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AVSIG: 30/90 degree intercept? wwswsigarch.jpg (7236 bytes)

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AVSIG Discussion Sections >> Air Traffic Control

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Stephen Loeffler (SoCal TRACON)
Public Guest


Reged: 11/07/04
Posts: 366
Loc: San Diego, CA
30/90 degree intercept?
      #440852 - 07/24/17 01:34 AM

So we all feel stumped sometimes and it's always good to go back and review the rules. But this time I feel stuck. I am training a new guy, and he gives an aircraft direct to an IF on an ILS and clears them for the ILS. I ask him about the intercept angle (it was about 60 degrees), and he says he can do it because it's within 90 degrees of the final approach course. I was always taught 20/30 degrees for normal approaches, and 90 degrees for RNAV approaches.

So we go back, review the 7110, and he agrees he should not have given the aircraft that turn because it was only an IF, but still believes he can give the aircraft up to a 90 degree turn to a straight in final if it's at the IAF.

4-8-1
Paragraph a section 4 and 5:

4. Where adequate radar coverage exists, radar facilities may vector aircraft to the final approach course in accordance with Paragraph 5-9-1, Vectors to Final Approach Course, and Paragraph 5-9-2, Final Approach Course Interception.
5. Where adequate radar coverage exists, radar facilities may clear an aircraft to any fix 3 NM or more prior to the FAF, along the final approach course, at an intercept angle not greater than 30 degrees.


4-8-1
Paragraph e:

e. If a procedure turn, hold-in-lieu of procedure turn, or arrival holding pattern is depicted and the angle of intercept is 90 degrees or less, the aircraft must be instructed to conduct a straight-in approach if ATC does not want the pilot to execute a procedure turn or hold-in-lieu of procedure turn. (See FIG 4-8-3)



EXAMPLE: Aircraft 1 can be cleared direct to XYZ VORTAC, or
SECND because the intercept angle is 90 degrees or less.



For our example, I will use a local ILS we use a lot.

KPOC ILS plate (sorry, couldn't get an image of it)

The trainee believes you can be east of PDZ and clear the aircraft direct to LIZZE for a straight in ILS, no procedure turn. To me, this doesn't seem right, and goes against what I have read elsewhere (and earlier in this chapter!)

5-9-2 Paragraph a

a. Assign headings that will permit final approach course interception on a track that does not exceed the interception angles specified in TBL 5−9−1.


I wasn't able to find an image of table 5-9-1, but it basically says, either 20/30 degree intercept angle based on how far you are from the final approach gate.


I feel like the diagram in 4-8-3 is misleading, or I am wrong in thinking that the intercepts for an ILS need to be 20-30 degrees. Hopefully I can pull some of you out of the shadows to help me understand what the 7110 is trying to say. Of course, I can always provide more information if needed.

Stephen


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Ray Tackett
Top Gun


Reged: 04/30/04
Posts: 8892
Loc: Philadelphia, USA
30/90 degree intercept? [Re: Stephen Loeffler (SoCal TRACON)]
      #440854 - 07/24/17 02:02 AM

I was always taught and received 30 degrees or less, except ...

I have requested and gotten a more challenging intercept to save myself some
time or to help make sequencing work. I also would not take it amiss if ATC
asked whether it would be possible to fly such an intercept.

The 90 degree RNAV deal gets a lot of GPS help. The GPS looks at the ground
speed and figures out the turn lead for the pilot. I've never tried such on
an ILS, but have mentally worked out a conservative* turn lead.

* Conservative == a lead where I might have to shallow the bank angle to
complete the intercept rather than possibly getting into an excessively tight
turn to avoid an overshoot.

That said, the cockpit, especially solo IFR, is not a good place to be doing
math. Easy intercepts, e.g. "Join the localizer" when I'm fifty miles out
(has happened when coming from the right direction), can be a big help to the
pilot.

--------------------
Ray,

Owner, Lake Wood Be Gone

Turning quality lumber into sawdust and noise since 2013.


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sreyoB yrraL
AVSIG Member


Reged: 05/16/04
Posts: 9442
Re: 30/90 degree intercept? [Re: Stephen Loeffler (SoCal TRACON)]
      #440859 - 07/24/17 03:17 AM

Quote:

The trainee believes you can be east of PDZ and clear the aircraft direct to LIZZE for a straight in ILS, no procedure turn. To me, this doesn't seem right, and goes against what I have read elsewhere (and earlier in this chapter!)



The 7110.65 quotes say "assign heading". If you're cleared direct to a fix you're using some kind of RNAV which will lead the turn to provide a smooth intercept. On a vector, you are waiting for the localizer to come alive and will likely overshoot with higher intercept angles.

Are vectors and direct-to-fix treated differently with regard to the intercept angle?

I forgot you were in SAN. Great layover, and always fun doing the full LNAV/VNAV arrival like the LYNDI4 to the RNAV Z Rwy 27 (though the 3.50deg path makes unexpected speed reductions challenging). Went to a Padres' game in '15.


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Stephen Loeffler (SoCal TRACON)
Public Guest


Reged: 11/07/04
Posts: 366
Loc: San Diego, CA
Re: 30/90 degree intercept? [Re: sreyoB yrraL]
      #440864 - 07/24/17 04:04 AM

Quote:

Are vectors and direct-to-fix treated differently with regard to the intercept angle?




For as long as I've been working, direct to a fix was the same as a vector. It's just a bit more accurate.


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Stephen Loeffler (SoCal TRACON)
Public Guest


Reged: 11/07/04
Posts: 366
Loc: San Diego, CA
Re: 30/90 degree intercept? [Re: sreyoB yrraL]
      #440894 - 07/24/17 03:24 PM

Quote:


The 7110.65 quotes say "assign heading". If you're cleared direct to a fix you're using some kind of RNAV which will lead the turn to provide a smooth intercept. On a vector, you are waiting for the localizer to come alive and will likely overshoot with higher intercept angles.




Also says you can clear an aircraft to a fix 3 miles or more from the FAF, as long as the intercept angle isn't more than 30 degrees.

San Diego is a great place to live, for sure. The price of housing is humbling when you hear what other people pay for homes. It's getting out of control. Also, the 8 dollar beers are annoying.


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sreyoB yrraL
AVSIG Member


Reged: 05/16/04
Posts: 9442
Re: 30/90 degree intercept? [Re: Stephen Loeffler (SoCal TRACON)]
      #440896 - 07/24/17 03:35 PM

Quote:

San Diego is a great place to live, for sure. The price of housing is humbling when you hear what other people pay for homes. It's getting out of control. Also, the 8 dollar beers are annoying.



I love going to SAN. The weather is always "better". We stay at the Westin so lots of stuff in easy walking distance. USS Midway, Kansas City BBQ, Gaslamp, Werewolf for breakfast, Karl Strauss for beer, what's not to like?

Always have to remember to cross JETTI at or below 12,000' on the ZZOOO1 SID. (What do you have F16s departing there? That would be quite the climb rate!) What traffic is that protecting?


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Todd Alfes
AVSIG Member


Reged: 07/29/12
Posts: 41
Re: 30/90 degree intercept? [Re: Stephen Loeffler (SoCal TRACON)]
      #440915 - 07/25/17 02:08 AM

Stephen,

Yes, you clear an aircraft via LIZZE at courses up to 90 off of the localizer. The controlling paragraph is the one you mention, 4-8-1 e:

If a procedure turn, hold-in-lieu of procedure turn, or arrival holding pattern is depicted and the angle of intercept is 90 degrees or less, the aircraft must be instructed to conduct a straight-in approach if ATC does not want the pilot to execute a procedure turn or hold-in-lieu of procedure turn.

That's not the way I would have written that paragraph, but ask yourself "how can it NOT be legal" given what the paragraph says.

LIZZE is an IF/IAF, which qualifies the fix for use as either type of fix.

The chapter 5 stuff that refers to 20/30 degree intercepts does not apply in this case because the pilot is not receiving vectors to final - the pilot is being cleared for a pilot navigated approach with a little twist, the twist being that ATC is doing away with the holding pattern and clearing the aircraft straight in.

Clearing the aircraft this way allows the controller to move on to other priorities without blowing the turn-on later.


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Todd Alfes
AVSIG Member


Reged: 07/29/12
Posts: 41
Re: 30/90 degree intercept? [Re: sreyoB yrraL]
      #440920 - 07/25/17 02:33 AM

Yes, vectors are different than direct.

A vector is an assigned heading while direct is an assigned course. This is particularly important when we're applying our "passing or diverging" rule whereby we're allowed to discontinue vertical separation once we've established a minimum course divergence - which can be as little as 15 degrees. We can establish this by clearing aircraft to fixes whose courses are at least 15 degrees apart or by assigning two vectors that are at least 15 degrees apart.

The thrilling misapplication of this rule occurs when we apply a course to one and a heading to the other and then discover that we don't have 15 degrees. Fifteen degrees isn't much to begin with, and when you have the wind blowing it down to five degrees it gets people excited in a hurry.


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Scott Dunham (RDU)
Top Gun


Reged: 04/29/04
Posts: 6470
Loc: Chapel Hill, NC
Re: 30/90 degree intercept? [Re: Stephen Loeffler (SoCal TRACON)]
      #440932 - 07/25/17 06:37 AM

Reading the TERPS manual late at night always makes my head hurt, but yes, at an IAF that works. DME arcs are a good analogy that might make you feel better about it - the intercept angle is 90 degrees if the DME transmitter is on or in line with the final approach course, but the DME source can also be offset from the FAC. If it's on the other side of the localizer from the aircraft, the intercept angle can actually be more than 90 degrees and still legal. Because of the distance from the runway and the wider localizer course further from the airport, the intercept angle is less critical than when you're vectoring closer to the FAF, hence the 20/30 degree limits approaching the FAF.

The key is that the intercept has to occur within obstacle protected airspace, and far enough out to give the crew a sporting chance to get laterally established and stable before the FAF. TERPS establishes minimum distances for the intermediate segment from the arc/LOC intercept to FAF to do that. In the 7110 example, the critical distance would be from SECND to the FAF, and at POC it would be LIZZE to HAWNN. The holding pattern template and protected trapezoid at LIZZE should cover the obstacle clearance at the turn, but I need to look a bit more to see what guarantees that the IF length is adequate to get stabilized in all non-DME cases.


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sreyoB yrraL
AVSIG Member


Reged: 05/16/04
Posts: 9442
Re: 30/90 degree intercept? [Re: Scott Dunham (RDU)]
      #440937 - 07/25/17 02:10 PM

Quote:

I need to look a bit more to see what guarantees that the IF length is adequate to get stabilized in all non-DME cases.



I did my initial instrument training and rating in a C-310E without DME. Seemed 'normal' at the time. Looking back, it seems like having nothing better than sharpened rocks and bear skins for tools!


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