to HSI or not to HSI

"Michael" wrote in message
om...
"Richard Hertz" no one@no one.com wrote
Can you explain why that is the one advantage (BC)/revers on localizer,
and
why that is so?
Do you mean to say that people confuse which color sector they are in on
a
localizer due to "reverse needle"?

Yes, that is exactly what I mean to say.

If so then it is a training issue, not a
technology issue.

Oh man, here we go. You've just touched off a religious debate.

In real life, I run a department that designs instrumentation for
process environments. What that means is that engineers design it,
but generally non-engineers (plant operators, meter readers,
technicians) install and use it. These days, most instrumentation has
software in it, so it should not come as a surprise that I rose into
that position from software engineering.

In the process, I learned a lot about user interfaces. There are two
kinds of user interface bug. There is the kind where the user
interface acts contrary to design, in a useless or unpredictable
manner in a given situation (coding error) and there is the kind where
it acts as designed (intentionally or unintentionally), in a manner
that is predictable and useful but, in certain situations,
counter-intuitive to the operator (design error). The first kind is
unusuable in those given situations. The second kind is usable,
provided you read the manual and are aware of how the system will
behave. There are those who believe that this means it's not an error
- that you should simply RTFM. In other words, that it is a training
issue. They are wrong.

I do not disagree.


The "reverse" indication of a conventional CDI is a design error. You
can work around it. I have. I had to shoot a LOC BC approach with
engine failure at leveloff (simulating a failure of the engine to come
up on the powerup for leveloff) followed by a single engine missed
approach. I passed - meaning I executed the approach and miss to ATP
standards, and I have the certificate to prove it. Nonetheless, a
couple of times I found myself, with the needle half a dot off,
applying the incorrect contol inputs before I "caught" myself. So
clearly the training worked - I corrected before I deviated beyond ATP
standards - but that doesn't mean that the design is correct. It's
not. On an approach, you're used to correcting towards the needle.
Under normal conditions, you should have the situational awareness to
know what you are doing, rather than just correcting by habit.
However, in emergency conditions where the workload becomes high,
there is a tendency to revert to habit. In other words, the operation
becomes counter-intuitive.

Here disagree - the current use appears to imply a "design flaw" but that
is only because of imporper use and instruction of the instrument. It does
not "point" to the course, rather it shows what sector of the localizer
course you are on. (shaded or unshaded.)

It is a back course - meaning you are coming from the other way. You know
this. Clearly the instruction should be corrected - it is a lot cheaper
than everyone buying HSIs. I have no difficulty with localizer front or
back course, though I am certain I would mix things up if I had been taught
the improper (but more common) use of the needle pointing to the course
(except for BC which is different)



Sometimes this is unavoidable, but where this is done for no good
reason, it's simply bad design. It's really quite simple to modify a
conventional CDI for "reverse" sensing - all it takes is the addition
of a simple DPDT switch, and the needle will act correctly on the BC.
Thus I have to say it's done for no good reason. Only in aviation is
somehting like this not done - because this is how we've always done
it (and because the FAA would make such a modification
cost-prohibitive).

The UK (and I believe other nations) will not certify LOC-BC
approaches because the potential for error is believed to be too high.
I don't agree with this - I consider the potential for error to be
adequately low with proper training - but the addition of a cheap,
simple, and reliable part to the CDI (or replacement with an HSI)
eliminates the potential for error - and is thus clearly an advantage.

And what is thins part?

I contest that ensuring the blue and yellow colors on the instrument and
proper training would avert the confusion.


Michael