Author Topic: 25 Dec: Skywest #3567 received terrain warning on approach to Medford Airport  (Read 6221 times)

Offline GeoffSM1

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I am grateful to kyle0440 for bringing this incident to my attention.
My audio clip also includes relevant Tower and Ground feeds.


https://www.reddit.com/r/flying/comments/7qp7ta/skywest_flight_receives_terrain_warning_after/

Quite apart from who was at fault in this instance, does the divergence of opinions expressed in the comments in the reddit.com thread suggest something seriously needs attention in such situations to reduce the scope for confusion? Just wondering.

« Last Edit: January 17, 2018, 04:55:58 PM by GeoffSM1 »



Offline kb4tez

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this is an awesome post, and reading thru the Reddit comments, I'm not sure if I learned something, or am more confused.
very eye opening. (moral of the story, trust but verify)

Offline Rick108

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Yeah, this one got me thinking, too.  If I were told to cross a fix "at or above 7800", I'm cleared to descend to 7800' - even if that leg of the approach is charted at something higher. This happens all the time. Controllers have MVA charts that tell them how low they can vector a flight in a given area, and those numbers are usually lower then the published minimums on the charts, so I wouldn't have thought twice about it. The only explanation I can come up with is that the controller just messed up here.  Yes, maybe the pilot should have questioned it if he was (and should have been) aware of the surrounding terrain. But in my opinion the clearance was just fundamentally flawed.  Yeah, trust but verify - especially when the descend-to altitude is below the published numbers for the approach. A nice lesson here - thanks Geoff!

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Offline tyketto

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Yeah, this one got me thinking, too.  If I were told to cross a fix "at or above 7800", I'm cleared to descend to 7800' - even if that leg of the approach is charted at something higher. This happens all the time. Controllers have MVA charts that tell them how low they can vector a flight in a given area, and those numbers are usually lower then the published minimums on the charts, so I wouldn't have thought twice about it. The only explanation I can come up with is that the controller just messed up here.  Yes, maybe the pilot should have questioned it if he was (and should have been) aware of the surrounding terrain. But in my opinion the clearance was just fundamentally flawed.  Yeah, trust but verify - especially when the descend-to altitude is below the published numbers for the approach. A nice lesson here - thanks Geoff!

Not necessarily.. If you are told to cross a fix at or above 7800, that doesn't give you clearance to descend immediately to 7800; the call means exactly what it says: "I don't care when you have to start your descent, but be sure to cross this fix at or above 7800ft. You can cross it at 8500, or even 15000; as long as you cross it at or above 7800ft, you're good. However, if the chart has published minimums for the segment of the chart, and the fix you are told to cross is below the published minimums on the chart, than that would be a problem.

The rest, you are absolutely correct, but it should be stressed that a clearance to cross a fix at a given altitude does not necessarily mean that you are given clearance to descend directly to that altitude, especially if you have traffic along that segment, for example, converging at the same fix.

BL.

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Offline Rick108

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Certainly the "above" part gives me the option to cross higher if I choose to, but that's not the point here.  If I'm trying to get down, I could choose to descend to the lowest altitude allowed, which is 7800 in the example.  Are you saying that I would need to plan my descent to hit my altitude (or above) right at the fix?  In other words, I can't reach 7800 before I get to the fix?

Another question long these lines, what would be the difference between the clearance given, and "descend at pilot's discretion to 7800"? They seem to mean the same thing here - I own all the altitude between where I am and 7800.

Offline tyketto

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Certainly the "above" part gives me the option to cross higher if I choose to, but that's not the point here.  If I'm trying to get down, I could choose to descend to the lowest altitude allowed, which is 7800 in the example.  Are you saying that I would need to plan my descent to hit my altitude (or above) right at the fix?  In other words, I can't reach 7800 before I get to the fix?

Basically "cross <fix> at or above 7800" means this: I don't care what you do, or when you start your descent; you need to cross <fix> at 7800ft or above. If you immediately descend, that's okay. If you take your time, that's okay. To be honest, you could cross that fix at FL210, and would still meet the requirement for what you were told to do.

What it doesn't mean is that you are immediately cleared to descend to that altitude. Take, for example, "reduce your speed to 230kts, then cross <fix> at or above 7800."

You wouldn't be immediately able to descend (which would basically be at pilot's discretion) until the speed restriction is met.

Quote
Another question long these lines, what would be the difference between the clearance given, and "descend at pilot's discretion to 7800"? They seem to mean the same thing here - I own all the altitude between where I am and 7800.

Are you on a STAR? If so, what "cross <fix> at/maintain" would be primarily used for is to meet a crossing restriction as depicted on a STAR. Take for example, the KEPEC6 or the CLARR3 arrivals into KLAS:

http://155.178.201.160/d-tpp/1801/00662KEPEC.PDF
http://155.178.201.160/d-tpp/1801/00662CLARR.PDF

Both have the 13000ft and 250K crossing restriction at CLARR. And seeing that you'd be joining the STAR anywhere from 60 to 120nm away, you definitely would not want to be immediately descending to 13000, especially if it may take you below the MVA/MOA for the segment in question.

If not on a STAR and receiving vectors, "cross <fix> at/maintain" wouldn't be a good call to give, because you'd either have to be properly equipped to know where <fix> is, or check if it is a non-RNAV fix, meaning that it should be defined by a radial from a given VOR and DME from that VOR. Regardless, you "should" be on a published chart to be given that call to cross a given fix.

BL.

Online wiedehopf

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tyketto they were cleared to cross cegan at or above 7800
quite a bit after that comes the mountain with 7300 feet elevation.

so your logic does not apply here.

Offline tyketto

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tyketto they were cleared to cross cegan at or above 7800
quite a bit after that comes the mountain with 7300 feet elevation.

so your logic does not apply here.

Then something else is completely off, because they would have to be on a published IAP or STAR or airway to get the call for such a crossing restriction. CEGAN is on V122, which has an MEA of 8500ft westbound and 9000ft eastbound, with an MVA of 7900 in the area. The call in this case to cross AoA 7800 would be questionable, because of the MVA in the area. You should not receive a crossing restriction that would be below the MVA. That's asking for trouble. However, with "at OR ABOVE", the call is still valid. I would have questioned the altitude, especially with the MVAs in the area.

The mountain in question is southwest of CEGAN, roughly on the OED R-141, at 7265ft. The problem here is that after crossing CEGAN, if they were truly at 7800ft, they were below the MVA for the area. The question here is what was the call AFTER they crossed CEGAN.

Hence why I said that while given a "cross <fix> at/maintain or at/above <altitude>" doesn't necessarily mean you should immediately descend. You can remain at your given altitude for as long as you want as long as you cross that fix at the restrictions given.

BL.



Online wiedehopf

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tyketto
please just click the reddit thread linked in the first post.

there is atc transcript and the approach plate :)

so yes technically the controller might have done nothing wrong i agree.
still bad form at least.

pilots should have followed the procedure and told ATC they were doing exactly that.
or at least check the topographical map included in the approach plate ... and then they would've stuck to the procedure anyway.

oh well GPWS saved the day wooohooo (i wonder if they would have cleared the mountain top without cold temperature correction on that day)

Offline tyketto

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tyketto
please just click the reddit thread linked in the first post.

there is atc transcript and the approach plate :)

so yes technically the controller might have done nothing wrong i agree.
still bad form at least.

pilots should have followed the procedure and told ATC they were doing exactly that.
or at least check the topographical map included in the approach plate ... and then they would've stuck to the procedure anyway.

oh well GPWS saved the day wooohooo (i wonder if they would have cleared the mountain top without cold temperature correction on that day)

This is the IAP I was mentioning, which is how they came to the "cross CEGAN AoA 7800" call. Additionally, the chart now shows the tale of the tape, especially as to why you don't immediately descend to the altitude given with the "cross <fix> at/AoA <altitude>". If the pilot was immediately down to that altitude, they not only descended below the MVA for the area, but were below the minimums for the approach. If you look at the VOR/DME-C:

http://155.178.201.160/d-tpp/1801/00251VDC.PDF

Forget the topographical portion of the plate; look at the descent profile. They were to cross BRKET AoA 10000, SERTE AoA 8500. then further down. The problem here is if given "Cross CEGAN AoA 7800", not only were they below the MVA for the area as the MVA in the area was 7900, they were below the MDA for the segment of the approach. in fact, they wouldn't have been above the MDA until they crossed SERTE.

This is a perfectly good example as to why you wouldn't immediately descend to cross a fix at the lowest altitude given by the controller, as not only would you still have to be aware of the MVAs and MSAs in the area, but if you're below the MDAs of the IAP being used, you're going to have a problem.

In this case, this would be pilot error, as they did not adhere to the descent profile of the chart. The controller would essentially be in the clear, as they did include "at OR ABOVE".

BL.

Online wiedehopf

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for the interested another few opinions on the matter:
https://www.pprune.org/rumours-news/604467-near-cfit-medford-oregon.html

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