3 ms·

In general I agree with this. (Without respect to this particular crash.) If GPS signal is sufficiently degraded or lost, TAWS is probably unavailable (highly

by cmurf 2mo ago

In general I agree with this. (Without respect to this particular crash.) If GPS signal is sufficiently degraded or lost, TAWS is probably unavailable (highly dependent on what navigation system is on board). TAWS might have saved them. But needing TAWS implies loss of situational awareness. This flight crashed in the mountains ~6 miles north of the ILS 24 approach course. At ~12 miles out on the approach course, the ILS course width is ~1 mile. A full needle deflection translates to 1/2 mile off the course. A flight is not established on the approach course with a full needle deflection. Course intercept with localizer or VOR is basic instrument work - GPS not required. The flight requested ILS 24, later they indicated airport in sight, requested and received a clearance for a visual approach. That means pilots have full responsibility for terrain and obstruction clearance. The visual approach requirement is to maintain contact with with airport. It seems to me they went off course, by a lot, for reasons I'm not clear on, and then descended prematurely into hostile terrain at night, which is likely why they lost visual contact with the airport. The terrain blocked their view and by the time they realized they were descending into hostile terrain they crashed. If that's what happened, GPS signal loss is perhaps an instigating source of distraction and confusion but I do not see how it's a primary factor. If the signal becomes degraded, navigation should indicate it (auditory and visual) and that source is ignored entirely. Falling back to ground based navigation should be familiar, it doesn't need to be as precise as GPS, the instrument approach procedure has terrain clearance baked into it. Best practice is to follow the instrument approach procedure to the final approach fix, then do the visual approach - especially at night which many have argued constitutes instrument flight conditions - mountains will be invisible. Pitch black on black.

cmurf 2mo ago
Ok after rereading the preliminary report, ATC gave them a vector north and clearance to a named fix. They were not off course. While on a vector clearance, terrain and obstacle clearance is ATC's responsibility. Once the accident flight requested and received a visual approach clearance ~30 miles out, terrain and obstacle avoidance becomes pilot responsibility. It seems likely to me that whether GPS was working or not, whether RNAV 24 or ILS 24, they'd have requested a visual approach. And the pilots did not adequately account for the terrain between the flight and the airport when they initiated the descent, and descended below minimum safe IFR altitude. Further due to this flight operating under 14 CFR 135, there's a regulatory bridge to bind the flight to the (Part 135) Certificate Holder's operation specifications (OpSpecs). Pilots cannot operate outside of OpSpecs. It's possible the medivac company's OpSpecs require mandatory direct visual contact with terrain and obstacles - something that could not be achieved with 0% illumination due to no moon. Some OpSpecs will prohibit visual approaches at night to non-towered airports. SRR airport has no tower. I'm not familiar with this operator's OpSpecs.

by cmurf 2mo ago

cmurf 2mo ago

Ok after rereading the preliminary report, ATC gave them a vector north and clearance to a named fix. They were not off course. While on a vector clearance, terrain and obstacle clearance is ATC's responsibility. Once the accident flight requested and received a visual approach clearance ~30 miles out, terrain and obstacle avoidance becomes pilot responsibility. It seems likely to me that whether GPS was working or not, whether RNAV 24 or ILS 24, they'd have requested a visual approach. And the pilots did not adequately account for the terrain between the flight and the airport when they initiated the descent, and descended below minimum safe IFR altitude. Further due to this flight operating under 14 CFR 135, there's a regulatory bridge to bind the flight to the (Part 135) Certificate Holder's operation specifications (OpSpecs). Pilots cannot operate outside of OpSpecs. It's possible the medivac company's OpSpecs require mandatory direct visual contact with terrain and obstacles - something that could not be achieved with 0% illumination due to no moon. Some OpSpecs will prohibit visual approaches at night to non-towered airports. SRR airport has no tower. I'm not familiar with this operator's OpSpecs.