VOR approach- when to descend from the FAF

On Tue, 19 Feb 2008 05:22:18 -0800, Sam Spade
wrote:

Les Izmore wrote:
On Mon, 18 Feb 2008 07:24:39 -0800, Sam Spade
wrote:


Terence Wilson wrote:

On Sat, 16 Feb 2008 19:01:14 -0800, Sam Spade
wrote:



Terence Wilson wrote:


When flying a VOR approach in which the FAF is defined by the VOR, is
it appropriate to start the descent to the MAP, or next stepdown,
immediately after overflying the VOR or should I wait until I have
positive course guidance for the final approach segment?

I ask because it can sometimes take 1-2nm for the CDI to move, which
may require a rushed descent to the MAP on some approaches.

Are you a simulator only pilot?

I ask, because that is a question that would be answered during
instrument training.


Sam, I think you asked me that question before and I answered .

I'm a RW instrument student.

Forgive me my lousy memory. I figured your CFI-I would have covered
this by now.

When the two-from indicator indicated "from," you should turn to the
appropriate heading and begin descent at that time. You are effectively
on-course because you are over the station where the courses are very,
very small. Sort of like lines of longitude at the North Pole.



Now, the question is, when you are 2 miles from the VOR and the needle
still has not moved (because of bad navigation, winds, whatever),and
you have descended xhundred feet, what do you do?


Are you going back up? Probably not.

Starting down without at least some indication that the needle is
moving back towards center can very well be the beginning of the
pernicious and well-known accident chain.

Don't do it.



That would represent issues of lack of competency to be 2 miles from the
station and not have a needle moving back towards center. In fact the
needle should be moving back towards center far sooner than that.


Well, that is precisely the reason why it is bad advice to start down
with no indication of movement of the needle.

If it does indeed start moving far sooner than that, then there is
minimal if any penalty for waiting.

If it does not start moving sooner than that, there is good reason
(issues of incompetency being one) not to have already begun the
descent.

So I'll say it again for emphasis. Until there is some indication
that the needle is moving toward the center, indicating a return to
the desired course, don't descend below the FAF altitude.

(The 2 miles, incidentally, was what the poster used as a basis for
his question).

Sam Spade wrote:

I ask, because that is a question that would be answered during
instrument training.

Do you think that every instrument instructor has the knowledge to
provide informed opinions on subjects like these?

Les Ismore wrote:

Well, that is precisely the reason why it is bad advice to start
down
with no indication of movement of the needle.

I think your advice is good and would be more easily defensible if you
were being evaluated by a check airman. Very few of them are TERPS
experts.

pstanley wrote:

You do not need pos course guidance to start descent.

Such a statement needs documentation, IMO.

Greg Esres wrote:
Sam Spade wrote:

I ask, because that is a question that would be answered during
instrument training.

Do you think that every instrument instructor has the knowledge to
provide informed opinions on subjects like these?

No, but the good ones know.

On Tue, 19 Feb 2008 08:07:11 -0800 (PST), Greg Esres
wrote:

pstanley wrote:

You do not need pos course guidance to start descent.

Such a statement needs documentation, IMO.





This is being treated like some kind of academic
angels-on-the-head-of-a-pin question. It is not.

Stuff happens. I have seen enough "competent" pilots screw up a
course intercept to know that it doesn't take all that much for a
pilot to end up well below the FAF altitude with no positive course
guidance, if he starts his descent with none. Distractions can mount
up in a hurry, and one can find himself in a situation with a high
pucker factor in a hurry. Few pilots who find themselves in this
situation then take the proper corrective action, in my experience.
They usually "try harder" to reintercept the course while going still
lower. This practially always ends up in a situation that could be
disastrous in actual conditions.

Advising someone new to instrument flying that it is an OK thing to
start a descent with no positive course guidance is a really bad
thing, in my opinion, and I base that opinion on watching a lot of
pilots, many of them experienced and instrument rated, screw
approaches up pretty badly, for any number of reasons.

I don't care what the "book" says.

Les Izmore wrote:

This is being treated like some kind of academic angels-on-the-head-
of-a-pin question. It is not.

Not to you, maybe. People ask questions here for a variety of
reasons: 1) legality, 2) safety, 3) passing checkrides, 4) curiosity,
etc.

Regarding safety, no doubt you're 100% correct.

We have a local VOR approach with a course change at the FAF and a
high descent gradient on final. Without descending immediately pass
the VOR, the odds of getting in shrink rapidly.

Peter wrote:
For what this is worth, on the European IR one is not supposed to
descend (from the platform altitude) unless one is established within
5 degrees of the track. I don't know the reference but this is what I
was always taught. To me, it makes sense and the only time I would
deviate from it would be


ICAO standards for "established" are 1/2 scale deflection. The US
doesn't have a similar definition, unfortunately. Sam Spade was
arguing that, over the VOR, that standard doesn't have much meaning.
For a staying-in-protected-airspace point of view, I'm sure he's
correct.

On Feb 19, 2:53*pm, Les Izmore wrote:

This is being treated like some kind of academic
angels-on-the-head-of-a-pin question. *It is not.

It is academic.

Two choices, very simple. Either you are on the final approach
course, or not. If not, you go missed, nothing complicated.

They usually "try harder" *to reintercept the course while going still
lower. *This practially always ends up in a situation that could be
disastrous in actual conditions.

Then that pilot is behind the plane. See above for choices of
action. Really very academic. It may not be a training issue, but a
currency issue, thus the IMPORTANCE of staying current.

and I base that opinion on watching a lot of
pilots, many of them experienced and *instrument rated, screw
approaches up pretty badly, for any number of reasons.

At anytime a pilot "screws up" as you alleged, missed is always that
option. Very academic. I personally experience quite the opposite
for IA pilots when I was their safety pilot, they get it right and
never gone full deflection. I am not a CFI by any means for what this
is worth.

I don't care what the "book" says.

Book is very clear. If you are not on the final approach course
within the established tolerances, you go missed. What part of the
book is hard to understand?

The orignal question is a very elementary question for IA flying.

For what it's worth, in my 600 hours of instrument flying, it takes a
hell of a lot more then 1 or 2 miles for the CDI to start moving. If
that is happening to the original poster, or you experienced that,
then there is something wrong with that station signal or airplane
equipment and a missed approach is in order. KMBO has the VOR alpha
approach and I can pick up the VOR signal strong on both my NAV1 and
NAV2 within 20 miles.

When you are within 1 or 2 miles of a final approach fix (I.E. VOR) it
is normal to get a full deflection from that "zone of confusion" for
the CDI. The needles will recapture within a mile or two of station
passage.

But you do not descend before getting the from flag on your NAV1 or
NAV2 AND being on the final approach course. If you don't get a from
flag you go missed even if you are on the final approach course as
something is radically wrong equipment wise.. Note the word AND, both
being on the approach course AND from flag, BOTH must be present.
VERY SIMPLE!

The original question was so basic, that it sounded like an Mx
question..

Allen

Greg Esres wrote:
Peter wrote:
For what this is worth, on the European IR one is not supposed to
descend (from the platform altitude) unless one is established within
5 degrees of the track. I don't know the reference but this is what I
was always taught. To me, it makes sense and the only time I would
deviate from it would be


ICAO standards for "established" are 1/2 scale deflection. The US
doesn't have a similar definition, unfortunately. Sam Spade was
arguing that, over the VOR, that standard doesn't have much meaning.
For a staying-in-protected-airspace point of view, I'm sure he's
correct.

Correct. The protected airspace for a VOR IAP overhead the VOR when
it's the FAF is 1 mile each side of the station. This width expands as
the aircraft departs the VOR/FAF. The maximum course change permitted
at the VOR/FAF is 30 degrees.

Descent shouldn't be made unless the pilot is tracking on course inbound
to the VOR/FAF. If he is on course as station passage occurs, and a
course change is required, a turn to intercept while commencing descent
is expected in the design of the criteria; i.e., the descent gradient is
calculated from the VOR/FAF, not from some imaginary post-facility point
of intercept, and the maneuver required to reintercept is minimal
assumming reasonable comptetency.