics unlike PMDG where everything stops.

The first two digits of the metric altimeter tip is just a rule of thumb guide, the FMC TOD will be more accurate but I still find PMDG to be a bit speedy in descent and sometimes it's hard to stay ahead of the plane. When I fly FS flightplans that crazy chick in ATC is always telling me to "expedite", I'd like to expedite her from the PC box!

[BillTee]

Hi guys,

Yes, during descent the N1 is around 35%, I figure that that is windmilling. I check the FF to see if there is A/T input, I see no call for power from the A/T.
Again, I use the PMDG aircraft with no panel, PM's MCP, CDU and ND.

Bill T.

[mauriceb]

So, it looks like the consensus is to forget about VNAV during descents and manage things manually. As I said before, this will likely make things less 'boring' at times, but I still wish VNAV worked the way it is supposed to. I guess I'm foolishly hoping for perfect software. :roll: :)

Bill & Westozy, good point about the windmilling effect. I should have seen that when I disengage the A/T and go to idle power. The engines never go to ground idle power. But I was really talking about seeing close to full power on some of my descents when the aircraft was already at its top safe speed and I get a warning from FS9 about over-speeding. This is what is really frosting my moustache. What the heck is commanding full or almost full power at that time on a descent? Is it the CDU, the MCP, FS9, PDMG flight model or a combination of some or all of these?

I guess before my moustache does freeze & fall off, I should really just accept the fact that laziness in the cockpit is not conducive to longevity. :mrgreen:

Thank you all for your inputs,
Maurice

[Jackpilot]

To calculate TOD here is another real world super easy method.
DIVIDE YOUR FL ALTITUDE BY 3 and make it the distance in NM between the airport and the TOD.
EX: AT fl300 start descent at 100 NM DME from the runway
AT 24 000Ft start descent at 80 NM DME from the runway
Piece of cake

To refine it if the IAF or a close point has a DME make it your altitude minus the altitude mandatory at the IAF and divide by 30

FL240 and IAF at 3000= 24000-3000=21000 ....TOD at 70NM from IAF


AND AFTER THAT FLY YOUR STAR MANUAL AND FORGET THE CDU...AND SWEAT AND HAVE FUN!!!!

[mauriceb]

[QUOTE=Jackpilot

AND AFTER THAT FLY YOUR STAR MANUAL AND FORGET THE CDU...AND SWEAT AND HAVE FUN!!!![/QUOTE]

Sometimes I wonder if this is really fun or just self-flagellation. We do it because it hurts so good :D

Maurice

[dodiano]

Ok guys a word abput Speed Brake, it is mainly used for increasing the Rate of descent in flight in case you get High, also of course it helps you to slow down... But on Jets you cannot go down and slow down... You go down and then slow down or you plan your descend and begin your descent before to get down slow or... if you are asked to mantain a speed in descent the speedbrakes help you mantain that speed and increase your rate of descent... For example yesterday in Miami they asked us to mantain 300 knots or less for spacing from FL 370... Now from that level down to 10,000 without speed brake you would not be able to mantain that speed and an apropiate descent so you use the speed brakes... So don´t think that when you apply it it will decel you immediatelly... Now I fly an Airbus and not a Boeing but it is the same for all planes.

Regards

Roberto

[mauriceb]

Thanks Roberto for that explanation. One more question. When you start the descent from cruise altitude, is that at your discretion based on the flight plan you filed, or do you need clearance from ATC before you start your descent? My guess here is no.

And if you don't need clearance to start the descent, can you decide to start your descent early for any reason such as a strong tailwind for instance? In that case, would you require clearance to descend early since you changed your initial flight plan? My guess here would be yes.

Are my assumptions correct?

Thanks,
Maurice

[dodiano]

Remember any level Change on IFR airspace you have to report it to ATC now you have two choices... Normally in some places almost all of them they know when you´ll start your descent so they either clear you at your discression to certain flight level or you ask descent at your discression and they´ll give you a Flight level...

Actually descent start is displayed on the TOP of Descent normally an arrow in the Bus... Now this is a computer based top of descent it is not mandatory... For example I do it a couple of miles after the Top of descent why cause I save fuel by not leveling of in certain places where you are expected to do so... Why cause I know the airspace I´m flying too is all Technique... Plus we are humans we tend to do things different to the machine now strong tail wind is an isseue but at lower levels normally not at higher ones cause when you are closer to the grounds due to speed constraints is much easier to be high!!

Regards,

Roberto

[Michael Carter]

The 727 is another slippery bird. As Roberto said, spoilers will increase the descent rate, sometimes dramatically if you leave control up to the Sperry 150 on the way down.

You can always pitch up a degree or so to slow down, but also like Roberto said, if you need them in the descent, you didn't plan properly.

Speedbrakes are for ATC, not pilots.

About the only place where I have to use them regularly is on the STAR from the southeast into Paris after TRO. You are held at FL140 at OMAKO for 08L/R or 09L/R. At CLM you may descend to FL120 to MOSUD, max speed 220KIAS.

At MOSUD, it's a dive in the downwind to get down to the intercept altitude and control your speed and get it down to the flap schedule speed.

You're going down about 8,000' in about 16 miles all the while the controller is asking you to keep your speed up for trailing company traffic. With a little skill you can have it at 190 before PG503 which is the turn to base. Sometimes they'll extend you out a bit though.

But I like the challenge and it's all done by hand.

[NicD]

This is great information from eveyone ... thank you all!

I asked the question over on my VA forum. We're an all 737NG VA with many real world Virgin Blue employees - including flight crew. So these guys either jump seat, or crew seat, all the time and know the NG like an old mistress :)

So ... on the spoiler issue...


The PMDG speedbrake is about as effective as the real thing.. in the -800 she is just too aerodynamic and just doesn't want to slow down when heavy...

Make sure you enter descent winds however .. this makes a big diff to descent calculations especially for e.g. into SYD via CULIN/RIVET8 because of the tailwinds you get on the way down...

And on descent management ...


Many real NG drivers have told me (during descent in the jump seat) that the VNAV PTH does not really hold the descent profile exceptionally well, meaning constat attention and corrections may be necessary. Whilst the profile may be correct, the speed is not. VNAV SPD will hold the speed fairly well on descent but could leave you high or most commonly low on profile.

LVL CH is used frequently, especially in SY. Vert Speed mode used a lot too, allowing manual setting of the speed on the MCP whilst controlling descent to maintain profile (which is displayed on your ND)

No simple answer for a good descent unfortunately. It needs to be assessed constantly and appropriate action taken. I see many pilots in RL, and I myself use a combination to get a good descent and good speeds.

And this advice from a RL 737 skipper:


The 737-800 and the -700 to a lesser extent does have trouble with speed control in VNAV PATH. Its a lot better than it was since Boeing released a software update about 2 years ago, but will still have trouble avoiding an overspeed particularly with a tailwind in excess of about 30 knots.

Its important to note that that - due to the non-linear effect of high altitude descents (too complex to go into here) - that this overspeed generally occurs in the upper third of the descent (say between FL350 and above). Its up high that the barbers pole (MMO or maximum mach number) is perilously close to the typical 0.77M/300kt speed. So, usually once youre established below FL300 you are pretty well home and hosed, with there being a hefty buffer above MMO which climbs to 340 knots.

The idle speeds in flight on descent should see N2 at 59% (ref Boeing manual), with N1 at around 22%. Both figures go up if Anti-Ice is on, and even more when "Approach Idle" kicks in when gear and flaps are out.

I'll try to remember next time i fly to get some video footage of the airspeed runaway during a tailwind descent. I'll try to show the N2/N1 values at the same time.

Finally, remember, VNAV path is putting "Path" as the absolute priority, and this is tricky to achieve in a glide when there are so many variables. VNAV Speed (because it focusses on speed control at the expense of path) does a much better job, but you have to intervene to control divergences from path.

[mauriceb]

Thanks NIC for this extensive 'tutorial'. Lots of very good info there.

One thing I never do actually is enter the winds (descent or other) in the CDU. I always thought the CDU would know what they were since the wind values are displayed on the ND. I actually don't even know where to enter that on the CDU (goes to show my ignorance I guess ;)

Having said that, how would you ever manage to enter the wind value in the CDU when it is constantly changing, especially on the last legs since the headings are changing quite often at that time and you are quite busy in that phase of the flight? Forgetting about VNAV at that time & reverting to manual altitude, speed and V/S control would seem to be the only sensible thing to do then.

By the way, just for fun yesterday, I tried flying the stock Boeing 777 in FS9 in my 737 flight deck. I was amazed at how effective the speed brakes are on that bird and I would actually be shocked if they were that effective on a real 777. It was like I stepped hard on the brakes in my car. The speed dropped dramatically and the rate of descent also increased dramatically. It made me wish I had chosen a 777 instead of a 737.:mrgreen:

Thanks,
Maurice

[Michael Carter]

...Forgetting about VNAV at that time & reverting to manual altitude, speed and V/S control would seem to be the only sensible thing to do then....



It's definately the FUN thing to do.

I can't imagine an airplane doing all of the work for me. That's why I learned to fly to begin with because it's FUN!

[Jackpilot]

Way to go...
see my post on TOD...

CDU is fantastic ..but as a tool, not as a pilot.
Has to be your slave not your master.

[mauriceb]

It's definately the FUN thing to do.

I can't imagine an airplane doing all of the work for me. That's why I learned to fly to begin with because it's FUN!

I can't imagine that either, but wouldn't it be nice if everything worked like in a real airplane (within reason of course)?. Aren't we all striving to build something that closely emulates the real thing? If not, why would anyone add a cabin pressure gauge or engine fire extinguisher controls. It's really all a make believe world and the closer it resembles reality, the more enjoyment we get even if we do not really use all the functionality all the time or any time.

I understand what everyone is saying, but I would also bet my next year salary that real Boeing 737 drivers do not have to fight the systems as much as we have to. I want to work at flying & I enjoy real problems & challenges, but not fake ones caused by poor code or flight model or whatever.

And that is what I am talking about & I'm not just trying to be lazy and let the hardware do everything for me, far from it. We already have incredibly complex systems available that we could only dream about only a few years ago and I'm thrilled with what I have. I'm just trying to understand its limitations by finding out if others are experiencing the same problems I have seen.

Anyway, this has been a very good discussion I think and it's nice to be able to do that & get all the different inputs and opinions and this is what makes this forum really good.

Maurice

[Jackpilot]

Way to go too, Maurice....
HEY you are the one who sold me the CDU idea, remember?...and I bought it.
I repeat , it is a wonderful gizmo and like most of us I am far from using all of its potential.

[NicD]

Funny thing is ... Boeing bring out new software for their systems too. New 'builds' just like PM :). And when that happens some RL pilots say they are edgy .. unsure of how changes in functionality will perform in the air ... and even worried about bugs. So I guess that feeling we have is a bit like real life. Altho I reckon Boeing do a little more beta testing than PM etc. :D

[Michael Carter]

I guess that was a small part of why I chose to build an older aircraft. I really don't want the automation the new aircraft provide.

I have an autopilot, very crude by todays standards, but it takes the workload off of me enough so that I can concentrate on navigating and systems monitoring.

And get the occasional Hofbrauhaus from the galley.;)

[Jackpilot]

"..I guess that was a small part of why I chose to build an older aircraft. I really don't want the automation the new aircraft provide."....

I basically agree with U but even a real Airbus can be flown "sort of" manually.
One has to fight a bit with the systems though.
I was very tempted by the steam gauges of a 737-200 but I must admit that the NG screens get addictive....and the whole beast can be 100% manually handled as long as the flight model is stable,
I use the 800 from Steve Reyling,... really enjoyable to handfly especially on approaches.
PM software used that way is close to perfect as it gives very precise information, like any precision instrument, that the pilot can use as needed.
And the automation can be called upon, on and off, by stages , when the workload is overwhelming, especially with an empty FO seat!

We must not forget that the rationale behind all this automation (CDU mainly)and reduction of crew from 3 to 2 is SAVINGS and Fuel Economy, far before safety....modern airliners are now always driven in a boring "economy mode" .
We do not have to abide by those rules.

Well, whatever the methods...this is a GREAT HOBBY, and as Maurice said, a great way to share ideas...
Have a good one Guys.