zarniwoop
Posts: 255
Joined: Wed Apr 27, 2005 11:43 pm

When To Start Turning?

Fri Dec 09, 2005 6:19 pm

I was wondering.....

How do you know when to start a turn?
For example when you are cruising and you are coming up to a waypoint and you need to make a turn at the waypoint to take a different jetway. How far from the waypoint do you start your turn to intercept the different course?
Would it be any different when you are flying slow on an instrument approach?

I guess it is not hard to judge in a small light aircraft but in a large airliner travalling at M0.8 you have to think about it a bit more?

Is it all based on a standard 2 minute turn?

Thanks in advance......
 
Pihero
Posts: 4248
Joined: Mon Jan 31, 2005 5:11 am

RE: When To Start Turning?

Fri Dec 09, 2005 8:46 pm

Quoting Zarniwoop (Thread starter):
Is it all based on a standard 2 minute turn?

A "rate one" turn as it is (still ? ) called would call for a bank angle roughly equal to 15% of the ground speed. For passengers' comfort, airliners restrict this bank angle to 30°.At cruising speed of 500kt, the rate one will be achieved with an angle of 75°, which is above the design load factor.

The turn anticipation can be computed. A rate one turn and for a course change of 90°, it is worth 20 sec of flight :A combat airplane at 480kt would start turning some 3 Nm before the waypoint.An airliner under autopilot, restricted to 25° of bank woult start at 9 Nm.
Note that they won't fly over the waypoint.
Contrail designer
 
joness0154
Posts: 650
Joined: Sat Nov 19, 2005 12:56 am

RE: When To Start Turning?

Fri Dec 09, 2005 11:49 pm

Quoting Pihero (Reply 1):
Note that they won't fly over the waypoint.

It depends. Some approaches (GPS comes to mind) have waypoints specified that you must fly over.

Quoting Zarniwoop (Thread starter):
Is it all based on a standard 2 minute turn?

Well, flying IFR you don't want rate of turn outside the standard rate. But the faster the aircraft, the slower the rate of turn

Small slower aircraft have 2 minute turns
A lot of military aircraft have 4 minute turns, so that bank angle isn't overly excessive.
I don't have an attitude problem. You have a perception problem
 
Pihero
Posts: 4248
Joined: Mon Jan 31, 2005 5:11 am

RE: When To Start Turning?

Sat Dec 10, 2005 1:12 am

Quoting Joness0154 (Reply 2):
Quoting Pihero (Reply 1):
Note that they won't fly over the waypoint.

It depends. Some approaches (GPS comes to mind) have waypoints specified that you must fly over.

You're right but see the question. It has to do with anticipating the course change.
Contrail designer
 
joness0154
Posts: 650
Joined: Sat Nov 19, 2005 12:56 am

RE: When To Start Turning?

Sat Dec 10, 2005 1:19 am

Quoting Pihero (Reply 3):
You're right but see the question. It has to do with anticipating the course change.

Yeah, I understand now, sorry I misread it the first time.
I don't have an attitude problem. You have a perception problem
 
CosmicCruiser
Posts: 2051
Joined: Tue Feb 22, 2005 3:01 am

RE: When To Start Turning?

Sat Dec 10, 2005 2:09 am

In most modern jets the FMS has computed the turn and won't exceed 25 deg of bank. Unless the specific waypoint is a mandatory flyover point the turn will "undershoot" the waypoint so as not to "overshoot" wide on the airway, assuming a large change in course heading.
 
User avatar
zeke
Posts: 10374
Joined: Thu Dec 14, 2006 1:42 pm

RE: When To Start Turning?

Sat Dec 10, 2005 2:50 am

Quoting Zarniwoop (Thread starter):
How do you know when to start a turn?

Follow the flight director.

Quoting Zarniwoop (Thread starter):
Would it be any different when you are flying slow on an instrument approach?

Depends on how you program the FMC/FMGC, still follow the flight director.
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