When to Descend
The flip side of the descent error is when the applicant knows there is a lower altitude available but doesn’t know when to initiate the descent. The problem centers on the word “established”. The controller is giving you vectors and tells you to maintain X-thousand feet until established on the final approach course; or, upon intercepting final out of the procedure turn you can descend to the intermediate altitude when you get established inbound. But at what point are you “established”?
When I learned instrument flight back in the mid-seventies I was taught that I could descend as soon as the needle came alive (off the peg at full-scale). However, navigation receivers will sometimes cause their CDI display needle to begin to quiver some distance from the desired course, and then maybe fall back to the full-scale deflection again for quite a while before becoming truly alive. In order to prevent the overly anxious IFR pilot from initiating a descent (unsafely) at the first misleading movement of the needle the guidance was later changed to the needle being at half scale (2.5 dots) from the center before the airplane is considered to be established on the course.
Some applicants believe the needle has to be centered on the final approach course before they are officially “established” and can therefore descend. If, say because of a strong crosswind, they take a long time to get the needle centered they will be at or near the final approach fix at way too high an altitude. As instructors we must tell our students to do the 6 T’s at every fix or intercept, and that they have intercepted (become established on) the new course when the CDI needle reaches half scale. That way the applicant can descend while making the final correction to get exactly on course (needle centered). Then they will cross the final approach fix at the correct altitude and be able to further descend to the MDA with time to spare, as it should be.