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tonymerry

Difficulty in slowing down

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I am convinced that either the drag coefficient (in the programming) or the air brake efficiency are incorrect for the 737.

I am aware that it is a slippery beast, but so are the 777 and the Airbus series. Slowing down the 777 & Airbus is no great problem, whilst at descent rates of anything over 1,000 f.p.m the 737 actually accelerates & constant use of the inefficient air brakes is required. This is not true to real operation of a 737. Yes I do use F1 to check the throttles are fully retarded & the problem remains if fully using Vnav.

Does anyone have  any suggestions re the problem, as it spoils the realism of an otherwise fantastic aircraft.

Tonym

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Even the actual 737 pilots that visit here have said that they routinely use speedbrakes.  You didn't mention the variant, the B738 a lot more slippery than the B737.

 

I think you are pointing the finger at the simulation for your inexperience. Relax, as you gain hours in type in various conditions you learn to anticipate what the aircraft is going to do. For example a typical let down may exceed my FMS set speed but as long as we are on VNAV PATH and not going into VNAV SPD all is well, unless you have ATC restrictions. Then you stay ahead of the aircraft and pop up the brakes just as required to help the VNAV PATH slow it to the restriction.  Another tip:  If a high altitude restriction is 280 and the FMS descent speed is something like 284/0.789 just change the FMS to match the constraint to 280/.79 to reduce one deceleration point.

 

Footnote:  The PMDG NGX has been extensively vetted by pilots and mechanics with experience in type. It's not perfect but it's pretty good for the price you pay. Something as major as drag coefficients has been thoroughly looked at by the guy at PMDG with a PhD in aeronautics or fluid mechanics or something like that, pretty sure it's not biology.

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I am convinced that either the drag coefficient (in the programming) or the air brake efficiency are incorrect for the 737.

 

Based on your many, many hours in the type, I'm assuming...?

 

I get that it might feel different, but it's actually remarkably close - the sim engine isn't perfect, after all. Still, unless you fly the type, then I think it might be best to start troubleshooting your own technique before casting aspersions.

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I am not type rated in the 737, or in any other jet for that matter, but I have noticed that most arrival procedures on the NGX as soon as you hit 250/10,000 feet you need at least Flaps 1 or better 5 to be able to carry them out. Another thing that bothers me and please enlighten me--is that after setting up an arrival in the FMC I get a message that says "no descent path after XXXXX"

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but I have noticed that most arrival procedures on the NGX as soon as you hit 250/10,000 feet you need at least Flaps 1 or better 5 to be able to carry them out.

 

Haven't noticed this. What are you referring, specifically...?

 

 

 


Another thing that bothers me and please enlighten me--is that after setting up an arrival in the FMC I get a message that says "no descent path after XXXXX"

 

This isn't a problem. It's simply telling you that there is no descent path after whatever fix it references. This is a real message:

 

NO DES PATH AFTER XXXXX: This message appears whenever satisfying the entered path restrictions requires a descent angle larger than the limit angle. The limit angle is defined as the idle angle at maximum DES speed which is Vmo minus 5 above 10,000 and restriction speed plus 10 below 10,000. What it's telling you is that you're going to be in danger of a reversion to VNAV SPD due to these conditions - it's not telling you that it can't calculate a vertical path or anything like that. Also, if you pass the "global" restriction altitude (10,000 usually) and the FMC detects that you're approaching the +10 limit, it will level out to allow it to decelerate (back to 240 usually) before continuing the descent. The limited DES angle based on speed feature was introduced in FMC Op Program 10.6+. In previous FMC versions the angle values used to trigger the message were 7 degrees above 10,000 and 6 degrees below. Boeing changed this due to pilot complaints where VNAV would constantly disconnect because that was too . VNAV in 10.6+ can't disconnect like this anymore, it just reverts to VNAV SPD. The 737-800 in particular is extremely slippery and hard to slow down and often requires 4:1 or greater when descending, especially if slowing down is also commanded. 

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Isn't there still a bug with the ngx, but fixed in the 777, with idle thrust still producing too much thrust? Correct me if I'm wrong. - David Lee

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Thanks for replying Kyle. About the first quote, when doing most STARs as soon as you hit 10,000/250KIAS I have to deploy flaps 5 in order to add the necessary drag to slow down a bit and meet the subsequent altitude constraints. I have heard that the 738 with winglets is particularly slippery as you point out.

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Thanks for replying Kyle. About the first quote, when doing most STARs as soon as you hit 10,000/250KIAS I have to deploy flaps 5 in order to add the necessary drag to slow down a bit and meet the subsequent altitude constraints. I have heard that the 738 with winglets is particularly slippery as you point out.

 

Just got to MIA on an AAL 737-800. We rode the speed brakes off and on from cruise on down, and under 10,000, we had Flap 1 and 5 soon after. Speedbrakes used off and on all the way until turning onto the LOC.

 

http://flightaware.com/live/flight/AAL1083/history/20160308/1100Z/KIAD/KMIA

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Yeah, arriving KMIA from the Q routes over the gulf are the same way especially landing East. It's interesting that the pros are using Flap 5 for drag since that's still in the buoyant range, Flap 10 hits the drag.  I've always relied on dropping gear rather than flaps so yes, very interesting.

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Energy management is one of the big struggles pilots have with the 800. I've jumpseated on the 737 a number of times, and it's been something that the captain comments on every time in the approach briefing. I've also seen the speed brakes used extensively in the descent. If they're planning on a short approach, then they'll sometimes drop the gear early to help in getting slowed up.

 

High energy approaches are also something I keep in mind as a controller, and we've been told to be keenly aware of the workload that it can put on pilots. If I have a sequence where I can put a 737 on a short approach, I'll look at the type. If it's a -400 or -700, then I can usually get them in pretty tight without any problems. If it's a -800 or -900, then I'll think twice before trying to set them up for a tight approach. 

 

If you want to get a taste of how energy management works in the real world, next time you fly an approach hold 210 kts to a 15 mile final, and then 170 to the FAF. This is typically how we set aircraft up on the finals here at IAH. It can be challenging, but I find it to be a satisfying way to fly approaches.

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If you want to get a taste of how energy management works in the real world, next time you fly an approach hold 210 kts to a 15 mile final, and then 170 to the FAF. This is typically how we set aircraft up on the finals here at IAH. It can be challenging, but I find it to be a satisfying way to fly approaches.

 

I want to say I think you guys are great.  Everytime I came into KIAH in a C-414 Chancellor you always squeezed me in, it helps that you can drop me on a short final and I can maintain 120 Kts until over the lights.

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It's interesting that the pros are using Flap 5 for drag

 

Remember, flaps are used to slow down the aircraft and speedbrakes are used to increase drag. We do not use flaps as drag devices, therefore use “Flaps to Slow Down and Speedbrakes to Go Down!”

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Am I doing something incorrectly, because I descended as per the FMC profile from 35k down to threshold 3000 without having recourse to the speedbrake once, using the Lvl Chg to input the speeds as per flaps once descent had commenced below 25k?

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Am I doing something incorrectly, because I descended as per the FMC profile from 35k down to threshold 3000 without having recourse to the speedbrake once, using the Lvl Chg to input the speeds as per flaps once descent had commenced below 25k?

 

Nope - certain STARs are steeper than others. Add slippery plane and you get a need for more speedbrakes, possibly...

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Remember, flaps are used to slow down the aircraft and speedbrakes are used to increase drag. We do not use flaps as drag devices, therefore use “Flaps to Slow Down and Speedbrakes to Go Down!”

We? You're a pro now?

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We? You're a pro now?

 

Yeah I'm not understanding that logic either. I fly US Navy aircraft in sim and speedbrakes are used for slowing down. Flaps are for increasing lift at slower speeds.

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This is not true to real operation of a 737.

If you would like the opinion of a Real World 738 pilot with thousands of hours in command who fly`s for Qantas and also has the PMDG NGX, he will no doubt give you an indication of how close PMDG have modeled this to the real deal.

 

https://www.facebook.com/Flap30greenlight/

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Just got to MIA on an AAL 737-800. We rode the speed brakes off and on from cruise on down, and under 10,000, we had Flap 1 and 5 soon after. Speedbrakes used off and on all the way until turning onto the LOC.

 

http://flightaware.com/live/flight/AAL1083/history/20160308/1100Z/KIAD/KMIA

 

That's just another typical day in MIA.  Quite often it's 230 knots, flaps 5 with the speed brakes out all the way down.  Worse case scenario there is coming in from the west and landing straight-in to the east.  They like to keep you high, but that's OK because they also keep you fast.  :)

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Nope - certain STARs are steeper than others. Add slippery plane and you get a need for more speedbrakes, possibly...

Thanks for that, Kyle. Learn something everyday.

 

Rick Wilson, thanks for that link. Interesting read.

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That's just another typical day in MIA.  Quite often it's 230 knots, flaps 5 with the speed brakes out all the way down.  Worse case scenario there is coming in from the west and landing straight-in to the east.  They like to keep you high, but that's OK because they also keep you fast.  :)

 

ha - yep. I appreciated it a little more having flown it several times in the sim. Luckily, it was HILEY6, landing East, so they had the long downwind and vectors to get down and slow down.

 

Coming out of cruise:

IMG_0177.JPG

 

Next step down in altitude:

IMG_0178.JPG

 

Flaps out to drag down under 10:

IMG_0180.JPG

 

Rollout pic for completeness (C-32 in the background for effect):

IMG_0186.JPG

 

[Full size version of the last pic in case anyone wants to see the C-32 more clearly - be aware, it's 3024x4032]

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Energy management is one of the big struggles pilots have with the 800. I've jumpseated on the 737 a number of times, and it's been something that the captain comments on every time in the approach briefing. I've also seen the speed brakes used extensively in the descent. If they're planning on a short approach, then they'll sometimes drop the gear early to help in getting slowed up.

 

High energy approaches are also something I keep in mind as a controller, and we've been told to be keenly aware of the workload that it can put on pilots. If I have a sequence where I can put a 737 on a short approach, I'll look at the type. If it's a -400 or -700, then I can usually get them in pretty tight without any problems. If it's a -800 or -900, then I'll think twice before trying to set them up for a tight approach. 

 

If you want to get a taste of how energy management works in the real world, next time you fly an approach hold 210 kts to a 15 mile final, and then 170 to the FAF. This is typically how we set aircraft up on the finals here at IAH. It can be challenging, but I find it to be a satisfying way to fly approaches.

 

I live near Atlanta airport and listen to Atlanta Final Approach on Live ATC, and those are the usual instructions, 210 to around a 15 mile final slowing to 170 at the FAF. Repeated day in and day out. 

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Remember, flaps are used to slow down the aircraft and speedbrakes are used to increase drag. We do not use flaps as drag devices, therefore use “Flaps to Slow Down and Speedbrakes to Go Down!”

 

Flaps are used for:

  1. Lift
  2. Drag

There is a crossover point where the flap will produce more drag than lift, but an increase in lift means there's a corresponding increase in drag. See induced drag. For a more basic example, think of why we need a rudder for turns (the increased lift on the up-side wing requires rudder force to counteract the increased drag caused by that increase in lift).

 

The speed brakes on most of the jets of today are not speed brakes as much as spoilers. As such, they're not there for drag as much as killing lift, which steepens descents simply by causing the wing to not do its job as effectively.

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This is an interesting topic and luckily I am meeting couple of my coworkers tomorrow who are 777x wing engineers and I shall ask them just how much those speed breaks have in reducing airspeed in flight. Although, from Kyle's photos of the speed breaks being deployed in flight, I can't think it would slow you down that drastically with only two panels. From my flight training, slowing down an aircraft is always throttle back and nose up if you want to really slow down.

 

And by the way, flaps are never to be used as a way of slowing an aircraft down, the real purpose of the flaps are to change the camber of the wing so to lower your stall speed so you can land at a slower speed, or to provide more lift at take off depending on flap settings.

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