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747 "classic" Descent Mode  
User currently offlineOli From France, joined Jun 2005, 24 posts, RR: 0
Posted (4 years 8 months 3 weeks 1 day 13 hours ago) and read 2910 times:

Hi,

I have a question regarding the descent mode to use on a 747/200 or 747/300.

I suppose that, at high altitude, you would use the IAS mode with the throttles at idle position.

Now, what would you use let us say below 10000 feet - same mode, or the Vertical speed mode + autothrottle controling speed?

Thanks in advance for your input.

Olivier

4 replies: All unread, jump to last
 
User currently offlineFabo From Slovakia, joined Aug 2005, 1219 posts, RR: 1
Reply 1, posted (4 years 8 months 3 weeks 1 day 11 hours ago) and read 2873 times:

Whatever pilot prefers, I imagine. I'd use M hold until usable IAS comes by and switch to IAS hold when it does. Then, shortly above 10k, set 245 or minimum clean+5 or 10, whatever higher. But that is just me. Then, if you had a set objective (say, cross DEDIS at FL090) why not use VS?


The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
User currently offlineCCA From Hong Kong, joined Oct 2002, 836 posts, RR: 14
Reply 2, posted (4 years 8 months 3 weeks 17 hours ago) and read 2737 times:

I would usually use V/S to start the descent and maintain the descent Mach No until intercepting 300kts and hold 300kts in VS for a bit until it was stable and then engage IAS hold.

Mach hold on descent wasn't very good it tended to chase the Mach No once it was disturbed not very comfortable, the same thing could happen in IAS hold.

ie if accelerating through say 280 kts towards 300 in VS and you selected IAS hold as you got to 300 the A/C would continue to accelerate to say 310-320 pitch up to get 300 back but then go through it to 290-280 pitch down again and so forth. I loved the fact that the Autopilot wasn't perfect and it took a little skill to make things happen nicely.

I tried several different options to see what worked and what didn't.

Once descending at 300kts in IAS hold and nearing 10,000FT I would select VS reduce to 500FPM and slow to 250kts then increase the VS to hold 250kts for a bit then reselect IAS hold.

Ideally an idle descent would be made and VS just to reduce the descent rate when low on profile. IAS hold and advancing thrust to reduce the decent works but was never as tidy as VS and using the thrust to control speed.

However there is more than one way to skin a cat and I'm sure there are plenty of other versions.



C152 G115 TB10 CAP10 SR-22 Be76 PA-34 NDN-1T C500 A330-300 A340-300 -600 B747-200F -200SF -400 -400F -400BCF -400ERF -8F
User currently offlineOli From France, joined Jun 2005, 24 posts, RR: 0
Reply 3, posted (4 years 8 months 2 weeks 5 days ago) and read 2544 times:

HI guys,

Thanks very much for your answers - it is very clear indeed.

Happy flying,

Olivier


User currently offlineFabo From Slovakia, joined Aug 2005, 1219 posts, RR: 1
Reply 4, posted (4 years 8 months 2 weeks 4 days 23 hours ago) and read 2526 times:



Quoting CCA (Reply 2):

So you are basically doing what I would, only you have a method to smooth the descend. I actually have no idea of 747 Classic autopilot response, so it never occured to me it could be problematic to just hold the speed. Thanks for interesting input.



The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
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