Spoilers & 737NG behaviour

[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.

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.

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

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.