Merged: APNG Twin Otter Missing

[gulliBell]

Chimbu chukles, regarding:

"...In the mountains GPS is of very limited value (in fact it is near useless) and I point blank, absolutely refuse to believe this crew was fcking around with TAWs/EGPWS..."

Can I be brave enough to suggest otherwise?

I think they must have been, and if they weren't they should have been, and here's why. If I recall correctly, and from the outset I stand to be corrected here, the accident aircraft was fitted with a Garmin GNS530 equipped with the TAWS option. [If eyewitness accounts as reported by the Australian media are correct] the aircraft was in cloud when it hit the mountain, or at least until moments before it hit the mountain at which point impact was unavoidable. If you find yourself inadvertently in cloud in the mountains, or even if visual but with deteriorating weather and IMC is likely, I am quite sure any crew who had TAWS available would be keeping an eagle eye on its indications. Quite possibly TAWS is the best, maybe only, thing going for you in this situation once the opportunity to turn around has passed.

The crew knew the mountain was where it was because, [according to the operator], they had flown the route many times before. This leads to the obvious conclusion that they were not exactly sure where they were in relation to the mountain [because they flew into it]. We [might assume] they couldn't determine their position visually because they were in cloud. This means they must have been entirely reliant on GPS indications to determine where they were and what was around them. In other words, in this case the GPS information was extremely valuable because that's all that had, but was it reliable? (I think at the time of the accident RAIM was available in the area, however in very heavy rain and moutainous terrain I have seen degraded GPS position information). So the GPS and the crews interpretation of the GPS information will be absolutely critical to the investigation.

There's no "fcking around" as such with operating TAWS and the GNS530, it takes only a few seconds to call up the TAWS page from whatever page you might be on at the time. And in any event, even if you don't have the TAWS page up you still get the TAWS pop-up and aural alerts whatever other page you have displayed.

(For those who are unfamiliar with Terrain Avoidance Warning System, the TAWS page provides a graphical presentation on the GPS display of the terrain around you relative to your present height. Simply: black being space where there is no danger of impact, yellow being space where you might hit something, and red being space you will hit something).

TAWS can be disabled on the GNS530, but not completely disabled (the Premature Descent Alert and Forward Looking Terrain Avoidance alerts can be disabled, but not the Excessive Descent Rate Alert). However EDR alert only triggers if the closure rate with terrain is determined about 1000 fpm or more, meaning you could be climbing, but not out climbing rising terrain, and if the closure rate was less than 1000 fpm, theoretically you might not get a TAWS aural warning before impacting terrain. Again, I stand to be corrected here.

TAWS warnings and graphical presentations vary depending on the phase of flight, whether enroute, terminal, approach or departure. We don't know what the TAWS Phase of Flight was just prior to impact, it was most likely to be in enroute phase because the required criteria for the other phases was likely not present. In enroute phase the minimum clearance value for alerts is higher, meaning for a giving situation, you could get a TAWS alert if you were in enroute phase, but not in terminal or the other phases. (The TAWS logic and when warnings do and don't get triggered is really only fully understood by those people who wrote the algorithms, it's not something a pilot could ever hope to fully understand).

My experince with the GNS530 is you can get lots of eronious terrain warnings with TAWS either on or off, and very very rarely you don't get any warning when there is a clear collision risk ahead.

It is quite likely the crew had TAWS disabled whilst in the mountains to minimize annoying false warnings, but the TAWS page still shows surrounding terrain, and you still get EDR alerts if you're not on the TAWS page. So fair chance if the crew didn't have the TAWS page up they were getting aural and visual terrain warnings even if TAWS was disabled. However, if TAWS was disabled and they were on the TAWS page, as far as I know (perhaps??) you don't get the aural warning.

I think the key to finding out what went wrong here is in the GPS, and understanding the human/GPS interface factors. Another possibility is of course they were in cloud trying to get up the valley towards Kokoda gap [we now know they weren't, they were to the north of Kokoda slightly right of the extended runway centreline], relying on the TAWS graphical presentation for terrain avoidance, but they had an engine failure and from which point their fate was unavoidable (not being able to turn around in the space available, and not being able to out climb the rising terrain). The other possibility is degraded GPS position information due to the heavy rain, and they were half a mile away from where the GPS said they were.

In summary, TAWS is great. If you've left the decision to turn around too late or you find yourself in cloud in mountains, only TAWS, and luck, is all you have going for you to save your life. What is not great about TAWS is some might rely on it to do stuff they wouldn't be doing without it. Don't lose sight of the caveat that TAWS should only be used as an aid to visual acquisition - do not use terrain information to navigate or maneuver to avoid terrain. Unless you are inadvertently in cloud in mountains then you have no other choice.

Let's hope the GPS data can be recovered. Also, the CVR is going to be helpful to unravel what the crew intentions were [if it had one installed, not that a CVR was required for the class of operation]. Whatever happened, and why, whatever the investigation finds, nothing will change the fact this is a very tragic event.