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Speed brakes efficacy?

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Am I wrong in thinking that the speed brakes don't seem to do anything below flight detent? I've used them a few times on a rather slow descent (100-1500 fpm), however, the plane doesn't slow down at all unless I pass flight detent? This can't be right?Chris

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"however, the plane doesn't slow down at all unless I pass flight detent? This can't be right?"The PMDG 737 shouldn't be allowing you to go past Flight Detent in the air. Sounds like something's not working properly or is out of sync??? On the real aircraft it's possible to go past Flight Detent, but not allowed (except, I believe, in emergencies).Do you hear the speedbrakes operating when you pull the lever back?Rgds.Ian.

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Yes, just prior to flight detent, the noise kicked in. Everything else works fine, I can't understand why it would be so poor at slowing the bird down.Chris

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Hi ChrisAs far as I know the speed brakes are indeed working even before you reach flight detent. Please remember that this aircraft is hard to slow down though. Also note that you shouldn't be able to go pass the flight detent when the PMDG bird is off ground as stated above. If you can something is screwed up.Also make sure your A/T is not working against the speed brakes when they are deployed. Typical rates of descent in a M0.78/280/250 descent for the real bird is 2200 fpm clean and 3100 fpm with speedbrakes deployed. And as far as I've seen the PMDG NG hits these values pretty much on the money.Hope it helps,Cheers,

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Hi ChrisWhat about an emergency decent? Shouldn't it be possible to fully deploy the speed brakes (as they do on the ground)? I never managed to do so with the PMDG 737NG. Or is it just normal not to fully deploy them even in an emergency?-Andreas

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Hi Matts,A/T is surely working against it. What is the procedure? Turn of A/T?regards,Samim

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Hi Samim,Well. If the A/T is working against the speed brakes there is no real use of the speed brakes, now is it? This means that the NG is flying at commanded speed, and that is what we're aming for. What you should do in that case is to decrease the FMC or MCP speed (whichever target speed the AFS is trying to keep, check the FMA). The A/T will decrease to idle and the speed will drop. If you need a faster decrease in speed, deploy speed brakes as necessary.Hope it helps,

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Chris,I know exactly what you mean ! They behave more like popping a hand up from the side window don't they ? Much less efficient than the speed-brakes we were all used to from other freeware/payware products right ? But this is what they are in real life. I stood corrected by our pilot advisors when I initially modelled their efficiency (already too low compared to what circulates on the net). Yet speed brake efficiency had to become even less and was promptly re-adjusted to the levels you see now. To give you an idea below 10000 ft usually you maintain 250 at a VS of say 1700 fpm (dependes on weight) and idle thrust. With full speed brakes out you can do 2300 pm at most. That is the real figure and that is the way they are tuned. I repeat the twin-parachute effect featured by FS default and other aircraft DOES NOT exist in the real world.Incidentally, over a speed limit speed brakes should become even less efficient due to speed blow-down. Regrettably, (or thankfully in view of your post ?) this cannot be modelled within FS.Regards,Vangelis===================================== Evangelos M. Vaos Precision Manuals Development Group www.precisionmanuals.com=====================================

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It's actually a misnomer to think of them as "speed brakes" - they are in fact "spoilers". The primary function of the spoilers in flight is to reduce lift, thereby allowing the flightcrew to increase the rate of descent without gaining excessive speed"Gaining" speed is the key. The spoilers really are not intended to be used as a speed-management device in the sense of bleeding off excess airspeed. Airspeed management should be done first and foremost with a combination of engine power and pitch. There are times when a higher-than-standard descent rate is called for - meeting a crossing restriction with a higher than expected tailwind for instance, or to comply with last-minute ATC instructions. If a descent is already in progress at flight idle, the only way to further increase descent rate (without spoilers) would be to decrease pitch (point the nose down), which might well cause airspeed to increase beyond a desired limit. (Such as 250 kts below 10,000 feet.) Deploying spoilers will usually give an additional 1000 FPM in the descent rate, without having to decrease pitch.They will act as "true" speed brakes only when deployed in level flight, and while there may occasionally be a need to use them in that fashion, it really isn't all that common.The spoilers' other main function of course, is as a lift-dumping device on landing, when they are deployed fully. Again, they are not functioning as "speed brakes" during the landing rollout - any braking action comes almost entirely from a combination of wheel brakes and reverse thrust. The fact is that the "speed braking" effect of spoilers is way over-done in many FS aircraft. I think PMDG has it about right on the 737NG.Jim Barrett

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Okay, well thanks for the replies. I'm used to an extremely strong slowing effect with other FS planes. I'm just surprised that the slowing doesn't even seem to be there when descending. How then, does one slow down when drag is required to obtain the speed desired per FMC or my own limits? I know from watching that A/T isn't working against the spoilers when I use them. Engines are around 35% N1 and the speed bug is around 15 - 20 knots below. I usually combat this by programming the FMC to descend early and slow, and also putting in speed restrictions in much earlier than I normally would.Chris

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