Oli
Topic Author
Posts: 25
Joined: Mon Jun 20, 2005 5:43 pm

747 "classic" Descent Mode

Fri Jan 01, 2010 7:10 pm

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
 
Fabo
Posts: 1151
Joined: Tue Aug 16, 2005 1:30 am

RE: 747 "classic" Descent Mode

Fri Jan 01, 2010 8:50 pm

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 avatar
CCA
Posts: 774
Joined: Wed Oct 02, 2002 7:29 pm

RE: 747 "classic" Descent Mode

Sat Jan 02, 2010 2:50 pm

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.
P1 in A330, A340, A346, B742, B744, B748.
 
Oli
Topic Author
Posts: 25
Joined: Mon Jun 20, 2005 5:43 pm

RE: 747 "classic" Descent Mode

Tue Jan 05, 2010 7:45 am

HI guys,

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

Happy flying,

Olivier
 
Fabo
Posts: 1151
Joined: Tue Aug 16, 2005 1:30 am

RE: 747 "classic" Descent Mode

Tue Jan 05, 2010 9:09 am



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.
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