Zarniwoop From United Kingdom, joined Apr 2005, 265 posts, RR: 0 Posted (8 years 8 months 3 weeks 4 days 11 hours ago) and read 2247 times:
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?
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.
CosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 5, posted (8 years 8 months 3 weeks 4 days 4 hours ago) and read 2112 times:
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.