IR written Primary/Secondary instrument questions

(Andrew Sarangan) wrote
I believe strongly in the primary/supporting instrument concept.

I don't.

For
the experienced pilot, this may not make any sense because they look
at the whole panel and figure out what's going on. But a beginning
student needs clear directions.

I agree with this part completely.

The primary/supporting concept provides that direction.

It absolutely does. I just think it provides suboptimal direction.

If you let a new student loose without
teaching the primary/supporting, they will eventually learn the
correct technique, but it will take longer. There is also the
potential for omitting one instrument in the scan.

I think everything you say would be 100% correct if the only choices
were "teach the student a scan based on the primary/supporting method"
or "let him flounder around and find his own way." I do not in any
way disagree with the value of teaching a scan. I simply do not agree
that "primary and supporting" is the best scan to teach. Neither does
the military, which teaches the performance/control method
exclusively.

From a control theory viewpoint, the primary/secondary method is
primarily a feedback method, and the performance/control method is
primarily a feedforward method. Of course this is a gross
oversimplification - there are feedback and feedforward components to
each - but it's a matter of emphasis.

In the performance/control model, the emphasis is on the attitude of
the airplane. This is either read directly off the AI, or inferred
from the TC and ASI in partial panel flight. That centers the scan.
If you keep the wings level, heading will stay approximately constant.
If you keep the wings banked, you will turn. If you keep the nose on
the horizon, you will hold altitude. If you raise it, you will climb.
Lower it, and you will descend. The correct pitch and bank angles
vary based on the performance of the airplane, but they are pretty
well known in advance, so the method is more about acting than
reacting.

On the other hand, the primary/secondary method is based on reaction.
In level flight, the primary pitch instrument is the altimeter. So
let's say the altitude goes up. Why did it go up? Did you let your
nose come up? How much? By the time you saw the result on the
altimeter, it might have been a lot or a little. How much should you
correct? What if your nose is still on the horizon, and you simply
entered rising air?

Of course this is more important when flight attitude changes - at
which point the FAA gives up and makes the AI primary, making the
methods equivalent. But why should it not be primary all the time?

Michael