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Good day for GPWS

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Good day for GPWS

Old 01-20-2018, 05:44 PM
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Default Good day for GPWS

Incident: Skywest CRJ9 at Medford on Dec 24th 2017, GPWS alert on approach
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Old 01-21-2018, 07:55 AM
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This goes back to the pre-TEM days of swiss cheese layers.

They punched thru 3 layers of cheese and they just missed the hole in the 4th layer. Good thing the GPWS was there as a 4th layer.

The 5th unintended cheese layer of 7800 feet may have saved them by a few hundred feet, but maybe not at at 6 degrees celsius?

Big fat Awe-Crap on ATC for a LET THE LAWYERS SORT IT OUT clearance. I'm not a controller, so I will defer judgement.

Aw-crap on the crew for taking the ambiguous ATC clearance bait.

Big fat Atta-boy to the crew for following the GPWS. Many FOQUA events reflect crews disregarding GPWS or being dangerously slow to react.
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Old 01-21-2018, 08:24 AM
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Originally Posted by Mercyful Fate View Post
Is the GPWS something that is always required to be operational for a 121 operated flight?
No, but IIRC it's a 1 or 3 day MEL.
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Old 01-21-2018, 08:35 AM
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Not that I have a dog in this particular fight, but I am seeing a trend with ATC here. I have been getting cleared for approaches into nontowered airports where they “allow” me to descend below a published segment. I say allow because those clearances are extremely misleading. A simple, cleared at or above “fix minimums” cleared “approach “ would suffice. Not clear us for anything lower than the segment altitude.

Just my .02
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Old 01-21-2018, 08:45 AM
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In US domestic ops, RJ drivers get used to quickly dancing to arbitrary and constantly changing ATC instructions in large terminal areas. If you fail to respond instantly, you'll likely create a conflict.

This leads to the mindset of comply now, think about it later. In this case the crew should have at least double checked what they were doing after they initiated the descent.

But these guys have been trained by the system to react quickly... foreign aircraft are handled with kid gloves, and the locals are assumed to be familiar with the area, experienced, with a good command of english so they get used and abused when ATC needs buffers and adjustments.

They are also very accustomed to getting vectored in below published altitudes... even on complex non-precision approaches in the mountains. Most of them are probably not aware that a "cross at or above" clearance could legally be issued with an altitude below safe altitudes on the approach. Most would tell you (based on common practice) that you can immediately descend to the at or above altitude.

What the controller did may or may not have been legal, but it sure wasn't consistent with what his buddies do in practice
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Old 01-22-2018, 09:51 AM
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Default SKW CRJ has near CFIT:

Incident: Skywest CRJ9 at Medford on Dec 24th 2017, GPWS alert on approach
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Old 01-22-2018, 09:53 AM
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Old 01-22-2018, 06:28 PM
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Default GPWS saves the day

A Skywest CRJ9 was cleared to "at or above 7800'" on an approach that specified an altitude of 10000'. Down they went to 7800, but the GPWS prevented collision with the mountains.

Any thoughts on the appropriateness of the ATC clearance?


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Old 01-22-2018, 08:09 PM
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Several things spring to mind.

The ATC clearance to cross CEGAN at or above 7,800' implies that the crew is safe all the way down to 7,800.' It appears that ATC took the position that the procedure required 10,000 at CEGAN, and therefore a clearance to cross at or above 7,800 didn't prevent them from following the published numbers. This is a very dangerous slippery slope, and I don't buy it.

What if ATC had said "at or above 1,000?" Clearly it's not possible, but it implies that the crew may commence the procedure at the ATC altitude. ATC frequently provides vectors at altitudes lower than procedure altitudes, such as vectors to a localizer or ILS. The approach guidance with a glideslope is nearly always intercepted from below.

In this case, I'd be leary of beginning the approach at CEGAN at anything less than 10,000. That said, one might look at the arc and the high points along the arc to note that The highest point to be encounered is joining the inbound course from the arc, and that altitude is 7655. One might be forgiven for hearing the altitude restriction, glancing at the chart, and believing that based on the presented data, the clearance and arc imply guidance safe from terrain.

At the same time, neither crewmember had flown this procedure or routing before?

While the arc is beyond the 25 nm MSA radius, the radius for that area varies between 8,800 and 10,700; a crucial clue that 7,800 may be unwise, despite the potentially disasterous clearance.

Another important clue is that BRKET (27 DME OED) is published at 10,000, the next fix inbound is SERTE (21 DME OED) at 8,500. Clearly at that point in the approach one is still restricted well above 7,800, and a review of the procedure and briefing of the procedure should be enough to convince to fly the published altitudes.

Another observation is that both crewmembers heard the clearance and both agreed to the decision to descend below published altitudes, apparently. The narrative doesn't suggest one crewmember voicing an objection.

This reminds me somewhat of the B757 mishap in Colombia years ago, American 965, in which the crew was cleared via an NDB, but entered the wrong one. Both crewmembers bought into the decision, and ultimately they did get a GPWS alert prior to impact.

If this procedure was database selectable, were they not seeing the segment altitudes in more than one place; procedure charts or displays as well as on the FMS/FMC?
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Old 01-23-2018, 05:44 AM
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Holy crap....
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