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CajunRon

Numbers on STAR chart legs

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I'm trying to get a better understanding of STAR charts.   I've search on Google but can't find the answer.   On the Navigraph STAR charts each leg of the STAR shows four numbers.  (I'm looking at the CHINS 3 arrival into KSEA)  First the heading for that leg, then what looks like an altitude which I understand is the minimum allowed altitude for that leg, then a number like 6600T and then the length of that leg (I think).

Question 1:  What is the meaning of 6600T?

Question 2:  If the STAR contains altitude restrictions at certain waypoints (12000 ft at AUBRN for example), why would the minimum altitude shown for the leg to that waypoint (7000 in this example) be less than the altitude restriction?   What is this number (the altitude for each leg) used for?

Thanks for any replies.

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The 6600T is a lower, minimum alritude for terrain ckearance. The other higher minimum altitude also assures navigation signal coverage while the 6600T does not. The 12000 crossing restriction is procedural, for atc reasons such as traffic, route, airspace, etc.

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Thanks for the reply.

I thought the STAR is a procedure so I'm still not sure I fully understand when the the nav signal altitude limit and terrain altitude limit would be used instead of the procedural altitude restriction.   I'm guessing the chart can be used as a reference if ATC doesn't instruct use of the procedure?

Edited by CajunRon

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It depends if it's a conventional nav, or an RNAV arrival.

If it's a conventional nav STAR, then the pilots would still need information regarding the altitudes for navaid reception. 

If it's an RNAV, they dont technically need it but it's never a bad idea to include an MEA (minimum reroute altitude). 

 

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2 hours ago, CajunRon said:

I thought the STAR is a procedure so I'm still not sure I fully understand when the the nav signal altitude limit and terrain altitude limit would be used instead of the procedural altitude restriction.

They wouldn't normally. But what if you had an engine failure and had insufficient performance to maintain altitude, or a depressurisation, for instance?

Now imagine you're inthe clouds and you can't see the terrain around you. 

Then it would be really handy to know the minimum safe levels you could descend to on a particular route without bumping in to some cumulo-granitus, wouldn't it? Especially if you're in mountainous terrain where the grid MORA (the minimum safe altitude across the whole of a grid square) might be significantly higher than the MSA on a particular defined route (which will only need take terrain a couple of miles either side of the route in to account rather than everything in the grid square).

Likewise, if ATC descend you below the STAR altitudes (very possible) how would you be able to verify your terrain clearance (yes, technically ATC have a responsibility... but at the end of the day it's not the controller who dies if you end up in a mountain)? Again, having those minimum altitudes published gives you much better situational awareness.

Remember - instrument procedures and charts are designed to be able to get you from A to B virtually without ever being able to see the ground, except for a brief moment during the takeoff roll and a brief few moments before touchdown (if at all, in the case of CAT III approaches). Obviously on a day to day basis in most parts of the world we tend to become accustomed to flying these procedures mostly in VMC where we can see the terrain around us, but when it's dark and cloudy knowing exactly where you are and how low you can go at any given point becomes of the utmost importance.

Also - not all procedures will have hard altitudes published all the way through and in these cases it may be very useful to know what the minimum level on any particular segment is.

Edited by skelsey

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7 hours ago, CajunRon said:

I'm trying to get a better understanding of STAR charts.   I've search on Google but can't find the answer.   On the Navigraph STAR charts each leg of the STAR shows four numbers.  (I'm looking at the CHINS 3 arrival into KSEA)  First the heading for that leg, then what looks like an altitude which I understand is the minimum allowed altitude for that leg, then a number like 6600T and then the length of that leg (I think).

Question 1:  What is the meaning of 6600T?

Question 2:  If the STAR contains altitude restrictions at certain waypoints (12000 ft at AUBRN for example), why would the minimum altitude shown for the leg to that waypoint (7000 in this example) be less than the altitude restriction?   What is this number (the altitude for each leg) used for?

Thanks for any replies.

I don't see a T after 6600 on the latest CHINS3 arrival.  I see an asterisk prior to 6600 (*6600).  The asterisk indicates MOCA, or minimum obstruction clearance altitude.  This would include towers and other obstacles, not just terrain.  The 7000 is your MEA, or minimum enroute altitude.  In other words, you should not be below 7,000 feet, but if you are, 6,600 is the lowest possible altitude you could go without risk of CFIT. 

The 24 indicates NM from SEA VOR.  The (16) is NM between AUBRN and HUMPP. 

As to question #2, the altitude restriction at AUBRN of 12000 and 250 knots is required for turbojets landing south.  You would head 343 degrees downwind and received descent instructions and vectors to land on RWY(s) 16.  For KSEA landings, you would likely be cleared lowered before AUBRN.  Look at the Husky Visual approach plate, it shows you be at or above 5000 feet 13 NM from SEA VORTAC.  It's not impossible, but slowing to a reasonable and safe speed within the aerodrome at SEA and cutting off 7000 feet seems like some work in a 737 or Airbus.

Keep in mind, CHINS3 is also a STAR used for landing at four airports.  So it serves as a template for pilots and controllers to guide traffic into a region.  Most will be vectored off the STAR at some point and given different altitude restrictions. 

Around here (KPHX), jets are typically at 210 knots descending through 7,000, then told to slow to 170 closing in on 4,000. At 8 miles out they are vectored to final approach course and told to start slowing to approach speed.  If they are flying downwind into KPHX, the altitude is usually 4000 feet for the downwind leg.  You aren't going to slow and descend a jet that fast unless you want a long downwind.   It also seems that most visual approaches in the US are shorter than what a plate will represent.  They will usually turn base around 8-9 NM and around 2000 feet AGL.  I realize that changes depending on location, noise rules, and other factors, but I rarely see a jet carry a downwind and base turn 20 miles or so from an airport.  Weather permitting, tighter patterns are usually flown than what a lot of charts seem to represent.  Again, it depends on other traffic, noise rules, terrain, aircraft limitations, and weather.  STARs and SIDs (DPs), as well as IAPs serve are a guide for IMC weather.  In VMC, ATC and pilots may do completely different things, like vectoring off a STAR or even IAP, including descending below certain published restrictions.  Published restrictions aren't necessarily MEAs or MOCAs.

Edited by Orlaam

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56 minutes ago, Orlaam said:

I don't see a T after 6600 on the latest CHINS3 arrival.  I see an asterisk prior to 6600 (*6600).  The asterisk indicates MOCA, or minimum obstruction clearance altitude.

This is one of the differences between the FAA charts and the Jeppesen charts. Navigraph are Jeppesen.

 

8 hours ago, CajunRon said:

I'm trying to get a better understanding of STAR charts.   I've search on Google but can't find the answer.

Here are links to both the Jeppesen and FAA guides:

 

https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/aero_guide/#TPP_Symbols

http://ww1.jeppesen.com/documents/aviation/business/ifr-paper-services/glossary-legends.pdf

Edit: SID and STAR legend starts on page 65 of the Jeppesen PDF.

Edited by BrianW

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5 hours ago, BrianW said:

This is one of the differences between the FAA charts and the Jeppesen charts. Navigraph are Jeppesen.

 

Here are links to both the Jeppesen and FAA guides:

 

https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/aero_guide/#TPP_Symbols

http://ww1.jeppesen.com/documents/aviation/business/ifr-paper-services/glossary-legends.pdf

Edit: SID and STAR legend starts on page 65 of the Jeppesen PDF.

Oh I see.  I don't like Jeppesen charts.  I know, I'm weird.  Just very used to the free NACO charts, operative word, free lol

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