Wednesday, December 28, 2011

This continues from my last two (non-reindeer) posts, so read down to catch up.

I got a whole whack of information in this post from AvCanada, specifically the accident speculation thread. Stuff in quotes is taken verbatim from the discussion thread.

This is the ILS approach chart for Runway 35T into Resolute Bay. Do NOT use this to navigate, it's out-of-date now.

This is what the ILS into Resolute Bay looks like. The ceiling in this approach was 1,300':

On August 20th 2011, First Air flight 6560 was a 737 travelling from Yellowknife to Resolute Bay with 15 people on board, including four crew members. They called about 4 miles final for the runway, and then crashed a few minutes later. The aircraft appears to have been under control, and the surviving passengers reported nothing unusual right up to the point of impact.

The accident report hasn't come out yet, and the following stuff is entirely speculation - I want to make that clear. But I'm going to speculate.

METAR CYRB 202000Z 18009KT 8SM VCFG SCT003 OVC005 07/07 A2986 RMK

The weather at the time of the accident was foggy with low (300') scattered and (500') overcast layers. People on the ground couldn't see much at all, but flight visibility can be very different from ground visibility.

We do know some things: The crew was still in control on three mile final, the engines were running, the aircraft is oriented parallel to the runway and level with the horizon, and the wreckage is strewn across a large area. If it had stalled or a mechanical failure had caused it to rapidly descend it would leave a much smaller impact area. That has all the hallmarks of classic CFIT, or 'controlled flight into terrain'.

Here's a Google earth view of the area around the accident. The wreckage trail is represented by the yellow line. The gully leading down from the nose of the aircraft at the end of the wreckage appears to contain the same washed out sort of area in google and the photo. The two red circles outline similar geometric forms. One is the approach end of 35T. The other is a road structure on a slightly sloping flat area elevated above the airport by about 325 feet and one mile to the right of the centerline of the runway. The lake to the southeast of the wreckage trail could appear in certain cloud arrangements to be similar to the shoreline on the approach to 35T.

The aircraft would have been between two layers of clouds with approximately 200 feet between layers, with the bottom scattered layer at 475 ASL and the top broken layer at 675 ASL. Some of what was a scattered layer from the weather observation point may have been broken in the hill above.

Now here's the thing: Even with the local terrain being a bit confusing, there should have been no problem finding the runway and landing with any normal kind of ILS approach, with a 500 foot ceiling. ILS tolerances are pretty tight, and it would be really hard to conceive why the aircraft would have been a mile to the right of its course, and more or less on runway heading without any abnormal indications to the crew.

Let's add some more information: here's another picture of the accident site, this time with the Resolute VOR location plotted. This one chills my bones.

It turns out the ILS was functional on that day, but suppose the crew didn't get a glideslope indication and decided to fly the Localizer only approach. That takes them down to 540' ASL. The ridge they hit was 653' ASL.

The step down approach (localizer) really comes into the equation if the aircraft is mistakenly inbound on the 167T radial from YRB. In that scenario, the crew believes it is tuned to the ILS, can't get glideslope, and switches to a localizer approach with the final drop only 160 feet four miles back. They keep tracking the 167T radial as if it were the localizer. They don't have glideslope because they're tuned to 112.1 instead of 110.3. Their DME is coming off the VOR.

"In the mistuned VOR scenario, they have to have POKAN on the 167 radial at 4 DME, a mile east of proper track. Then do the procedure turn and fly back inbound on the false localizer. Maybe they left the VOR eastbound with 167 already set on the OBS, flew it needle centered to the false POKAN, did the PT and re-intercepted inbound. Get no GS, maybe call it in U/S, and fly the localizer approach instead. That scenario ends exactly where the accident happened.

I can't see a late tuning of the VOR and nobody noticing that the needle moved when the OBS was turned. If they'd been tracking 167 outbound to Pokan, and then set up the inbound course on the CDI, they wouldn't have noticed it so much as they would have been in the PT where they would expect it to be deflected."

An added factor now: Has anyone noticed that there is no missed approach point for the LOC/DME approach on the ILS/DME 35T plate? Not on the DND copies anyway. Take a look.

A second factor: Notice how the VOR isn't even depicted on the ILS chart? Why on earth would the crew have it tuned in? Well, a couple of possibilities exist. There was a temporary military control tower at Resolute that day, coordinating aircraft that were participating in a mock search-and-rescue exercise. The control tower was asking other aircraft for their radial and distance to the airport, and the accident aircraft had reported that information to the control tower fairly late in their approach. In order to report that information they would have had to tune in the VOR. The other possibility is that maybe they were using the VOR to navigate to the airport before commencing the approach.

"Another Canadian carrier used to have an unofficial procedure on the 737-200. If you wanted to retain the DME display while doing an ILS, you would tune #1 to the ILS and #2 to the VOR. Then you would transfer the display [overhead switch] to "both on 1" Now, both pilots would have their HSI displaying the info from the #1 radio, and the dme would readout from the vor still tuned on the #2 radio. This practice was banned after a crew mistakenly switched "both on 2" during an approach to Prince George. Thinking they were tracking the LOC, the aircraft descended towards the YXS VOR and very nearly had an accident. [With both radios tuned to the ILS you would not have a DME readout and there is no DME hold switch on the 200] The practice of transferring the display was then banned and to be used only in case of radio failure. Keep in mind that was another air carrier, not First Air."

Here's a pic of the panel from the actual accident aircraft.

Hmm, same switches.

But wouldn't their GPWS (Ground Proximity Warning System) have saved them? The 737 in question was equipped with an older model, which basically gives no warnings once the landing gear is down. GPWS will only give you two calls "500'" and "Sink Rate", whereas the newer Enhanced GPWS will give 1000', 500' 100', 50', 40', 30', 20', and 10' above ground calls. This late in the approach, the gear would have been down.

So here's my speculation: With the knowledge that the military was asking for radials/DME bearings prior to the crash, maybe the VOR was tuned in, thinking they had the ILS frequency up. Now to those of you who fly IFR, how often have you made a late change in the approach? It happens to me once in a while. If you thought you had the ILS tuned in and once on the approach you had a G/S flag, it would be a fast and easy brief to re-brief for the LOC only approach and continue to the higher minumums (especially in an environment such as the Arctic airports)...if that was the case here, the LOC only minimums still would have put them into a hill if they were tracking the VOR.

One final pic:

This was taken on the LOC DME BC approach to rwy 17T (the accident runway, just landing in the opposite direction) in 2009. The distance to the rwy threshold is less than 1.5NM. Can you spot the runway? Imagine looking for that in low cloud and fog, let alone adding being on the wrong approach frequency.

Lots of links in the accident chain on this one, and again the final report hasn't come out yet - but if it went even remotely close to how I think it went, you can see the tragedy that resulted from a bunch of different factors that added up all at once.


Paul Tomblin said...

You know, my instrument instructor always had me do a couple of things that I'm not sure if they're safe any more.

1) He had me turn the OBS to the inbound course on a LOC or ILS approach just as a visual reminder of what course you're flying. Maybe if I turned it 90 deg off, it would be more obvious if you were accidentally turned to the VOR instead of the ILS.

2) He always wanted me to be ready to switch from a ILS to a LOC approach if I didn't receive the glide slope or if the glide slope failed. For single pilot IFR, it seemed to me at the time and it still seems to me now that in a case like that it would be better to go missed, take stock, and do the LOC approach from start to finish instead of trying to salvage something half way down.

Sulako said...

I agree with point one completely - on an ILS the OBS should be set to the inbound course, especially so if you are using an HSI.

The second part - going missed if the G/S fails - is a little more difficult. It really depends on a number of factors, like where you are in the approach, but also a few other things, which I'll quickly touch on. If you fly for a living, you are usually flying equipment that costs a fair amount of money to keep in the air - our cost to operate our baby jet is somewhere in the neighborhood of 25 bucks a minute, and that's just for a baby jet - my guess is the DOC on an old 737 is closer to double that - also keep in mind that transport jets burn truly staggering amounts of fuel down low and especially on a missed approach. They were up in Resolute and alternates are few and far between, so going missed might make the difference between having enough fuel to safely make the approach and having to immediately divert to your alternate because you are at bingo fuel. Also, if you go missed approach, you are adding more flight time (and more cost) to the trip, which is something the boss usually frowns upon. It's not hard to rebrief an ILS into a LOC only approach, it would only take a minute or two. I see your point, but I also understand the pressures that working pilots face.

david said...

"Blood chilling" is exactly -- literally -- the best description of this scenario. My whole body felt cold reading it.

I could easily picture myself, fatigued, stressed, and mildly hypoxic at the end of of a long IFR flight, ending up in the same situation in my little Warrior.

I have no regrets now for buying my Garmin 696 last year (which gives me ground warnings based on the elevation data and GPS position). So far, it mainly warns me that the CN Tower is close to Toronto Island Airport (surprise!), but it only has to save me from lost situational awareness once to prove its worth.

Ted said...

I saw the runway in your picture because of 2 things: i knew from the ils plate there would be buildings to the right of the rw (easier to spot a blue hangar than a gravel runway), and I assumed you were lined up for landing, thus the runway being straight ahead; then i saw the vasi to the left of the runway and at the very end the runway itself. had you been flying on a different course or in poor visibility, i doubt i could see it (i admit, i don't have the practice a pilot has)

Bobermo said...
This comment has been removed by the author.
Bobermo said...

First of all, really awesome blog! Discovered it today, so quite some reading left;)

As a pilot who got his IR 5 months ago, this post is really "bone-chilling"! In particular the Google Earth picture which shows the VOR position, and the crash site exactly on radial 167..

Kudos to you for not hiding "the Teterboro mistake", but instead to make a blogpost about it.

Will learn a lot of this blog, thank you!

Sarah said...

There's a minor update released Jan. 5th. They're still looking at the navigation data. I think you nailed it with the "Both on 1 / Both on 2" switch snafu - they had to be mistaking the VOR for the Localizer.

So close..

"The crew initiated a go-around 2 seconds before impact. At this time, the flaps were set to position 40, the landing gear was down and locked, the speed was 157 knots and the final landing checklist was complete."

Anonymous said...

So, in your honest opinion, do you feel that this incident could very well have been avoided had there been a more current/up to date GPWS unit in the aircraft?

Anonymous said...

I did not unferstand anything! :O