Complete Reversal or Not?

[Greg Esres]

Can anyone suggest any reasons why you shouldn't use GPS or DME to
lead your turns when approaching a fix, such as an IAF, on a non-GPS
approach? (Meaning that you don't wait for complete reversal of the
nav instrument.)

The AIM has a couple of requirements for a "complete reversal" of the
TO/FROM indicator, but they aren't instructions about how to fly an
instrument approach. Regardless, those injunctions may be outdated in
a GPS world.

[John R. Copeland]

Here's a reference to a 19-page paper by Jim Terpstra of Jeppesen.
On page 6, there's a description of "Fly-Over" vs. Fly-By" fixes.
http://www.bluecoat.org/reports/Nav2001Bluecoat.pdf

Basically, he says that 'Fixes are charted as fly-over fixes only when
specified by the governing authority.'
I think that says there's no fixed rule, and you dare not guess.
If Jeppesen places a circle around the fix, you must fly over it.
---JRC---

"Greg Esres" > wrote in message =
...
> Can anyone suggest any reasons why you shouldn't use GPS or DME to
> lead your turns when approaching a fix, such as an IAF, on a non-GPS
> approach? (Meaning that you don't wait for complete reversal of the
> nav instrument.)
>=20
> The AIM has a couple of requirements for a "complete reversal" of the
> TO/FROM indicator, but they aren't instructions about how to fly an
> instrument approach. Regardless, those injunctions may be outdated in
> a GPS world.
>=20
>

[Greg Esres]

John:

Thanks for the link. However, fly-over vs. fly-by is a GPS only
concept. On a GPS approach, normally only the missed approach point
and the missed approach holding points are fly-over waypoints.

My present question focuses exclusivly on traditional approachs using
a VOR or NDB, which don't chart the fixes that way.

[Barry]

> Can anyone suggest any reasons why you shouldn't use GPS or DME to
> lead your turns when approaching a fix, such as an IAF, on a non-GPS
> approach? (Meaning that you don't wait for complete reversal of the
> nav instrument.)

The AIM discusses this in the section on En Route Procedures, but doesn't say
anything about instrument approaches:

5-3-5. Airway or Route Course Changes

a. Pilots of aircraft are required to adhere to airways or routes being
flown. Special attention must be given to this requirement during course
changes. Each course change consists of variables that make the technique
applicable in each case a matter only the pilot can resolve. Some variables
which must be considered are turn radius, wind effect, airspeed, degree of
turn, and cockpit instrumentation. An early turn, as illustrated below, is one
method of adhering to airways or routes. The use of any available cockpit
instrumentation, such as Distance Measuring Equipment, may be used by the
pilot to lead the turn when making course changes. This is consistent with the
intent of 14 CFR Section 91.181, which requires pilots to operate along the
centerline of an airway and along the direct course between navigational aids
or fixes.

[Michael]

Greg Esres > wrote
> Can anyone suggest any reasons why you shouldn't use GPS or DME to
> lead your turns when approaching a fix, such as an IAF, on a non-GPS
> approach? (Meaning that you don't wait for complete reversal of the
> nav instrument.)
>
> The AIM has a couple of requirements for a "complete reversal" of the
> TO/FROM indicator, but they aren't instructions about how to fly an
> instrument approach. Regardless, those injunctions may be outdated in
> a GPS world.

Yes, I can suggest a reason. What you are proposing is very precise
and very easy, and unless one is careful to maintain proficiency in
doing it the old-fashioned way, it will lead to a very insidious form
of GPS dependence.

In fact, I generally do what you suggest in real life because:
(a) ATC RADAR is not good enough to tell anyone I'm doing it, so I
don't care too much about the legality - and in any case, like you I
can't find anything regulatory to say I can't.
(b) It allows me to become established much more quickly and
precisely.
(c) It's just much easier to anticipate the turn than it is to wait to
cross the fix, and then wind up with a needle out of place that has to
be returned.

This is all great until the GPS goes TU. Suggest you goole r.a.ifr
for a previous thread on the topic: GPS Dependence - more insidious
than I thought...

It's not exactly on point but covers a lot of this same ground.

Michael

[C J Campbell]

"Greg Esres" > wrote in message
...
| Can anyone suggest any reasons why you shouldn't use GPS or DME to
| lead your turns when approaching a fix, such as an IAF, on a non-GPS
| approach? (Meaning that you don't wait for complete reversal of the
| nav instrument.)
|
| The AIM has a couple of requirements for a "complete reversal" of the
| TO/FROM indicator, but they aren't instructions about how to fly an
| instrument approach. Regardless, those injunctions may be outdated in
| a GPS world.

I was taught as an Air Force navigator back in the 1970's to always begin my
turn before actually reaching the fix in order to avoid overshooting the
outbound (from the fix) course. I have never heard anything since then that
would suggest a better course of action.

[[email protected]]

"John R. Copeland" wrote:

> Here's a reference to a 19-page paper by Jim Terpstra of Jeppesen.
> On page 6, there's a description of "Fly-Over" vs. Fly-By" fixes.
> http://www.bluecoat.org/reports/Nav2001Bluecoat.pdf
>
> Basically, he says that 'Fixes are charted as fly-over fixes only when
> specified by the governing authority.'
> I think that says there's no fixed rule, and you dare not guess.
> If Jeppesen places a circle around the fix, you must fly over it.
> ---JRC---

RNAV approaches have fly-by waypoints, except for the missed approach fix, which is usually a fly-over
waypoint.

[Roy Smith]

In article >,
"C J Campbell" > wrote:

> "Greg Esres" > wrote in message
> ...
> | Can anyone suggest any reasons why you shouldn't use GPS or DME to
> | lead your turns when approaching a fix, such as an IAF, on a non-GPS
> | approach? (Meaning that you don't wait for complete reversal of the
> | nav instrument.)
> |
> | The AIM has a couple of requirements for a "complete reversal" of the
> | TO/FROM indicator, but they aren't instructions about how to fly an
> | instrument approach. Regardless, those injunctions may be outdated in
> | a GPS world.
>
> I was taught as an Air Force navigator back in the 1970's to always begin my
> turn before actually reaching the fix in order to avoid overshooting the
> outbound (from the fix) course. I have never heard anything since then that
> would suggest a better course of action.

What were you flying? In a spam can doing 120 kts (which is the
environment most civilian instrument is done in), the turn radius is
pretty small. If you're flying a jet, leading the turn may be the only
way to avoid blowing out the side of the airway on sharp turns.

Another factor is that up until a few years ago when GPS started to
become ubuiqitous, most instrument trainers had no DME so a full TO/FROM
reversal way the only way to be sure you had reached the fix. If you
have DME or GPS and thus accurately know your distance to the fix, I
don't see any reason not to lead the turn.

[Teacherjh]

>>
Another factor is that up until a few years ago when GPS started to
become ubuiqitous, most instrument trainers had no DME so a full TO/FROM
reversal way the only way to be sure you had reached the fix.
<<

Or you could set a crossing radial in a second VOR receiver, and use that.
Some approaches even have a "lead in" radial charted.

Jose

--
(for Email, make the obvious changes in my address)

[C J Campbell]

"Roy Smith" > wrote in message
...
| What were you flying? In a spam can doing 120 kts (which is the
| environment most civilian instrument is done in), the turn radius is
| pretty small. If you're flying a jet, leading the turn may be the only
| way to avoid blowing out the side of the airway on sharp turns.
|
| Another factor is that up until a few years ago when GPS started to
| become ubuiqitous, most instrument trainers had no DME so a full TO/FROM
| reversal way the only way to be sure you had reached the fix. If you
| have DME or GPS and thus accurately know your distance to the fix, I
| don't see any reason not to lead the turn.

I was in C-130s. Now I fly a C-206, almost twice as big. :-)

Or, as my brother says, "Once a trash hauler, always a trash hauler." (He
prefers sleek little experimental.)

Even at 120 knots your turn radius is more than half a mile
(0.63661977236758134307553505349006 mile, to be a little more accurate).

By the time your TO/FROM flag has flipped you have probably gone at least
half a mile beyond the fix. So now you are a mile off course. In a C-130 you
could probably triple that -- I always planned my turns for a 2 mile radius
and it worked out pretty well. In the Cessna 206 I start my turn about half
a mile before the fix if I can. Those GPS units that give you turn warning
and show the turn radius on their moving maps seem to lead the turn about
half a mile, too.

[[email protected]]

Greg Esres wrote:

> Can anyone suggest any reasons why you shouldn't use GPS or DME to
> lead your turns when approaching a fix, such as an IAF, on a non-GPS
> approach? (Meaning that you don't wait for complete reversal of the
> nav instrument.)
>
> The AIM has a couple of requirements for a "complete reversal" of the
> TO/FROM indicator, but they aren't instructions about how to fly an
> instrument approach. Regardless, those injunctions may be outdated in
> a GPS world.

"5-3-5. AIRWAY OR ROUTE COURSE CHANGES" states you can lead a turn.
Whether you can descend to the altitude for the next segment during the
turn has been addressed at FAA/Industry meetings but the feds won't
resolve the issue.

With GPS, the correctly flown fly-by waypoint will never cross over the
fix, rather the arc of the turn will depart the inbound course at the same
distance as it intercepts the outbound course. The consesus at the
aforementioned meeting is that descent to the altitude of the outbound
segment should be able to commence at the bisector of the turn, provided
groun speed is limited to a value to assure that the aircraft remains in
protected airspace.