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WarpD

Steep / Shallow descent

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Hi all,there are two planes I really like flying. Wilco's A330 and PSS' 757.But both have some issues with their descent phase from high altitude.A330 - I know that the Airbus wing is really efficient at high AOA's, thus making them hard to slow down, but when you're really light and still can't get to green dot speed (about 200 knots) with a vertical speed of -600 is a bit much.757 - To maintain a descent speed of abou 300 to 310, you have to maintain a descent rate of close to -4500. That's way to much. You either have to start descent very close to the airport, or use the autothrottles to maintain speed. Again unrealistic.It seems both these aircraft can benefit from the same value albeit in different directions. I was thinking along the lines of raising the idle thrust on the 757 (you need a lot of power to taxi too) and I was thinking of raising the induced drag scalar on the A330.My question is, how do you raise the idle thrust? And does it affect the cruise thrust fuel flow settings?For the A330, I guess raising the induced drag will raise thrust required to maintain speed at altitude, so to keep the same flow, I would need to lower the flow scalar correct?I have it somewhat figured out, I just need someone more experienced to tell me if I'm right on or if I'm talking out of my arse.Regards,Xander

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A330: 200kts at what altitude?757: No, it's not "too much". It all depends on altitude and weight and a bit of throttle is actually normal for the 757/767.I notice you're not actually stating if you're manually flying the descents or using an autopilot to do so.

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A330: Out of 10000' with a V/S of -600. The plane stays frozen at 230 knots. And this is with a very light gross weight. Would be worse then at MLW.757: Out of FL200 I still need more than -4000 to maintain 300 to 310 knots. And at mid weights.I fly MD-80's for a living and at mid weights, you start TOD at -2800 to -3200 depending on weight. This gradually decreases to -2200 to -2400 just before deceleration to 250 at 10.000'. To decelerate you should get ample decel at -700. Once at 250 knots, a rate of-1600 keeps you around that speed. Now I can understand the A330 with it's weight being a little hard to slow. But I think it may need a little extra drag down low. And as for the 757, I think for any heavy jet, -4500 at FL200 to maintain 300 knots seems a bit much.Don't you agree?Xander

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I'm not a pilot, nor am I an expert. I also know very little about the Airbus line of aircraft.However, you indicate you're doing both descents at idle throttle. I don't know if that is the correct method for the Airbus... but I do know that for the 767 idle thrust during descent is not unusual. I also know that the wing on the 757/67 is extremely efficient.I believe for the 757 your descent airspeed is a bit high. At that point in your descent your target speed should be closer to 270kts. The rule of thumb for the 757/67 is slow down, early. It's an aircraft that can get away from you speed wise during approach. If you're not at approach speed at the right time... you won't make it during approach at all. Unless you toss out a boat anchor. ;) I would recommend Mike Rey's checkride handbook for more information regarding flying the 757/67.I don't know who's 757 you're flying, so I can't comment on accuracy of it's flight model.

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The descent rate for the 757 at mid-weight would be in the order of 2500fpm at mid-altitudes. To give you a specific example; at 180000lbs/290kts/20000ft idle descent would give you 2300fpm.In this example descent rate would be highest during the mach phase of the descent, reaching a maximum of about 3500fpm just before the transition to CAS descent speed. Descent rate would then range from 2500 to 2100 down to 10000ft, and then about 1500fpm at 250k for the remainder of the descent.I've taken these numbers from real world data.

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> as for the 757, I think for any heavy jet, -4500 at FL200 to maintain 300 knots seems a bit much.That's an L/D of 6.76, and it's way too low. Boeing would never let specs like these off the drafting table!For the 757 I guesstimate best L/D of 27 @ 180 KIAS, so conservatively L/D @ 300 KIAS could be around 15, i.e. a descent rate of 2,027 fpm.Cheers,-jahman.

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I don't know much about the 757, but I know the Airbus aircrafts, including the A330.What you describe here is not a bug nor a bad aircraft behavior, it is absolutely normal. As I already explained on other forums, the A330 has a specific descent mode called "managed descent". In this mode, the aircraft will reduce engine thrust to idle and use its weight to maintain the required speed. Depending on the weight, the V/S may be very important, up to 5000 ft/min !!This is normal. The FCU is just doing what you would do manually: reduce to idle and fly the aircraft to maintain to target speed. If you believe the descent rate is too high, you can use the V/S mode instead of the managed descent (the engines will be used to maitain the required V/S and the target speed) or you can reduce the target speed. By default, the FMGC has a descent target speed that is the same as the cruise speed (something like 300kts/M0.82), it is quite high.My advice is that you should NOT increase the drag. If you do this, the aircraft will have to take even more important V/S to maitain the speed!!For more information, read this:http://www.wilcopub.com/downloads/Airbus_Series_2_FAQ.pdfEric

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